Midwest States Pooled Fund Program Consulting Quarterly Summary

Midwest Roadside Safety Facility

04-01-2011 to 07-01-2011


Downstream Anchorage for One Way Roadway

Question
State: WI
Date: 04-05-2011

I have answers for your questions regarding the end anchorages for the MGS.


1. The post bolt, nut, and washer hardware specification is according to the Hardware Guide. Note that the specifications and the diameter are consistent for all post bolts, but the length of the bolt varies depending on the blockout and type of post used. For the BCT posts in the end anchorage, the specs should be:

a. 5/8" diameter x 10" long ASTM A307 guardrail bolt galvanized according to ASTM 153 (AASHTO M232 Class C) or ASTM B695 (AASHTO M298 Class 50)

b. 5/8" diameter A563 DH heavy hex nut galvanized according to ASTM 153 (AASHTO M232 Class C) or ASTM B695 (AASHTO M298 Class 50)

c. 5/8" diameter F436 flat washer galvanized according to ASTM 153 (AASHTO M232 Class C) or ASTM B695 (AASHTO M298 Class 50)

2. As a side comment, your details should show a 6" long, 2" Schedule 40 pipe sleeve in the BCT hole.

3. The post bolt hole in the BCT post should be 7 1/8" down from the top of the post. The detail you have shows a second hole at 10 3/8". This hole is for use with standard W-beam mounted at 27 ¾".

4. Another side comment, page 2 of your detail shows two different cable end fittings. We would prefer that you use the one shown on the bottom as it is what we test with. The end fitting should also be Grade 5 material in order to have sufficient ductility. Some people have ordered Grade 8 cable end fittings, but these are too brittle and can fracture under loading.

5. The cable anchor bracket is a standard part from the Hardware Guide (FPA01). I have attached the details from the hardware guide with the remaining dimensions.

6.


One last item to discuss was your desire to adapt the end anchorage to use white pine posts. Ron and I discussed this and we believe that it is possible, but it will require some further investigation. From the CRT work that Scott did in the white pine report, we could expect that the white pine BCT post would increase in size by around 2". This in turn would increase the size of the foundation tube and the angle of the cable to the guardrail. The larger foundation tube would increase the soil resistance of the tube, and it might need to be made shorter in order to prevent excessive loading of the anchorage. We think that this kind of change can be accomplished, but we would recommend component testing of the anchorage prior to recommending its use. The component test required would be a simple jerk test on the redesigned anchorage to verify its force vs. deflection properties.


Attachment: https://mwrsf-qa.unl.edu/attachments/8372ba093bf8b38818e8207fe11ae5b9.pdf


Response
Date: 04-05-2011

Here is a PDF with some question about the type 2 end treatment.


Attachment: https://mwrsf-qa.unl.edu/attachments/8372ba093bf8b38818e8207fe11ae5b9.pdf


MGS behind 6" curb

Question
State: IA
Date: 04-05-2011

What is your current recommendation regarding the maximum offset of the MGS behind a 6" AASHTO Type B curb for TL-3 conditions?
I am specifically interested in the case where the top of rail height is 31 inches relative to the gutter elevation.



Response
Date: 04-05-2011

I am enclosing a copy of a presentation that I gave a few years ago at a TRB AFB20 summer workshop. This presentation was given prior to conducting the failed TL-3 test at the 8 ft offset location. At that time, we had critical lateral locations where we believed one would need to transition from 31" to 37" MGS relative to the road. However, testing would be needed to evaluate these limits.

Originally, the research study was geared toward a performance limits study where we would increment through critical test conditions and locations. However, the project was refocused by the sponsors in the middle of the study where the performance limits portion was replaced with testing at practical locations and then later a lower test level following a failed TL-3 test. As such, we later obtained a successful TL-2 test at the 6-ft lateral offset but still were unable to explore all of the critical locations.

We really are unable to provide much guidance beyond our original MGS testing with the 6" offset at TL-3 and extrapolate some guidance at TL-2 due to the 2270P test. No small car testing was performed with combination curbs and barriers.


Attachment: https://mwrsf-qa.unl.edu/attachments/5f5b5e1b008ff7e01d29749dbe768ebd.ppt


CRT Posts Adjacent to Slopes for MGS Long Span

Question
State: WA
Date: 04-14-2011

I found your e-mail on the "Midwest Guardrail System for Long Span Culvert Applications" and was hoping you could offer some quick advice.


I'm using the Washington State Design Manual which provides the following installation cases (See Attached Figure 1.jpg).


I'm spanning 25' and need to install 3 CRT posts on each side of the culvert.
What are your thoughts on using 11' long CRT posts on each side as shown in CASE 6 above? I'm trying to stay within my R/W and not have to spend money on a retaining wall.


Attachment: https://mwrsf-qa.unl.edu/attachments/04e0e40b8c67901b713ea33d4dee2888.jpg


Response
Date: 04-15-2011

MwRSF has successfully developed and crash tested two W-beam guardrail systems to span across long concrete box culverts, such as those measuring up to 25 ft in length. For the first system, the metric-height W-beam guardrail was configured with a 27-3/4-in. top mounting height, while the Midwest Guardrail System (MGS) was utilized for the second configuration with a 31-in. top mounting height. For both designs, three 6-in. x 8-in. by 6-ft long wood CRT posts were placed adjacent to the long span using the 6-ft 3-in. post spacing. Beyond the CRT wood posts, the guardrail system was transitioned into a steel post, wood block, semi-rigid barrier system which also used 6-ft long posts and a 6-ft 3-in. post spacing. For both crash-tested systems, a region of level, or relatively flat, soil fill was provided behind the CRT wood posts.


We recommend providing 2 ft of level, or mostly level, soil grading behind the wood CRT posts. However, we understand that this can be difficult. As such, your inquired as to whether the wood CRT posts could be lengthened to account for the reduction in soil resistance resulting from an increased soil grade behind these six posts, especially when placed at the slope break point of a 2:1 fill slope.


Recently, MwRSF performed limited research to determine an acceptable MGS post length for a 6-in. x 8-in. solid wood post installed at the slope break point of a 2:1 fill slope. MwRSF determined that 7.5-ft long wood posts are an acceptable alternative when considering the 31-in. tall MGS placed at the slope break point of a 2:1 fill slope using 6-ft 3-in. post spacing.


The MGS Long Span system utilizes six CRT wood posts. A CRT post's moment capacity about its strong axis of bending is approximately 81 percent of that provided by the standard wood post. In the absence of dynamic component test results, it is believed that the six CRT wood posts could also be fabricated with the 7.5-ft length when used in the MGS Long Span system. If the steep fill slopes continue beyond the location of the CRT posts, then the guardrail would transition to the MGS for 2:1 Fill Slopes using either 6-in. x 8-in. by 7.5-ft long wood posts or W6x9 by 9-ft long steel posts.


Thus, for the cases you sent, we believe Case 2 is acceptable and that Case 1 and Case 3 would be acceptable if 7.5 long CRT posts were used. We cannot recommend the use of extended length CRT posts on steep slopes as you have shown in Cases 4-6. Determination of proper CRT post lengths for this type of installation would require additional analysis and testing in order to ensure proper function of the CRT's in the long span system.



MnDOT Questions Regarding Bridge Barriers

Question
State: MN
Date: 02-16-2011

We've had several meetings within Mn/DOT to discuss various options and criteria regarding traffic barriers on bridges and barrier/guardrail transitions and would like to have a conference call w/ either or both of you to get your opinions and insights on these issues (see specific details below). We're proposing a 2 hour telephone or video conference call the week of March 7th or 14th.


