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W-Beam bridge barrier transition posts in concrete

Question
State MO
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  The attached picture shows a low-cost, TL-3, W-Beam bridge barrier transition that we have chosen to use on about 500 bridges that we are rebuilding.  

A small number of these were inadvertently built as shown, with posts 1-5 set in concrete.  One of our inspectors noticed the condition and asked me if we should insist on a retrofit.  My initial reaction was to order the retrofit, given the increased stiffness introduced into the system, and the resulting increase in the likelihood of  beam rupture.

Yesterday I spoke to the contractor and he pointed out that both MoDOT's internal policy, as well as that of the Roadside Design Guide point to special post settings in solid rock and pavement for regular runs of W-Beam guardrail only, not thrie beam, or stiffened W-Beam transition sections. After a little research of my own, I could only agree with his logic and told him I would render an opinion based on your advice.

My questions follow:

1. Does the concept of special post setting, i.e. a slot cast in the slab and backfilled with aggregate, apply to all posts, or just regularly spaced W-Beam posts in a roadside guardrail run?

 


2. If so, can an exception be made for a situation like the one shown here wherein the entire system is intentionally being stiffened anyway?



Approach Guardrail Transitions (AGTs)


Date December 7, 2011
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In response to each question:

1. Does the concept of special post setting, i.e. a slot cast in the slab and backfilled with aggregate, apply to all posts, or just regularly spaced W-Beam posts in a roadside guardrail run?

 

Concrete or asphalt leave-outs could also be considered for closely-spaced posts found in approach guardrail transitions. These leave-outs should allow for greater post rotation within the compacted soil region which should be more representative of the post-soil behavior observed in the actual crash testing programs. Since most transition posts rotate less than that observed for standard guardrail with 6 ft " 3 in. post spacing, it would be appropriate to utilize the recommended leave-out sizes and placement guidance if comparable post cross sections are used.


2. If so, can an exception be made for a situation like the one shown here wherein the entire system is intentionally being stiffened anyway?

For posts placed in concrete pads, guardrail performance could be greatly affected in terms of increased wheel snag on posts and increased vehicle pocketing farther upstream as well as greater propensity for rail ruptures. Even though AGT posts deflect less than standard guardrail posts, changes in stiffness and strength from that used in actual testing programs could potentially degrade barrier performance at various locations. It would be my general recommendation to sawcut the concrete, jackhammer out the non-desirable concrete pad around AGT posts, and configure appropriate leave-outs for AGT posts. Then, soil, crushed rock, or other low-strength leave-out material could be placed around the posts at the upper surface. How thick is the concrete around the posts?
Date December 7, 2011
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The concrete around the post is 4" thick.

 
Date December 7, 2011
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With a 4" thickness, it would be relatively easy to sawcut the concrete to provide a leave-out length similar to that provided for guardrail posts assuming similar post positioning in hole. If post cannot be made close to front of the leave-out, then increased length would be needed on back side of posts. It would be acceptable to cut one large opening for all 4 or 5 posts versus individual cutouts.
Date December 7, 2011
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