Midwest States Pooled Fund Program Consulting Quarterly Summary

Midwest Roadside Safety Facility

01-01-2012 to 04-01-2012


Mitchell Interchange Standpipe Protection

Question
State: WI
Date: 01-02-2012

Dear MwRSF,

Below is an email from one of our major project teams who are building some tunnels as we speak. They are having an issue with the stand pipes. Do you have any suggestions


_____________________________________________

Subject: Mitchell Interchange Standpipe Protection



As discussed last week, the Mitchell Interchange Construction team is looking at options to enhance standpipe protection within Tunnel construction at the request of the City of Milwaukee Fire Department. MFD is requesting "the maximum, reasonable,
protection and/or isolation of the standpipe outlets from vehicular damage".

Currently the standpipe system has been protected with 7" (+/-) lateral offset to connection fittings in Tunnel 3. Tunnel 1 and 2 will have nearer to 10" of lateral offset to standpipe that is located about 18" above the top of barrier. Our construction team has dismissed the potential to change to vertical face barrier due to potential increased damage to vehicles involved in a crash.


Construction team has identified that we have met NFPA requirements:

9.3.3 " Fire department connections shall be protected from vehicular damage by means of bollards or other approved barriers.

9.4.3 " Hose connections shall be located so that they are conspicuous and convenient but still reasonably protected from damage by errant vehicles or vandals


Team has discussed locating bollard, or some physical protection on top of the barrier at the same offset as the standpipe fittings. You offered some hesitation with that alternative in our phone call last week, and also potential to look at other alternatives. Can you look into, and get me an assessment on physical protection alternatives " bollard and other?


Appreciate your help in advance.



Attachment: http://mwrsf-qa.unl.edu/attachments/b356354daed958a76c6523a410167e27.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/759ee16d7de6a00bd4351160d852d918.jpg


Response
Date: 01-09-2012

We are unaware of any special protection barrier systems that have been designed and tested for use in shielding water values which extend off of the tunnel side walls and above the vehicular barriers. However, if safety treatment is desired, it would seem possible to design steel or reinforced concrete structures which anchor to the top of the concrete parapet and possibly to tunnel wall in order to prevent vehicle snag, and even occupant snag, on the pipe hardware.

If this barrier option is considered, then the upstream and downstream ends should be sloped to mitigate snag concerns as the additional protective barrier which falls within the zone of intrusion. As noted above, these systems could be attached to barrier or tunnel.

Alternatively, it may be reasonable to consider design changes to the pipe system, such as to recess more the structure (i.e., 2 outlets and handle for valve) within the tunnel side walls, thus greatly reducing concerns for vehicle snag, and even occupant snag, on the pipe structure.

Please let me know if you have any further questions or comments regarding the information provided above.



Response
Date: 01-11-2012

After looking at the standpipe issues further, we are concerned that the standpipe can be impacted based on the ZOI for the barrier in questions. In order to address this, I came up with the concept attached. It consists of telescoping tubes that shield the stand pipe. Access to the stand pipe would only require removing the drop pins and sliding the middle tube into the larger side tubes.

This design should protect the standpipe from interaction with impacting vehicles. I have not fleshed out the anchorage details yet, but I wanted to get some feedback from you on the concept.


Thanks



Attachment: http://mwrsf-qa.unl.edu/attachments/7257adccfdfbdec1fd4fd4865196d0ed.PDF


Response
Date: 02-24-2012

The fire department didn't like the standpipe design. Construction team is asking if we could use the top portion of our combination rail design. I've attached a link to the railing details.

<http://on.dot.wi.gov/dtid_bos/extranet/structures/LRFD/standards/3005.pdf>




Response
Date: 03-05-2012

I believe that the railing design shown can prevent interaction with the stand pipe as desired. However, we have seen potential for horizontal railings to promote vehicle instability when mounted on single slope barriers. In previous pedestrian rail testing on a 32" single slope, we found that the horizontal railings provided vertical restrain on the front corner of the vehicle which caused it to roll towards to the barrier and become unstable enough to rollover. Please refer to the attached report.

We would have concerns for the railing shown potentially affecting vehicle stability. I don't recall the speeds in this areas. Obviously, if the speed were limited to TL-2 type speeds, then the concern becomes much less.

This was the rationale behind the design we sent previously having a solid front face.



Short radius

Question
State: NE
Date: 12-28-2011

Q about the w-beam perpendicular to the lanes of traffic beyond the short radius.

Should the CRT posts be placed on the portion of the guardrail which is perpendicular to the traveled way within the clear zone?

Or can these be full strength posts?



Response
Date: 01-04-2012

The NDOR variation of the short radius guardrail system appears to be somewhat based on the original Yuma County (YC) curved barrier system that was crash tested and evaluated by SwRI researchers. In terms of CRT posts, it would be appropriate to carry out the CRT posts along the secondary length at least as far as utilized in the original YC crash testing program.

More recently, TTI researchers conducted an LS-DYNA analysis and investigation of a modified YC system according to the NCHRP Report No. 350 TL-2 impact conditions. Following this effort, the modified YC system was submitted to and accepted by FHWA. Copies of the acceptance letter and research report are attached.

At this time, neither the YC or modified YC designs have been adapted to 31" tall W-beam guardrail systems nor accepted by FHWA.


Attachment: http://mwrsf-qa.unl.edu/attachments/9f9c7edd970a68761aa2e153c5d7ba62.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/9f9c7edd970a68761aa2e153c5d7ba62.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/9f9c7edd970a68761aa2e153c5d7ba62.pdf


MGS Steep Slope

Question
State: IA
Date: 01-11-2012

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail? Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?


Thanks
Attachment: http://mwrsf-qa.unl.edu/attachments/c3d06842442ba2752ba64f14344c3342.pdf


Response
Date: 01-11-2012

See my comments below!


Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.





Response
Date: 01-12-2012

See my comments in blue below!

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT. Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that. Would you be able to provide a ballpark estimate of how much time such an analysis might take?

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out. We should modify our standard to show the curb ending under the thrie beam. Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements. When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches. Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...




Response
Date: 01-13-2012

See comment's below in green.

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT. Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that. Would you be able to provide a ballpark estimate of how much time such an analysis might take?

**I suspect 1-2 days would be adequate to acquire and review prior bogie testing data and perform simple hand calculations. However, if we cannot find sufficient information and results from prior bogie tests, then a bogie testing program would be needed. At this point, staff could look into this issue later this month.

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out. We should modify our standard to show the curb ending under the thrie beam. Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements. When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches. Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...

**Small car wedging started to occur with 1100C vehicle on stiffness transition project. However, the test results were satisfactory. Now, if we add a lower curb, it is our opinion that performance could be potentially degraded as wedging and snag could be accentuated. Also, the 2270P vehicle would contact MGS slightly higher while reaching stiffer region. We believe 2 tests would be needed to evaluate curb in advance of asymmetrical part. Also, it would be beneficial to test Iowa transition near bridge end but without curb. Future research would be extremely helpful to investigate whether or not these potential concerns are real.



Strength Properties Of Guardrail Posts

Question
State: WA
Date: 01-17-2012

We've been asked if we could provide a material strength comparison between steel and wood posts. This is about the material properties rather than the guardrail system performance. Dave and I looked at this briefly in a few resources. One of these we looked at particularly was the "Task Force 13 Standardized Hardware Guide" which lists inertial properties of these post materials and also a stress grade for the wooden posts (see below links).

https://www.aashtotf13.org/Files/Drawings/pwe01-04.pdf

https://www.aashtotf13.org/Files/Drawings/pde01-08.pdf

We are wondering how we can formulate a meaningful comparison between these post material types and their associated performance. Is there anything in your past work with pendulum testing etc. that would help simplify a response to this question? Also, please share any studies/reports you may know of.

We predominately use 6 x 8 Douglas fir Grade No. 1 or 6 x 8 Hem Fir Select Structural grade as a comparable post to the W 6 x 9.

Thanks for any help.



Response
Date: 02-02-2012
I have some comments and information regarding wood and steel posts with respect to the MGS below. Hopefully it will help you with your decision process.

As with all strong-post W-beam guardrail systems, the MGS system dissipates energy through the deflection and deformation of the rail and the rotation of the posts in the soil. If the posts have do not rotate in the soil and absorb energy, the bulk of the impacting vehicle's energy will be absorbed by the W-beam element, thus increasing the tensile force in the rail. If the force increases beyond the capacity of the rail, it will fail, allowing the impacting vehicle to pass through. Therefore, the posts must have sufficient structural capacity to displace founding soils and absorb energy. Wood and steel posts can both serve this function, but they do have inherent differences.

Numerous bogie tests have been conducted on steel, rectangular wood, and round wood guardrail posts in both soil and a cantilever sleeve. In addition, many full-scale tests have been conducted with both types of posts using standard W-beam and the MGS. I have attached a thesis done in the past here at MwRSF that has a pretty complete literature search on steel and post testing up until 2005 for your reference. The general trend was that the two types of posts behaved very similarly, with some tests suggesting steel posts were better and others suggesting wood posts were better. So previous research suggests only minimal performance differences.

That said, the section and material for the steel and wood posts create distinct differences that should not be ignored. W6x8.5 steel posts have very distinct strong and weak axis bending capacities due to the "I" shape design of the section. However, steel posts do not fracture and tend to bend and absorb energy when impacted in either the strong or weak axis if the surrounding soil is sufficiently strong. Wood posts tend to generate slightly higher soil rotation forces. Wood posts also tend to fracture if the soil resistive forces exceed the capacity of the post. Wood posts also has a higher degree of material variability due to splits, checks, knots, etc...

So how do these differences translate to their performance in guardrails system? That takes further explanation. First, there is a difference in wood post and steel post behavior in the weak axis. In the case of the steel post, the weak axis impact would tend to have some limited rotation in the soil and then yield and bend the post. In a wood post weak axis impact, the post would also rotate in the soil to some degree and then would tend to fracture. Looking at the post capacities, the wood post would likely tend to generate higher peak loads during a weak axis impact, but the energy the post absorbs would be largely dependent on how much the post rotated in the soil prior to fracture. For the steel post, the peak loads would be somewhat lower based on the weak axis capacity of the two posts in question, but the energy absorbed may be higher than the wood post due to the post deformation developing more consistent load over the weak axis deflection. So while the wood post may generate higher weak axis accelerations, the steel post may absorb more energy and create larger changes in velocity. Thus, both posts have some competing negative aspects in their weak axis behavior, but testing has not indicated that either post has a significant advantage or disadvantage or that these effects are detrimental to overall system performance.

In a strong axis post impact, the post behavior is different for steel and wood posts. In a strong axis loading of the post, both posts will tend to rotate through the soil. Wood posts have been shown to have slightly higher soil rotation forces in the strong axis, but the effect is minimal on performance. If the soil forces do not exceed the capacity of the post section, then the two posts performance should be fairly similar in the strong axis. If the post in embedded in a very strong soil or frozen soil, then the performance of the post varies more depending on the type of post. When post-soil interaction forces exceed the capacity of a steel post, the steel post yield and deforms. This deformation of the steel post continues to dissipate energy, although the forces and energy are higher than those seen with soil rotation. A wood post will fracture if the post-soil interaction forces are high enough to exceed the capacity of the post The lack of fracture is important to the performance of the wood post. If post-soil resistance forces exceed the capacity of a wood post, the post fractures and ceases to dissipate energy during an impact. That said, we have run numerous full-scale crash tests with wood posts where the post fractured in the impact area and the performance of the system was still acceptable.