Could you please respond by indicating 2-3 times/dates that work for you? Do you have video conference capabilities?


Specific issues we'd like to discuss are outline below;


1). Our past/present policy is to place traffic barriers on bridges "plumb" or "level", regardless of the adjacent shoulder slope. (See Figure 1.jpg);


Any comment on this? Do you know if other states use a similar detail?


2). Our current policy on when to use a TL-5 barrier (42" high) on a bridge (in lieu of a TL-4, 32" high) includes the following criteria; Degree of curvature > 5 degrees (radius of 1145 ft) and speed > 40 mph. An incomplete survey of nearby states indicates they use the following TL-5 criteria;


Illinois

a). Structures with a future DHV (one way) x % trucks greater than 250

b). Structures located in areas with high incidences of truck rollover accidents.

c). Structures with a radius of 1000 ft. or less with truck traffic


Nebraska

All interstate structures, expressways, and over railroads.

They use a 34" (2" taller than Mn/DOT) TL-4 barrier on all other "on system" bridges.


Iowa

"Most interstate projects due to higher truck traffic"


Michigan/North Dakota/South Dakota

No set policy for use of TL-5 barrier.


Any guidance, criteria, or opinions on when to use TL-5 barriers on bridges?


3). At the end of a concrete barrier, where it transitions to a guardrail connection, Mn/DOT details a slight slope (5V:12H) to the top of the barrier (see top sketch below).
This guardrail connection/transition has been crash tested and approved for TL-3. What is the appropriate slope or taper length that should be used when transitioning from a 42" (or taller, glare screen barrier that is 4'6" tall, 6V:12H taper) barrier to a guardrail connection? (See Figure 2.jpg and Figure 3.jpg)


4). Based on recent test results regarding the New Jersey shape and the new MASH criteria do you have any recommendations or considerations for what shape and height of barrier should be used on new bridges going forward? We're considering single slope, vertical face, etc, and looking for advice. Which states (if any) do feel are headed in the right direction and may have standards that we can review and compare? FYI, our version of a vertical face bridge
barrier is shown in Figure 4.jpg.




Attachment: https://mwrsf-qa.unl.edu/attachments/2cdbb1ae4f1af2305dbaa1955b83204d.png

Attachment: https://mwrsf-qa.unl.edu/attachments/d2ab5f3385bfaee13dea84bbd3c7e4f5.png

Attachment: https://mwrsf-qa.unl.edu/attachments/09fed34d78897acab2de2dcfbd921a78.png

Attachment: https://mwrsf-qa.unl.edu/attachments/ab2359c8939a69b15c192ad3e30e7d1b.png


Response
Date: 04-14-2011

MnDOT had a conference call with MwRSF to discuss various questions regarding barriers on superelevations and cross slopes. A summary of the discussion is attached.


Attachment: https://mwrsf-qa.unl.edu/attachments/0bc31be142f86da6b0b4986d10164b8f.pdf


Highway sign support structures

Question
State: NE
Date: 04-21-2011

Can the universal breakaway steel post use the bolts placed in the bottom post/ plate using a keyhole slot?


We have access to the top of the bottom post - for repairs of the top post.


If we slide the bolt against something that keeps the bolt from rotating we can repair it from the top side without digging under it. I'm thinking of something similar to the slot on the flange under my toilet. The keyway does not need to be this long.


For a hex head the bottom would need ribs on each side or a channel from the larger opening back to the left under the small opening to hold the head from turning. (See attached Figure 1.jpg)


Attachment: https://mwrsf-qa.unl.edu/attachments/ac24705454736c95bcda630d7a3b19bf.jpg


Response
Date: 05-04-2011

It may be possible to utilize this concept to reduce the clearance on the bottom side of the lower plate to that required to place wrench on bolt head versus to place hand under to hold a nut/wrench. With slots, one would likely need to use thicker plate to account for the weakened region around the hole. However, it may be possible to do this with bogie testing in each direction to compare with final modified design. Bogie testing on final design would also be needed since changes were made between prior crash tests.


Maybe in an upcoming Pooled Fund year and/or if other states are interested in a simplified design, we could investigate this option.



Iowa Transition and Curbs

Question
State: IA
Date: 04-22-2011

You have alluded several times over the years that the Iowa guardrail transition might not need the 4-inch curb installed below it. Could you please follow up with FHWA on this issue to see if the 4-inch curb requirement can be waived? There are many situations where installation of the curb would be difficult.

Here is a link to our current bridge end post styles and guardrail attachments (Types A, B, C):

http://www.iowadot.gov/design/SRP/IndividualStandards/eba202.pdf

And here is a link to our current transition:

http://www.iowadot.gov/design/SRP/IndividualStandards/eba201.pdf



Response
Date: 05-02-2011

I have been asked by the State of Iowa to follow up on an old issue pertaining to approach guardrail transitions. In the mid to late 90s, MwRSF successfully developed, crash tested, and evaluated two thrie beam approach transition systems for use in shielding and attaching to the ends of safety shape and vertical concrete parapets. A series of three acceptance letters were prepared on this topic - B-47, B-47a, and B-47b - based on the NCHRP Report No. 350 guidelines. Crash tests were performed on both steel and wood post systems which were spaced on 1 ft - 6 3/4 in. centers near the bridge/parapet end. During the R&D effort, the Pooled Fund States stated that they may desire to utilize a curb at the ends of the bridge rail to better accommodate water drainage. As such, MwRSF incorporated a 4-in. tall wedged shaped concrete curb under the thrie beam region. MwRSF believed that the curb's presence would provide a critical evaluation as impacting front wheel/rim could become wedged under and upward into the tighter space and/or increase vehicular instabilities. If testing with the curb was found to be satisfactory, then MwRSF researchers believed that the non-curb option would also be acceptable.


Later, TxDOT and TTI cooperated to further investigate some changes to the approach guardrail transition, including an elimination of the steel connector plate under the thrie beam end shoe, the elimination of the curb, a different geometry to step back the lower concrete toe near the barrier end using steeper flare rate, and non-use of special chamfer at parapet end.

In the MwRSF testing program, a special connector plate was used to allow the thrie beam and show to remain vertical while attached to a sloped face of the NJ safety shape. In the TTI testing program, the steel connector plate was eliminated, and the rail/end shoe was twisted backward near the top region to lay on the upper sloped face. Subsequently, the modified transition was crash tested under NCHRP 350 using a pickup truck. During this test, the pickup truck rolled over. TTI researchers later concluded that the curb was necessary to provide acceptable safety performance even though several other changes were incorporated. In MwRSF's opinion, the other system changes could also have contributed to the poor barrier performance. Unfortunately, this testing program was stopped and only continued with the development and testing of a TL-2 W-beam transition.


During NCHRP Project No. 22-14(2), MwRSF conducted another pickup truck crash test into the approach guardrail transition system noted above in Paragraph 1 but using the forthcoming MASH criteria. One crash test was performed on an identical transition system which included the curb underneath. Following testing, the barrier system was judged to provide acceptable safety performance. In this effort, higher lateral barrier deflections were observed as compared to those found in the prior successful MwRSF testing program. Once again, MwRSF considered the system to be acceptable both with and without curb. However, State DOTs are unable to use the system without curb due to the language provided in the original B-47 series of acceptance letters.


To date, no full-scale crash testing has been performed on an identical approach guardrail transition system that excluded the wedged-shaped curb.

As such, I am inquiring as to whether FHWA continues to maintain that the approach guardrail transition must be installed with the curb located underneath or whether there has been any changes in stance on this issue.

Regardless, I will forward your FHWA's response to the State of Iowa and the Pooled Fund Program member states. Thanks again for any clarifications on this issue in advance!



