Midwest States Pooled Fund Program Consulting Quarterly Summary

Midwest Roadside Safety Facility

07-01-2010 to 10-01-2010


Tie-Down Strap for Temporary Barrier

Question
State: WI
Date: 07-02-2010

I was reviewing MwRSF's detail for the temporary barrier tie-down strap. I have a question about a plate installed near the bottom of the connection pin (see attached drawings).

How is this plate installed? If it is welded to the connection pin, I don't think the connecting pin can be installed. I don't think that the plate can be installed after the tie-down strap and the barriers are installed ( not enough room to work).

Looking at Iowa's concrete barrier detail, they don't show the plate on the connection pin.

If the plate is needed what is its' purpose? I don't believe that it is needed for the double shear connection (i.e. the double shear is provided by the second set of loops).

Any information you can provide would be greatly appreciated.


Attachment: https://mwrsf-qa.unl.edu/attachments/c615f8a23e79526dec1e46733f22721e.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/a269732e3cc2bca7256c57b38272308f.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/017ecc98bc6e7a6a1ee7f6633e3323b7.pdf


Response
Date: 07-02-2010

The plate on the bottom of the connection pin is not welded. It slides onto the bottom of the connection pin and is held in place by the bolt.

In order to install it with the strap tie-down, you have to lift the strap up, put the plate and bolt on the connection pin, and then lower the strap back down. The strap can then be bolted to the drop-in anchors or secured with wedge bolts.

The system was tested with the bolt and plate in place. Because this system relies on loading of the connection pin to restrain the barrier, we believe that the retention plate and bolt are necessary to prevent the pin from pulling out of the loops. I don't believe that the bolt has sufficient capacity to prevent the pullout of the pin under high loads. Thus, the plate is likely necessary.



ZOI

Question
Date: 04-21-2010

I was wondering if you'd be able to send me your report (Guidelines for Attachments to Bridge Rails and Median Barriers: regarding the ZOI) for consideration in my review of a recent submittal for a continuous CRB median barrier that tapers up to cast-in-place 1350mm high (with a vertical face) near the location of bridge piers behind the median. I am no longer with Equilibrium and am now working on a major bridge project reviewing engineer's submittals for a different project.

The divided highway is a 90 kM/hr high use one, and I have personally never seen a Vertical face barrier of 1350 high with a 453 minimum clearance (measured from traffic side to face of pier behind) ZOI behind it (610 is noted as being preferred).

In general cases, should the geometry of the vertical 1350 height face beyond the physical obstructions and the taper zone back to the typical CRB height be defined on drawings? Is 453mm an acceptable minimum ZOI?

If you can send the document by PDF, it'd be appreciated. Let me know if you have any questions, or if the above is unclear.



Response
Date: 04-27-2010

I have enclosed a copy of the requested report. Please note that the ZOI information mostly pertained to test levels 3 and 4. Information for TL-5 was not determined nor provided therein. However, as barrier height is increased, the ZOI would decrease for TL-3 and 4 conditions.

Various height for rigid parapets have been used across the U.S. For TL-5 barriers, it is common to use 42" tall parapets. In addition, it is not uncommon for States to use 51 to 54" tall parapets when shielding objects or for additional glare screen protection.



Response
Date: 07-07-2010

My responses to your recent email are in red


Some questions related to ZOI and traffic barriers;

  1. Treatment of CRB placed up against MSE (concrete panel) walls parallel to traveled highways (I.e. Are barriers even needed, should the MSE wall be designed for Impact, or just be designed for repair, panel replacement)

**Does CRB stand for a permanent or temporary concrete barrier " either precast or cast-in-place? Regardless, MSE walls would not need to be shielded unless done so to: (1) prevent vehicular impacts into MSE walls located within clear zone if the crash results in serious safety risks to motorists; (2) prevent significant repair costs to MSE wall panels, if found to occur; or (3) prevent structural damage to highway/roadway infrastructure located above as well as to surrounding motorists
- adjacent and above.

**It should be noted that TTI researchers are currently conducting a research study pertaining to vehicular impact into MSE walls. I do not have any results from this study but would recommend that you contact Dr. Roger Bligh at TTI for further details.

  1. Some of our drawings show a 1.0m sliding distance for divided highway precast CRB's. If the sliding distance is reduced at overpass columns in the centre, should there be a transition detail from free to fixed? (Current details seem to show a rising of the height to vertical 1300mm high barriers).

**If temporary or portable concrete barrier are installed in a free-standing manner, then the location of discrete fixed objects on the back side could have serious consequences. Free-standing. portable concrete barriers move laterally when impacted. Vehicle redirection occurs as a result of the inertial resistance of the barrier, the axial tension developed throughout the long, inter-connected barrier system, and the friction developed between the barrier base and the support surface. If barrier movement is restricted at discrete locations, vehicle could pocket into the barrier, snag on barrier components, override the barrier, become unstable upon redirection, etc. Depending on the location of the fixed object, transitioning of the barrier system from free-standing to fixed may be required. Some barrier systems may have options for transitioning the lateral barrier stiffness, others may not.

**I am not sure how the rise in barrier height corresponds to the placement of hazards and free-standing and rigid barriers. Can you provide further details regarding the situation to which you refer?

  1. Do you have any information on the California 60G barrier Design, and what levels of Crash testing it meets (I.e. CAN/CSA-S6-06)?

**CALTRANS has conducted significant research on a family of single-slope concrete barriers. The research results from these crash testing programs are contained on two different locations of their website. Actual research reports and crash videos are available. I will ask that one of my colleagues sends to you the links if you are unable to locate them.

http://www.dot.ca.gov/research/researchreports/dri_reports.htm

http://www.dot.ca.gov/research/operations/roadsidesafety/index.htm

**Scott " do you have any additional information on the Type 60G barrier?

  1. Are you familiar with the ZOI TL-4 of 230mm from Keller, Sicking, Polivka, and Rohde, feb 26-2003 document: do any of your findings disagree with this?

**I do not understand your question. MwRSF prepared a TL-4 ZOI chart for concrete parapets based on a review of research findings available at that time. No new study has been performed to review and/or update the prior findings. As such, they stand as prepared until further research is funded.

  1. Is it normal practise to reduce shoulder widths at underpass column support locations on divided highways (>80kM/hr): what is the absolute unsafe minimum that should be accommodated in these types of situations.

**Unfortunately, I do not have an answer to this question and must defer to any guidance provided within the AASHTO document entitled, "A Policy on Geometric Design of Highways and Streets."



Cable to W-beam Transition

Question
State: IA
Date: 06-29-2010

I would like your opinion regarding a construction issue with Iowa's cable guardrail to w-beam transition (which is now voided). The standard drawing for this transition (http://www.iowadot.gov/erl/archives/2009/april/RS/content_eng/re84.pdf) is based on the South Dakota design.

"‹Ã...“Case A' on the drawing allows one of the transition brackets to be placed on the w-beam end of the w-to-thrie transition piece. As is clear from the drawing, especially in the plan view on sheet 2, this configuration has proven very difficult to construct; the downstream post and blockout interfere substantially with the path of the cables as they travel from the transition bracket to the end anchor.

In your opinion, should we allow "‹Ã...“kinks' in the cables as they travel around the post and blockout? If not, would we be able to adjust the location of the transition bracket and the end anchor to provide a straight line of travel for the cables?



Response
Date: 07-08-2010

We have looked at your details and have a few comments and responses to your questions. We looked at several options for Case A in your details.

A total of four solutions were investigated. The first solution consisted of allowing the wire rope to bend around the post at the midpoint between posts at standard spacing. However, analysis of the degree of bending of the wire rope around the posts, in combination with concern that the wire ropes will either lose tension during post deflection or be pulled from the terminal, indicates that this alternative is likely not an acceptable solution without crash testing to prove crashworthiness.

The second solution proposed by the Iowa DOT was to shift the downstream transition bracket further upstream which would decrease the effective angle to the anchor bracket and allowing the cables to bypass bend locations around the post. While this design would help alleviate cable interference with the post, it is not known what the effect of shortening the overlapped cable length would have on the design. Changing the position of the transition bracket would changed the angle of the cables to the ground anchor. One of the concerns in the original design of this system was the potential for snag of the vehicle in the area where the cables angle down towards the ground anchor. Thus, I am leery of changing the transitioning of the cables or the location of the anchorage without further analysis.

