View Q&A

I would appreciate your thoughts and recollection about the
design and testing of the base plated w-beam system on shallow (low fill)
concrete box culverts. Both of the designs shown below from TTI (Report No.
405160-23-2 - MASH Test 3-11 of the W-Beam Guardrail on Low-Fill Box Culvert)
and from MwRSF (TRP-03-114-02 - NCHRP 350 Development And Testing Of A
Guardrail Connection To Low-Fill Culverts) were designed and tested with 9” of
cover. Page 25 of MwRSF report (TRP-03-114-02) states that “since zero or
minimal thickness of soil fill is generally not an option for most culvert
designs, a 229-mm layer of soil fill was selected for the research study and
was believed to still provide a critical safety performance evaluation on the
new barrier system”. The TTI design duplicated the 9” cover used for the MwRSF
design.

I am trying to confirm whether these base plated designs
would be acceptable for installation on an exposed culvert deck (or bridge
deck) without the 9” of fill (cover) that both MwRSF and TTI evaluated and
tested. I did not see any other details in either report suggesting the 9” of
cover is actually required for the based plated systems to perform acceptably
during an impact, but both show 9” cover as the minimum.

If the 9” of cover is required for these designs, are you
aware of another MGS or thrie beam system that could be base plated to the top
of an exposed concrete deck that meets MASH TL-2 or higher? I am also dealing
with an existing curb in front of the barrier system, which I could remove or
partially remove, but would prefer to retain it. I was thinking about
potentially doubling up on the 8” offset blocks to keep face of w-beam within
5” of face of curb (similar to MGS with 12” offset blocks with w-beam set 5”
behind 6“ high barrier curb) if you did not have any concerns. Proposed
installation is on a narrow low volume low speed road, so TL-2 would be
acceptable.

MwRSF TRP-03-114-02 – Based plated W-beam, NCHRP 350 TL-3

TTI Report No. 405160-23-2: Base plated MGS, MASH TL-3

At this time, we would not recommend installing either of these guardrail systems without a minimum of 9” soil fill on top of the culvert.  Over the past few years we have observed multiple instances of rail tearing (both partial tears and complete rupture) related to MASH testing of stiffened guardrail systems.  In fact, both of these systems experienced partial rail tearing as part of their MASH testing and evaluation.   Eliminating the soil fill results in a reduction to the post length and the corresponding moment arm (distance from base plate to center of rail), which will effectively stiffen the barrier system.  This additional stiffening may lead to further tearing of the rail and even complete rupture.  As such, we would recommend testing of these systems with reduced soil fill prior to installing them in the real world.

TTI has developed and crash tested a top-mounted, weak-post W-beam bridge rail.  Both MASH TL-2 and MASH TL-3 variations have been tested and differ only by the spacing between posts.  I have provided a few links to this system below.  Links to the testing reports should be at the bottom of the respective web pages.

https://www.roadsidepooledfund.org/longitudinal-barrier/txdot-t631-bridge-rail/

https://www.roadsidepooledfund.org/longitudinal-barrier/txdot-t631-bridge-rail-2/

The TTI system was based off of the MGS bridge rail developed here at MwRSF (a socketed, side-mounted system).  We have also developed socketed, side-mounted barrier options for culvert headwalls, and we have recently developed a new side-mounted socket attachment that better distributes deck loads, thereby preventing damage.  If you are interested in utilizing any side-mounted options, let me know.

I was not aware of the two TTI deck mounted MGS systems, which are perfect for one side of project (and others), whereas we are considering the MwRSF side mounted MGS system for the other side (which is shown below from TRP-03-277-14 - Concept D2: Side-Mounted, Epoxy-Anchored). If you have another option we should consider, please advise.

Currently both sides of the triple cell culvert/dam has 10” high x 8” wide curbs (one side at edge of deck, other in front of dam control lifts for control gates. Would it be potentially acceptable to use 8” or 12” offset blocks with the top and side mount weak post systems to get face of rail in front of existing curbs for this low speed low volume single lane structure, or could we consider reduction in curb height to 4” or 6” with sloped face (similar to MGS with 12” blocks offset 5” behind 6” barrier curb)), or is complete removal of curb our best option.

Thanks again for your quick response and references.