Response
Date: 06-07-2011

We have reviewed the crash tests videos of subject testing and offer the following appraisal of that review.


Via careful review of crash tests provided by TTI, we conclude the decision which TTI has rendered from their crash test is correct (i.e., transition w/o curbing failed).


We however remain open minded on any additional testing brought forward to our attention in the future.


Therefore and until additional testing is presented, FHWA continues to maintain that approach guardrail transition must be installed with the curb and/or rub-rail located underneath rail element to be considered acceptable installations constructed on NHS system.



Response
Date: 06-07-2011
Thank you very much for the additional review and consideration on behalf of the State DOTs. If any new crash tests are performed in the future which are believed to potentially alter FHWA's opinion, we will bring this material and results to your attention. Once again, thank you for the additional examination. I will forward FHWA consistence stance on this matter to the Pooled Fund Program member states.

Low Tension Cable Barrier Adjacent to 2:1 Slope

Question
State: NE
Date: 04-26-2011

I have a question about 16' spacing. See from email exchange. From below "¢â‚¬Ã...“ it was stated -"However, I know there are states that would install with 16' spacing if the cable was 2' from a 2:1 slope as you show below and I don't believe that is a major concern."

I prefer to use the 16' spacing 2' from a 2:1.

Would you confirm that this is the current practice and reference any testing to support this?

The following email correspondence summarizes the discussion:

Roadside Cable Guardrail:

We are updating our cable plans to include the in-line anchorage with 3 cables.

What is the current guidance about slopes behind the cable guardrail?

We have many installations of the cable guardrail on the shoulder of our highway 2' from a 2:1 slope

I have considered using 4 cable as tested for the median 14" to 34", what are the pros / cons of this.

MGS 31":

What guidance can you give for implementation of 31" including the Bridge Approach Section?

****************************************************

Cable barrier " Are you using the generic (low tension) cable barrier where you install an anchor every 2000'? Most states have changed over to require the high tension systems that don't require the intermediate anchors.

In general, placing the cable 2' in front of a 2H:1V slope is considered acceptable. I know Brifen tested a system at the edge of a vertical drop and it worked and the folks at MwRSF tested a system 4' in front of a 1.5H:1V slope with 4' post spacing.

http://trid.trb.org/view.aspx?type=MO&id=855946

http://www.brifenusa.com/

I like the systems that are now being tested on 4H:1V slopes. The MwRSF is testing a generic system and several manufacturers have tested their systems. In general, these systems have 4 cables that cover a wider range of heights to help catch the smaller vehicle that might try to go under as well as the larger vehicle that might try to go over. Having a system that was tested on a 4:1 slope gives a better tolerance to the slope issues that we have seen. The approach being taken at MwRSF is to test the barrier at the worst location and then it should work anywhere in the median. There are some debates on this but we hope to reach agreement on the number of tests for median testing so that everyone is approaching it consistently (this will be discussed in May).

The biggest con of the 4 cable systems tested on 4:1 slopes is the additional cost but I think it will be worth it.

31" Guardrail

I believe that states should switch over to the 31" guardrail for new installations. It doesn't cost any more (both Washington and Illinois found no additional cost) and provides much better performance in a number or areas. In Washington, we adopted it because it allowed us to reduce the height as a result of overlays, and still have a crashworthy system (standard guardrail at 28" height passes MASH testing).

For transitions to bridges there are a couple options. My preference is to use a "stacked" W-Beam. You maintain the 31" height of the rail but add a w-beam rub rail. The stacked W Beam was tested under NCHRP 350 for a 28" guardrail and when WSDOT was developing the plans, Dick Powers with FHWA HQ confirmed that we didn't need to retest with a 31" height. Attached is a link to the WSDOT standard plan

http://www.wsdot.wa.gov/publications/fulltext/Standards/english/PDF/c25.18-01_e.pdf

The other option is to use a thrie beam transition that was developed by MwRSF.

http://www.wsdot.wa.gov/publications/fulltext/Standards/english/PDF/c25.20-04_e.pdf

I attached a couple of photos of these transitions.

*************************************************************

Low tension cable:

I like the low tension cable because of its safety record & for the anchorage in the sandy soils we have in Nebraska.

I agree that the low tension (generic) system is a very good system. Most states have gone to the high tension to reduce deflection distance, reduce maintenance, and reduce the number or anchors needed. To me the biggest of these was the maintenance because we often had to convince the maintenance folks in order to get it installed.

We have implemented this 3 cable low tension system cable w/ heights @ 30" 27" & 24".

Most Low tension systems I am aware of have the bottom cable at 21". Some states have installed it with the top cable at 33" and with 6" spacing between the cables. For medians, lower cables are being used to address the underride issue. I am not aware of a crash tested system with the lower cable at 24".

How close can this be placed to a 2H:1V? And with what post spacing?

The testing of 4' post spacing @ 1' from a 1.5H:1V failure, and 4' post spacing @ 4' from a 1.5H:1V pass has us considering 4' post spacing at 4' from a 2H:1V " this is impossible in some locations with 2H:1V at the edge of shoulder.

I am not sure I have an absolute answer for you on this so I will give you some opinion.

I believe the slope behind the cable is much less of a concern than the slope in front of the cable. The critical concern for cable is having the bumper of the vehicle engage at least one cable.

I have seen tests of a high tension system in front of a vertical drop-off. While the tires went over the edge, the cables engaged the body and brought it back.

I am aware that MwRSF tested a system in front of the 1.5:1 slope with ¼ post spacing. However, I know there are states that would install with 16' spacing if the cable was 2' from a 2:1 slope as you show below and I don't believe that is a major concern.

Our past implementation (See sketch.jpg);

This sketch with 16' post spacing has seemed to work very well.

Now we are updating our plans to the inline steel I beam end treatment & I'm wondering " is this the best system?

Is this still the generic system but with a proprietary end treatment like what is used on the high tension systems?

I want to implement the 4-cable 14" to 34" " the system MwRSF tested & met 350 in the median 4:1 down to 4:1 up section.

This system will improve catching the variety of bumper heights.

Would this be acceptable to implement this system for roadside use?

I think the systems that are being tested on 4:1 slopes would be excellent for roadside use and may change the way we treat slopes and roadside objects. I was thinking the MwRSF system was higher than 34" and I know they are looking at lowering the bottom cable. I think the considerations on the roadside make it easier since you wouldn't have a back slope where the vehicle might bottom out.

Let me know if I created new questions or not.

******************************************

MwRSF said they are talking 13 inches for the bottom cable and 45 inches for the top cable for a high-tension four-cable system. Their research is showing that early cable-release from the post is critical to prevent the cables from causing serious damage to impacting vehicles.

I suggest you contact UNL/MWRSF for the latest scoop on their recommendations.


Attachment: https://mwrsf-qa.unl.edu/attachments/f606278bab4a8cca2573fdd055fbd880.jpg


Response
Date: 05-02-2011

Based on the low-tension cable barrier research I conducted, I had no instances of testing in the last 30 years of a cable barrier 2 ft from a 2:1 slope.
There was sedan testing conducted in 1965 by New York where a low-tension cable barrier was placed 18" from the break point of a 2:1 slope; however, this was never tested with a truck and the truck performance on this configuration would be necessary for approval according to 350, much less MASH.


Based on my research I would be unable to recommend that configuration.
Unless some research which I have not seen turns up somewhere, this would be considered untested.



Response
Date: 05-02-2011
We were unable to find any prior pickup truck research/testing demonstrating that the 3-cable, low-tension barrier with 16-ft post spacing is crashworthy when installed 2 ft from the slope break point of a 2:1 fill slope. Please let us know if you locate the supporting research so that we can add it to our literature review.