An additional option proposed was to drill a hole in the blockout of the post which interfered with the cables. This design option has several advantages, in that the positioning of the bracket and the W-beam do not change relative to each other, minimizing the potential for snagging, pocketing, and loss of cable tension. However, the required size of the hole required to pass the cable through the blockout would be very large, which could lead to lower compressive strength of the blockout, greater propensity for twisting, and the cables would be subject to post rotation or fracture in the soil. Damage to the post at the point of cable routing could interfere with the cable's tension and could potentially cause catastrophic release of the cable from the end terminal. Furthermore, the additional labor required for field drilling holes in the blockout and the potential to cause unexpected damage are high; therefore this is not an optimal solution.

The final design option is to add an additional 12-6" of guardrail between the the flared crashworthy end terminal and the approach transition. By introducing an additional span of guardrail, transition bracket interference issues, cable tension concerns, and field operations are maintained. In addition, this options allows the cable transition to be completed before the approach transition to the bridge rail begins. Though this may be the be slightly more expensive option, it is nonetheless the most crashworthy from a design standpoint, and will most likely result in acceptable performance of the transition design.

An additional issue which was brought to my attention was the standard plan design of the cable anchor. This cable anchor, a 4" x 4" anchor angle, does not have sufficient strength to maintain the loads from the cables during a crash event. Cable loads on anchors can, in TL-3 crash conditions on low-tension cable guardrail systems, rise as high as 60 kips with peak loads from a single cable as high as 25 kips. It is conceivable that higher-energy impacts may cause tension increases in excess of this number. The angle bracket anchor shown in your detail will most likely not be sufficient to maintain these loads without a large degree of deformation, which may compromise the performance of the anchorage. It is recommended that Iowa adopt the design tested in the test report prepared for the South Dakota Department of Transportation entitled, "Crash Testing of South Dakota's Cable Guardrail to W-beam Transition", by Faller, Sicking, Rohde, Holloway, Keller, and Reid, MwRSF Research Report No. TRP-03-80-98. Anchor bracket design details tested in the report are attached. This design uses a gusseted anchor plate that is significantly stronger.


Attachment: https://mwrsf-qa.unl.edu/attachments/317b0be6df04499b8435df6c07010b63.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/46f344d8a8cddce89c91f080cab58956.jpg

Attachment: https://mwrsf-qa.unl.edu/attachments/4477943bfe48882d283d6755630ce943.jpg


Type 2 Downstream End Terminal - Illinois DOT

Question
State: IL
Date: 07-13-2010

I received the following message from a colleague in my department

"<<0593_001.pdf>> Have you ever placed 2 Type 2's at the end of SPBGR Type D? See the attachment. There is a ramp merging into the mainline which is one way, so the Type D is on the departing end on both sides. Could I use this or use something like a CAT 350, even though it would never be hit head on."
I'm not finding a record of sending this to you before, as I'd promised my colleague in our District 2.


The Type 2 we refer to is our downstream anchor terminal, and we're wondering about applying it on the downstream end of double face guardrail.
It's our Standard 631011-06. http://www.dot.il.gov/desenv/hwystds/rmpdf211.html


We'd welcome any opinion or observations on using this for anchoring the downstream end of guardrails. Per one of our earlier discussions with you or Karla, we do plan to correct the length of the soil tube to 6' from the 7' shown.

Attachment: https://mwrsf-qa.unl.edu/attachments/ad4b5bebeebf99ef37f96a9149b5645b.pdf


Response
Date: 08-24-2010

I am enclosing a pdf file which compares your downstream anchor hardware to that currently used by MwRSF. Within the file, corrections are noted that show which dimensions are actually used within our CAD details. For our general guardrail testing programs, we utilize a standard end anchorage system on both the upstream and downstream ends of our 175-ft long W-beam guardrail installations when terminals are not being evaluated. These anchorages were adapted from the modified BCT, also consider the MGS rail height and cable anchor increased rise, and include a channel strut, two lengthened foundation tubes, and a common cable anchor with bolted attachment plate and bearing plate on each end.

In addition, you inquired as to whether the trailing end hardware noted above could be utilized in a double rail or median-type configuration where reverse-direction impacts could not be achieved on the end spoons. In such installations, we believe that this trailing end terminal hardware in combination with the MGS would likely provide sufficient capacity to successfully contain and redirect most passenger vehicles impacting at high speeds and angles. Unfortunately, no full-scale crash testing programs have yet been performed on most trailing end terminal systems.

Currently, there have been concerns with many different non-crash tested trailing end terminals that the small car vehicles could become snagged or wedged under the anchor cable on the downstream end if the end post is not fractured or does not release in a timely manner.

Two prior crashworthy box beam guardrail end terminals have utilized a post breaker system to ensure post fracture and cable release prior to snagging the small car vehicle. However, the current generation of energy-absorbing and flared W-beam guardrail end terminals do not utilize post breaker features for releasing the cable anchor end located near the groundline of post no. 1. As such, there could be an argument for not utilizing post breakers in trailing end guardrail terminals if similarly configured to current W-beam terminals in terms of anchorage. This opinion would be based on the design, prior crash testing performance, and in-service experience of most crashworthy W-beam guardrail end terminals.

Using engineering judgment and in the absence of crash testing, we believe that the downstream trailing end terminal hardware, similar to that used at MwRSF and shown herein, could be utilized in a double rail, median-type configuration. However, full-scale crash testing is the only true way to determine the safety performance of the downstream trailing end terminal system. In addition, it should be noted that future testing may provide a basis for modifying our opinions on this issue.

If one were to have significant concerns regarding the potential for small car snag or wedging under the cable anchor, then a slight design change may be considered. First, it may be advantageous to incorporate blockouts with the end posts in foundation tubes, thus allowing a 8-in. lateral shift of the post, strut, and anchor cable. Such a design modification would likely require a longitudinal stagger of the anchor posts combined with a single post installed between the two blockouts. Unfortunately, there are also concerns with this design variation, such as little or no experimental experience, lack of prepared design details, unique loading on anchor posts and foundation tube, and potential for inadequate cable length with the 8-in. lateral shift.

At this time, MwRSF has received research funding from the WisDOT to examine, test, and evaluate a standardized downstream anchorage system for the MGS. With this project, I am hopeful that we will be able to provide design guidance for both roadside and median applications, including for double, median-type W-beam guardrail systems.


Attachment: https://mwrsf-qa.unl.edu/attachments/2c558f6e62d35a7574f787f7ff4a1f4e.pdf


Steel Bridge Railing Question

Question
State: IA
Date: 06-03-2009

What would be your opinion of installing a steel bridge railing (Illinois 2399 curb-mount) at standard post spacing (6'-3" as tested), but increasing the post spacing at four locations on the bridge in order to accommodate some structural members? Our consultant feels they can limit the maximum post spacing at these locations to 7'-6". Do you think allowing the larger post spacing at these locations would be feasible without additional testing, or should we be investigating other options?


There would be only 1 spacing of 7'-6" at each of the four locations on the bridge.



Response
Date: 06-15-2009

MwRSF feels that increasing the post spacing from 6'-3" to 7'6" in only a few non-adjacent spans is a possible task. However, the bridge rail must be stronger to accommodate the 20% increase in moment due to the elongated post spacing. As such, we recommend the following:

Replace the 4"x4" bottom tube with another 8"x4" tube (the top tube). Thus, the bridge rail would consist of 2 8"x4" tubes. Assuming the top and bottom rail carry equal loads (which it really doesn't " top takes more load), this small change would provide a 30% increase in rail strength - enough to accommodate the 20% increase in moment.

This rail combination should be used throughout the bridge to ensure rail continuity and prevent snag points

Also, keep the bottom of the lower tube at 14" above the roadway. Thus the top of the lower tube is 22" above the roadway (2" gap between rails). This will allow the lower rail to better interact with an impacting vehicle and absorb more of the impact load.



Response
Date: 06-16-2009

We have an additional question to follow up the attached email which recommended that an 8" x 4" tube be used on the bottom rail throughout the bridge.