With respect to the MGS system, there are several types of post loading occurring. Some posts are loaded primarily in the lateral direction like the bogie testing. Most of the posts are undergoing a combined load that involves mainly lateral load with some twisting and longitudinal loading of the post. We believe that the majority of the posts in the system are undergoing the combined loading. Finally some posts are being loaded directly along the weak axis of the post due to the vehicle impacting it. In addition, most of the posts impacted along the weak axis will have deflected laterally along the strong axis prior to being impacted by the vehicle.

Under combined loading, steel posts tend to twist during impact and fail due to lateral/torsional buckling of the section. When we have conducted simulation analysis in past projects comparing wood and steel post versions of the MGS, we have found that a 10-15 percent reduction in the strong axis moment capacity of the steel posts accounts for the twisting of the steel posts. This reduction has correlated very well with our full-scale crash testing results. We don't see the effect of the steel post twisting as being very different from the wood post when deflected laterally. In addition, the use of a wood post that generates slightly higher lateral resistive forces would not be a concern for the performance of the MGS. We have tested several systems which would bear this out. For example, the original MGS system tested with W6x8.5 posts worked very well and had a dynamic deflection of 43.9" when tested according to MASH. When we tested the MGS with ¼ post spacing, it generated a safe redirection with a much lower deflection of 17.6". Thus, a small increase in post lateral stiffness for wood posts would not be cause for concern.

We have tested several wood post MGS systems including round wood posts made from ponderosa pine and Douglas fir, and 6"x8" white pine posts. These systems all performed similarly to the steel post MGS and no issues were observed with occupant risk values or vehicle stability. In addition, conducted testing of the MGS with 6"x8" SYP posts in the past year with both the 1100C and 2270P vehicles. Comparing the 2270P tests with 6"x8" SYP and W6x8.5 posts, we observed very similar performance in terms of vehicle stability and redirection. Dynamic system deflections were 43.9" for the steel post system and 40" for the wood post system. Very similar performance. I have attached videos at the link below of the steel and wood post testing for you to compare.

The file 'Wood vs steel MGS.zip' (189.0 MB) is available for download at

http://dropbox.unl.edu/uploads/20120216/ef4e8177e06a0405/Wood%20vs%20steel%20MGS.zip

for the next 14 days.

It will be removed after Thursday, February 16, 2012.

In summary, we believe that there is little difference in system performance for the MGS with respect to steel and SYP wood posts. While the post sections have some differences in terms of how the perform, these differences do not seem to have a large effect on the overall performance of the system.

The above discussion refers to SYP posts. You had a comment below regarding Douglas fir and Hem fir posts. The Hem Fir Select Structural and Grade 1 Douglas fir have very similar strength to SYP post, albeit around 11% lower. We have successfully tested the MGS with 6"x8" White Pine posts that were roughly 37% lower strength than the SYP posts. Thus, I would see no issues with using the Douglas fir and Hem fir materials in a 6"x8" wood post in the standard MGS system. Other specialty systems, MGS on slopes, or long span for example, might require further analysis and thought prior to using the alternative posts.

Let me know if you have further comments or questions.


Attachment: http://mwrsf-qa.unl.edu/attachments/f33501f76016b0a8099061918191480f.pdf


Tall Curb at Bridge Approach

Question
State: IA
Date: 01-23-2012

We would like to update the approach guardrail to MGS at these existing bridge ends. Would we still be able to install our standard AGT given the height of the existing curbs? If not, would you recommend grinding the curb down, or should we investigate the use of a w-beam (rather than thrie-beam) AGT here?


Attachment: http://mwrsf-qa.unl.edu/attachments/4c0409e56544bb18abb8f3ac72920a6c.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/e28b3a041eb74093f5061823e39b5785.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/4ae2e2d5a611e338c99e01f5b6386058.jpg


Response
Date: 02-14-2012

Considering the existing concrete curb measures up to 12 in. tall, we would recommend that you remove it and replace it with the standard 4 in. wedge curb. Then, it would be appropriate to implement the modified thrie beam approach guardrail transition with new MGS stiffness transition. For now, we are also guiding the State DOTs to end the concrete curb on the thrie beam end of the asymmetrical section in order to reduce concerns for vehicle snag under the sloped rail. Please let me know if you have any more questions or comments on this matter. Thanks!



Response
Date: 02-15-2012

We can remove the 12-inch curb on the approach to the bridge and replace it with a 4-inch curb. However, the 12-inch curb will still be present on the bridge. Should we transition the 4-inch curb up to 12 inches say, in the last foot prior to the bridge? Or is there another method we should employ to mitigate the snagging potential? Or is snagging here not a concern?



Response
Date: 02-16-2012

I think there are two options.

First, you might consider tapering the brush curb back toward the face of the vertical wall as long as rebar does not get in the wall of the grinding. Then, there would be no curb at the end of the parapet but instead a vertical face. You may also need to taper the end of the wall to reduce concerns for wheel snag there. We prepared draft CAD details for this which are contained in a FHWA approval letter associated with the original Iowa transition but directed to TxDOT. Let me know if you need these details.

Second, you might consider adding a 1-2 ft long RC buttress and foundation which eliminates the curb in front of the vertical parapet and has a tapered end to reduce wheel snag concerns, similar to above.

Can you accommodate any of these ideas?



Charts from Guardrail Need paper in Transportation Research Record 1599

Question
State: WI
Date: 01-25-2012

I'm thinking of adding some of the charts from Guardrail Needs: Embankments and Culverts by Dan Wolford and Dean Sicking (Transportation Research Record 1599) to our FDM. Does MwRSF have these charts in a spreadsheet?

If not I'll have to try replicate it myself (not as accurate).

After thinking about it some more. I believe that Dr. Sicking put together a report for NDOR on this subject. Would it be possible to get a copy of the whole report?

Thanks



Response
Date: 01-27-2012

I am attaching an electronic copy of the MwRSF-NDOR research report. Please let me know if you need anything else.


Attachment: http://mwrsf-qa.unl.edu/attachments/183aefe4b972a8b16478f526a696309c.pdf


Concrete median barrier on bridge

Question
State: MN
Date: 01-27-2012

We have a bridge joint repair project coming up and could use some advice on how to address our concrete median barrier situation. Hope you can help.

This bridge is a 4 lane bridge (2 lanes each way) divided by an 32" F-Shaped median barrier (non-reinforced). See attached file: fig7130e.pdf. During the repair project, traffic will be diverted to one side of the bridge with 2 way traffic. We expect the posted speed will be 45 mph. Shoulder width in the median lane is 1'.

Approximately 6' of concrete median barrier, 3' on either side of joint, will be temporarily removed for joint repair access. It's our intention to install a 12.5' length of standard thrie beam, plus terminal connectors in this span. The thrie beam and connectors will be located flush with the top of the barrier. The concrete median barrier will be replaced after the joint is repaired. See attached file: BarrierJointRepairDetails.pdf for details.

Does this work as an acceptable solution to maintain work zone and driver safety during construction? If not, do you have (or know of) any other potential solutions to this situation? Any comments you have would be appreciated.

Thanks for your help in this matter.


Attachment: http://mwrsf-qa.unl.edu/attachments/f4998f5ff02ee547b6e373941144e6c4.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/88ecb969584d99652bd6612943171c77.pdf


Response
Date: 02-13-2012
Attached is a proposed revision to your installation for spanning the gap in your median barrier.

Just to recap our phone conversation today regarding the design.

1. We concur that if the traffic on the barrier is on one side only and the work crews need access to the back side, then the nested thrie beam and lower angled plate are only needed on the traffic side face of the barrier.

2. We would limit speeds in this area to 45 mph and would prefer lower speeds than that if possible.

3. We prefer the use of nested 12 gauge thrie beam over the use of a single 10 gauge section as it provides for increased bending strength and capacity.

Let me know if you have any further comments or concerns. I have attached a revised detail showing the system with the hardware on only a single side.

Thanks


Attachment: http://mwrsf-qa.unl.edu/attachments/f34a79b1c36c26b4d6f3b169a5dce382.pdf


Temporary Barrier Thrie Beam Connection

Question
State: WI
Date: 02-09-2012

Below is an email from one of our contracting associations.

I would like MwRSF to review question #2.

___________________________________

2) Temporary Barrier Thrie Beam Connections
A) 1st detail showing the use of 7/8" bolts and bolting back to back.
AA) Problems of field drilling with rebar interference
B) 2nd detail showing the use of 3/4" anchors and back to back Thrie
Rail Assemblies offset
C) Why is single sided Temp. Thrie Connections not acceptable for
single side traffic
D) Use of 3/4" Concrete Anchors


Attachment: http://mwrsf-qa.unl.edu/attachments/8bdfc38fe767db5b5ecaa07a1a0d9b9d.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/5955730fe3aa883ecb6edcad03f75be1.pdf


Response
Date: 02-10-2012

I will try to address the questions under number 2 below.

2) Temporary Thrie Beam Connections
A) 1st detail showing the use of 7/8" bolts and bolting back to back.
AA) Problems of field drilling with rebar interference

- There may be issues with rebar interference when through bolting the thrie beam pieces on the end of the TCB approach transition. The degree of interference will depend on the reinforcement of the barrier that is attached to. I believe that we were able to avoid interference on the PCB sections. We would allow the thrie beam to be adjusted a foot or so longitudinally along the barriers to prevent rebar interference if necessary.


B) 2nd detail showing the use of 3/4" anchors and back to back Thrie
Rail Assemblies offset

- The offset shown is used to prevent interference of the mechanical anchors. If the ends of the thrie beam on each side are not offset, the anchors will interfere.


C) Why is single sided Temp. Thrie Connections not acceptable for
single side traffic

- The double sided thrie beam is used even with single sided traffic to increase the stiffness of the connection between the PCB and the rigid barrier. It also serves to prevent snag, but obviously only one side of the thrie beam is effective for that. The use of both sides is primarily for stiffening of the joint.


D) Use of 3/4" Concrete Anchors

- Not sure what the question is here. The system was tested with ¾" dia. x 6" long Powers Fasteners Wedge Bolt anchors.



Cable Terminal Anchor Bracket

Question
State: NE
Date: 06-30-2011

We are trying to get this fabricated and need some changes discussed at your level.
(See Figure 1.jpg)


Attaching the cable plate to the base plate:

What is the weld symbol at the bottom right of this sketch referring to?

Can I remove the weld symbol? I think it is redundant from the one below on the 1/8" / 3/8" on the bottom right. (See Figure 2.jpg)


Lever Retaining Cable 3/8" is shown in the report: should this be smaller/ more flexible?

I seem to recall this being a fairly limp cable, 3/8" would be stiff.

Smaller would hold the lever to keep it from flying into traffic, and breakaway if snagged on the impacting vehicle.