Minimum Rail Height for MGS Long Span and MGS Transition to Rigid Barrier

Question
State: WI
Date: 04-28-2011

Does MwRSF has any recommendation for the minimum rail height for the MGS long span or MGS transition to rigid barrier?


On overlay projects, the can overlay the bridge or the roadway. These overlays will reduce the effective height of the thrie beam.

I believe that we will have to accept that the transitions to rigid barrier may be lower than 31".



Response
Date: 04-29-2011

With regards to the MGS long span, we recommend that the system be installed at 31". The MGS long-span guardrail was tested with a top rail height 31.0 in. Previous reports on the standard MGS system have allowed installation at heights as low 27.75 in. However, the reduced post embedment of the MGS system was a major factor in the performance of the MGS long-span design. Thus, in order to retain the improved load distribution and reduced rail strains, the MGS long span should be installed with a top rail mounting height of 31.0 in. Reduction of the rail height below this value will likely reduce the performance of the barrier system.

With respect to the MGS transition, the answer is a little less clear. We would expect that a slightly lower transition height would allow for redirection of the vehicle. However, all thrie beam transition testing that I have seen was conducted at 31" or at the metric height of 31 5/8". In addition, lower transition heights would expose more of the concrete barrier at the downstream end of the transition and create a potential snag hazard. Thus, we would generally recommend a height of 31" for the MGS transition as well.

We understand the need for states to overlay their roadways, but it can have detrimental effects on barrier performance. With respect to transitions, the issue has not been adequately addressed with respect to the current MASH vehicles and impact conditions.

We cannot say with certainty that overlays adjacent to the MGS transition will not affect its performance. Adding a 2"-3" overlay in the transition area will increase the impact load height on the post and the relation of the vehicle front to the barrier. This in turn could increase the moment on the posts and affect the lateral deflection and stiffness of the system. In addition, there would be concerns for vehicle stability as well as an increased potential for wood posts in the transition to fracture.

Soil Gaps Around Foundation Tubes

Question
State: WI
Date: 04-29-2011

If I remember correctly, during our meetings with MwRSF staff, there was some discussion about beam guard failing because the gap between the soil tubes and the soil surrounding it was 10 inches.

WisDOT is putting the final touches on its existing barrier guidance and would like to place some statement in our design guidance to instruct our designers to review soil tubes and the soil around the tubes. What gap distance between the soil tube and the soil around the soil tube should corrective action be taken (e.g. 10" of gap).



Response
Date: 04-29-2011
We would not recommend allowing a minimal soil gap at any existing anchorage foundation tubes. As general guidance, we would recommend that there be no visible soil gaps around foundation tubes. Visible soil gaps of 1/3" or less could be repaired by tamping and recompacting the soil around the foundation tubes. For soil gaps larger than 1", we would recommend that the anchorage be pulled and reinstalled with the tubes plumb and reset.

Wildlife Crossing Holes in Barriers

Question
State: MO
Date: 04-29-2011

Our environmental specialist that handles threatened and endangered species has proposed a small wildlife crossing for single-slope barrier (See attached Figure 1.jpg). He met with a contractor who ensured him the construction of the barrier as shown is entirely feasible, even for slip forming.


Given the diameter of the opening, I suspect this would neither add a snag point nor in any other way significantly decrease the crashworthiness of the barrier. I'm basing that on 3.4.2.1 of the Roadside Design Guide which states that single cross drainage structures with diameters less than or equal to 36 in. are traversable. Of course that is referring to inslopes and not barriers.


The dimensions shown below were those given to me, but I would also like to know if I could extrapolate your opinion to an 18 in., 21 in., or 24 in. CMP. Any diameter larger than that would begin to conflict with the reinforcing steel in the barrier.


Do you concur with my analysis of this situation?


Attachment: https://mwrsf-qa.unl.edu/attachments/ae34140fd72f817fc5ccf2e28ba7a63f.jpg


Response
Date: 05-02-2011

It is well known that the structural capacity of a barrier is controlled by its ability to safely contain and redirect the heavy, taller vehicles which impact at high-speeds. Thus, a structural analysis could be performed to modify the barrier reinforcement in regions where drainage holes are desired. Drainage openings in concrete parapets should be acceptable as long as the hole does not interfere with the structural steel reinforcement and the barrier capacity is not weakened in this region. If the 42-in. tall, single-slope concrete barrier section is weakened with a lateral hole, then the longitudinal and/or vertical steel reinforcement surrounding this area may need to be increased in order to provide equivalent or greater barrier capacity.

In general, it would be my first impression that the size of the half hole (i.e., 15 in. diameter by 7.5 in. tall) through the barrier would not significantly degrade barrier performance as long as the vertical steel anchorage could be placed at its normal locations adjacent to the opening, adequate longitudinal steel is provided, and as long as the small car's front wheel does not negatively interact with the downstream side of the opening. Small cars have wheel center heights ranging from 10 to 12 in. As such, it would be necessary to ensure that the wheel/steel rim does not snag on the downstream side of the hole, cause increased vehicle climb up the barrier, or result in barrier override or vehicle instability.

For rigid, vertical shapes, it would seem reasonable to assume that a hole height less than or equal to 50% of small car wheel center height, or 5 to 6 in., would not post too much concern for small car instability, climb, or snag. For rigid, safety shape parapets, the safety risks would be accentuated due to increased lateral snag distance at the downstream side of hole caused by a sloped, front face on a barrier system. As such, the maximum allowable hole height would likely be lower for safety-shape parapets as compared to vertical parapets. Previously, MwRSF provided guidance as to a 3-in. maximum allowable hole height for safety-shape parapets. Single-slope parapets would likely fall somewhere between the two noted above, or between 3 and 6 in. However, it should be noted that no official safety guidance or criteria exists for configuring the size and height a drainage holes in rigid, concrete barriers.

Safe half-hole heights for single-slope parapets are believed to fall somewhere between 3 to 6 in. using my best engineering judgment. It would seem possible for them to work 7.5 in. as well. However, my concerns for small car rim snag on hole edge, wheel climb on hole, instability go up as we increase hole size. If I were picking a recommended upper limit for the single slope barrier, I would say 5-6 in. for half-hole height.

Based on the enclosed information, I am unable to extrapolate the noted guidance to the larger opening sizes shown in Figure 1.jpg.



Retro-fitting a Single Slope Concrete Barrier

Question
State: WI
Date: 04-29-2011

An existing project has had to remove a section of single slope concrete barrier to work on a structure. The existing single slope barrier was cut in the middle of the run. Now the existing single slope barrier has a free end. The region is asking what should they do when the match a new single slope barrier into the old single slope barrier.

As I see it we have two options:

1. Let the existing single slope barrier have a free end and place the new single slope concrete barrier per standard specifications.

2. Drill into the existing single slope barrier. Place reinforcement steel to transfer impact forces into the new single slope barrier.

Our Standard Detail Drawings are located at:

I'm concerned about transferring impact energy from the old barrier to the new. But I am not sure that we can easily drill into the existing barrier to place reinforcement.

Could MwRSF provide a recommendation on which way to proceed? If installing reinforcement is the desirable option, could MwRSF provide a design?



Response
Date: 04-29-2011

I would use dowels to connect the two sections. The dowel bars should be the same size and at similar locations as the existing longitudinal steel. Further, the dowels should be embedded into each end (old and new) far enough to develop the full load of the bar (splice/development length of bar or specified by the epoxy used to secure dowels in old barrier section.).



