View Q&A

1.     We've had several questions/issues come up recently regarding bridge end sections:

a.     The amount of reinforcing required in our "standard" end section is believed to be contributing to voids in the concrete (this is only speculation). Would it be feasible to reduce the amount of steel without affecting the crashworthiness of the guardrail connection? If so, what is the minimum strength requirement for the end section?

** TL-3/TL-4 bridge end sections have traditionally been designed to withstand lateral impact loads of approximately 100 to 120 kips and longitudinal impact loads of approximately 90 to 110 kips. From your question, it appears as though the IA DOT would like to reduce the number of vertical bars near the end sections, although I am making this assumption. This can be completed as long as the the structural capacity remains the same use different combinations of bar sizes and/or more or larger longitudinal bars. A yield-line analysis at the end of the barrier would need to be performed to replicate your capacity of the crash worthy design. We likely cannot reduce its strength and obtain approval of the design without testing unless it provides equivalent or greater capacity.

b.    How much variability are we allowed when it comes to the shape of an end section? Specifically " how much can we vary the overall length of the end section and the angle of the leading chamfer? Looking at other states' standards, there appears to be quite a variety of shapes used. Am I to assume they have all been crash tested?

** When making changes to the shape of the end section, one should follow accepted crashworthy details such as the slope of the toe (could be flatter but not steeper), the coping or cutback at the end to reduce wheel snag on corners, etc. Yes, other variations may be acceptable, but one should discuss with us your proposed changes first so that we can provide feedback in advance of making such changes. It may also be helpful for your staff to provide to us the desirable details used by other states so that we could determine whether they have been crash tested or not.

c.     Due to the proximity of our first guardrail post to the leading edge of our end sections, some contractors have complained that they cannot drive that post and must instead excavate and backfill by hand. Is it possible to increase that gap distance or must it stay constant?

** There are other transition designs available that do not use the 18.75-in. post spacing and a short span from post 1 to the buttress end. If desirable, we show provide the IA DOT with a list of other crashworthy transition details that may eliminate with field installation problem. It is not recommended to change post locations of the crashworthy designs without testing.

d.    I have seen other states lengthen or shorten the "overlap" of the guardrail connection at the bridge end based on some conditions. Do you know what this is accomplishing? What are your thoughts on this practice?

** I have a personnel opinion that some states are providing extra overlap of the thrie beam guardrail (part of the transition) onto the concrete buttress end because they may feel that there reinforced ends have insufficient strength if impacted directly in this region. If adequate structural capacity of the RC buttress exists, there is no reason to have significant overlap of the concrete end  other that to bolt the terminal connectors in place and have adequate steel placed around this connection to prevent its pullout and premature wall fracture.

2.     Are you aware of any research regarding what effect, if any, low temperatures would have on the crashworthiness of steel bridge rails?

** Unfortunately, I am not aware of any specific research into this topic.

3.     On some of our construction jobs I have seen where a contractor has overlapped freestanding Temporary Barrier Rail directly in front of a permanent concrete barrier or bridge rail as a means of terminating the TBR. Typically this has been accomplished with only one or two sections of TBR resting in front of the permanent barrier. Would you consider this an approved practice? If not, would there be a minimum number of TBR sections that would need to be overlapped in front of the permanent barrier in order for it to function properly?

** I would not consider acceptable the practice of using two free-standing, 12-ft long, TBR sections for downstream anchorage near and directly in front of a permanent barrier installation. Instead, our best engineering judgment would be to overlap at least 8 TBRs past the end of the permanent barrier and maintain a clear gap of 24 in. between the barrier bases. Our concern with freestanding TBRs directly in front of permanent barriers is that the freestanding barrier could pocket at the end of the permanent barrier. Thus, the provision for 24 in. allows for some normal deflection without pocketing over the first 2-ft of barrier translation.

4.     Finally, do you have a method for attaching the MGS directly to a paved sidewalk or through the top of an intake? If not, are we able to develop our own based on a current design with standard guardrail?

** At this time, we have not considered how to attach the MGS to paved sidewalks, but it is reasonable that rectangular leave-outs placed around the posts would allow for adequate post rotation, similar to what was used for the MwRSF post in rock study and the TTI guardrail in paved mow-strip study. Second, it would be acceptable to span over the intakes if they are 25-ft long or less. Recall that the MGS was adapted to long-span culvert applications using 3 CRT posts on each side of the long span.