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Dr. Faller is placing some stacked W-beam reports in a Box folder for you to reference.  The design tested at TTI under test 404211-12 has an FHWA eligibility letter, see attached.   This transition design was a modified version of a NCHRP Report 230 tested design, which was approved by FHWA in a technical memo – see link below

https://safety.fhwa.dot.gov/roadway_dept/countermeasures/reduce_crash_severity/barriers/techadvs/archive/t504026/

Scott

Scott:

Crash test shows metric height from ground to center of top W-rail at 550mm (top of guardrail).  Our structures use 21” (533mm). I assume this is acceptable.

In general, are bridge anchor sections still acceptable when approaches are overlaid that would reduce effective height of rail?  Specifically, is this transition acceptable when approach is overlaid?

Gregory Sanders, P.E.

There are a few small differences between your old transition standard and the one crash tested to NCHRP Report 350 and approved by FHWA.  However, I believe the ¾” height difference would have a minimal effect on performance. Thus, you could make the argument that your existing transitions are NCHRP Report 350 TL-3 compliant.

During our conference call this week, you expressed that you will be attaching MGS to the upstream ends of these existing transitions.  The bridge railings and parapets in which the transitions are attached to are only 29” tall, so you will not be able to raise the height of the transition rails to match up with MGS.  Note, TTI recent conducted a study in which the height of the upper W-beam rail was increased to 31”, but the system failed to satisfy MASH criteria during crash testing.  Thus, you will need a height transition between 31” MGS and the 27” transition, and your transition section will not be MASH crashworthy.

In order to create a MASH crashworthy transition, you will need to remove/replace the ends of the concrete barrier to match a MASH tested system.  This retrofit would be costly.  You had stated that you have not observed safety issues with the existing transitions, so upgrading the transitions  would have limited benefits as the existing system is NCHRP 350 crashworthy. As such, I recommend leaving the transitions in place (as part of the bridge rail) when the adjacent guardrail is replaced with MGS.

To transition between the existing transitions and MGS, you will need a height transition – from 27” to 31”, respectively.  MwRSF has been recommending that such height transitions be done gradually over a distance of 50-ft.  Additionally, the height transition should start upstream of the stacked w-beam transition, which would include rub rail and reduced post spacing. So, the height transition should begin at the 9^th post upstream of the bridge rail.

I do not think these transitions would remain crashworthy if an overlay raised the height of the roadway 2-3 inches.  These transitions are already short (at 27”), and further reducing this height could have major effects on the performance of the system.  At this time, it is not known if a 31” tall guardrail transition would maintain its crashworthiness if the height was reduced.  I have not found any thrie-beam transitions to pass TL-3 criteria (either NCHRP 350 or MASH) with a height less than 31”.  Most of the shorter height transitions resulted in rollovers.  If an overlay is necessary, you may want to grind down the pavement prior to the overlay so that the roadway height remains the same. 

For future installations, MwRSF and NDOT are currently finishing a project to develop a 34” tall guardrail transition system – which would make it crashworthy as installed and after a 3” overlay is added.  The system has been successfully crash tested to MASH TL-3. The report is not yet completed, but I can supply you with further details if you are interested.