Cable Guardrail on Slope and End Terminal Questions

Question
State: NE
Date: 03-14-2011

The new in-line cable end treatment requires post 3 through 7 to be spaced @ 16'. What is the offset to a fixed object in this area?


When we design a long run of guardrail in the past we have used an intermediate anchorage section. Is this still necessary?


If so, is there a design for the new in-line intermediate anchorage section?


The spacing in front of a 1.5:1 slope requires 4' post spacing. Is it acceptable to have 16' post spacing then 4' spacing?


Or, is there a suggested length of transition of 8' post spacing?


Have you been able to run a simulation when our slope is 2:1, with a 2% lane and 4% shoulder slopes? I think this will keep the front tire on the slope and not require the 4' post spacing.

Response
Date: 05-03-2011

See responses in red.

The new in-line cable end treatment requires post 3 through 7 to be spaced @ 16'. What is the offset to a fixed object in this area?


**A 2000P pickup truck was crash tested at the length-of-need of the end terminal at the TL-3 conditions of NCHRP Report No. 350. The vehicle impacted post no. 3 which was 15 ft downstream from the upstream steel anchor post. For this crash test, the working width was reported to be approximately 84 in. when using a 254-ft long installation.


**Please note that the target impact angle for this test was 20 degrees, as required by NCHRP Report No. 350. The new MASH guidelines now utilize an impact angle of 25 degrees. With higher impact angles, one would expect higher angle loading and slight increases in anchor movement, thus resulting in greater barrier deflection and working width near the system ends.


When we design a long run of guardrail in the past we have used an intermediate anchorage section. Is this still necessary?

**As noted above, the test installation was 254 ft long. For longer test installations than denoted above, dynamic barrier deflections and working widths would be expected to increase.


**A prior Pooled Fund R&D program resulted in the successful development, testing, and evaluation of three alternative anchor systems in lieu of the large cast-in-place reinforced concrete anchor blocks. However, the R&D program did not evaluate changes in anchor spacing. As such, we would recommend that NDOR continues to utilize an anchor spacing equal to or smaller than that currently specified, especially since barrier deflections and working widths could be greater with the use of the alternative anchor options.


If so, is there a design for the new in-line intermediate anchorage section?

**The alternative anchor options were developed for terminating and anchoring the ends of the three cables. I am unclear as to the difference between end anchor hardware and the anchor hardware used at intermediate anchor sections. Please forward those details to us for review as I am unaware of prior crash tests performed to evaluate the safety performance of the overlapped cables with two intermediate anchor sections crossed in opposite directions.


The spacing in front of a 1.5:1 slope requires 4' post spacing. Is it acceptable to have 16' post spacing then 4' spacing?

**The SdDOT three-cable guardrail to W-beam transition utilizes a cable barrier with 16-ft post spacing that transitions into a cable barrier with 4-ft post spacing in advance of the BCT W-beam terminal. No intermediate post spacing was integrated into this original SdDOT design. More than 60 ft of cable barrier with 4-ft spaced posts was used to prevent pocketing near the BCT end. No testing was performed upstream of the 4-ft post spacing design. However, I do not believe that the reduction in post spacing would create a significant pocketing concern for large vehicles or penetration concern for small cars when used in combination with the standard cable hook bolt.


**For the three-cable barrier with 4-ft post spacing in front of a 1.5:1 fill slope, MwRSF performed a 2000P crash test according to the TL-3 conditions of NCHRP 350. An 820C small car test was not performed nor deemed necessary by the MwRSF team. The successful 2000P crash test resulted in nearly 125 in. of dynamic deflection when placed 4 ft from the slope break point, thus resulting in the vehicle extending nearly 6 ft off of the slope. The vehicle's lateral extension off of the slope further accentuated the barrier deflections observed in the 2000P test.


**TTI crash tested a 3-cable barrier on level terrain with a 16-ft post spacing at TL-3 of NCHRP 350. This testing resulted in 3.4 m (134 in.) of dynamic deflection, which was slightly larger than the deflection observed above in the ditch. Since it is uncertain where the 4-ft post spacing will end w.r.t. the ditch start/finish, it would be reasonable to expect the 4-ft spacing to overlap regions of level terrain. When the 4-ft post spacing is installed on level terrain, dynamic deflections would likely be reduced below 125 in.


**Although it would not be deemed necessary at this time, one may consider the use of 4 or 5 spans with posts spaced on 8 ft centers prior to reaching the 16-ft post spacing region.


Or, is there a suggested length of transition of 8' post spacing?

**See comments noted above.


Have you been able to run a simulation when our slope is 2:1, with a 2% lane and 4% shoulder slopes? I think this will keep the front tire on the slope and not require the 4' post spacing.


**No work on this project has been performed. This work was included in a Pooled Fund study that was not funded in the Year 21 final program. I will copy this request to John Reid and Bob Bielenberg to determine what level of effort would be required to conduct this specific request



Sand Barrel Attenuator Guidance

Question
State: IL
Date: 03-25-2011

Is there any later guidance about an apparent discrepancy between the design procedure in the 2002 RDG and results from crash testing? I've seen that a version of the RDG is available with a 2006 update to Chapter 6, but do not understand that Chapter 8, including the sand barrel design issue is changed.

Table 8.3 in the 2002 RDG gives a transfer of momentum method. It gives lower predicted deceleration than observed in crash tests. The example attached uses data from Test 3-41 in FHWA acceptance memo CC-29 (6/28/1995). The calculation predicts a maximum deceleration of 8.0 G's, while the crash testing produced 14.2 G's.
I've also included an example based on test 3-41 from acceptance memo CC-52 of 7/10/1998. The latter calculation predicts 7 G's, while testing documented 9.5 G's.

By changing the distance over which the impulse acts to about 1.7', the first (CC-29) calculation can be calibrated to match the crash test result. I tried a similar approach for tests 3-41 in acceptance memo's CC-28 and CC-52 and found calibration of this parameter ranged from 1.3 to 2.2 feet.
The lower value of 1.3 feet for test 3-41 of memo CC-28 involved a heavier pickup truck (4934#) and higher speed (68 mph). In general, it appears that the more energy in the impacting truck, the higher the calibration factor is needed.

Because the transfer of momentum is elastic, some energy is going to other actions "¢â‚¬Ã...“ breaking the drums, deforming the vehicle, and bulldozing the sand.
The sand and the drums have some variations from test to test.
If the systems were inelastic and frictionless, the transfer of momentum equations would correctly predict the change in velocities, but accelerations would be excessive.
This appears to be a more complex problem than presented in the RDG methods.

I'm looking into this because we would like to adopt sand barrel arrays to comply with MASH, and don't find any crash testing under these criteria.

Response
Date: 05-05-2011

think there are a couple of items here that are leading to your comparison issues.

First, the simple inertial procedure in the RDG generates a change in velocity based on and impulse momentum calculation. From this, you can estimate an average deceleration due to the impact of the vehicle with each row of barrels. This is an average deceleration and not an instantaneous acceleration like we measure in testing. Thus, the estimation procedure will always yield a lower acceleration value than the full-scale test. To deal with this, we have typically limited the average deceleration values for our sand barrel array calculations to 12 g's or less. This is conservative, but it is done to account for the difference between the actual and average calculated accelerations. The other mechanisms you mention have some effect on the acceleration as well, but the issue you are seeing is due mostly to the calculation of average acceleration rather than instantaneous.

I noted that you are attempting to vary the deceleration distance to yield better results. We would not recommend this. As noted above, you are only calculating average decelerations, so we cannot expect the values to match. Instead, design the arrays with the 12 g limit noted above. We typically use 36" or 3' for the deceleration distance which is the typical diameter of the sand barrels.