Since this bridge is relatively long, using an 8" x 4" tube for the bottom rail over the entire length would result in a significant increase in the steel quantity and cost. (The length of bridge to receive new rail is about 3,000 feet and the weight difference between a 8 x 4 x 5/16 tube and a 4 x 4 x 1/4 tube is 11.14 pounds per foot. Thus there would be an increase in steel of about 2 x 3,000 feet x 11.14 lb./ft. = 66,840 pounds.) Also, we would like to minimize the additional total dead load that is added to the bridge since the weight capacity of the bridge is an issue. (We are even planning to use lightweight concrete for the curbs on this project.)


In view of this, would it be possible to strengthen the rail at only the few areas where the span would exceed 6' 3"? In order to accomplish this, could the rail be strengthened at just those longer rail spans and any necessary adjacent spans, while using a 4 x 4 x 1/4 tube for the bottom rail throughout the rest of the bridge? The following are some ideas for your consideration to accomplish this:


Increase the wall thickness of the standard top and bottom rails in order to get a 20 % or greater increase in the section modulus (S) for bending. This would result in no change in the outside railing geometry.


Install a tubular member inside of the standard top and bottom rails in order to get a 20 % or greater increase in the section modulus for bending. For example, a 4 x 4 x 1/4 tube has a S of 3.90 inches^3. If a 3 x 3 x 3/16 tube (S = 1.64 inches ^3) were inserted inside of the 4 x 4 tube, the total S for the bottom rail would be increased by 42 %. This would result in no change in the outside railing geometry.


Add another 4 x 4 x 1/4 tube directly above the standand 4 x 4 x 1/4 bottom rail to increase the bending strength. In order to avoid a snag point, this section would need special fabrication at the ends for a transition down to the typical bottom rail.


Replace the bottom rail with a 8 x 4 x 5/16 tube as recommended in the attached email, except fabricate a special transition down to a 4 x 4 x 1/4 tube at the ends in order to avoid a snag point.

Please let us know if any of the above concepts would be acceptable, and if so, we will ask the consultant to investigate further.




Response
Date: 06-24-2009

We do feel that we can strengthen the rail in the areas surrounding the extended post spacing only. With a 3,000 ft bridge, using the increased rail size for the entire system would be wasteful. Comments on the proposed solutions are discussed below.


(1). Using a thicker / stronger rail in certain areas will result in abrupt stiffness transition points at the connections between the two rail types. These stiffness transitions could lead to vehicle instabilities or snagging.


(3) & (4) Altering the shape of the rail in these locations can lead to more vehicle interaction problems (snagging, instabilities, wedging, etc...). As such, we do not favor the option of transitioning between different rail geometries without testing these transitions.


(2) MwRSF does like the tube-in-a-tube idea for strengthening the rail. The inserted tube should fit relatively snug inside the original tubes, so that the smaller tube develops load before the rail suffers larger deformations. The 3x3 tube inside of the lower rail (4x4x1/4) tube is a good fit. However the upper rail should also be reinforced. The same 3x3 tube could be used if its position could be centered inside the 8x4 (perhaps resting it between the attachment bolts, bolting through the 3x3 tube, or using spacers to position the 3x3 tube inside the 8x4 tube.


The inserted reinforcement tubes should be extended out from elongated spacing, though the adjacent spacing of 6'-3", and to the nearest ¼ spacing. The 1/4 points of the rail are recommended for the stiffness transition to prevent the tube end from occupying a point of maximum deflection / deformation (midspan) or a stress concentration point (at the posts). Thus, the inner tubes should be extended 94 inches past the posts of the longer spacing (6'-3" plus 19"). Total length of the inner tubes would then be 188 inches plus the length of the longer post spacing (approximately 7'-6" from your previous e-mail.



Response
Date: 07-14-2010

I've got one more (hopefully the last) request for you regarding our I-74 bridge rail replacement. Apparently our consultant, rather than incorporating your previous advice, has developed an alternate method for spanning the wide expansion joints on the I-74 bridge. This method places specially-designed posts on either side of the joint, spaced 5 feet apart.


Could you please review and comment on the attached drawings showing the proposed design? Just as before, this will be used at a total of four locations on the bridge - on both sides of the road at each of the two suspension towers.


The post spacing varies in order to avoid the vertical stringers located just beyond the edges of the bridge deck.


The consultant felt that he needed to space the corbels (and therefore the posts) in order to avoid the vertical trusses due to the tight tolerances (see the attached picture of the current bridge). The vertical trusses are located approximately 1'-5" behind the face of rail. Would you agree that even if a post were placed at a truss location, that the truss would lie outside the working width of the barrier?


The proposed spacings have not been analyzed. Do you feel the abrupt changes in post spacing throughout the bridge is concerning enough to warrant a possible redesign? If we could somehow reduce the depth of the corbels, perhaps that would allow them to be installed at truss locations?



Response
Date: 07-15-2010
The full scale testing on the original Illinois steel tube bridge shows a maximum dynamic deflection less than 3 inches. Also, although the working width of the system was not specified in the summary pages, the vehicle does not appear to extend more than 12 inches past the face of the rail. Thus, the 1'-15" of clear space between the face of rail and the vertical trusses provides enough room to minimize the risk of vehicle snag on the truss members. Further, the 17 inches of space matches that of the recommended offset from the head ejection envelope developed in TRP-03-194-07 for the 95th percentile passenger (14 in. + 3 in. = 17 in.).


With a maximum dynamic rail deflection < 3" with the post spacing at a constant 6'-3", I am not concerned about pocketing of this rail when the spacings you have shown are all between 5'-6" and 3'-6". All the extra posts will only stiffen the bridge rail. Therefore, the post spacings you show are acceptable and the splice at the tower locations should work.

Two Loop PCB Connection

Question
State: NE
Date: 06-19-2010

Has MwRSF tested a 12.5' Concrete Protection Barrier with two loops?

I'm thinking this is from the 2001-2003 era.

Have we tested two loops in the end of a concrete bridge rail or median rail?

I thought we tested this with the Kansas style steaked-down with 3 stakes on the traffic side.

Then 2 barriers staked down with 2 stakes each, then 1 or 2 staked down with 1 stake.

What was the name of this research study?

Would the tied down barrier move less than the free standing barrier and put less force on the loops?



Response
Date: 07-20-2010

The original 350 testing utilized 2 loops per end. Later, we added the third loop to get double shear " top and bottom.

The only 2-loop TCB system that was crash tested and evaluated while anchored corresponded to the steel tie-down strap system. And, this TCB used a version where each loop was configured with 3 small bent rebar. All of tied-down systems and transitions used the Kansas version with 3 rebar loops per barrier end.

Without detailed analysis, I believe that an anchored TCB would encounter reduced tension within the loops as compared to a free-standing TCB. However, the loops would potentially experience increased shear and moment at the concrete interface if one barrier shifts relative to the other. This shifting has been observed in the anchored barrier testing. Please note that no directed study has been made for comparing the various loop configurations under free-standing and anchored installations.



Bridge Rail Retrofit

Question
State: WI
Date: 07-28-2010

WisDOT has a project where an existing thrie beam bridge rail was installed too low. Regional staff has asked if:

1. The existing longitudinal channel on top of the bridge rail could be removed.

2. A small box beam or steel tube could be bolted to the existing post (i.e. to get the correct rail height)

3. Existing longitudinal channel is reinstalled.

From what I understand, the existing deck bolts and nuts are very rusty and difficult to remove. This will make it difficult to remove the existing post and replace with new taller posts. In addition the taller post are more expensive to fabricate than the smaller box beam/steel tubes.

An example of the retro fit is attached (W Rail Retrofit.pdf)

An example of our current thrie beam retro fit is also attached (3002.pdf).

The rail used on the existing bridge is also attached.


Attachment: https://mwrsf-qa.unl.edu/attachments/28512cd0f5bfc72c5969b343d44ea512.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/3a51aff78a5d81ef3821b41b90e5d86e.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/87462e84fb39314b81880a3bf71c895a.pdf


Response
Date: 08-02-2010

The Wisconsin design is similar to the Missouri thrie beam and channel bridge rail tested by TTI. However, the bridge rail plans show the system as 4 inches shorter than it was tested at previously. Thus, the addition of the 4 in. tall spacer block to the top of the shorter post allows the rail to be installed at the correct height. The use of four 5/8 in. diameter bolts to connect the block to the post should provide more than enough strength to prevent shear failure during an impact.