Unsure of size of cable (See Figure 3.jpg) " found in Pooled Fund Progress 2005 V3.ppt

3/8" was used on the short radius system (See Figure 4.jpg)


The ¾" hole used in the small gusset plates out front is too large to place at the location shown & still allow a weld on the bottom side, the metal gets too thin.

The bolt used to retain the lever we don't see dimensioned: can I change this to a ½" bolt and use a 5/8" hole?

If so I would raise it 1/8" and move 1/8" right- this will allow enouph metal to weld too.



Attachment: http://mwrsf-qa.unl.edu/attachments/fc20b9add7acfee5a8fa31c7a17fa278.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/2fef786004fdc5df16f3539f24e85786.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/fd207d031adbbef299ebc79c464907c3.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/aa319f1a2c0e6c3ceec7952243ddf0e1.jpg


Response
Date: 02-09-2012

Responses are shown below in red.

Attaching the cable plate to the base plate: What is the weld symbol at the bottom right of this sketch referring to? Can I remove the weld symbol? I think it is redundant from the one below on the 1/8" / 3/8" on the bottom right.


While I agree that the top weld symbol is redundant, the weld symbol on the lower drawing has the top and bottom welds reversed. The 1/8" fillet weld should be on the bottom of the weld specification. The arrow side of the detail is shown on the bottom, while the opposite side is detailed on the top.

Lever Retaining Cable 3/8" is shown in the report: should this be smaller/more flexible? I seem to recall this being a fairly limp cable, 3/8" would be stiff. Smaller would hold the lever to keep it from flying into traffic, and breakaway if snagged on the impacting vehicle.


Your first attached photograph corresponds to a low-tension, three-cable end terminal test, test no. CT-3. The lever retaining cable was added to the system between test nos. CT-2 and CT-3 to address the occupant compartment penetration caused by the free-flying cable release lever. While the report states that the cable was 3/8", the initial as-tested cable size was smaller (if I remember correctly, it was likely 5/16") and utilized different clamping methods. However, during test no. CT-3, the lever retaining cable ruptured, thus allowing for the cable release lever to travel downstream with the vehicle. The lever retaining cable was increased to 3/8" for test no. CT-4. During that test, the cable again ruptured allowing the cable release lever to travel downstream but without occupant compartment problems.

The lever retaining cable was also used in test no. SR-5 for the R&D effort pertaining to the short radius guardrail system, where a 3/8" cable was utilized and did not rupture. For test no. SR-5, the cable release lever was retained.

Based on the hardware used in test no. CT-4, we believe that the 3/8" size should be maintained within the actual system. I can attach the FHWA acceptance letter CC-111 which contains additional CAD details regarding the retainer cable hardware.

The ¾" hole used in the small gusset plates out front is too large to place at the location shown & still allow a weld on the bottom side, the metal gets too thin. The bolt used to retain the lever we don't see dimensioned: can I change this to a ½" bolt and use a 5/8" hole? If so I would raise it 1/8" and move 1/8" right- this will allow enough metal to weld too.


Response:
On the first page of the cable guardrail plans and near the top-left corner, the retainer bolt is specified as being a 5/8" diameter, Grade 5 hex head bolt, 10" long. Based on the bending strength of the cable, I would not recommend lowering its diameter to a ½" bolt. Technically speaking, the 3/8" wire rope could impart a bending load to the middle of the bolt that exceeds the yield and plastic bending capacities. The shear capacity of 1 or 2 planes would be adequate with 5/8" bolt. A ½" bolt would not have sufficient shear strength if shifted to one side. Bending strength is also much weaker. At this time, I would not recommend using a smaller diameter bolt. We may need to re-examine the bolt strength for a cable loop positioned in the center of the bolt as well. As for the ½" gusset plates, the current bolt placement does interfere with the weld. We have drawn a second line in the shape of the gusset but inwardly offset by 3/8" to show the interference. By adjusting the hole position, one can minimize the interference without having to alter the hole and bolt specifications. For this configuration, the hole was moved down 1/16" and to the left 3/16". The proposed location for the hole is shown in the attached detail.


Attachment: http://mwrsf-qa.unl.edu/attachments/86d54827cad89ced97a0826b8a6286c5.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/86d54827cad89ced97a0826b8a6286c5.jpg


Thrie Beam Bullnose Post Length after Post 9

Question
State: KS
Date: 02-10-2012

What is the allowable post length for the crash tested options for thrie beam guardrail systems? We wlould like to know what post length is acceptable beyond post 9/10 in bullnose. We would like to use 6 ft.



Response
Date: 02-11-2012

We reviewed the prior research and found that three different thrie beam configurations have met the 350 safety standards. They are listed below. Upon inspection, it would seem appropriate to maintain the use of 6.5 ft long posts for the standard wood post thrie beam guardrail.

 

________________________________________________________________

 

Summary

 

1.       Standard Thrie Beam (G9) system did not pass 350 - 6.5 ft W6x9 posts with 21.5" long W6x9 blockouts w/ 32" rail height

2.       Modified Thrie Beam = 6'-9" long W6x9 posts with 49.5" embedment and 18" deep M14x18 blockouts w/ 33.6" rail height

a.       Tested to TL-3 and Tl-4

3.       Thrie beam with 6'-9" long W6x9 posts with 49.5" embedment and 6"x8"x21.8" long routed wood blocks passed NCHRP 350 w/ 31.65" rail height

4.       Thrie beam with 78" long 6"x8" SYP posts and 6"x8"x21.8" long routed wood blocks passed NCHRP 350 w/ 31.65" rail height

 

Short answer, 81" long posts for steel and 78" long wood have passed.

 



Temporary Barrier advice

Question
State: OH
Date: 02-14-2012

Sending this question to you since your name is on the paper...

 

Based on the June 18, 2003 research report, Deflection Limits for Temporary Concrete Barriers, Ohio has followed a 2' offset general standard.

 

However, we have an upcoming project on our Cleveland Innerbelt bridge on I-90 (main freeway through downtown), that has some additional criteria/concerns:

Temporary Concrete Barrier will be separating 6 lanes of opposing traffic: 2 Eastbound, 4 Westbound.  50mph. This phase of traffic will be on a new bridge deck, waiting for a 2nd bridge to be constructed for the traffic in the opposite direction. 

Project duration may be DECADES depending on funding for the 2nd bridge.  Thus we are facing a potential semi-permanent installation using temporary barrier.

 

The discussion is whether or not to anchor the temporary barrier.  Obviously, the bridge guys would prefer not to put anchor holes in their brand new deck.

·         Your report based on Iowa's Temp Concrete Barrier...Roadside Design Guide indicates NCHRP 350 deflection 45"; while Ohio's barrier deflected 66" when tested.

·         Another point in the report, bottom of page 1 "traffic lanes of less than 3-m (10-ft) wide are rare, and a 600-mm (2-ft) lateral barrier displacement would not intrude significantly into the paths of oncoming traffic."  I interpret this to mean that even if the path of oncoming traffic is reduced (temporarily until the barrier is moved back) to as little as 8' wide, the risk of an accident from opposing traffic is still low. Was that the intention?

·         Another point to consider is the length of temporary barrier on the bridge " approx. 4200 feet has quite a bit more mass holding it in place as compared to the couple hundred ft test section. Yet with 4 lanes of westbound traffic, we anticipate the potential for increased impact angles

·         Cross section sketch of barrier placement attached

 

Based on all of that, would you advise to anchor the barrier in order to limit deflection during the duration of the project?  Any other points to consider?

 

Thanks!


Attachment: http://mwrsf-qa.unl.edu/attachments/d93a06ddf415be0453c3fbcb5edd9f57.jpg


Response
Date: 02-15-2012

Will you be using the Iowa Temp Barriers with the 45" of deflection or Ohio's with the 66" of deflection?



Response
Date: 02-16-2012

We are using Ohio barrier, 66" deflection.



Response
Date: 02-17-2012

First, we want to caution you on using the Ohio PCB in a permanent application. Recall, PCB's should only be used in short-term temporary applications. If you are looking at the project duration being DECADES, we would recommend placing a permanent barrier especially since it is separating 2-way traffic with lane width reductions. Recall, the more exposure you have the more permanent the barrier should be.

If the PCB's will only be used for a short-term duration, we ran a primitive set of calculations to help us get a hand on the deflections that may be encountered by the Ohio PCB at 50 mph. Free-standing PCB (Iowa/Kansas) impacted at 62 mph deflected 45". From the reference report (Deflection Limits for Temporary Concrete Barriers), at 36 mph free-standing PCB (Iowa/Kansas) was predicted to deflect 24". Therefore, at a 42% speed reduction there was an approximate 47% deflection reduction. For simplicity sake (and erring conservatively), we can say it is a 1 to 1 reduction. With the Ohio PCB when impacted at 62 mph in the free-standing configuration, it deflected 66". Therefore, at 50 mph (approximately 19% reduction) we can estimate the deflection of the free-standing Ohio PCB to be approximately 54" (4.5'). From your sketch of the barrier placement, this would mean there would be a potential for 1' of intrusion into the lane adjacent to the 3.5' temporary shoulder (2 lane side) and 2' of intrusion into the lane adjacent to the 2.5' temporary shoulder (4 lane side).

From this and if it will only be a short-term application, we recommend centering the free-standing PCB in the "median". Thus, you would have 3' temporary shoulders. Which would mean that the potential intrusion into either direction of traffic would be 18". In addition, we also recommend using lots of speed signs and even reduce the speed if possible.

Note the 4200' length of the temporary barriers on the bridge will have minimal effects (if any) on the deflection the system could sustain. When the Iowa/Kansa PCB was tested in a free-standing configuration with a length of 200', the longitudinal movement of the system was only an inch or so.

In addition, since the application you are using the Ohio PCB is a median application, we have concerns with anchoring the Ohio barrier. I am attaching the email correspondence that Bob Bielenberg had with Michael Bline in July 2009 in regards to tied-down Ohio PCB and the recommendations. A brief summary:

(1) Backside anchors cause concern since testing had not been conducted on tied-down PCB with anchors on both sides

(2) Ohio PCB anchorage insufficient to develop full strength of the threaded rods

(3) Ohio PCB anchor pockets reinforcement insufficient to develop full strength of the threaded rods.

(4) JJ-Hooks connection torsional rigidity is not sufficient for tied-down applications.

http://mwrsf-qa.unl.edu/view.php?id=377

Please let me know if you have further comments or concerns.

Thanks!

_________________________________________________



Openings in Concrete Median Barrier

Question
State: IL
Date: 01-30-2011

The IL Tollway has openings in the concrete median barrier to allow emergency vehicles to make a U-turn. These openings vary from 100' to 130' measured between the ends of the concrete barrier wall. Each blunt end is protected by an impact attenuator which is either a GREAT or a Quadguard. During construction projects when vehicles are riding on the inside shoulder there is a desire to fill in this median opening. In the past, several methods have been used. One way was to remove the attenuators and place precast temporary barrier wall sections in the opening. To completely fill in the opening, one section of wall had to be cut to fit. Making the connection between the temporary barrier wall and permanent median barrier was difficult because of the different widths.

