View Q&A

During your move I had asked whether or not you viewed the curved parapet treatment (shown on photos that I sent you) to be crashworthy.  You mentioned some research that had been done by TTI, but as you were mid-move, you were unable to put your hands on the data.  Have you been able to locate it and do you have an opinion?

Since then, another issue has come up.  We are replacing hundreds of bridges around the state in the coming few years, and in an effort to make the undertaking as cost-effective as possible, we have adopted a 350-approved bridge end connection that employs two double-nested w-beams for stiffness at the end.  This arrangement bears the federal approval number HMHS-B65 if you wish to view its test summary and drawings.  The FHWA approval letter cautions practitioners to taper the parapet wall to the top of the rail to avoid snagging potential.

Unfortunately, we've recently realized that about 30 bridges have been built without the taper (as shown below).  In fact, the condition in the field has the parapet height about 2 inches above the rail.  The wall does have a 3/4 inch chamfer around its top and sides.

My questions are these.  Do you see a snag potential in the as-built condition?  If so, is it drastic enough for us to fix it?  Grinding the concrete taper is not an option so the only remedy would be to raise the rail: an exercise we don't want to undertake.  Of course we will if there is a danger to the public.

I have reviewed a few prior development and crash testing efforts regarding the exposure of concrete buttresses above the rail element in approach guardrail transitions, more specifically two systems that were developed with funding from the Pooled Fund Program.

For the first barrier system, a thrie beam approach guardrail transition was developed and crash tested for use with a half-section New Jersey shape concrete barrier. The thrie beam's top mounting height was 31 in., while the top height of the concrete parapet was 32 in. The top horizontal edge of the concrete parapet's upstream end was not chamfered. As such, there was 1 in. of exposed concrete above the 31-in. tall thrie beam. For this design, the 1-in. of exposed concrete did not result in excessive vehicle snag on the upstream end.

For the second barrier system, a thrie beam approach guardrail transition was developed and crash tested for use with a single-slope concrete median barrier. The thrie beam's top mounting height was 31 in., while the top height of the end concrete parapet was 31 in. but with the concrete surface sloping upward at an 8:1. The top horizontal edge of the concrete parapet's upstream end was not chamfered. As such, there was no exposed concrete above the 31-in. tall thrie beam except for the upper 8:1 sloped surface.

For the approach guardrail transition system shown below, there is approximately 2 in. of exposed concrete above the upper W-beam guardrail. The original crash-tested transition system (link provided below) was configured with the concrete end flush with the top of the W-beam rail, thus mitigating any concerns for the engine hood and front quarter panel to snag on the concrete. Due to the lower W-beam rail height as compared to existing thrie beam transitions and the 2-in. exposed height, it is recommended that the transition system be retrofitted to mitigate the potential for vehicle snag on the upper concrete edge.

http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/barriers/pdf/b-65.pdf

MwRSF would be willing to discuss and brainstorm potential options for safely retrofitting the transition system to mitigate the vehicle snag concerns. I look forward to hearing from you in the near future.