The anchorage for the Wisconsin bridge rail seems to be a modification of the tested system as well. Tested used three 1 in. diameter A307 bolts, while the current drawings show 4 7/8 in. diameter A449 bolts. Noting that A449 provides a 20-50% increase in strength (depending on grade), the Wisconsin bridge rail design should provide equal or greater anchorage strength.

Therefore, the proposed bridge rail design appears to be of comparable strength and geometry to that of the tested Missouri thrie beam and channel system.



Cable Guardrail Questions

Question
State: NE
Date: 07-30-2010

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: 08-25-2010

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.



Temporary Sand Barrel Arrays

Question
State: WI
Date: 08-09-2010

I'm looking into providing additional guidance for our staff on the use of temporary crash cushions and sand barrel arrays.

During my reviews, I found NCHRP Report 358 Recommended Practices for Use of Traffic Barriers and Control Treatments for Restricted Work Zone (see attached). I have the following questions:



Response
Date: 08-16-2010

I have responded to your questions in red
below.


  1. The sand barrel arrays were designed for NCHRP 230 impacts. How would the layouts change for a MASH vehicle (e.g. offsets, barrel layouts...).


With regards to the sand barrel layouts, MwRSF could look at the barrel arrays and attempt to adjust them. However, we believe that it would be more appropriate for you to contact the sand barrel manufacturers in order to get their recommendations for the barrels arrays with the MASH vehicles.


  1. The guidance on what treatment to use to protect the blunt end of the temporary barrier was based on the barrier being installed for 1 year or less. If a project will have temporary barrier installed for more than a year what steps should be taken by a designer?


The end treatment guidance in NCHRP 358 was based on benefit/cost analysis. Thus, the longer the sand barrel array was installed, the more likely that a more robust, long term attenuator would be worth installing. That said, we do not believe that leaving the sand barrels in place for a period over one year hugely problematic. If the barrel array is installed for a much longer time than one year, then you may want to rethink which type of system you use.


  1. Would the charts (figures 4.17-4.24) have significant changes to the break points between different end treatments because of the new MASH vehicles? Or do these charts represent the most current state of the art for temporary barrier end treatment protection?

The charts mentioned in NCHRP 358 are currently the best guidance for barrier flare rates n the work zone. No further analysis has been done to update those tables with more recent accident data or to make considerations for MASH.





Structural Analysis of Approach Transitions

Question
State: WI
Date: 08-13-2010

I have a major project team that is challenging my requirement that they provide structural analysis for their transitions. They indicate the following:

  1. "AASHTO LRFD defines analytical procedures for structural design of barrier (aka bridge parapet) connection to bridge decks. The intent is to ensure that the connection to the deck, and the deck itself, offers greater resistance than the barrier (i.e., make sure the bridge deck is not the weak link). As far as we know, AASHTO does not establish analytical procedures for barrier design for purposes of load classification (e.g. TL-3) and physical crash testing is required. There is however some history of FHWA accepting analytical procedures (structural calculations) used to demonstrate that a customized bridge parapet will perform at least as well as a similar crash-tested version."

  1. If it is desired and/or required to adopt an analytical procedure for designing barrier transitions, what will the basis of those procedures be? From a structural engineering perspective, behavior of reinforced concrete barrier under static loads is predictable enough. Behavior of the foundation (structure interaction with subgrade below and pavement adjacent) is more difficult to predict and normally involves assumptions which are quite conservative. Structure response to dynamic loading (vehicular crash) is very complex and difficult to predict even when materials and construction are well controlled. Because of this complex behavior and variability in conditions, as well as unknowns associated with the crash vehicle itself, a purely analytical method to assess barrier performance may necessarily be very conservative. The adjacent pavement and subgrade would offer substantial resistance to overturning, but this is proven with confidence empirically (crash test) and not so easy to demonstrate analytically (as mentioned above). Could barriers be treated similar to gravity retaining walls, using the TL-3 equivalent static loading forces from the AASHTO LRFD.

  1. What precedents exist for either analytical methods or empirical methods for designing barrier and barrier transitions? I think the team would benefit from a historical perspective, and also perhaps a wider geographic (national) perspective, as well as local precedent.

In your opinion there is little difference between designing a roadside barrier and a bridge parapet(i.e. the impact forces and how to deal with them are about the same). The fact that one is in the soil, verse connected to a deck, may allow for different methods to handle overturning moments (e.g. a roadway barrier could be wedged between lifts of asphalt or tied into a footing).



Response
Date: 08-13-2010

Please see my comments in red!

I have a major project team that is challenging my requirement that they provide structural analysis for their transitions. They indicate the following:

  1. "AASHTO LRFD defines analytical procedures for structural design of barrier (aka bridge parapet) connection to bridge decks. The intent is to ensure that the connection to the deck, and the deck itself, offers greater resistance than the barrier (i.e., make sure the bridge deck is not the weak link). As far as we know, AASHTO does not establish analytical procedures for barrier design for purposes of load classification (e.g. TL-3) and physical crash testing is required. There is however some history of FHWA accepting analytical procedures (structural calculations) used to demonstrate that a customized bridge parapet will perform at least as well as a similar crash-tested version."

**The AASHTO LRFD Bridge Design Specifications provides guidance for designing bridge railings for use on bridge decks as well as those attached to bridge approach slabs. This guidance is intended to help engineers properly configure bridge railings as well as their attachment to reinforced concrete decks. Both solid and open concrete parapets can be configured as well as metallic beam and post systems. Combination concrete and metal systems are also addressed. Limited discussion is provided for timber railings. Yield-line analysis procedures have been provided for addressing the design of reinforced concrete parapets and railings. Inelastic design procedures are available for most metal systems. These rail design procedures were developed and/or documented in a 1978 study report by TTI researchers and have been consistently used for a large share of railing systems. Upon design, it has been common practice for the design to be verified through the use of full-scale crash testing. Actually, full-scale crash testing has also been used for demonstrating the system's structural adequacy and safety even when the prior noted design procedures were not used. If crash testing has been shown to corroborate a design based on the noted procedures, then these procedures have also been used to modify other parapets as long that they provided equivalent or greater strength and did not pose increased risk for vehicle snag, rollover, or override.

For reinforced concrete parapets, the noted design procedures have also been to ensure that sufficient strength is provided at critical locations within the barrier, such as at barrier ends and at expansion joints. At such locations, the number of yield lines that can be developed is much reduced, thus potentially lower the redirective strength of the parapet. Therefore, it is imperative that these equations be utilized to modify a barrier's capacity to ensure that an impacting vehicle can be safely contained and redirected along the entire barrier length. Basically, the entire barrier must act as though it is continuous even though weakened sections may exist therein. End buttresses that are used to anchored approach guardrail section must also provide adequate structural strength so as to not allow for vehicles to penetrate directly behind the bridge railing if the entire length plus AGT must shield the hazard.

  1. If it is desired and/or required to adopt an analytical procedure for designing barrier transitions, what will the basis of those procedures be? From a structural engineering perspective, behavior of reinforced concrete barrier under static loads is predictable enough. Behavior of the foundation (structure interaction with subgrade below and pavement adjacent) is more difficult to predict and normally involves assumptions which are quite conservative. Structure response to dynamic loading (vehicular crash) is very complex and difficult to predict even when materials and construction are well controlled. Because of this complex behavior and variability in conditions, as well as unknowns associated with the crash vehicle itself, a purely analytical method to assess barrier performance may necessarily be very conservative. The adjacent pavement and subgrade would offer substantial resistance to overturning, but this is proven with confidence empirically (crash test) and not so easy to demonstrate analytically (as mentioned above). Could barriers be treated similar to gravity retaining walls, using the TL-3 equivalent static loading forces from the AASHTO LRFD.

**As noted above, the yield-line and inelastic design procedures are appropriate for designing the barrier systems that are anchored to both the bridge decks and approach slabs. These procedures have also been used for designing similar parapets to soil grade beams. In most cases, full-scale crash testing has demonstrated that the procedures are effective. However, when we use such procedures, we use a load factor of 1 using our MwRSF loads and not necessarily the loads noted in AASHTO. In addition, we would use the appropriate reduction factor for determining the various capacities, such as bending of reinforced concrete. These equations may not always work in every case due the various types of anchorage or support. In such cases, approximations are sometimes made for certain parameters based on experience and historical crash testing results under review. In some special cases, the published dynamic design loads have also resulted in overdesigned moment slabs for concrete parapets placed on MSE walls when used in static overturn analysis and design.

