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Before I spend much time looking at your current standard, I wanted to make sure you were aware of some recent work we have done in developing the Standardized Buttress for Thrie-beam transitions.  The buttress was designed with an optimized geometry to limit vehicle snag while also laterally supporting the thrie beam.  It is compatible with a number of thrie-beam guardrail transition configurations, and may be utilized with or without a curb below the rail.  The report for this project has not been finalized, but there has been a Transportation Research Record journal paper on the development of the buttress.  I have attached a copy of this paper.

The standardized buttress has a vertical front face, but is intended to transition to match up with numerous other barrier shapes.  Details on how to transition to various shapes is discussed in the implementation section of the paper.  I have also attached a drawing set illustrating various shape transitions – including to single slope barrier on sheets 6-9.

I encourage you to review these materials and consider adopting the standardized buttress as part of SCDOT’s standards. MwRSF feels that is a superior buttress design to all others systems previously crash tested. Let me know if you have any questions.

Also, if you do not wish to incorporate the standardized buttress into you designs, then I can spend some time reviewing specific transition.

Thank you for taking the time to review our drawings. At this time a little background information may help explain our position.

In 2017 we began reviewing crash tests in order to develop standard drawings for MASH implementation. At that time we were not aware that a “Standardized Buttress” was in development.

Since then we have finalized and published standard drawings for MASH. Unfortunately, MASH testing criteria does not consider real world conditions for crash testing and as a result, most state DOTs are forced to make modifications to the as tested device in order to address specific state needs. One modification we made to the as tested AGT was to use a 6” vertical face curb instead of the as tested 4” curb (AASHTO Type G) within the AGT. Based on historical rainfall events in our state the hydraulic design requirements could not be achieved using a 4” curb. Since the curb is essential for controlling drainage and preventing erosion of the bridge embankment, we have specified on our standard drawings that a 6” VF curb is now required at all (AGT) bridge ends.

At this time SCDOT cannot adopt the standardized buttress. SCDOT will need time to evaluate the standardized buttress for any potential issues. The TRB Journal Paper indicates that the standardized buttress can be used with or without a curb present. However, according to the buttress geometry shown in Figure 5 (page 18) of the Journal Paper, there appears to be a conflict when a curb is used in combination with the standardized buttress (see attached 3D pdf). When the curb flow line is aligned with the flow line of the buttress there is approximately a 1/8” gap between the back of curb and face of guardrail post (when using a 12” offset block). If an 8” offset block is used the guardrail post would be in direct conflict with the curb. This post curb conflict could be addressed by pulling the curb flow line out (toward the edge of travel) but doing so also brings the face of curb out closer to the edge of travel which seems counterproductive.

We request that you complete the review of the details submitted for our bridge railings understanding some of the reasons why the buttress configuration will not be adopted by SCDOT.

First, a quick comment on the standardized buttress and curb issue you raised with the 3-D sketch.  The use of 12” blockouts to replace 8” blockouts within a guardrail transition has been recommended by MwRSF in the past as it should not affect the system performance .  Also, multiple states (Wisconsin and Minnesota come to mind right now) plan to install the face of the cub even with the front face of the buttress to keep a consistent flow line.  This would result in the curb being shifted forward 4.5” in your sketch.  The curb should extend further downstream and terminate adjacent to the lower tape of the buttress with a 4:1 flare rate.  I’m not saying SCDOT needs to adopt this, but wanted you to be aware of what others are planning to do.

I have reviewed your single slope concrete barrier transition detail. I do not see any safety concerns with this design.  The 9-ft long shape transition between Section A and Section C is plenty gradual for both the lateral tapers (we typically recommend 10:1 or flatter) and the vertical taper/height transition (we typically recommend 6:1 or flatter).  In fact, if you wanted to decrease the length of the transition, this 9-ft long transition segment could be reduced down to 5-ft.  This isn’t necessary, just wanted to let you know you could if you have limited roadside space or wanted to save a little on installation costs. The face of the curb is located even with the face of the rail, so that shouldn’t cause any stability issues.  Additionally, there is adequate longitudinal and vertical reinforcement to provide the necessary strength and anchorage for the barrier.