Another method was to use precast barrier wall sections placed on a diagonal within the opening so that the barrier did not need to be cut and also so there was no blunt end to protect. One drawback to this method was that the temporary barrier wall extends onto each inside shoulder. The temporary barrier was not attached to the existing median barrier or to the attenuators.
As you can see, each of these options has problems. I know MwRSF has tested several connections between temporary barrier wall and permanent concrete barrier. Is there a TL-3 system that we can employ to safely fill in these median openings for the duration of the project?
The system should accommodate:

1. 32" F-shape temporary barrier wall, 22.5" wide at base
2. 32" Jersey shape or 42" F-shape permanent median barrier, 36" wide at base
3. Possible presence of slotted drain in the center of median opening running parallel to roadway.


Response
Date: 02-15-2012
We do have a system for transitioning between free-standing PCBs and rigid, concrete median barrier. I have attached a report detailing its design and testing.

Refer to MwRSF Report No. TRP-03-208-10. I believe that this system can be used in your situation.

You may note that it might be more desirable to simply anchor or pin all of the barriers in the installation rather than use the transition. However, we have seen in past testing that pins on the backside of a barrier may cause excess rotation and tipping of the barrier which in turn can produce vehicle instability. Thus, we currently do not recommend pinning on both sides of the PCB when placed in the median except for the transition section which we tested.

This issue of anchoring barriers in the median comes up a great deal and is something that we need to test in order to be confident that it is safe.


Anchored PCB and Expansion Joints

Question
State: MO
Date: 12-15-2010

Do you have a piece of hardware that will handle the transition between two pinned-down temporary installations of pin & loop F-barriers over an expansion joint. The joint probably has a 7 to 10 inch throw. (See attached)



I spoke to Rory Meza and she showed me the Texas standard plan in which they simply cantilever one of the segments out over the joint. This would certainly allow for movement, but at full contraction, there would be a sizeable gap/snag point between the barriers, nor could the barriers be connected to one another. We've seen the state of Illinois use steel plates which cover the gap while one of the barriers slides independently. The plate is cast with studs into the other barrier.


This seems like a reasonable solution but I wondered how you have seen it handled.


Attachment: http://mwrsf-qa.unl.edu/attachments/c4f5c0c0a6e5cb6468396b8719228003.jpg


Response
Date: 12-15-2010
MwRSF has not developed a stiffened cover plate system for protecting a large gap formed between two rigid barrier ends with an interior expansion joint in the deck surface. However, I do recall that the Kansas DOT has developed a system but not sure of the details. We provided some feedback years ago, but their bridge division personnel performed the FEA analysis and design work. I suggest that you contact Rod Lacy or Scott King to acquire their details for such a connection. However, note that longer lengths of the gap could greatly degrade performance of the existing design.

Response
Date: 02-16-2012
Yes, we have an expansion joint barrier detail that was modeled by our bridge department like Ron indicated. Attached is that drawing in Microstation and Adobe formats. If you want to view the other concrete safety barrier details, you can download them for free at the website below in PDF or DGN format. Hope this helps!


KART service (free login):
http://kart.ksdot.org

Attachment: http://mwrsf-qa.unl.edu/attachments/a28d64af23e1d2174c9fc27dbd580c5f.pdf


Use of Wood Posts with limited fill behind Posts

Question
State: WY
Date: 02-17-2012

MwRSF has done some testing on SYP as well as white pine posts for Wisconsin. Some time ago the pooled fund issued recommendations for the use of 7' long W6X9 posts at 3'-1 1/2" spacing for conventional w-beam guardrail when the fill slopes off directly behind the post at a 1V:3H or steeper slope. Can wood posts be substituted for the W6X9 steel posts and of what length? Typically Wyoming permits the use of Ponderosa Pine which is probably similar to white pine, although we do get some southern yellow pine occasionally.



Response
Date: 02-18-2012

We have successfully tested the MGS with both SYP and white pine posts. We have also successfully tested round ponderosa pine posts with the MGS. These tests were all conducted on flat ground.

MGS has also been successfully tested with steel posts on slopes as steep as 2:1. This system used 9' long W6x9 posts at standard post spacing. The posts were located at the slope break point.

As part of that effort, a series of seven bogie tests were conducted on 6-in. x 8-in. (152-mm x 203-mm) SYP wood posts of 7.5 and 8 ft (2.29 and 2.44 m) lengths and 9-ft (2.74-m) long, W6x9 (W152x13.4) steel posts placed at the break point of a 2H:1V fill slope. The results from these tests were evaluated and compared. The results found that the 7.5-ft (2.29-m) long, 6-in. x 8-in. (152-mm x 203-mm) SYP wood post provided the best possible performance and the closest correlation to the 9-ft (2.74-m) long, W6x9 (W152x13.4) steel post used in the original design. Thus, it is recommend that the MGS system may be installed adjacent to a 2H:1V fill slope with either 9-ft (2.74-m) long, W6x9 (W152x13.4) steel posts or 7.5-ft (2.29-m) long, 6-in. x 8-in. (152-mm x 203-mm) SYP wood posts.

We addressed the issue of weaker post materials in the white pine MGS report. See the text below:

"Wood posts are often utilized in longitudinal barrier systems that are configured for special applications, such as in stiffness transitions, barriers adjacent to steep slopes, or barriers to shield the ends of transverse culverts. Within these special barrier applications, the dynamic behavior of an embedded post can greatly affect its safety performance. For example, premature fracture of wood posts within an approach guardrail transition may lead to an increased propensity for vehicle pocketing and/or snag on a bridge end. As such, MwRSF researchers have concerns regarding degraded barrier performance when considering the use of the weaker, 6-in. x 8-in. (152-mm x 203-mm), white pine wood posts in lieu of standard, SYP or DF rectangular wood posts in stiffness transitions and special MGS applications. However, it is possible for white pine posts to be used within approach guardrail transitions, guardrail end terminals, or guardrail anchorage systems. First, the geometry (i.e., width, depth, and length) of white pine posts could be modified to provide equivalent stiffness and strength to that provided by the original SYP or DF wood posts. Second, the post spacing could be modified to provide equivalent barrier capacity and energy dissipation characteristics to that provided by the original SYP or DF wood posts. Finally, full-scale vehicle crash testing may be used to demonstrate that unmodified, standard-size white pine posts provide acceptable barrier performance when used in combination with stiffness transitions or other special MGS applications.

As noted previously, W-beam guardrail systems have been developed for use in shielding various roadside hazards, such as fill slopes equal to or greater than 2H:1V and transverse culvert openings. Previously and based on full-scale crash testing, the Midwest Guardrail System (MGS) was successfully adapted for use at the slope break point of a 2H:1V fill slope using 9-ft (2,743-mm) long, W6x9 (W152x13.4) steel posts spaced on 6 ft - 3 in. (1,905 mm) centers. Later and based on dynamic component testing, a wood post version of the MGS system was configured with 7.5 ft (2,286-mm) long, SYP posts and for use in shielding a 2:1 fill slope. For the SYP wood post variation, the embedment depth was 58 in. (1,473 mm).

Unfortunately, WP posts would likely fracture prior to rotating in soil when installed with a 58-in. (1,473-mm) embedment depth on a 2H:1V fill slope, thus resulting in reduced energy absorption, increased system deflections, and a greater propensity for vehicle instabilities. As such, the post geometry would need to be altered in order to mitigate concerns for post fracture. For example, the post length and associated embedment depth could be decreased to reduce the post-soil resistance. Alternatively, the post's cross section could be modified to provide increased capacity and greater resistance to post fracture when using a 58-in. embedment depth. Further, full-scale crash testing could be used to demonstrate that the MGS with white pine posts would perform in an acceptable manner even with the fracture of a greater number of wood posts.

Based on the desire to maintain a standard cross section for 2H:1V fill slope applications, a reduction in post length was deemed more desirable. Unfortunately, a decreased embedment depth would result in a reduction in the lateral stiffness and strength of the MGS. Thus, the post spacing would likely need to be reduced to provide comparable barrier capacity and energy dissipation characteristics to that provided by the steel post and SYP wood post variations of the MGS for use on 2H:1V fill slopes. Further analysis, as shown in Appendix F, revealed that a white pine MGS system located adjacent to a 2H:1V fill slope should utilize 6.5-ft (1,981-mm) long, 6-in. x 8-in. (152-mm x 203-mm) wood posts at half-post spacing, or on 37½ in. (953 mm) centers. All other features of standard MGS remain the same."

With respect to conventional W-beam, we have not determined the proper post length for wood posts adjacent to 2:1 slopes. For non-MGS systems (i.e., standard metric height W-beam guardrail), the center of the W6x9 steel post is to be placed at the slope break point using 7-ft long posts spaced 3-ft 1 1/2-in. on centers. In order to make the argument for wood posts, I believe that it would be important to perform a limited number of dynamic bogie tests using 6, 6.5, and 7-ft wood post lengths in a sloped soil pit, similar to what was done with the MGS adjacent to slopes.

Let me know if this answers you questions or if you need more information or discussion.

Thanks



Roadside barrier with narrow shoulder

Question
State: IL
Date: 02-21-2012

Some locations in Illinois have narrow shoulders, often with outdated and deteriorated guardrail in place. Upgrading these locations with new roadside barrier is problematic because of the combination of narrow shoulder, and steep front slope. Often the front slope begins at the nominal shoulder width.

Crash testing at MwRSF has developed an option for cases where the steep slope begins at the back of guardrail post. However, the condition we are encountering is more severe. This photo is representative.

<>

In some cases, depending on right of way, depth of culvert, etc, it may be possible to improve the roadside by placing a grated culvert end. However, in many locations the right of way is also narrow and we are faced either with do-nothing, remove the roadside barrier entirely, or provide an improved roadside barrier.

With the face of guardrail at the edge of shoulder and hinge point to the 2:1 slope, what guidance can we consider in placing a guardrail (MGS)? Is it reasonable to consider placing the posts on the 2:1 front slope? The elevation of the ground at the back of these posts would be approximately 11" lower than at the face of guardrail.

In some cases the slopes may be steeper, perhaps as much as 1.5:1, with resulting elevation difference of the ground line of 14" between the face of rail and the back of post.

In crash testing the more severe angle of departure is usually taken as the critical case. What about this instance? A car or pickup departing the road at a flat angle might be more likely to drop a tire(s) over the hinge point and begin snagging posts.

We understand that the MGS without blockouts has passed crash testing. This would put the post much closer to the hinge point. Would this be a preferable/allowable application with the posts onto the front slope (back of post 9" from the hinge point, and approximately 5" lower than the ground at the face of rail) for this case? What about using the longer 9' posts with the MGS without blockouts? Can we make that translation from one system to the other?


Attachment: http://mwrsf-qa.unl.edu/attachments/b0803738fdf69d53a4544b7c4f9b0e07.jpg


Response
Date: 03-05-2012

From the attached photograph, it appears that the rock/gravel shoulder may be 1 to 2 ft wide and maybe conform to a 10:1 slope. The corrugated beam guardrail appears to positioned on a steeper slope, which in some cases may reach 2:1, or even 1½:1. Am I understanding the roadside geometry correctly?



Response
Date: 03-16-2012

The location is between Huey Road and Boulder/Ferrin Road (CH 787) on US 50 east of Carlyle.