  1. What precedents exist for either analytical methods or empirical methods for designing barrier and barrier transitions? I think the team would benefit from a historical perspective, and also perhaps a wider geographic (national) perspective, as well as local precedent.

**Both analytical methods, computer simulation, and full-scale crash testing have to be used by themselves, or in combination, when developing and verifying the safety performance of guardrails, transitions, and bridge railings/median barriers. In most cases, crash testing was used but not in all. After researchers, designers, and engineers have become familiar with these methods, the more experienced personnel know when to apply one or more than one method to ensure that a system is properly configured.

In your opinion there is little difference between designing a roadside barrier and a bridge parapet(i.e. the impact forces and how to deal with them are about the same). The fact that one is in the soil, verse connected to a deck, may allow for different methods to handle overturning moments (e.g. a roadway barrier could be wedged between lifts of asphalt or tied into a footing).

The procedures are generally the same. The foundation systems could vary between roadside and bridge applications.



Response
Date: 08-16-2010

I have to summarize your response to me about yield-line analysis. Am I on the mark with this comment? I want to say:


An errant vehicle imparts the same amount of force into roadside barrier or bridge parapet. Yield-line analysis has been used to develop both roadside and bridge parapets. Crash testing has proven yield-line analysis can provide a structural adequate roadside barrier or parapet. Some of these crash tests may have had failing crash test results because of the roadside barrier or parapet was not functionally adequate.


This design methodology provides that the barrier itself has:





How forces get absorb by a deck, footing or soil may be different. However, a structural design engineer should have the necessary skill set to develop a design.


Yield-line analysis is only required at special transitions and unique situations (e.g. sign bridge integrated into barrier...). A "normal section" of single slope barrier with end anchorages does not need to be analyzed. However, it does need sufficient longitudinal steel to prevent shrinkage cracking.



Is this correct? I'm having difficulties defending this topic because I'm not a structural engineer. An I know that structural engineers will be present at my meeting. So I want to run this past someone who knows more about barrier design than I do.



Response
Date: 08-16-2010

See my comments below in red!


An errant vehicle imparts the same amount of force into roadside barrier or bridge parapet. (This would be true if both barriers were rigid. If one barrier is allowed to displace, then the impact load would likely be reduced.) Yield-line analysis has been used to develop both roadside and bridge parapets. (If configured with reinforced concrete.) Crash testing has proven yield-line analysis can provide a structural adequate roadside barrier or parapet. (Yes.) Some of these crash tests may have had failing crash test results because of the roadside barrier or parapet was not functionally adequate.


This design methodology provides that the barrier itself has:





How forces get absorb by a deck, footing or soil may be different. However, a structural design engineer should have the necessary skill set to develop a design.



Yield-line analysis is only required at special transitions and unique situations (e.g. sign bridge integrated into barrier...). A "normal section" of single slope barrier with end anchorages does not need to be analyzed. However, it does need sufficient longitudinal steel to prevent shrinkage cracking. (Yield-line analysis is used at all locations, including interior regions, ends, gaps, special shape transitions, etc. However, experience may help determine if one really needs to perform the analysis at each location. The use of different types of footings may require that the certain terms in the yield-line analysis equations be neglected or minimized. Prior crash testing results may be used to support those changes.)





MASH Temporary Barrier Deflection Vs NCHRP 350 Deflection

Question
State: WI
Date: 08-16-2010

I believe that MwRSF indicated that the use of MASH crash test vehicles is increasing barriers deflection distances. How much has the temporary barrier deflection increased using MASH vehicles compared to NCHRP 350 vehicles?

Response
Date: 08-16-2010

We have seen an increase in the deflected of the F-shape PCB when impacted with MASH vehicles.


Free-standing TCB deflections were significantly higher when testing was conducted with the 2270P vehicle under the MASH criteria as opposed to testing conducted with the 2000P vehicle under the NCHRP Report No. 350 criteria. TCB deflections increased 25 to 76 percent when the F-shape TCB was tested under MASH impact criteria. See attached table.


Test No.

Vehicle

Mass (kg)

Speed (km/h)

Angle

IS (kJ)

Dynamic Delfection (m)

Static Deflection (m)

ITMP-2

2000P

2005

100.3

27.1

161.5

1.15

1.14

TB-1

2270P

2268

99.5

25.7

162.9

1.44

1.44

TB-2

2270P

2268

99.7

25.4

160.0

2.023

1.854



This increase in deflection is due to a couple of factors

  1. Higher mass = more inertia transfer and higher load
  2. Higher vehicle stability encourages less climb and vehicle rotation which allows the vehicle to directly load the barrier longer.


In addition, the photos
(Figure 1 and Figure 2 attached)
show the damage from the 2270P testing. In this tests vertical cracks were observed completely through the barrier section. This amount of barrier damage was not observed in the 2000P testing and again suggests that our impact loads have increased.





Attachment: https://mwrsf-qa.unl.edu/attachments/6a7dc20b4f5a30103b03f62d1ba4797d.xlsx

Attachment: https://mwrsf-qa.unl.edu/attachments/77565cc065e80d66bbcbd228920e041c.jpg

Attachment: https://mwrsf-qa.unl.edu/attachments/fa3ecf129e88959ac21cae2c1b412fa7.jpg


Mixed Guardrail Post Types

Question
State: OH
Date: 08-19-2010

I have a question about the deflection of our generic guardrail. Our current standard allows the use of four different post the 6x8 wood, 8" round wood, W6x9, and W6x8.5 with a rail height at 27.75". For new construction the contractor must use the same post for the entire guardrail run. When guardrail is being repaired after a crash contractor have asked if they can mix post types. Nick Artimovich said this is fine if the deflection distances are close. Where can I get a copy of the crash test reports for the 8" round wood post or the W6x9 steel post? Thanks for your time.

Post type Deflection

6x8 2.7'

8" round

W6x9

W6x8.5 3'



Response
Date: 08-20-2010

NCHRP 350 Testing

27¾" rail height, 7.25" round SYP post " 3.7 ft D.D. " 2000P TL-3 test by TTI for Arnold Forest Products

27"?? rail height, 6x8 rectangular wood post " 2.7 ft D.D. " 2000P TL-3 test by TTI (G4(2W))

27¾" rail height, W6x8.5 steel post w/ 6x8 wood blockout " 3.3 ft D.D. " 2000P TL-3 test by TTI (Modified G4(1s))



Bridge Rail Post Question

Question
State: IA
Date: 08-23-2010

Iowa DOT is currently working with Illinois DOT in detailing a bridge rail for the new I-74 bridge. The bridge will accommodate a multi-use trail behind the rail on one side of the bridge. For this application, we are considering using the Pennsylvania HT rail with a supplemental "sidewalk rail tube" attachment, as detailed in Pennsylvania's standard drawings (attached).

It is our understanding that the modified post used to support this rail has not been crash tested. Therefore, we request your assistance in assessing the effect, if any, the modified post and sidewalk rail tube would have on the crashworthiness of this rail configuration, and whether the presence of the sidewalk rail tube in a TL-5 impact could produce flying debris.

Note that we are interested solely in the use of the design shown in the "raised sidewalk section A-A" on sheet 1 of 3 of BD-615M. Also note that we will not be using the "sidewalk rail rod" or any of its associated hardware.


Attachment: https://mwrsf-qa.unl.edu/attachments/2169be447993e3b7aab3654a43e61b20.PDF

Attachment: https://mwrsf-qa.unl.edu/attachments/85963a3991d2e6dde87306c7d9772483.PDF


Response
Date: 08-23-2010

The TL-5, full-scale crash test of this barrier showed no evidence that the trailer would extend over the steel rail and contact either the back of the posts or the pedestrian rail. This is due in part to the rail height being 50 inches instead of the standard 42 inches for concrete barriers. Thus, the box leans on the rail, but the lower corner of the box never extends behind the rail.

Also, during the crash test, there was very little deformation of the rail. Therefore, I do not foresee any problems with the pedestrian rail being contacted and turned into flying debris.

The proposed system appears to maintain the crashworthiness of the original (tested) system.