I have attached a spreadsheet that we have developed to analyze sand barrel arrays. Feel free to use it to develop you MASH compliant versions. The sheet allows you to vary the array configuration for different speeds and vehicle masses and notifies you with colored formatting to indicate when the array has slowed the car sufficiently or exceeded the deceleration limits.

One thing I should note is that when designing these barrel arrays you should consider more than the head on impacts. In addition, you should address a coffin corner type impact similar to test designation 3-36 in MASH. This should be done to ensure that vehicles impacting downstream of the first barrel of the system do not experience excessive deceleration but are still brought to a controlled stop. In addition, if you the barrel array is used in installations where it can be impacted from the reverse direction, you should consider additional smaller barrels on the downstream end of the system to prevent excessive deceleration. Examples of these additional barrels can be found on page 97 and 98 of TRP-03-209-09.


Attachment: https://mwrsf-qa.unl.edu/attachments/5ab277dfd5c9ea899634508730098eb8.xls


High Tension Cable Guardrail with a 1V:3H Slope Behind

Question
State: WY
Date: 05-06-2011

We have a situation where we want to place guardrail on a 1V:8H slope which extends a few feet behind the guardrail, before breaking off on a 1V:3H slope. Would a high tension cable guardrail such as Trinity's TL-3 system perform adequately in this situation? The standard system deflects up to around 9 feet (16 foot post spacing), so an impacting vehicle would be traversing the 1V:3H slope during redirection. I believe this deflection would not extend beyond the 1V:3H slope. Please see the attached drawing.


Attachment: https://mwrsf-qa.unl.edu/attachments/823a5ec8a95611046bb0f403adb63b9f.pdf


Response
Date: 05-09-2011

We looked over your installation and compared it with the testing we conducted with the MGS on 8:1 slope and our current cable testing. In that testing, we found that with a 5 ft offset, the MGS (with a 31 in. top rail height) just captured and contained the 2270P vehicle. We don't have similar cable testing on 8:1 slopes to compare with, but we know that the cable can redirect vehicles on the 4:1 slope as long as the vehicle is effectively captured. Based on this comparison we would recommend that the Trinity system be installed with the 2 ft offset rather than the larger 5 ft offset. The decreased offset should compensate for the cable heights of the Trinity system. In addition, offset should allow for capture of the vehicle and redirection even with the 1:3 slope behind the system. We believe that if the Trinity system captures the vehicle then the system will redirect the vehicle even if it intrudes on the 1:3 slope to some extent.


This short offset recommendation is based on the fact that the Trinity system has not been tested on approach slopes, and thus it is difficult to make recommendations regarding its performance in those situations. In our experience, it is necessary to capture the front corner and rear corner of the vehicle with a cable system on slope to capture and redirect the vehicle. Therefore, the shorter 2 ft offset will gives more confidence that the system will perform safely.

Short Radius Guardrail Installation

Question
State: WI
Date: 06-01-2011

James Nall of Mesa County Colorado contacted me about WisDOT's short radius terminal. He then followed up our discussion by contact staff at TTI, and FHWA.


I was wondering if MwRSF could also take a quick look at this Mr. Nall's problem.


My first guess is to remove the beam guard and delineate, but this is just base on the fact that the installed system is not crashworthy and it may not be possible to install any crashworthy alternative without significant modification to the drainage canal or service road.



Response
Date: 06-01-2011

This would appear to be a low- to moderate-speed, low- volume crossing situation. From a roadside safety perspective, one would possibly argue for complete removal of the system if adequate end treatment and shielding of the hazard via the ends cannot be achieved. I realize the public wants the feeling of security with a rail for pedestrians and bicyclists, but rail height for those purposes is inadequate anyway. Without seeing the width and available space along service roads at ends, it is difficult to determine what other options may be available.



Alternative Approach Guardrail Transition

Question
State: IA
Date: 06-01-2011

Iowa DOT is considering the use of an alternative approach guardrail transition, based on the results of TRP-03-210-10. We currently use the wood-post version of the "Adapted Iowa Transition" that's shown on page 167 of the report. However, we were wondering if there might be a simpler design out there that we could use. Our preference is a design that uses larger post spacing near the bridge end, similar to the layout of the W6x15s used in the report. We are interested in both steel and wood post designs.

I have attached a drawing that shows the three most common bridge end post shapes that are encountered in Iowa. If you need exact dimensions on these, let me know. Type A on page one is the current bridge end post design and is the most common by far. If possible, we would like to be able to use the same AGT for all three types.

Our only constraint is that the transition cannot be longer than our current design (25 feet from the bridge to the end of the W-to-Thrie transition piece).

Can you see what other designs are available, if any, that could be adapted to meet our needs?



Response
Date: 06-01-2011

I have reviewed all of the FHWA accepted transition designs as well as the MwRSF tested transition designs. From this review, there seems to be two different styles of thrie beam transitions to concrete bridge rail:

  1. Use posts similar to the line posts (W6x9 or 6"x8" wood posts) but with increased embedment depths at ¼ post spacing (18.75"). A prime example of this style of transition is the transition you are currently using.
  2. Use significantly larger posts (W6x25, W8x24, or 10"x10" wood posts) at 37.5" spacing. These larger posts are typically chosen so that the first post adjacent to the bridge rail can be omitted for drainage purposes (see page 165 of the report you noted, TRP-03-210-10). However, do to the omitted post and the oversized transition posts need to compensate for the gap, the recommended stiffness transition had a length of 31'-6" to the US end of the W-to-thrie transition element. You expressed that you needed to stay at your current 25' length, so this probably is not the answer you were looking for.

The original approach transition utilized to developed the stiffness transition shown in report TRP-03-210-10 was designed for attachment to a steel post, thrie beam and steel channel bridge rail system. Thus, I would not recommend utilizing this transition for attachment to concrete barriers until further evaluation/crash testing proves its crashworthiness.

I have attached the recommended wood post alternative to the transition you noted from page 167 of the steel post transition report. The attached figure comes from the wood post transition report that is currently under in-house review at MwRSF and should be sent to the Midwest Pooled Fund States in the near future.


Attachment: https://mwrsf-qa.unl.edu/attachments/0749198f6d66d27fdfee824ea34bf4e3.jpg


Iowa Transition and Curbs - Blockout Length and Depth Inquiry

Question
State: IA
Date: 06-05-2011

I've got a follow-up question regarding steel-post versions of the Iowa transition: Are we limited to the version shown in the ITNJ report (6.5-foot W6x9s with tapered tube blockouts)? Or is there an allowable substitution we can make in order to use the 19-inch wood blockouts with steel posts?



Response
Date: 06-06-2011

The Iowa approach guardrail transitions were developed and crash tested with steel and wood post options. The systems were configured as follows:

Steel Post " Test ITNJ-2

6.5-ft long by W6x9 Steel Post w/ 49" embedment depth

17.4-in. long by TS 7"x4"x3/16" steel tapered blockout w/ 3.2 in. extending below lower thrie beam bolt

Wood Post " Test ITNJ-4

7-ft long by 6"x8" Wood Post w/ 52" embedment depth

18-in. long by 6"x8"wood blockout w/ 3.1 in. extending below lower thrie beam bolt

Several years ago, MwRSF developed and crash tested transition designs which utilized 19-in. long wood blockouts. In this case, two tests (STTR-3 and 4) were successfully conducted with a 4-in. extension below the lower thrie beam bolt when used in combination with a half-post spacing AGT.

More recently, MwRSF also developed and crash tested standard and simplified designs which utilized 19-in. long wood blockouts when adapting the MGS to a thrie beam transition which utilized a half-post spacing AGT. For these tests, a 4-1/8 in. extension was used below the lower thrie beam bolt.