I checked in our roadway inventory and find that the shoulders are listed as 2' wide, aggregate, except the reverse curve area has bituminous paved shoulders 3' wide. However, I suspect the shoulders were originally constructed to the width where the face of guardrail, and culvert headwalls are. Over time, resurfacing and aggregate wedge shoulders have reduced the usable width by increasing the cross slope.

A couple of additional photos are attached for a better overview. Right of way is narrow, and there are also concrete box culverts for entrances.


I have asked the district to take cross-sections of the roadway. My thought was to regrade and stabilize the shoulders (if necessary) and the foreslope and backslope with erosion control blanket. They could remove the existing guardrail completely and replace where necessary for things like culverts. My thought was where we needed to place guardrail we could place a no-blockout system, essentially such that we are NOT reducing the roadway width (due to farm equipment). The one question I had was what impact did that have on the post length requirement. At this point, I think that this approach would provide significant safety improvement over what is out there and would reduce liability to the agency. Any recommendations you have would be welcomed.


Attachment: http://mwrsf-qa.unl.edu/attachments/c3b117485634a2294e987b1328c51808.JPG

Attachment: http://mwrsf-qa.unl.edu/attachments/c3b117485634a2294e987b1328c51808.JPG


Response
Date: 03-21-2012

Thank you very much for the additional clarifications, information, and sample photographs. I like the options that you have proposed below.

Over the course of this discussion, I have received three photographs which appear to represent one particular site and both sides of the two-lane highway. Based on the information forwarded thus far, it is evident that the real-world scenario poses some difficult challenges, including whether to treat potential hazards within the clear zone, allow the current barrier system to remain in place, upgrade the overall barrier system, remove and replace all or segments of the barrier system, perform site grading of varying degrees, etc. Of course, a determination of an appropriate and/or a reasonable clear zone will likely help guide whether to shield or treat various roadside hazards, such as transverse culvert openings, parallel culvert openings which cross driveways or secondary roads, steep slopes and ditches, trees, utility poles, fences, etc. In addition, the existing W-beam guardrail systems more closely resemble prior ILDOT standards and technologies and were likely installed many years ago, including a non-blocked variety of guardrail with W-beam attachment bracket, steel-blocked W-beam guardrail option, and turned-down end terminations.

In terms of the current guardrail placement, there appears to exist two different sloped regions in front of the W-beam guardrail systems. The second steeper slope would appear to exceed 6:1. With that in mind and assuming that an improved guardrail is to remain in place to shield various hazards, then one may consider modifying the approach grading to provide a wider and flatter region between the roadway edge and guardrail face. If possible, one would provide 10:1 approach slopes; however, we have demonstrated that 8:1 approach slopes are acceptable for the standard MGS with blockouts and 31" mounting height. Depending on the grading behind posts, the guardrail configuration may vary. For example, if 2-ft of generally flat soil exists behind the posts, then a standard design can be used. Unfortunately, this may not be the case in your situation. Therefore, it may seem necessary to consider using a non-blocked version of the MGS when steep slopes exist behind the posts, say up to 2:1. In this scenario, I would consider re-grading the ditch to obtain a reasonably-flat approach slope in front of the W-beam rail (MGS) and then cut the ditch to 2:1 or as needed where you may desire the posts to be placed.

The MGS for 2:1 fill slopes was crash tested and successfully evaluated under MASH when installed with a 12" deep wood blockout and with the rail positioned at 31". However, the system was not successful when tested at 27¾". As such, we do not know the lower bound for this system variation when used with a blockout. As you know, the standard blocked MGS has a "recommended" lower bound at 27¾" for new construction based on the successful MASH testing of the modified G4(1s). Note that we recommend that it be installed at its nominal height of 31".

MwRSF previously tested a non-blocked MGS under MASH when placed on wire-faced MSE walls with 6-ft long steel posts and backup plates at the slope break point of a 3:1 fill slope. MwRSF has also successfully tested a non-blocked, steel-post version of MGS with backup plates on level terrain under TL-3 of MASH.

Based on the testing noted above, combined with engineering judgment and the best available information, we believe that a non-blocked version (with backup plates) of the MGS for 2:1 fill slopes could likely meet the MASH impact safety standards when installed at the 31" mounting height. However, it should be noted that no crash tests have been performed on this exact variation and that the lower bound would likely be affected, as was the case for the blocked version placed on 2:1 slopes with 9-ft long steel posts.

As a result and if deployed, it would be highly suggested that the MGS system only be installed with a 31" minimum mounting height when used in a non-blocked version on 2:1 fill slopes using the longer steel posts, backup plates, and an approach slope possibly ranging between 8:1 and 10:1. Of course, the proven crashworthy system which utilizes the blockout would provide the safest alternative out of the two options.

In summary and based on the best available information, we believe that a non-blocked, 31" MGS with back up plates, 9-ft long steel posts at the slope break point of a 2:1 fill slope, and a mostly flat approach slope (somewhere between 8:1 and flat) should also meet the TL-3 MASH impact safety standards. However, the MGS within this scenario has not been crash tested nor have we obtained FHWA acceptance for this variation. In addition, consideration for additional grading should be considered around the crashworthy guardrail end terminals.

Some situations may exist where guardrails can be removed if driveways and parallel drainage or transverse culvert structures are modified with flattened side and cross slopes in combination with any required grating to cover hazardous openings.

These are my initial thoughts for your scenario. I would welcome additional feedback regarding my comments contained above and am willing to continue this discussion. Thanks again!



Cable Guardrail Next to Slopes

Question
State: WY
Date: 11-04-2008

Since MwRSF did the testing on flat ground with the low tension cable system, would you change your recommendation if the cable was placed on 1V:6H or 1V:8H slopes in front of and up to 4 ft. behind the cable before starting on a 1V:2H fill slope. It would seem the vehicle would strike the barrier higher, so it may be necessary to constrain deflection even more.



Response
Date: 02-24-2012

I have reviewed you question regarding the use of the CASS adjacent to a steep slope with an 1:6H or 1:8H approach slope.


Previously we had given you guidance for using the CASS adjacent to a steep slope. We had suggested using a 4ft offset to the slope and reducing the post spacing of the CASS to 3 m. Your new question was whether or not these recommendations would hold true when the cable barrier was installed on a 1:6H or 1:8H approach slope. In order to address this issue, I looked into the performance of the CASS system, analysis of bumper trajectories for 2000P pickup trucks encroaching on approach slopes, and previous testing of cable barrier on approach slopes. Based on this analysis I have the following comments.


  1. We have concerns with placement of the CASS on a 1:6H approach slope adjacent to a steep slope. This concern is based on effective capture of the vehicle by the CASS system. Previous testing was conducted on 30" high, low-tension cable barrier placed on a 1:6H slope with a 6 ft offset from the edge of shoulder. Two tests were conducted. The first was a test (3569-5) of a 1974 Plymouth sedan that weighed 4500 lbs and impacted the barrier at 59.6 mph and an angle of 24.75 degrees. The second test (3569-6) was a test of a 1974 Chevy Vega that weighed 2250 lbs and impacted the barrier at 58.4 mph and an angle of 17.25 degrees. Both of these tests showed safe redirection of the vehicle. However, the sedan and small car vehicles in this testing had bumper heights of approximately 18". Typical bumper heights for the 2000P and 2270P vehicles are around 26". Thus, there is concern that the capture and redirection of the vehicles observed in these tests would not be as likely with the higher bumper heights of the current test vehicles and vehicle fleet. The RDG recommendations for approach slopes are based on this testing, but do not take into account the higher bumper heights and CG heights of the current vehicle fleet. The reference for this testing is given below.


Ross, H.E., Smith, D.G., Sicking, D.L., and Hall, P.R., Development of Guidelines for Placement of Longitudinal Barriers on Slopes, Research Report 3659-2 (DOT-FH-11-9343), Texas Transportation Institute, May 1983.


In addition, I reviewed some analysis that we conducted on bumper trajectories of 2000P vehicles running off slopes and compared these trajectories with the cable heights of the CASS System. A chart is attached. In the chart, the green lines are the cable heights, the pink line is the slope, and the navy blue line is the truck bumper trajectory. You can see from the chart that vehicle encroaching on a 1:6H slope could have bumper heights higher than the top cable height of the CASS system which could lead to the potential for override of the system. Similar analysis performed by TTI on 1:6H slopes with the 2000P vehicle also indicated bumper heights that would exceed the top cable height of the CASS depending on the barrier offset ( http://tti.tamu.edu/documents/0-5210-3.pdf ).


Based on the existing test data and analysis of vehicles encroaching on 1:6H slopes, we are concerned about the use of cable barrier adjacent to steep slopes due to uncertainty about the effective capture and redirection of the vehicle.


  1. No testing was available with cable barrier on 1:8H slopes. However, I did look at our bumper trajectory relative to the CASS system for an 1:8H slope. See the attached chart. In the case of the 1:8H slope, you can see that the bumper trajectory analysis indicated that there is an improved likelihood of vehicle capture as the bumper does not exceed the height of the top cable. This would indicate potential for capture and redirection. When using the CASS on an 1:8H slope adjacent to a steep slope, we would still recommend that you use the 4 ft offset from the cable barrier to the steep slope and also use the reduced CASS post spacing of 3 m.

Attachment: http://mwrsf-qa.unl.edu/attachments/cb36cd8858686ea4ddacc30139581125.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/71b885de3871fd38a04caa80bd5f5c54.pdf


Simplified Steel-Post MGS Stiffness Transition

Question
Date: 02-27-2012

What are the guidelines/limitations for using a curb in front on the MGS transition?

Also, what is the appropriate blockout depth?



Response
Date: 02-28-2012

I have some answers regarding your questions on the simplified approach transition for the MGS.

1. Using a curb in front of the MGS transition

a. Installing 6" curb in front of the MGS and upstream of the transition will cause a curb to be present throughout the length of the guardrail system. This will put us in a gray area. The MGS was tested with a curb, and the bridge transition was tested with a curb, but the upstream transition (or transition to the transition) has not yet been evaluated with a curb.

Adding a curb to the transition can lead to a number of problems for both the small car and pickup truck vehicles. The 2270P vehicle will be subjected to a vertical force component on the impacting side as the tire rolls over a curb. This vertical force combined with the changing stiffness of the transition may lead to stability issues (namely vehicle roll) as the vehicle is redirected. Theses factor could lead to vehicle rollover " similar to that seen for the recent MGS test with 6" curb placed 8 ft behind the rail. With the curb placed much closer to the rail (within a foot) the vertical force has less time to create instabilities, but this may still cause problems since the rail stiffness in the transition area is not constant (post spacing reduces and rail becomes larger/stiffer as you move downstream).

For 1100C vehicle, the small car bumper has recently shown a propensity to extend under the 31" high rail and snag on the steel posts (demonstrated by both the recent transition test, MWTSP-3, and the MGS test placed on a Gabion Wall, MGSGW-2). During the mentioned tests, the vehicle was able to bend the posts over and continue downstream without violating the ORA or OIV values. However, with inclusion of a curb and additional soil fill behind it, the post becomes stiffer and the moment arm for post bending is reduced. Thus, bending the posts over as the vehicle impacts the system will take more force and energy. Further, the as the tire rides up the curb the vehicle may become wedged between the curb and the bottom of the guardrail leading to further decelerations. The combination of these phenomena may lead to a violation of the ORA or OIV values (the transition test already saw a 14.7 longitudinal ORA and a 27.5 longitudinal OIV " recall maximum allowable values are 20.49 and 40, respectively.