Oregon PCB

Question
Date: 08-24-2010

Attached are 2 sketches for your consideration with the following queries:
Are you aware if this Barrier has been crash tested for highways of 80kM/hr to NCHRP Report 350, and/or S6-06? (Bridge Barrier)


For a separated highway (I.e. 2 lanes in each direction, barriers on each side), would it be acceptable to use the Single Pin-Shoulder Installation for the Fast lane Barrier protections for construction workers operating in the area between the highway (I.e. Median)?


Or should the Median Installation (2 pins) be used, even though there are "two layers of defense"? (Note 7 indicates that 2-pins are needed for a median of less than 8', but does this include 2 barriers?)


Does this detail in conjunction with a sliding distance of 500mm away from the impact side provide a crash tested barrier that should be considered safe for use in protecting median construction work and workers?


Is it necessary to have a design engineer seal the proposed design for a temporary use?


Your valuable input is appreciated if possible.
Attachment: https://mwrsf-qa.unl.edu/attachments/e9565ffc14ca9a783cc93d91dced1810.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/43c9cfb2a16ec17f2a919d47142ffa7f.pdf


Response
Date: 08-30-2010

I am not aware of any crash testing programs directed toward the evaluation of the Oregon TCB in anchored/tied-down configurations. However, some of the Oregon anchorage details are similar in concept to those used by MwRSF for an Midwest Pooled Fund F-shape TCB as well as by TTI in other TCB designs.

In general, anchorage options can be adapted over to alternative TCBs without actual crash testing if several conditions are believed to be met, including demonstration that equivalent or greater structural capacity is provided. Thus, one would require that the Oregon barrier provides equal or greater flexural capacity, shear strength, torsion resistance, etc. as compared to the as-tested TCBs where anchorage systems were evaluated. The Oregon joint detail should provide equal or greater strength between adjacent segments as compared to the as-tested TCBs. Barrier regions surrounding the openings where the vertical or sloped anchors are inserted in the Oregon barrier should provide equal or greater capacity as compared to the as-tested TCBs. The anchorage hardware should also provide equivalent structural capacity. Barrier lengths should also be similar. If these general conditions are met, then it would seem reasonable to adapt prior crashworthy anchorage options to similar TCBs.

MwRSF could provide you with research/test reports for anchorage systems that have been developed and crash tested at 100 km/hr for use with the Midwest F-shape TCB. TTI researchers would have similar information for those systems that were developed and tested by their personnel.

See my comments below in red in response to your questions.

Are you aware if this Barrier has been crash tested for highways of 80kM/hr to NCHRP Report 350, and/or S6-06? (Bridge Barrier)

**Comments provided above. I am not aware of the Oregon TCB being crash tested under NCHRP Report No. 350 when installed with various anchorage options. However, it should be noted that some options shown in the attachments have similarity to anchor methods used with other crash tested barriers.


For a separated highway (I.e. 2 lanes in each direction, barriers on each side), would it be acceptable to use the Single Pin-Shoulder Installation for the Fast lane Barrier protections for construction workers operating in the area between the highway (I.e. Median)?

**Yes, as long as the Oregon barrier provided equivalent or greater safety performance compared to the pinned TCB configured and tested by TTI.

Or should the Median Installation (2 pins) be used, even though there are "two layers of defense"? (Note 7 indicates that 2-pins are needed for a median of less than 8', but does this include 2 barriers?)

**TCBs pinned on both sides have only been subjected to limited crash testing when used with a median TCB transition between free-standing TCBs and permanent barrier.


Does this detail in conjunction with a sliding distance of 500mm away from the impact side provide a crash tested barrier that should be considered safe for use in protecting median construction work and workers?

**I am not sure that I understand the question.


Is it necessary to have a design engineer seal the proposed design for a temporary use?

**This is a question better suited for the DOTs. However, someone needs to ensure that the barrier hardware and its placement meet current safety practices.



Shielding of Cut Slopes

Question
State: MN
Date: 08-26-2010

Do you know if we need to protect the "cut slopes" steeper than 1:3 within the clear zone (everything I have seen is for "fill sections")? Could you please point me to some references on this topic?



Response
Date: 08-30-2010

The 2006 RDG provides limited discussion on the use on backslopes or cut slopes " see Section 3.2.2 and Figures 3.6 and 3.7. In general, the RDG states that the backslope may be traversable depending on its relative smoothness and the presence (or lack) of fixed obstacles.

Some time back, I recall briefly investigating this issue for Dave Little. His email (blue) and my response (red) is contained below, along with his original attachment. The noted references are NCHRP Report No. 158 and the 1996 RDG. From this, the guidance suggests the use of a maximum cut slope of 2:1.

What I see from the 1996 RDG is in Chapter 6, Subsection 6.4.1.9 Earth Berm (P. 6-8,9), which says that "slope rates should not exceed 1:2, although steeper slopes can be used if they are smooth and liberally rounded at the base."

But I don't see any such information in the 2002 RDG, so it apparently got removed in that revision. Also don't see that there are any references identified for this information in the 1996 RDG.

I have reviewed the results presented in NCHRP Report No. 158 which was also discussed at the spring Pooled Fund meeting. I have also reviewed the guidance in prior RDGs. Basically, the NCHRP authors do not recommend using slopes beyond a 2:1 back slope when the foreslope is flat. Front end bumper/vehicle snag into the slope was a noted concern. Dean and I are also concerned with a 1:1 slope as it would be the worst situation for causing vehicle rollover, especially for higher center of mass vehicles found on the roads today and as compared to the test vehicles used in the early 70s.

Therefore, we recommend treating the 1:1 back slope situation by one of the following options. First, as you mentioned, a reinforced concrete parapet could be installed close to the base of the back slope but actually cut into it to match the wall height slightly above the soil grade. A vertical parapet would be preferred, although single slope or other approved shapes could be used. Alternatively, a smooth MSE or block type wall could be constructed at the same cut back location, thus producing a smooth vertical parapet for redirecting vehicles. Both of the barrier options would be backed up (i.e., supported) with soil over most of the vertical height.



Breakaway Devices Behind the MGS

Question
Date: 08-31-2010

Is there guidance for clearance behind the MGS for hardware that has a breakaway base/devices such as light poles or ground mounted signs?

Have any of these devices been tested with the MGS?

On many locations along the IL Tollway the typical section consists of a v-shaped gutter at the edge of outside shoulder, a noise abatement wall that is 5' from the back of gutter. The guardrail post is set back 6" from the back of gutter, so the distance from the back of post to face of noise wall is 4'. This 4' space is where light poles and single post ground mounted signs are placed. If the light pole is centered in the 4' space, the distance from the back of guardrail post to near edge of light pole is approx. 1'-7". This is obviously less than the 28" minimum clearance distance recommended in the MGS documentation.



Response
Date: 08-31-2010

There exists guidance for use in placing the MGS in front of various obstacles. In the absence of crash testing with poles, trees, supports, etc., it would be recommended that the Working Widths be used. The working width is measured from the original front face of the barrier system. I have enclosed a table with available working widths from the MwRSF crash testing programs.

If pole placement is desired within the published WW values, then full-scale crash testing would be necessary to verify acceptable safety performance with the alternative pole placement.


Attachment: https://mwrsf-qa.unl.edu/attachments/1d1c68856e1b49cb29446fabced8724d.pdf


PCB Transition

Question
State: OH
Date: 08-31-2010

I would appreciate any comments you have on the barrier transition drawing below. There is a need for Ohio to improve the way we are transition from PCB to rigid structures. I have included some pictures from construction projects in Ohio. We know that additional testing is needed but until then we would like to do the best we can. We took the results of the K barrier report and created layouts of our Jersey shaped PCB to different rigid structures. The anchor holes are 1.125" in diameter so we changed the soil anchors to 1" in diameter.
Attachment: https://mwrsf-qa.unl.edu/attachments/7c4dc50a2aa761dbc97200fcc0948141.JPG

Attachment: https://mwrsf-qa.unl.edu/attachments/d5cfc19087a5c6edbaed6d792c1b550d.JPG

Attachment: https://mwrsf-qa.unl.edu/attachments/0d5d1ae625f7fe96ba02ac9dc258a8c5.PDF

Attachment: https://mwrsf-qa.unl.edu/attachments/9931b1ebb9fe6f98cac5ba3a431b7e07.pdf

Attachment: https://mwrsf-qa.unl.edu/attachments/14371a55a9962d7382117cdb38771ea4.JPG


Response
Date: 09-09-2010

I have gone through the transition plans that you sent and I have a few concerns.


