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1)      The Traffic Barrier Terminal transition from a bridge rail or concrete parapet wall is usually a thrie beam apparatus.  In IL, when a curb extends under the first section of the barrier, it can be affixed to the end of the parapet (TYPE 6).  If there is no curbing, the thrie beam extends an additional section onto the structure (TYPE 6B).  So I draw the conclusion that the curb is also a safety feature over just a drainage diversion – I assume it possibly help redirect and contain an errant vehicle.  So, is there a curbing depth and width that is required, especially if it was test crashed for acceptance?  Sometimes, it will be connected to the approach pavement, and other times it will be next to an asphalt shoulder.  If it is part of the pavement, it will be doweled in; if it is next to asphalt shoulder / pavement, in order to retrofit at this point in time, they will have to excavate it and form it up presumably with no reinforcement. 

So, a) are my assumptions correct in that the curb is actually a safety feature adding to the deflective capabilities?, and b) what kind of curb is specified / depth / width / curb and gutter (B6-18) as “crash tested”?

-          I have reviewed your details and believe that your current type 6 transition is a combination of the MGS upstream stiffness transition that we developed in TRP-03-167-07 and the Iowa transition that used quarter post spacing. However, it appears that you may have modified things lsihgtly in terms using longer posts in the quarter post spacing region.

-          For your information, we have developed newer versions of that upstream stiffness transition that can be used that are shorter and use fewer post types. I have attached reports dealing with the steel and wood post versions of those newer transition designs (TRP-03-210-10 and TRP-03-243-11) You may want to consider these details in order to simplify your approach transition details.

-          You are correct that the curb has an effect on transition performance. However, the effect varies depending on the transition. Some approach transitions such as the Iowa transition with ¼ post spacing were designed and tested with a curb. In these cases, the curb served to help limit vehicle snag on the end of the concrete parapet. Some tests have shown that removal of these curbs can degrade the performance of the transition. That said, it is our belief that improved design of the concrete end section through shape and tapering can reduce snag to a manageable level without the need for a curb in most cases. However, the exact design and geometry of such an end section have not been formalized, so the use of the curb is still recommended for certain transitions such as the one you currently use. We are working on coming up with improved concrerte end section details as part of the Pooled Fund consulting effort in the near future.

-          The curb that we currently have used in most transitions is a 4” tall wedge curb. We have recently tested a version of the MGS approach transition with this curb extended past the W-thrie transition rail as shown in the attached detail. We have not done testing with the more vertical shape curbs you have shown in your details. We would recommend the wedge curb if possible, but we believe that the more vertical curbs you have shown would have the potential to perform acceptably for a 4” height. We would not recommend curbs over 4” tall as their effect on the approach transition has not been evaluated.

-          Note that the 4” wedge curb is believed to be acceptable as long it terminates prior to the W-thrie transition piece in the details you have shown. As noted above, we have tested extended curbs, but we found that nested W-beam rail was required to prevent rupture as shown in the attached detail.

-          As far as omitting the curb, removal of the curb depends largely on the design of the transition, its dynamic deflection, and the shape and geometry of the concrete parapet end. As noted above, we believe that transitions can be made to work with and without curbs, but for now, we are generally recommending that systems tested with curbs keep them in place.

2)      In installing sand barrels for permanent protection at bridge piers, I was told that some are allowing the barrels to remain on wooden pallets (4” height), as an end loader cannot get access to them from underneath with the pallets.  Is that acceptable / other states doing that / what about the safety performance and crash worthiness with the pallets in place?  Do you see a safety problem with leaving them atop the wooden pallets?

-          We have not heard of other states placing 4” pallets underneath the sand barrels. It was our understanding that the barrels were put in place unloaded and then filled with sand. There is some potential concern that placement of the barrels on a raised platform may affect the performance of the system by affecting vehicle stability and the location of the barrier CG’s relative to the impacting vehicles. However, you would need to contact the sand barrel manufacturers to get a definitive answer on that.