Any curbs that accompany a given thrie beam transition design should remain part of the "new" system. However, the curb should be terminated ( via sloped or flared end) without extending into the w-to-thrie transition element. Again the concerns were that the addition of a curb could (1) lead to further snagging of small cars between the curb and the downward slope of the bottom of the w-to-thrie transition element, and (2) cause vehicle instabilities due compressing the suspension and creating vehicle climb. If the curb used in your existing system already terminates prior to the downstream end of the w-to-thrie-transition piece, then use it as previously designed. If not, the upstream end of the curb should be altered to meet this specification.

After identifying these potentially critical mechanisms, MwRSF is hesitant to recommend the use of the upstream transition with a curb until further evaluation is conducted (most likely full-scale crash testing). However, I can point out a few design elements that would minimize the increased risk of adding a curb.

1. Extending the 4" triangular curb throughout the upstream transition would incorporate less of a vertical force to the vehicle than would a 6" high curb. Therefore, the 4" curb should be extended upstream at least 12.5 ft (2 full post spacings) past the first 37.5" or ½ post spacing. The transition to 6" curb can then be made over the next post spacing upstream of this point.

2. To mitigate some of the increased snag potential for the small car, it may be wise not to fill in the soil behind the 4" curb in the upstream transition area (specifically from the beginning of reduced post spacings to the first 6.5 ft long post. This would eliminate the extra force and energy required to bend the posts over if the vehicle bumper gets under the rail.

2. Blockout depth in the transition

a. The blockouts used on the W6x9 posts in the transition were 12" deep. The W6x15 posts used 8" deep blockouts. 12" deep blockouts would be acceptable for use on the W6x15 posts as well if that was desired. We would not recommend the use of blockout depths shorter than those used in the tested transition.

Let me know if you have further questions.

Thanks



Low Tension Cable Slopes

Question
State: NE
Date: 02-28-2012

Low Tension Cable Guardrail

We are implementing the 30" high low tension cable guardrail with 27" & 24" lower cables.

The proposed standard installation for given slopes:

- S3X5.7 " 5'3" with 4' post spacing at 4' from a slope less than 1V:1.5H " tested to 25 degrees 62 mph by MwRSF.

- S3X5.7 " 5'3" with 8' post spacing at 3' from a slope 1V:1.5H to 1V:2H " transition between practice & tested.

- S3X5.7 " 5'3" with 16' post spacing at 2' from a 1V:2H or flatter standard practice & TL-2 tested in the 60's & 70's (possibly with 27" high top cable).

The explanation behind this implementation is the failed test which used 16' spacing 1' from a 1V:1.5H, where the front tire did not contact the soil before flipping, the tire is in contact with the soil on a flatter slope.

My thought is that the tested; flat concrete to 1V:1.5H slope, is different from the real world 2% lane, 4% shoulder, 1V:10H surfacing under guardrail to a 1V:2H slope, that this is enough difference to keep the front tire on the slope in testing conditions & that this force on the vehicle will keep it from flipping over the top of the cable.

Can the 2' from a 1V:2H be modeled with the slope rounding which Ken Opiela spoke of at TRB?



Response
Date: 04-12-2012

Over the past couple of years, the placement of low-tension cable barriers near steep slopes has garnered much discussion. Some of this discussion has been archived in the following Pooled Fund Consulting Summaries:

http://mwrsf-qa.unl.edu/view.php?id=233

http://mwrsf-qa.unl.edu/view.php?id=234

http://mwrsf-qa.unl.edu/view.php?id=328

The recent questions found below pertain to the potential transitioning of the crashworthy low-tension, cable barrier system to variations in the configuration when placed near alternatives fill slopes.

Previously, a low-tension, cable barrier system with three cables at 30, 27, and 24 in. provided acceptable TL-3 safety performance under NCHRP Report No. 350 when placed 4 ft away from the slope break point of a 1½:1 fill slope. In this test, maximum dynamic barrier deflection reached approximately 125 in. (10.4 ft) with a vehicle traveling on level terrain prior to impacting the barrier system. Using this information, the 2000P vehicle extended approximately 77 in. (6.4 ft) beyond the slope break point. For a 77 in. (6.4 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 51.3 in. (4.3 ft).

Previously, TTI researchers crash tested a low-tension, 3-cable barrier on level terrain with a 16-ft post spacing at TL-3 of NCHRP Report No. 350. This 2000P crash test resulted in a 3.4 m (134 in.) maximum dynamic barrier deflection, which was slightly larger than the barrier deflection observed by MwRSF in the ditch test.

If a 16-ft post-spacing design were placed 4-ft away from the slope break point of a 1½:1 fill slope, the 2000P vehicle would likely extend 86 in. (7.2 ft) over the slope terrain. For a 86 in. (7.2 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 57.3 in. (4.8 ft). If a 16-ft post-spacing design were placed 4-ft away from the slope break point of a 2:1 fill slope, the 2000P vehicle would likely extend 86 in. (7.2 ft) over the slope terrain. For a 86 in. (7.2 ft) lateral offset on a 2:1 fill slope, the corresponding vertical drop from a horizontal datum is 43 in. (3.6 ft). >From this simple investigation and comparison, one may infer that the 16-ft post spacing design at a 4-ft lateral offset may result in slightly reduced safety performance as compared to the 4-ft post spacing design at a 4-ft lateral offset for 1½:1 fill slopes. For 2:1 fill slopes, one may infer that similar performance would likely be obtained. However, another factor may influence whether or not the 16-ft post-spacing design would adequately perform when placed at a 4-ft lateral offset. Recall that the 4-ft post spacing design has many more cable hook bolts and posts for which to support and contain the three cables. As a result, the 16-ft post spacing design may allow for increased cable drop and vehicle roll motion with reduced cable support from decreased hook bolts and posts. Thus, it still may be difficult to infer whether comparable vehicle and barrier performance would be obtained with the 16-ft post spacing design.

If a 16-ft post-spacing design were placed 3-ft away from the slope break point of a 1½:1 fill slope, the 2000P vehicle would likely extend 98 in. (8.2 ft) over the slope terrain. For a 98 in. (8.2 ft) lateral offset on a 1½:1 fill slope, the corresponding vertical drop from a horizontal datum is 65.3 in. (5.4 ft). If a 16-ft post-spacing design were placed 3-ft away from the slope break point of a 2:1 fill slope, the 2000P vehicle would likely extend 98 in. (8.2 ft) over the slope terrain. For a 98 in. (8.2 ft) lateral offset on a 2:1 fill slope, the corresponding vertical drop from a horizontal datum is 49 in. (4.1 ft). >From this simple investigation and comparison, one may infer that the 16-ft post spacing design at a 3-ft lateral offset may result in moderately reduced safety performance as compared to the 4-ft post spacing design at a 3-ft lateral offset for 1½:1 fill slopes. For 2:1 fill slopes, one may infer that similar performance would likely be obtained. Once again, differences in system design may influence whether the 16-ft post-spacing configuration adequately performs when placed at a 3-ft lateral offset. As noted above, the 4-ft post spacing design has many more cable hook bolts and posts for which to support and contain the three cables. Thus, the 16-ft post spacing design may allow for increased cable drop and vehicle roll motion, thus making it difficult for the 16-ft post spacing design to provide similar vehicle and barrier performance.

Similar comparison could be made for a 16-ft post spacing design at a 2-ft lateral offset with either slope condition. However, the results noted above would lead one to believe that further degraded performance may be obtained as compared to the 4-ft and 3-ft lateral offsets. In summary, it is difficult to know to the degree at which barrier performance is potentially degraded with increased post spacing and reduced lateral offset from steep fill slopes.

As noted above, the physical crash testing programs were performed with impacting vehicles traveling on level terrain prior to striking the cable barrier systems. You noted that in real-world applications, highways are typically constructed with cross slopes, shoulder slopes, slight grades or rounding near barrier and at slope break point in advance of the steel fill slopes. However, these geometric conditions may not exist at the outside of curves which protect steel slopes. In any event, it was theorized that these deviations from level terrain testing may potentially mitigate some of the degrading effects noted above. As a result, you inquired as to whether transitions to an 8-ft post spacing at a 3-ft lateral offset and then a 16-ft post spacing at a 2-ft lateral offset may be considered in advance of steep fill slopes. Based on our review of test results, simple investigation and comparisons above, and engineering judgment, we feel that it is difficult to support such transitions without additional R&D. This R&D could begin with computer simulation with LS-DYNA or other vehicle dynamics simulation codes, but it may eventually require compliance testing to verify that such real-world conditions can mitigate other degrading effects and prevent pickup truck rollovers.



Tapered Bridge Rail Approach Transition

Question
State: NE
Date: 02-29-2012

I am working on a bridge rehab project that is going to remodel the bridge rail over the Loup River in the Bessey National forest. Due to the geometrics of the approach roadway we can't attach standard guardrail without encroaching on the driving surface (see third attachment). Because we can't attach standard guardrail, the speed limit is 35 mph, and this is the off end of the bridge rail, we are proposing to build a tapered bridge rail transition (see first & second attachment).

Are you aware of any testing or guidance on this subject? Would this be acceptable? Thanks for your time.


Attachment: http://mwrsf-qa.unl.edu/attachments/3f9cf9ee4fb646ab3ecee57fe3394093.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/52eeb4f4fae8a7019f27776a2c878572.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/92912def0bbfaf24a381db14bbf6549d.pdf


Response
Date: 03-01-2012

I need to gather some background info regarding your question.


1. Is the road curving "south" (per drawings you sent and traveling from right to left or "west to east"), does the road continue straight, or both? This would help determine likelihood and possible angle of impacts

2. If road is curving, how tight in the radius?

3. When you state "off end of bridge rail" I'm assuming you mean downstream end for "eastbound" traffic, correct? Can you clarify number of lanes in each direction please?

4. What is the hazard you are protecting? Just the bridge rail end, or is there an non-traversable slope adjacent to the bridge?


5. There appears to be guardrail on the "north and south" sides of the roadway (even though it's drawn with the rail facing away from the road). Why not extend this to the bridge rail? As drawn with the tapered end, there is a blunt end of the guardrail near the downstream (low) end of the concrete which would require a terminal.

6. When stating that the "geometrics of the road" prevent guardrail installation, are you saying that the radius is too tight for a guardrail installation? Or are you saying that the concrete of the roadway was poured wide enough that it covers the area where the guardrail posts would be placed?

7. Is the speed limit 35 only do to the lack of a complete transition, so only near the bridge, or is the road 35 mph for a long distance.





Response
Date: 03-02-2012

Scott,

Please see my response to your questions below in red.

1. Is the road curving "south" (per drawings you sent and traveling from right to left or "west to east"), does the road continue straight, or both? This would help determine likelihood and possible angle of impacts

As you can see from the image above, our spur road, S86B, curves to the west. There is an intersection that continues to the south so the answer would be both. This is a State/National recreational area with the ranger station being just east of the curve.

