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We’re considering the use of the PennDOT PA bridge barrier (MASH TL-5, see attached) for an upcoming river bridge project. The bridge is a truss, and we’re wondering what an appropriate minimum setback distance to the superstructure would be if this barrier were employed. Posted speed would be no more than 45 MPH. Can you point me to some MASH-compliant guidance that would indicate minimum setback to structure?

From your email, it appears that you are looking for guidance on the Zone of Intrusion (ZOI) for a TL-5 impact. The TL-5 impact for test no. 5-12 is only 4.7 mph slower than your 45 mph posted speed.

I recently gave some guidance on TL-5 ZOI values to Joe Hall from West Virginia. Review the response I gave him below and see if that helps. If you want to discuss the setback further, let me know.

Thanks

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MwRSF is currently looking at ZOI for TL-5 in NCHRP 22-34 which is tasked with updating the ZOI numbers for MASH. Reviewing the most recent data from that study, it appears that the numbers we are currently getting are consistent with our original guidance for both the 42” and 54” barriers under TL-5. None of the additional tests collected as part of 22-34 demonstrated greater ZOI values than the ones I discussed with you previously.

Note we still do not have updated TL-5 numbers for any parapets taller than 49.25” as no tests have been conducted. We will be attempting to simulate some of the gaps in Phase II of 22-34.

Thus to reiterate from the previous guidance.

The following were the ZOI measurements from a 42” tall TL-5 bridge rail and a 42” tall TL-5 median barrier tests conducted at MwRSF between 2004 and 2007.  The critical ZOI value was found to be 76.5" - measured from the median barrier crash test.

Bridge Rail - Working width = 79.5"

                     Top corner offset from front of barrier = 5.5"

                     ZOI = 74"

Median Barrier - Working Width = 86.5"

                     Top corner offset = 2" (lower slope)+ 8" (head offset) = 10"

                     ZOI = 76.5"

The ZOI values were calculated from the working widths (measured from high speed video) and the offset distance between the top-front corner of the barrier and the front of the barrier.  Both of these barriers incorporated a head slap prevention geometry, so the top of the barrier was offset above 35" in height.  If you wanted to apply these ZOI numbers to a vertical parapet, conservatively you could add back in the offset distances noted above for a ZOI of 86.5”. Note that these values are the upper values only. We have not calculated the lower values.

For 54” tall TL-5 barriers, we only have data from the Manitoba bridge rail work that we did recently. That barrier was a near vertical single slope with a height of 49.25”. Based on the working width of that system, which was 37.4” as measured from the bottom of the face of the barrier, the ZOI would be 31.65” as measured from the top front corner (barrier had a top corner offset of 5.75”). Note that this number would be conservative for a 54” tall TL-5 barrier. Similar recommendations to those made for the 42” tall TL-5 vertical and safety shape barriers would apply here as well.

Thanks for the information on TL-5 ZOI.

Our situation is that we’re using a TL-5 barrier only to reach minimum bicycle railing height (due to Complete Streets policy guidance) but in a context that looks more like TL-2, at least from a speed perspective. Additionally, accommodating the full working width from crash data would make the bridge so much wider than required for the roadway that we probably couldn’t afford to build it. What we need is a determination of the critical clear distance to near structure behind the barrier. Normally, we’d assume 3 feet from the top traffic face corner would be sufficient, and looking at some recent bridge projects confirms this range (see attachment).

Do you have any further thoughts or can you point us to guidance that is more targeted to this situation?

I am not sure how best to answer your question.

Are you planning to base the offset from the superstructure based on TL-2 passenger vehicle impacts alone? If so, the setback is essentially zero for a 54” tall barrier.

We have previously run a MASH TL-3 test of a 2270P vehicle on a 42” tall single slope barrier. That had a ZOI of 10.8 inches. As the barrier was 12”wide a the top, the vehicle did not extend past the top of the barrier. We would expect significantly less or no extension of the vehicle over the top of the barrier for a 54” tall barrier at TL-3 impact conditions. For TL-2, their would certainly be no concern for vehicle extent over the barrier. Small car vehicle extent over the barrier would be considered less likely than the pickup truck, or again equal to zero.

That said, if you need to consider  the potential for heavy truck impacts on the railing, then the ZOI I noted previously would generally apply. The TL-5 impact speed for test no. 5-12, the 36000V tractor-trailer vehicle, is 50 mph. As your speed limit is within 5 mph of that, you may want to consider potential intrusions as large as the ones I noted previously if you are considering the potential for heavy truck impacts on the superstructure.

I understand your confusion about what we are looking for. We’ve chosen the PennDOT PA barrier for a reason other than crash performance (i.e. bike height), but we also want the extra height it provides to help protect truss superstructure behind it. It’s not a straightforward problem.

I think perhaps the basic problem may lie with how “working width” is determined. If it is the full extent of intrusion of a semi-trailer box during test 5-12, then it’s not indicating the intrusion of just the cab, which we assume would be the critical component in engaging a piece of superstructure behind the barrier. It appears when looking at the attached 5-12 Summary of Results for the PennDOT PA, that it was the semi-trailer box, not the cab, that established the 51.9 inch working width.

Is there MASH guidance for minimum setback to superstructure for the various test levels, or is this a matter that allows for some engineering judgment?

Thanks for the additional information.

Just for clarification I thought I would define a couple of the terms.

Zone of Intrusion is defined as a measurement of the farthest protrusion of a critical vehicle component behind the top front corner of a barrier during a crash. Critical vehicle components are structural components that may pose a risk to vehicle occupants and/or may impart a significant load to fixed objects located within the ZOI. These critical components included the engine hood, fender, door, roof, cargo box, etc., but they do not include non-structural components, such as plastic bumpers and side mirrors.

Working width is measured from the farthest forward point on the barrier to the maximum system or vehicle extent beyond this point. Thus, when the vehicle does not extend beyond the back face of the barrier, the working width is equal to the barrier width. When the vehicle extends beyond the back face of the barrier, working width is the distance from the farthest forward point to the vehicle extent or maximum lateral deformation of a barrier component. Additionally, working width is usually measured as the maximum extent of any vehicle component that is still attached to the vehicle, which includes both structural and non-structural components.

We would generally consider the semi-trailer box a critical structural component and would have the potential to impart significant load into the truss structure of an adjacent bridge. We have used ZOI in past recommendations regarding shielding of bridge piers which is somewhat similar in that we want to limit damage to the bridge pier. Based on the report data you sent it would appear that the ZOI would be around 52” for the Penn DOT barrier, and that the ZOI was established by the trailer box.

The more difficult question is whether or not that level of potential contact needs to be addressed in your barrier offset from the superstructure. There may be engineering analysis that could be performed, but it would need to include consideration of the structural capacity of the trailer and the truss structure to ensure that the truss structure would not be compromised in the event of contact by the trailer.

LRFD is somewhat noncommittal on the subject. It notes in chapter 3 that offsets from a 54” tall barrier to a pier should be 10 ft or less, but does not define “less” clearly.

There is some other guidance for TL-5 barrier offsets for piers that was produced in NCHRP 12-90. It suggested lower offsets than those indicated by ZOI values. As such, you may be able to apply this study for your application. The offset in that study were estimated using simulation of TL-5. However, they do not necessarily align with the vehicle intrusion and ZOI data from existing TL-5 testing under NCHRP 350 or MASH which used the same vehicle and impact conditions. See section 3.3.5.