1. The overall layout of the transition looks acceptable based on the design that we have developed here using the F-shape barrier. However, the barrier alignment will need some adjustment for some of the installation cases you have shown. The approach transition design was tested with the 42-in. tall, CA single-slope median barrier because this barrier was identified as the most critical barrier design for the transition. However, there are other permanent concrete median barriers that can be attached to the approach transition as long as the following guidelines are applied.

a. If the permanent median barrier is 32-in. high, the sloped, steel transition cap is not required for the transition. For barriers with heights greater than 32-in. high, the steel transition cap if required. The cap design can be adjusted for different height and shape barriers as long as adjusted cap provides equivalent slope, permanent barrier coverage, barrier overlap, structural capacity, and anchorage as the original design.

b. Alignment of the temporary barrier system with the permanent barrier may also change when the transition is applied to different permanent barrier geometries, as shown in below. When attaching to a single-slope barrier profile, the slope break point between the toe of the barrier and the main face of the barrier should be aligned flush with the oncoming traffic side of the single-slope barrier. For safety shape barriers, the toe of the temporary barrier should be aligned flush with the toe of the oncoming traffic side of the median barrier. Vertical median barriers require that the toe of the temporary barrier segments on the reverse direction traffic side be aligned with the base of the permanent barrier on the reverse direction traffic side. These alignments will prevent vehicle snag for oncoming traffic on the permanent median barrier while preventing snag on the toe of the barrier for reverse direction impacts. (See Figure 1.jpg)

c. The thrie beam sections that span the gap between the end of the temporary barrier and the permanent median barrier should be used in all instances.

2. I would recommend that you check taper on the vertical transition section to higher median barriers. The system we tested used a vertical taper of 11.4 degrees. We would not recommend tapering to the taller barrier at a rate faster than that.

3. I have concerns with the size of the anchors used in the design. Asphalt pins in your system are much smaller than tested (1" diameter versus 1.5" diameter pins). This may cause significantly lower pin reaction forces and thus lower constrain of the barrier.
I understand that you use a different barrier section, but the pins you are using will are not likely to perform similarly to those used in the testing.

4. Similarly, the threaded rod anchors smaller shown in your details appear to be smaller diameter than the A307 threaded rod anchors we have used in the past, but they are listed as high strength. What specifically is high strength? These may be acceptable.

5. In reviewing your barrier details, it appears that the barrier reinforcement is insufficient around the pin or bolt pockets. The F-shape barrier we tested with had specific reinforcement loops for those areas. During testing, those loops have been shown to be the primary restraint that contains the pins. Without that reinforcement in place, I do not believe that the tie-down system will function.

6. The barrier you use appears to be a NJ shape barrier. We cannot recommend this barrier for use with the tie-down system without further testing. Testing of the tie-down and transition systems has shown that the sloped face and toe of the barrier can rotate back during impact causing vehicles to ride up the barrier and increase instability. We believe that the NJ shape will make this behavior worse with its higher toe section. As such, we would not recommend NJ shapes with the tie-down systems shown.

7. We also have concerns with JJ-hooks connections with a tie-down system. We would not recommend this connection for use in an anchored barrier system. The JJ-Hooks connection is fine for free-standing systems. However, to be safely used in an anchored barrier or approach transition, the barrier joints must have comparable or greater torsional rigidity about the longitudinal barrier axis when compared to that of the as-tested configuration. JJ Hooks connection is not similar in torsion to the Kansas barrier joint, and the JJ Hooks connection is also non-symmetric in that it has different capacities depending on the direction it is loaded.


There are certain barrier types out there that use cable loops or other types of connections such as JJ Hooks that have significantly different torsional capacity than the F-shape we were working with. Thus, we feel the need to warn that applying tie-down anchorages to a barrier with less torsionally stiff joints could promote vehicle instability through barrier rotation or snag.


Attachment: https://mwrsf-qa.unl.edu/attachments/52468bcc69326130dd57710265aabe59.jpg


NU Rail

Question
State: NE
Date: 07-01-2010

NDOR has had a few impacts to our new (NU) rail that have resulted in the bottom of the slab falling off under the posts.  We are currently considering the following changes to the post and slab reinforcing:

 

Move the horizontal leg of the back post bars down from just beneath the top layer of longitudinal slab steel to just above the bottom layer of longitudinal slab steel.

 

Move the horizontal leg of the front post bars up from just beneath the bottom layer of longitudinal slab steel to just above the bottom layer of longitudinal slab steel.

 

Flare the horizontal legs of both the front and back post bars from perpendicular to the post for the middle bars to 45° for the outside bars.

 

Extend the horizontal legs 1' for both the front and back post bars from 3' " 2" to 4' " 2".

Add additional longitudinal # 5 bars in the bottom of the overhang (from 12" spacing to 6").

 

Could you please review the attachment showing our existing design and these proposed changes?  We would like your opinion on whether or not these changes would require new crash tests for acceptance and any other comments you would wish to share.



Response
Date: 09-01-2010

We have reviewed your proposed changes to the deck reinforcement for the TL-4 version of the NDOR open concrete bridge rail. We have also reviewed photos from 3 different crash sites that were sent to MwRSF over the last 9 months or so. However, most of these photos show only the damage to the rail itself and do not illustrate the deck cracking that you are attempting to mitigate. Therefore, the following comments are based on general structural design and the understanding that cracking is occurring in the deck. They are not specific to damage at an individual crash site.

I am unclear as to the benefits of fanning the transverse steel. It may cover a greater area, but the lateral stress that each bar can take is reduced. Further, it may be a pain to layout/tie the steel in a consistent manner when all of the angles are changing. If you are wanting the transverse steel to cover a greater area and protect the deck at the post edge locations, I would recommend that you instead place an extra transverse (lateral) steel bar in the deck on both side of the post. These bars would be parallel to current legs and could extend 4'-2" into the deck to match the new proposed length of the legs.

The additional longitudinal steel (along the length of the bridge) near the bottom surface of the deck should provide additional resistance to bending and punching shear. These bars in combination with the additional transverse bars described above should help mitigate cracking.

Lowering the bag leg of the post reinforcement to bottom level of the deck may provide some additional steel near that surface to resist shear cracking, but it may also cause punching shear problems. Those back legs are carrying a compression load during impacts, and moving the bend to the bottom of the deck leaves the bottom of the deck susceptible to compression force punching through that surface " creating cracks and possible concrete spalling. Therefore, I would recommend leave these back legs in their current position.



Design Considerations for Prevention of Cargo Tank Rollovers

Question
State: WI
Date: 09-07-2010

I received the attached memo from FHWA today (INFORMATION: Design Considerations for Prevention of Cargo Tank Rollovers - September 3, 2010). I have only skimmed through the document, but I am concerned that FHWA is recommending the use of taller vertical barrier without considering the effect of head slap on smaller vehicles.

If MwRSF could review view this document and provide comments it would be appreciated. I wish to send a letter back to FHWA indicating my concerns about the use of taller barrier wall without considering head slap.


Attachment: https://mwrsf-qa.unl.edu/attachments/8512ee4061f9cfc33a50e5b3be6fa083.pdf


Response
Date: 09-17-2010

I have briefly reviewed the attached information. From my review, FHWA has noted the concern for head slap against taller TL-6 barriers such as a 90-in. configuration tested at TTI years ago. However, this same concern does not appear within the guidance to correspond to TL-5 barriers which have commonly been configured with 42-in. high RC parapets. You have correctly pointed out that 42-in. parapets can also pose risks of head ejection and contact against taller barriers. I believe that Dean has uncovered the risks associated with ejected passengers resulting in serious injuries and/or fatalities after analyzing accident data for the State of Kansas as part of the median barrier study.

I believe that the roadside safety community needs to be careful about blindly placing a large number of tall, rigid barriers in more locations in an effort to contain the rare occurrence of a tractor-tank trailer into piers and other structures, especially if it results in much greater risk of injury/fatality for occupant of passenger vehicles to have partial body/head ejection against tall, rigid barriers. If deemed necessary, it would seem reasonable to utilize TL-5 barrier designs which can both prevent catastrophic crashes as well as reduce/prevent head slap against tall parapets for the occupants of passenger vehicles.