2. If road is curving, how tight in the radius? The radius for the spur road curve is 205'.

3. When you state "off end of bridge rail" I'm assuming you mean downstream end for "eastbound" traffic, correct? Can you clarify number of lanes in each direction please? One lane in each direction with the tapered rail proposed for the southwest corner of the bridge which would be the off-end for the southbound traffic.

4. What is the hazard you are protecting? Just the bridge rail end, or is there an non-traversable slope adjacent to the bridge? As you can see from the attached image, looking south from the bridge, there are trees on a steep bank. The end of the tapered rail will be 50' beyond the end of the existing concrete bridge rail.

5. There appears to be guardrail on the "north and south" sides of the roadway (even though it's drawn with the rail facing away from the road). Why not extend this to the bridge rail? As drawn with the tapered end, there is a blunt end of the guardrail near the downstream (low) end of the concrete which would require a terminal. As you can see from the attached image the existing guardrail is constructed out of timber and will be removed.

6. When stating that the "geometrics of the road" prevent guardrail installation, are you saying that the radius is too tight for a guardrail installation? Or are you saying that the concrete of the roadway was poured wide enough that it covers the area where the guardrail posts would be placed? The radius is too tight. A standard bridge approach section and end treatment encroach on the driving lane.

7. Is the speed limit 35 only do to the lack of a complete transition, so only near the bridge, or is the road 35 mph for a long distance. The entire 7 mile spur is posted at 35 mph.


Attachment: http://mwrsf-qa.unl.edu/attachments/3feb5af18f06eed7a3e264724f615637.jpg

Attachment: http://mwrsf-qa.unl.edu/attachments/3feb5af18f06eed7a3e264724f615637.jpg


Response
Date: 03-03-2012

Are changes also being made to the bridge rail? I'm asking because the rail shown in your attached photo looks short and the concrete structure you have proposed it 34" and 14" wide. Seems like a miss-match.

Also, the sketch you had provided showing how the transition would extend in to the roadway appears to be using a TL-3 transition design. With a posted speed limit of only 35 mph, have you given any thought to using a much shorter (and w-beam only) TL-2 transition? I know of one such TL-2 transition that is only 11'-6" long between bridge rail and standard guardrail.



Response
Date: 03-04-2012

See my comments below.

__________________________________

Are changes also being made to the bridge rail? I'm asking because the rail shown in your attached photo looks short and the concrete structure you have proposed it 34" and 14" wide. Seems like a miss-match.

This is a bridge rehab project that will remodel the bridge rail to current standards (See attached preliminary plan, tapered rail not shown).

Also, the sketch you had provided showing how the transition would extend in to the roadway appears to be using a TL-3 transition design. With a posted speed limit of only 35 mph, have you given any thought to using a much shorter (and w-beam only) TL-2 transition? I know of one such TL-2 transition that is only 11'-6" long between bridge rail and standard guardrail. The guardrail I showed was a TL-3 design. The TL-2 transition would be an option if it's non-proprietary.


Attachment: http://mwrsf-qa.unl.edu/attachments/aecf96d6d065c75f0b2b5e66fe67f723.pdf


Response
Date: 03-05-2012

Seeing that a TL-2 transition is an option for you, I would recommend that you use one of the TL-2 transitions that have been tested down in Texas. One is a shorter guardrail that utilizes only w-beam and was tested to NCHRP report 350 TL-2, while the other utilizes a short section of thrie beam and an asymmetrical transition piece and was tested to MASH TL-2.

I tried to attach the reports to this e-mail, but the files are too large. However, you can get them from the Texas Transportation Institute's website (tti.tamu.edu). Use the publications search and lookup these reports:

1. report no. 9-1002-8, "Development of a MASH TL-2 Guardrail-to-Bridge Rail Transition Compatible with 31-inch Guardrail", Bligh, R., et. al., December 2011

2. report no. 4564-1, "Evaluation of Guardrail to Concrete Bridge rail Transitions", Bligh, R., et. al., October 2003

If you have any problems finding the test reports, let me know.



MGS Adjacent to 2:1 Slope - No blockout

Question
State: NE
Date: 03-01-2012

We have implemented the 9' post on the edge of a 1V:2H slope.


We have also implemented the standard post no blockout.


Could we combine these items?

Ex. Using a 9' post at the break of a 1V:2H slope w/o a blockout?



Response
Date: 03-02-2012

I am copying my general thoughts that were recently provided for an identical inquiry about a month ago.

The MGS for 2:1 fill slopes was crash tested and successfully evaluated under MASH when installed with a 12" deep wood blockout and with the rail positioned at 31". However, the system was not successful when tested at 27¾". As such, we do not know the lower bound for this system variation when used with a blockout.

As you know, the standard blocked MGS has a "recommended" lower bound at 27¾" for new construction based on the successful MASH testing of the modified G4(1s). Note that we recommend that it be installed at its nominal height of 31".

MwRSF previously tested a non-blocked MGS under MASH when placed on wire-faced MSE walls with 6-ft long steel posts and backup plates at the slope break point of a 3:1 fill slope. MwRSF has also successfully tested a non-blocked, steel-post version of MGS with backup plates on level terrain under TL-3 of MASH.

Based on the testing noted above combined with engineering judgment and the best available information, we believe that a non-blocked version (with backup plates) of the MGS for 2:1 fill slopes could likely meet the MASH impact safety standards when installed at the 31" mounting height. However, it should be noted that no crash tests have been performed on this exact variation and that the lower bound would likely be affected, as was the case for the blocked version placed on 2:1 slopes with 9-ft long steel posts.

As a result and if deployed, it would be highly suggested that the MGS system only be installed with a 31" minimum mounting height when used in a non-blocked version on 2:1 fill slopes using the longer steel posts. Of course, the proven crashworthy system which utilizes the blockout would provide the safest alternative out of the two options.

In summary and based on the best available information, we believe that a non-blocked, 31" MGS with back up plates and 9-ft long steel posts at the slope break point of a 2:1 fill slope should also meet the TL-3 MASH impact safety standards. However, it has not been crash tested nor have we obtained FHWA acceptance for this variation.

Please let me know if you have any further questions or comments.



MGS without Blockouts

Question
State: WY
Date: 03-01-2012

I was looking through our stuff and I didn't see any reports about the MGS without blockouts and I didn't see any status for it in the quarterly report. I am pretty sure that testing was already done. Has a draft report (or final) been done? Also, do you know if it was tested on flat ground, or was there a 1V:2H slope break directly behind it?



Response
Date: 03-05-2012

That testing is completed and in the report stage. The system passed both the 1100C and 2270P testing on flat ground.

More thoughts on the non-blocked MGS.

The MGS for 2:1 fill slopes was crash tested and successfully evaluated under MASH when installed with a 12" deep wood blockout and with the rail positioned at 31". However, the system was not successful when tested at 27¾". As such, we do not know the lower bound for this system variation when used with a blockout.

As you know, the standard blocked MGS has a lower bound at 27¾" based on the successful MASH testing of the modified G4(1s).

MwRSF previously tested a non-blocked MGS under MASH when placed on wire-faced MSE walls with 6-ft long steel posts at the SBP of a 3:1 fill slope.

Based on the testing noted above combined with engineering judgment and the best available information, we believe that a non-blocked version (with backup plates) of the MGS for 2:1 fill slopes could likely meet the MASH impact safety standards when installed at the 31" mounting height. However, it should be noted that no crash tests have been performed on this exact variation and that the lower bound would likely be affected, as was the case for the blocked version on 2:1 slopes.

As a result and if deployed, it would be highly suggested that the system only be installed with a 31" minimum mounting height when used in a non-blocked version on 2:1 fill slopes using the longer posts. Of course, the proven crashworthy system which utilizes the blockout would provide the safest alternative.

Thanks



MGS Guardrail (Planing Blockouts)

Question
State: KS
Date: 03-20-2012

My name is Tom Rhoads and I am a design engineer in Scott King's squad at the Kansas Department of Transportation. I've been assisting Scott with some revisions to our MGS Guardrail Standard Drawings. Throughout our revision process we have been communicating with Ray Schacht at Midwest Machinery Supply and a question has arisen regarding a KDOT Standard Specification indicating the blockouts we use for our guardrail systems should be planed prior to installation (I've attached a copy of the Standard Specification with the area of interest highlighted). Ray indicated once the blockouts are planed the resulting dimensions are 5 ½ inches to 5 5/8 inches by 7 ½ inches to 7 5/8 inches rather than 6 inches by 8 inches. Our question for you is: Is the reduction in the blockout section acceptable for MGS Guardrail installations?

Please let us know your thoughts.


Attachment: http://mwrsf-qa.unl.edu/attachments/2982ef40f1cff69d886f8a16f1e62ce4.pdf

Attachment: http://mwrsf-qa.unl.edu/attachments/1724e4520cd1db7ac226d2cea12847c4.pdf


Response
Date: 03-20-2012

MwRSF has successfully crash tested the MGS in blocked (12" deep) and non-blocked (no offset block but with backup plates) applications. In these testing programs, improved performance was observed when using the offset blocks. TTI later successfully crash tested the MGS with an 8" offset block. As a result of these programs, I would not get too concerned with minor deviations in the block dimensions when comparing full-sawn, rough sawn, and dressed blocks. As a matter of fact, some fabricators start the process with larger wood sizes so that the dressed block actually measures 6"x12" or 6"x8".

Previously, manufacturers and other State DOTs also inquired into the MGS block tolerances. Since similar inquiries have been made and responses have already been complied, I am forwarding to you information from the 2007 discussions with the Pooled Fund members states. Hopefully, my response above and the attached information will sufficiently answer your question.


Attachment: http://mwrsf-qa.unl.edu/attachments/dbbe519c1a152833d849e4505062ee57.msg


Transition from Temporary to Permanent Barrier

Question
State: WI
Date: 03-26-2012

Bob,

I know that we talked about this a long time ago (This project keeps getting put on the back burner because of other issues. So, I lose my notes about it.).

What tension and shear values should I put on the anchors located on the attached drawing?

I know that the system was crash tested using asphalt stakes anchoring down the temporary barrier. I'm assuming that it is OK to use the same pattern for anchoring the barrier down on concrete as well.


Attachment: http://mwrsf-qa.unl.edu/attachments/7aef7f5187372aa1f2d0f3f7ae39ee10.pdf


Response
Date: 03-27-2012
There are two types of mechanical anchors specified in the attached detail.

For the 3/4" dia. x 6" long Powers Fasteners mechanical anchors, we would recommend that the manufacturers listed ultimate strengths be followed as guidance for any alternative anchor. Powers currently lists the ultimate shear and tensile capacities of those anchors in 4,000 psi concrete as 17.9 kips and 21.96 kips, respectively.

http://www.powers.com/pdfs/mechanical/07246BT.pdf

For the Red Head Multi-Set II RL drop-in anchor, the manufacturer lists the ultimate tensile loads of 9.48 kips for 4,000 psi concrete and ultimate shear loads of 10.48 kips for concrete strengths over 2,000 psi. We have tested these anchors dynamically to significantly higher loads, but we would stick with the manufacturers listed strengths if you are going to include them on your details.

http://www.itwredhead.com/pdfs/submittals/multi-set.pdf

We do allow the transition design to be used with the bolt-through tie-down option when applied to transitions on concrete surfaces using the configuration shown. The asphalt pin and bolt-through tie-down systems are believed to possess similar lateral restraint and thus can be interchanged in the transition design as needed.