Currently, the Iowa DOT is specifying a 19-in. long blockout with a 4¼-in. extension below the lower thrie beam bolt. The IADOT has inquired as to whether: (1) the 19-in. long wood blockout with a 4¼-in. lower extension is a acceptable alternative to the slightly shorter blocks having slightly smaller extensions and (2) it would be expected to provide satisfactory safety performance.

Based on my review of the prior systems and satisfactory crash testing results, I believe that a 19-in. long wood blockout with a 4¼-in. extension below the lower thrie beam bolt could be used in FHWA-accepted thrie beam approach guardrail transitions configured with either quarter-post or half-post spacings. This block length and lower extension should still allow the lower thrie beam corrugation to push back and/or fold under when impacted by vehicle wheels. In addition, it is my opinion that 7 or 8-in. deep wood blockouts could be substituted with 12-in. deep wood blockouts.



Response
Date: 06-14-2011
Would your recommendation remain the same if we were to increase the extension below the lower thrie beam bolt by 1/8 in. to 4-3/8 in.? See the attached drawing for proposed blockout dimensions.
Attachment: https://mwrsf-qa.unl.edu/attachments/4273e2ddfae26589b53241b464a113db.pdf


Response
Date: 06-15-2011
I do not believe the additional 1/8 in. of blockout length beyond the lower guardrail bolt would cause to change my opinion. As such, you should be fine using the slightly longer block.

Iowa Approach Guardrail Transition - Steel Post Alternative - Recessed Steel Post, Tapered Steel Block, & Full-Height Wood Block

Question
State: IA
Date: 06-07-2011

Chris Poole had a question regarding the length of the posts in the Iowa approach guardrail transition.



Response
Date: 06-07-2011

Your current inquiry pertains to the desire to utilize 7-ft long, W6x9 steel posts in lieu of the 6.5-ft long, W6x9 steel posts in an approach guardrail transition configuration which was based on the use of posts installed on a quarter-post spacing.

Background

In the late 1990's, two Iowa approach guardrail transitions with a quarter-post spacing near the concrete buttress were developed and crash tested under NCHRP 350 with steel and wood post options. The systems were configured as follows:

Steel Post " Test ITNJ-2

6.5-ft long by W6x9 Steel Post w/ 49" embedment depth

top of steel post recessed approximately 2.44" below top of tapered tubular steel blockout

Maximum (visible) dynamic deflection was found to be approximately 5.2 in.

Wood Post " Test ITNJ-4

7-ft long by 6"x8" Wood Post w/ 52" embedment depth

top of post flush with top of wood blockout

Maximum (visible) dynamic deflection was found to be approximately 3.9 in.

Recent Testing

Later and under NCHRP Project 22-14(2), the original steel-post AGT was retested under the proposed MASH guidelines using the new 2270P pickup truck. During this test, the maximum dynamic deflection was found to be 11.4 in., which was significantly higher than the magnitude of the visible dynamic deflections found in the prior NCHRP-350 crash testing programs.

Thoughts

The two transition designs noted above utilized slightly different post embedment depths " 49 versus 52 in. Although two depths were used, one may be able to argue for the standardization of this parameter.

For the steel-post, steel-blockout AGT, a tapered steel blockout was successfully used in combination with a slight recessed post. The tapered steel block and recessed post were used to reduce concerns for the pickup truck to extend over the thrie beam rail and snag on the metal elements as well as decrease the potential for vehicle instabilities. These features were originally implemented as design improvements to a thrie beam AGT for attachment to Missouri's single slope concrete median barrier. For the both the successful AGTs for both the Iowa and Missouri systems, the steel post was recessed in combination with a tapered steel blockout.

For the wood-post, wood-blockout AGT, a full-height post and blockout was successfully used in the crash testing program. In this test, the vehicle extended over the rail and contacted the wood components but was believed to more easily gouge the upper regions, thus likely reducing the resistance imparted to the pickup truck and potentially reducing concerns for vehicular instabilities.

Based on a review of the prior crash testing programs noted above, it would seem reasonable that a consistent embedment depth of 52 in. could be utilized in the steel post AGT. Unfortunately and in the absence of supporting test results, it would also seem appropriate to maintain the use of a recessed top of steel post in the region of quarter-post spacing as well as a tapered steel tubular blockout. However, it would seem appropriate to allow the use of a full-height wood blockout in combination with a recessed steel post.



Response
Date: 06-07-2011

What are your thoughts about using wood blockouts that were designed for use with wood posts (post bolt holes centered horizontally within the blockout), but using them with steel posts (post bolt holes off-center due to the flange)?



Response
Date: 06-08-2011
I would maintain the use of a centered wood blockout installed to a steel post. Thus, an off-centered bolt hole should be used in a wood blockout if attached to a steel wide-flanged post. A routed section in a wood block could be used to reduce block rotation. Non-routed blocks could be used in AGT designs which were configured with two guardrail bolts or if alternative anti-rotation methods (i.e., toe nails) were utilized.

Bolt Holes in Guardrail Posts

Question
State: IA
Date: 06-08-2011

We will be allowing the use of steel guardrail posts here soon, and I have a quick question. Does it matter to which side of the flange the holes are drilled? It seems that most states show the holes to the right of the flange on the elevation/front view. Would you see any benefit to drilling holes on both sides of the flange?

Response
Date: 06-08-2011

We typically place a hole on the upstream side of the front flange as well. However, I believe that the guardrail system would perform in an acceptable manner if the bolt were placed on the downstream side of the front flange as well. In most reverse direction impacts, the guardrail bolt would effectively be located on the downstream side of the front flange.

Many suppliers are fabricating guardrail posts with hole punched on both front and back flanges as well as left & right sides of each flange. The additional holes allow for the post to be more easily placed at a guardrail slot location without concerns for direction, side, etc. In general, it would seem reasonable to try to follow a practice of placing guardrail bolt on upstream side of front flange.

On another note, there may be a very slight increase in costs to punch all of the extra holes, depending on how the punching/drilling process was completed.

Extent of Lateral Encroachment 17-22

Question
State: WI
Date: 06-10-2011

Wisconsin requested information on the extent of lateral encroachment based on highway class as determined in the NCHRP 17-22 data for use in justifying clear zones.



Response
Date: 06-10-2011
Attached is the information you requested on the extent of lateral encroachment based on highway class. The data shows the median and various percentile lateral extents for different speeds.
Attachment: https://mwrsf-qa.unl.edu/attachments/ef87db291fcb7987de7c12466cbd1617.xlsx


Extra Blockouts

Question
State: KS
Date: 06-15-2011

We have a project that will be removing the bridge rails and upgrading the bridge rails to a 32" corral rail (Kansas type of bridge rail). Attached to the old bridge rail is what you see in the picture.
It appears that the contractor some time ago was allowed to use triple and double blockouts to avoid the pavement and curb and gutter. We are trying to minimize the project cost and avoid reconstructing some of the pavement and curb and gutter in order to provide a more typical single blockout guardrail installation at this location.

I recall that double blockouts can be used but limited to a certain amount of post locations. Also, what about the use of triple blockouts, limited to one post? I appreciate your help.


Attachment: https://mwrsf-qa.unl.edu/attachments/bbd1448e57aa24f23c79e53582981483.JPG

Attachment: https://mwrsf-qa.unl.edu/attachments/878b6468dface3940bdff4673ebd1f61.jpg


Response
Date: 06-21-2011

We have looked over the extra blockout issue that you sent. In the past, we have recommended no more than one triple 8" blockout installation very 50' for guardrail installations. This is based on concerns that the ability of the triple blockout to transmit load to the post would be compromised for large deflections. With regards to transitions, we have used a similar rationale and have limited the installation of triple blockouts to a single post in the transition at limited locations. For your installation shown, we believe that the number of consecutive triple blockouts is likely too many. In addition, the use of steel blockouts further complicates the issue, because they are more likely to buckle and fold under load during the impact and compromise the load transfer to the post.