MGS Steel Post Grades

Question
State: WI
Date: 09-14-2010

I was looking at some on line information about 6PWE06-07 post (I've attached a link). On the backside, there is a note that confuse me.

I may be mistaken, but isn't Grade 50W is weathering steel? If the bolt that connects the beam guard to the block and post is galvanized, wouldn't you want a galvanized post?


Attachment: https://mwrsf-qa.unl.edu/attachments/5879c386f6f3feff373a5edb2031f5e1.pdf


Response
Date: 09-16-2010

The AASHTO and ASTM steel specifications for determining the appropriate steel specifications for the W6x9 and W6x8.5 steel posts used within the MGS should be cleared up. Originally, we would have specified ASTM A36/A36M materials that used Fymin=36 ksi and Fumin=58 ksi. The TF13 Hardware guide designation page now also shows AASHTO 270 (ASTM A709) 50W in the 2006 version, which would denote Grade 50. Since there is a difference in the 50S and 50W designations, we need to be clear on whether or not weathering steel is specified. Also, the new W6x9 MGS posts have been being supplied with ASTM A992 (AASHTO ??) steel materials with Fymin=50 ksi. We may need to consider updating the TF13 details if the material specifications have changed.

Further clarification on steel grades:

A709 grade 250 is essentially the same steel as A36.

A709 grade 50S is essentially the same steel as A992

A709 grade 50W has nearly identical properties to 50S but is corrosion resistant (weathering steel)

So, perhaps the specifications should state: (note both 30 ksi and 50 ksi steel are already approved for use in specs)

For 36 ksi steel " A36/A36M or A709/A709M grade 250

For 50 ksi steel " A992/A992M or A709/A709M grade 50S. If corrosion resistant steel is desired, use A709/A709M grade 50W

I am not sure why we would have listed weathering steel other than it was copied from a prior specifications prepared for the steel guardrail posts already in the hardware guide. Mixing the two different components would likely be disastrous. We would like to not have anyone using corten guardrail steel or posts altogether. Thus, the specification should refer to A36, A992, A709 50S which provide options for using either Grade 36 (36 ksi) or Grade 50 (50 ksi) steel materials for prior guardrail systems.

We will need to look into why the note is currently written as it is and hopefully get it fixed. Karla will be meeting with AASHTO TF13 on Monday and Tuesday and can raise this issue with the entire guardrail community. We would not want to use up the zinc coating of the rail, bolts, nuts, etc. to first prevent A588 steel material from rusting the outer post layer.



Questions about Temporary Barrier Transition to Permanent Barrier

Question
State: WI
Date: 09-15-2010

I'm working on incorporating the transition from temporary barrier to permanent barriers. I've run across some questions (see attached) that I would like MwRSF to address.


Attachment: https://mwrsf-qa.unl.edu/attachments/3c36fbae1809b12c8f0402cec41293ed.pdf


Response
Date: 09-17-2010

With respect to the Red Head drop in anchor, we would recommend that any drop-in anchor that has equal or greater ultimate shear and tensile capacity (as listed by the manufacturer) when compared to the tested anchor would be allowable. The ¾" bolts used would remain A325.

With respect to the Wedge Bolt anchor, we would recommend that any mechanical screw in type anchor that has equal or greater ultimate shear and tensile capacity (as listed by the manufacturer) when compared to the tested anchor would be allowable. We would want to use the same diameter anchors as tested in order to be consistent.

The trimmed blockout used in the design was fabricated from an existing thrie-beam timber blockout.

The missing dimension on the top of the cap should be 7.25".

If you need to go to a different height, there are several factors to consider. First, you want to keep the slope of the cap the same as the tested design. Second, you will want equal gusset spacing to the tested design. Third, increasing the height while maintaining the slope of the cap may require increasing the length significantly. Thus, we would recommend extending the sides of the cap down another 3" along the side of the barrier to allow for placement of intermediate anchors that attach the side of the cap to the side of the barrier. The anchors can be the same 5/8" mechanical anchors used elsewhere for the cap. We would recommend having these intermediate anchors approximately every 50" along the barrier. These additional anchors will prevent the cap from disengaging from the temporary barrier and allowing snag on the permanent barrier as the length of the cap increases.



ZOI, Thrie Beam, and MGS 2:1 Slope

Question
State: IA
Date: 09-21-2010

I've got a couple of questions for you:

1. Could you send me a picture or description of the current guidance for ZOI for a 42" F-shape concrete barrier at TL-3, TL-4, and TL-5 conditions (if available)?

2. Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?

3. Has an equivalent wood post design been determined for use with the MGS at the breakpoint of a 2:1 slope?



Response
Date: 09-21-2010

1. Could you send me a picture or description of the current guidance for ZOI for a 42" F-shape concrete barrier at TL-3, TL-4, and TL-5 conditions (if available)?

** I will do some digging on this one. I may need to review the past 26 consulting summaries to find the answer.

2. Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?

** I am not aware of the performance limits for this barrier in terms of maximum and minimum height. However, I will seek comment from my colleagues and review a prior TTI report to see if testing at variables heights was included.

3. Has an equivalent wood post design been determined for use with the MGS at the breakpoint of a 2:1 slope?

** Yes, the wood post equivalent is a 6-in. x 8-in. by 7.5-ft long SYP post. A draft research report has been initiated but remains incomplete at this time.

You may be a little surprised that the wood post equivalent to a 9-foot long W6x9 is a whole 1.5 feet shorter. However, the 9-ft and 8-ft W6x9 post lengths were fairly close when we selected the 9-ft length. I also believe post-soil forces are higher now than before " due to MASH installation procedures that were finalized in the latter years of the MASH document and performance specification for soil. Thus, the 7.5-ft wood post length is not that far off of the original two options. We re-conducted some steel post tests on a 2:1 slope so that we would have a comparison of the two material types with newer MASH installation procedures and soil conditions. Those comparisons are contained in the draft report.



Thrie Beam Height Guidance

Question
State: IA
Date: 09-21-2010

Are you aware of any minimum and/or maximum allowable height guidance for thrie-beam guardrail?

Response
Date: 12-13-2010

I am not aware of a documented height tolerance for thrie beam guardrail systems. Initially, one may attempt to argue that the minimum height could be as low at that corresponding to W-beam guardrail systems. However, I would suspect that mounting thrie beam with a top height of 27¾ in. would potentially increase the propensity for vehicle climb, barrier override, and/or rollover upon redirection due to the increase face below normal W-beam rail with same top height.

At this time, the roadside safety community has considered the minimum top height for W-beam rail to be approximately 27¾ in., while the maximum top height for the MGS is 32 in. At the minimum W-beam top height, a thrie beam element would extend downward to 7¾ in., thus potentially creating new safety risks. Selected thrie beam guardrail systems have successfully met crash testing guidelines when installed with a top height of 34 in. As such, it is my opinion that the minimum height tolerance for modified thrie beam guardrail may be somewhere around 31 in. for NCHRP 350, while the top height tolerance may be closer to 39 to 40 in. at the TL-3 impact conditions.

Thrie beam has been successfully crash tested over the years. Below, I have provided a few of the test results but not those for the T-39 thrie beam guardrail system.

Test No. System Description Top Rail Height Result

404211-5a Modified Thrie Beam w/ 81" Steel Post & Tapered Block 34" Passed 8000S TL-4 test

404211-11 Strong-Post Thrie Beam w/ 81" Wood Post & Wood Block 31.65" Passed 2000P TL-3 test

404211-10 Thrie Beam w/ 81" Steel Post and Routed Wood Block 31.65" Passed 2000P TL-3 test

471470-31 Thrie Beam (G9) w/ 78" Steel Post and Steel Block 32" Failed 2000P TL-3 test

471470-30 Modified Thrie Beam w/ 81" Steel Post and Tapered Block 33.6" Passed 2000P TL-3 test

Recent Test Thrie Beam (G9) w/ 78" Steel Post and Full-Depth Wood Block 31.5" Failed 2270P TL-3 test (MASH)

Although I have yet to see the results of the recent failed MASH test, it would seem reasonable that improved safety performance could be obtained by using a shortened wood blockout or the modified steel tapered (collapsible) blockout " both of which reduced climb and allow the lower corrugation to fold back.