The only recommendation I have is to slightly alter the shape of the taper on the upstream end of the transition.  Currently you show a 2” lateral offset over a 12” longitudinal length.  In our review of previous transition testing, we thought that a 3-4 in. lateral offset resulted in the best snag mitigation for the edge of the buttress behind the thrie beam.  2” may work fine, but I would feel better utilizing a 3” lateral offset to match the standardized buttress offset.  The standardized buttress has a 3”x4” chamfer.  You may elect to utilize the same dimensions, or keep your 12” length and use a 3”x12” taper.

The note on the guardrail attachment says “Thrie Beam Guardrail” – I am assuming this is actually nested thrie beam, but specified on another standard.  All of the MASH crashworthy transitions have utilized nested thrie beam to connect to rigid parapets. 

Let me know if you have additional questions.

Thank you for this response.  Our Bridge Engineer, can review your comments to determine if any of your suggestions would provide additional benefit on the submitted bridge typicals.

Since these are all bridge parapet details, and my group is looking for some solutions for barriers on top of retaining walls, I am curious if you have any data or suggestions on how many states use vertical face barriers on their bridges/walls vs states that try to incorporate a shape transition like the drawings we sent.

I have discussed with several engineers from FHWA and other states, but have never been able to find definitive guidance on vertical face vs sloped or safety shape barriers on bridges and retaining walls.  My concern involves balancing the occupant risk factors of the more frequent passenger vehicle impacts with the less common but not infrequent impact concerns of larger vehicles on the road when there is an immediate drop-off hazard directly beyond the barrier as well as a much less common but possible condition of above-deck superstructure hazards on some of the larger bridges.

As I understand them, sloped and safety shape barriers offer reduced impact forces on the passenger vehicles compared to vertical face barriers, but that is at the expense of higher instability, more risk of ride up, more risk of rollover, and more risk of overtopping the barrier – particularly for taller vehicles.  Now that so many rigid shapes have been tested under 350 and MASH conditions, is there a source where we can obtain comparative data on these factors to see if my previous statement is correct or if the research shows a different set of performance characteristics?

If there is no source, could you provide some recommendations on this topic?

Have vertical face barriers passed MASH testing?

Would you have any concerns in using vertical face parapets on all bridges and at tops of retaining walls in order to minimize the overtopping and instability issues with larger vehicles? 

If so, what would be the preferred minimum height for the vertical face barrier? 

How does selection of the 9.1 degree CalTrans/Manitoba Single Slope barrier compare to the other available options for the different risk factors I mentioned?

For structures that have above-deck superstructure, would Roadside Design Guide’s Zone of Intrusion style 54” TL5 barrier or even the Texas or Ottawa TL6 barriers be appropriate?  All of these barriers are of significant weight and would add additional wind loading, but could be reviewed if guidance suggests that they are appropriate. 

I do not know that a definite source for the comparison of various barrier shapes exists, but there are a few reports that I am aware of that discuss the effects of barrier shape on performance for passenger vehilces.  The following can be found on our website and should serve as a good start to your search:

TRP-03-194-07  (chapter 4)

TRP-03-259-11

Those two reports will have a lot of references than may also prove useful in assembling a barrier shape comparison.

In general, you are correct about the shape performance. Safety shape barriers are associated with decreased accelerations and vehicle damage, but increased vehicle climb and rollovers.  Vertical barriers are associated with increased (somewhat) accelerations but optimize vehicle stability.  Single slope barriers are somewhere between the first two categories and will depend on the slope/angle of the barrier face. MwRSF is on the side of preventing vehicle instabilities during redirection, so we recommend vertical or steep single-sloped barriers (5° or less).

Vertical barriers have passed MASH crash testing, and minimum height for the barrier would depend on test level.  32” is typical for TL-3, 36” is necessary for TL-4, and 42” is typically the minimum for TL-5.  However, taller versions for any of these barriers have been installed previously.

For bridges with above-deck superstructure, yes we would recommend using ZOI guidelines to prevent vehicle contact with the superstructure elements.  MwRSF currently has an NCHRP study to define the ZOI for MASH impacts and vehicles.  So, more data on this topic should be available in a couple of years.

I hope this helps.  Let me know what else I can do to assist