Double 8" blockouts pose much less of an issue as we have tested them in certain systems with good results and the depth is only 4" more that the MGS blockout depth. That said the use of double steel blockouts still poses an issue due to block collapse. We would recommend that the steel blockouts be gusseted to prevent collapse under load. This would apply to the triple block installation as well.

I should also note that when steel blockouts are used, we are recommending the use of backup plates to reduce the potential for guardrail rupture.

For the installation shown, we would recommend moving up the posts closer to the curb if possible to eliminate the triple blockouts. In addition, if you are planning on replacing the bridge rail, we would recommend realigning the transition and bridge to reduce the number of extra blockouts needed.



Low Tension Cable Guardrail Tension Settings and Other Items

Question
State: NE
Date: 06-16-2011

I am reviewing the research for low tension cable guardrail.

Cable guardrail heights: 30", 27", & 24"

Line Posts: S3x5.7 " 5'-3" with

Inline anchor Includes Posts 1-7 & spring compensators on one end when less than 1000' on both ends run of guardrail is between 1000' to 2000'.

15' is the length of need from the anchor base plate.

Cable compensator table from our old plan unsure of origin:

(See Figure 1.jpg)

Design guidance:

Grading protected by cable guardrail:

For grading 1:1.5 and steeper: slope requires using 4' max. post spacing & 4' minimum grading @ 10:1 max. behind the cable"¢â‚¬Ã...“ as tested MwRSF

For grading 1:1.5 to 1:3 slope requires using 16' max. post spacing & 2' minimum grading @ 6:1 max. grading behind the cable as stated in the Roadside Design Guide.

Distance to fixed object:

Post spacing 4' " no items closer than 9'.

Post Spacing 8' - no items closer than 11'.

Post spacing 16' " no items closer than 12'.

Short radius placement:

Post spacing of 16' may be used on radii longer than 715'.

For radii 443' to 715' use 12' post spacing.

Cable should not be installed on radii less than 443'

Do you concur with this implementation of cable guardrail?


Attachment: https://mwrsf-qa.unl.edu/attachments/4c262f20ccd94107f715177940326974.jpg


Response
Date: 06-16-2011

The table contents contain the revised spring compression settings for cable guardrail tensioning based on research performed by the New York DOT in 1985. These tension values match the tabulated tensioning guidelines presented in Table 1 of that report. The reference for that report is as follows:

Kenyon, W.D., Cable Guiderail Tension, Interim Report on Research Project 166-1 to the Federal Highway Administration, Research Report 124, Engineering Research and Development Bureau, New York State Department of Transportation, State Campus, Albany, New York, July 1985.

I am reviewing the research for low tension cable guardrail.

- Cable guardrail heights: 30", 27", & 24"

Correct

- Line Posts: S3x5.7 " 5'-3"

Correct

- Inline anchor Includes Posts 1-7

Correct

- Spring compensators on one end when less than 1000' and on both ends for run of guardrail between 1000' to 2000'.

Correct, according to the New York DOT report referenced above

- 15' is the length of need from the anchor base plate.

Correct

Other items will be addressed in future. Thanks!



Foothills Parkway Aluminum Railing and a Modified Version of the Foothills Parkway Railing

Question
State: WI
Date: 06-21-2011

Our structures department received a bridge plan with a unique bridge rail (see pages 2-4 of attached PDF). It appears to be similar to the Foothills Parkway Aluminum Railing that MwRSF crash tested in 1994. However, we don't have a good copy of the crash test report (most of the pictures are gone).

The modified rail is being used on a raised sidewalk and needs to be tall enough to prevent pedestrians/bikes from falling off the bridge (that is why they added an extra railing). It will be used on a roadway with a posted speed of 35 mph.

Would it be possible for MwRSF to provide some feedback about the rail modifications. I know that I do not like what they are planning to due by the light pole. Some concerns have been expressed about the reinforcement of the concrete under the rail.


Attachment: https://mwrsf-qa.unl.edu/attachments/6c3f2a11d8ce8fd4a9dc6357d610085d.pdf


Response
Date: 06-22-2011

I have briefly reviewed your enclosed materials. From my best recollection, MwRSF crash tested the 2-tube Foothills Parkway Aluminum Bridge Railing (FPAR) in the early 90s according to the 1989 AASHTO Guide Specifications for Bridge Railings using Performance Level 1 (PL-1). In this testing, MwRSF conducted two pickup truck tests and one small car test. After an initial failed pickup truck test, the longitudinal aluminum tubes were sized up to double the wall thickness to 7/16 in. and modify the post anchorage system.

The proposed 3-Line Aluminum Railing utilizes one additional tubular rail section above the top rail provided in the FPAR system. Although the additional rail will provide increased barrier capacity, it may also result in higher vehicle loading to the barrier system, including the anchorage hardware within the concrete curb. At the present, no reinforcement details are provided for the concrete curb and deck to help demonstrate that equivalent or greater anchorage capacity is provided. Further, the new design concept contains three cables which pass through the web of the posts in addition to new structure on the face of the posts. Discontinuities have been incorporated into interior regions of the tubular railing system in order to accommodate vertical light pole systems. It is uncertain as to how these modifications will affect the safety performance of the railing system under the new MASH impact safety standards. With so many changes, one should consider testing at gaps and ends, something that was raised to the sponsor but not addressed or funded in the 1990 FHWA Guardrail Testing II program. These concerns remain for the ends as well as at any gap where poles exist.



Cable Hanger Post Tab Issue

Question
State: NE
Date: 06-30-2011

Another change requested to the Cable Guardrail:

The slot cut to hold the cable on the end post breaks off "every time" in the field.

If we allow a hole in the cable bracket & a light weight clip placed in the hole, is this still a system which will meet NCHRP 350? (See Figur 1.jpg)

Would a 3/16" - 4" brass rod bent in a U and bent over on the back side after installation work as a light weight holding device?


Attachment: https://mwrsf-qa.unl.edu/attachments/625863b110f3d96cd48c1e072eacbbfa.jpg


Response
Date: 07-07-2011

It would appear that the original FHWA approval letter used 5 mm brass rods to hold each cable within the slot. Thus, a similar pin design would be acceptable.



Curbs Under Transitions

Question
State: WI
Date: 06-30-2011

We are using the thrie beam transition to rigid barrier developed in TRP-03-210-10. This transition was crash tested without a curb under it. Some other thrie beam transitions that MwRSF has crash tested used a sloped 4-inch curb.

Is it possible to use a 4" sloped curb similar to the previous crash tests with the transition in TRP-03-210-10?



Response
Date: 07-05-2011

I am assuming that you are referring to the curb detailed in TRP-03-69-98. If so, I do not believe that the addition of the 4" sloped curb would cause any adverse effects. However, be sure to use the same geometry for the curb, i.e., the height, slope, and length of the curb should not exceed that dimensions illustrated in the noted report. Note, this will keep your curb downstream of the asymmetrical transition piece and within the thrie beam rail sections for the newer transition. Further, the lateral placement of the curb must be as detailed in the original report (with the back of the curb adjacent to the face of the post).



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: https://mwrsf-qa.unl.edu/attachments/fc20b9add7acfee5a8fa31c7a17fa278.jpg

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

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

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

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