Response
Date: 04-02-2012

Bob,

As I was reviewing the drawing, I did find another mechanical anchor that I would like the strenght requirements for. The mechanical anchors are 5/8"x 4" power fasteners wedge-bolt anchors that attach the top of the cap to the permanent and temporary barrier.



Response
Date: 04-02-2012
The ultimate shear and tensile capacities for the 3/4" dia. x 4" long Powers Fasteners mechanical anchors in 4,000 pis concrete are 12.14 kips and 17.5 kips, respectively.

Anchoring Temporary barrier to Bridge Deck with Asphalt Overlay

Question
State: WI
Date: 03-26-2012

Dear MwRSF,

I know from previous discussions that the system to anchor temporary barriers to a bridge deck is not to be used on decks with asphalt overlays.

Is there a minimum amount of asphalt that MwRSF would consider an acceptable (e.g. 1" or 2" of asphalt ) or does all asphalt need to be removed?

Would there be an issue of removing asphalt from just underneath the barrier (i.e. the driving lanes would still have asphalt), and anchoring in the barrier to the bridge deck? Essentially the barrier would be keyed into the asphalt.

I know that the limit of this height reduction would be around 3 inches. But, are there other concerns about limiting the height of the temporary barrier based on current crash testing?




Response
Date: 03-27-2012
The tie-down system was intended for use on reinforced concrete bridge decks that did not include asphalt overlays. With overlays in place, the loaded barrier may allow the vertical anchor bolts to plow through the asphalt roadway material instead of being restrained by the rigid concrete deck, thus resulting in a longer moment arm and increased bending moment for the bolt region found at the asphalt-concrete interface. This change in loading and capacity may potentially decrease the safety performance of the existing, crashworthy tied-down barrier design. As such, we can not at this time recommend using this detail on bridge decks that contain an asphalt overlay. Although it may be possible that this design, or one very similar to it, may provide acceptable performance, we believe that future research is needed to investigate and evaluate various temporary tied-down barrier systems for this special application. Finally, we are not aware of any other temporary barrier solutions for bridge decks with asphalt overlays.

Even with only 2" of cover, the capacity of the bolt will be overcome without approaching the impact loads we expect.

Cutting away the asphalt in this area is an interesting option, but it brings barrier height reduction into concern. I don't believe there has been any TL-3 testing of safety shape barriers under 32" in height. There have been some shorter vertical barriers tested, but the concern with barrier climb much less when compared to safety shapes. The concern that arises is that the reduced height of the barrier combined with the sloped face could lead to increased vehicle instability and the potential for override of the barrier. Thus, we cannot recommend using this option as it would effectively reduce the height of the barrier to a level where the safety performance of the system is not quantified.

Thus, we cannot recommend reduction of the asphalt depth or cutting away the asphalt as solutions for the use of the bolt-through tie-down on a concrete bridge deck with an overlay without further analysis or testing.

Thanks


MGS Steep Slope

Question
State: IA
Date: 03-27-2012

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?




Response
Date: 03-27-2012

See my comments below! RED.


Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.




Response
Date: 03-28-2012

See my comments in blue below!

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT. Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that. Would you be able to provide a ballpark estimate of how much time such an analysis might take?

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out. We should modify our standard to show the curb ending under the thrie beam. Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements. When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches. Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...




Response
Date: 03-28-2012

See comment's below in green.

Due to right-of-way restrictions, we have a steep slope situation on a bridge replacement project where we will need to install steel beam guardrail. Design speed of the roadway is 60 mph and the traffic volumes are in the 1500 vpd range.

**For steep slope hazards, MwRSF previously developed two W-beam guardrail systems " one for metric-height rail and one for the MGS. In both scenarios, the steel post was centered at the SBP.

****OK

In your opinion, would it be satisfactory to install the MGS on a 10:1 pad such that a 1.5:1 foreslope begins 24 inches behind the face of rail?

**As noted above, the MGS option is being considered where a 10:1 roadside slope is followed by a steep 1.5:1 fill slope. For this configuration, the MGS could be installed with as little as 2¾ in. of mostly level terrain behind the steel post in advance of the SBP. This scenario would likely provide similar post-soil behavior to that of a steel post installed at SBP of 2:1 fill slope. Thus, it would be recommended to utilize the MGS System for 2:1 Fill Slopes for your guardrail system used in the application presented above.

****OK

Also, could this same cross section be used throughout our approach guardrail transition (see attached BA-201 drawing)?

**At this time, we do not have a design solution for approach guardrail transitions placed with the steel/wood posts located at or near steep slopes. For these scenarios, our first choice would be to modify the fill behind the posts in order to provide 24 in. of generally flat terrain behind the posts. If that cannot be provided, then we would need to investigate whether another surrogate post (larger and/or longer) could provide comparable post-soil behavior to the original transition post founded in level terrain. Although the later could be done, it would certainly require additional analysis and possibly some additional bogie tests. Please let us know whether you desire MwRSF to further explore the second option. Thanks!

****Our ROW is so restricted on this project that we are unable to provide 24 inches of flat terrain behind the posts throughout the AGT. Having you conduct some additional analysis and/or testing would be desirable, but I'm doubtful our project timeline would allow for that. Would you be able to provide a ballpark estimate of how much time such an analysis might take?

**I suspect 1-2 days would be adequate to acquire and review prior bogie testing data and perform simple hand calculations. However, if we cannot find sufficient information and results from prior bogie tests, then a bogie testing program would be needed. At this point, staff could look into this issue later this month.

P.S. " On another note, the CAD details provided in the attached pdf file depict the use of the wedged-shape drainage curb below the thrie beam rail. In the original testing program, the curb ended at the midpoint of the symmetrical W-beam to thrie beam transition section and started the taper to the ground at the thrie beam end of the section. All crash testing was performed near the bridge end, and no testing was performed near the start of the W-beam to thrie beam transition section. Later, the MGS stiffness transition was developed for use in combination to a thrie beam transition with half-post spacing but without a curb. This stiffness transition was adapted to other common AGTs. Your detail depicts the curb to end at the start of the asymmetrical transition section. Due to concerns for the small car to wedge under the rail, the concrete curb should preferably end at the thrie beam end.

****Thank you for pointing this out. We should modify our standard to show the curb ending under the thrie beam. Note, however, that extending the curb through and beyond the asymmetrical transition section is unavoidable in some cases due to drainage requirements. When a curb is required in this region, it has been a long-standing practice of ours to limit the height of the curb to 4 inches. Obviously, the slope at the bottom of the rail is more pronounced on the new asymmetrical transition compared to the old symmetrical one, but I'm not aware of any issues coming up regarding the wedging of small cars under the old transition (or the new one, for that matter). Of course, it might still be an issue. Maybe this is something we could investigate further (with pooled fund money perhaps). Seems like it would fit in well with a proposal to study the necessity of the 4-inch curb at the guardrail/bridge rail interface...

**Small car wedging started to occur with 1100C vehicle on stiffness transition project. However, the test results were satisfactory. Now, if we add a lower curb, it is our opinion that performance could be potentially degraded as wedging and snag could be accentuated. Also, the 2270P vehicle would contact MGS slightly higher while reaching stiffer region. We believe 2 tests would be needed to evaluate curb in advance of asymmetrical part. Also, it would be beneficial to test Iowa transition near bridge end but without curb. Future research would be extremely helpful to investigate whether or not these potential concerns are real.



MGS to low tension cable guardrail transition

Question
State: NE
Date: 03-29-2012

Can I use the steel pile anchor as used in the low-tension
cable barrier testing for the cable to MGS transition?

Test from reports TRP-03-155-05 & TRP-03-192-08

Or, should this be the concrete anchor block as used in the transition testing?

If we can use the steel I beam anchorge, where should it be placed?

Should I use the anchor in-line (under the w-beam) or should we place it a couple feet off set behind the w-beam like the transition testing used the concrete anchor?




Response
Date: 03-30-2012

I looked into your questions regarding adapting the low-tension cable barrier to W-beam transition over to the MGS and the type of anchorage to use. Looking at the details of the original cable to W-beam transition several issues arose in addition to the cable end anchorage you noted below.

1. The cable heights for the original system used a 27" top cable height with 3" cable spacing. This cable height and spacing correlated well with the W-beam barrier height used in the design and allowed the top cable to be run along the top of the W-beam and the bottom two cables to be run along the bottom of the W-beam as they were transitioned from one system to another. Connection of this same cable system to the MGS poses a greater challenge as the cable heights will have to be modified more to be moved behind the barrier using the existing design. The change in cable heights to match the MGS height could cause potential concerns for vehicles becoming snagged in the cable transition and may require changes to the cable bracket design.

2. The increased height of the cables in the transition as they meet and transition behind the w-beam will likely affect anchorage performance. The increased height on the cable anchorage will increase the vertical loading on the bracket which may act to pullout the anchorage, especially the driven pile anchor you referenced in your email. In addition, the change in cable anchor may require revision of the cable anchor bracket for attachment of the cables as well.

3. The driven pile anchorage you refer to below is capable of anchoring a cable barrier system. However, impacts near the end of the system would increase the loading of the anchor. As this anchor is designed to deflect in the soil as it develops load, the driven pile anchor may allow increased cable deflection as compare to the large concrete anchor used in the testing. This may be an issue that would need to be further investigated.

4. The original concrete anchor used in the cable to W-beam transition was allowed for use on sloped terrain behind the system. The driven pile anchorage would need to be re-evaluated if it was to be installed on slopes.

5. The original cable to W-bean transition was tested with both a BCT end terminal and a FLEAT end terminal. The BCT end terminal has not been tested with 31" high guardrail and would not be recommended for this application.

Based on these factors, we believe that further study is required to allow the cable to w-beam transition to be adapted for use with the MGS. It is also possible that the required changes to the design to adapt the transition to the MGS would require full-scale testing.

Thanks



Response
Date: 03-30-2012

Clarification: I am attaching 30" low tension cable to 31" MGS.

I see benefits to an increase in height of this system that should improve the system; as with the cable height raised from 27" to 30" & W-beam raised from 27 5/8" up to 31".

Please help me decide whether to use Concrete anchor as tested offset behind the w-beam or Steel.



Response
Date: 03-30-2012

I agree that there is much less concern for cable transitioning to the MGS when the cable height is 30".

However, the concern still exists that impacts near the end of the system would increase the loading of the anchor. As this anchor is designed to deflect in the soil as it develops load, the driven pile anchor may allow increased cable deflection as compare to the large concrete anchor used in the testing. As such, we would recommend that the tested concrete anchorage be used due to its larger size and increased capacity. .

We would also recommend that the anchorage be offset in the same manner as the test. The offset helps prevent the vehicle becoming wedged under the cables at the point where they attach to the ground anchor.

Thanks