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We are considering adopting this transition (MwRSF Report TRP-03-210-10, December 21, 2010), and have a question about transition to a concrete parapet.

Our current design uses a curb under the thrie beam transition to minimize wheel snagging on the end of the concrete parapet.  The new design was developed with a thrie beam version of the bridge rail.  Various adaptations are shown in Chapter 14 of the report, but these do not address the issue of the curb.  As it is not addressed, we are thinking that the curb would probably still be needed for cases where it is currently used.  If it could be deleted, or could be applied as needed for drainage that would provide considerable options for our bridge and roadway designers.

What is the recommendation for use of curbs when adapting the new stiffness transition to current designs attached to concrete parapets?

We have answered a similar question to this before.  From the 3^rd Quarter 2011 consulting summary:

Problem # 6 " Curbs Under Transitions

State Question:

Dear MwRSF,

We are using the thrie beam transition to rigid barrier developed in TRP-03-210-10. This transition was crash tested without a curb under it. Some other thrie beam transitions that MwRSF has crash tested used a sloped 4-inch curb. Is it possible to use a 4" sloped curb similar to the previous crash tests with the transition in

TRP-03-210-10?

MwRSF Response:

I am assuming that you are referring to the curb detailed in TRP-03-69-98. If so, I do not believe that the addition of the 4" sloped curb would cause any adverse effects. However, be sure to use the same geometry for the curb, i.e., the height, slope, and length of the curb should not exceed that dimensions illustrated in the noted report. Note, this will keep your curb downstream of the asymmetrical transition piece and within the thrie beam rail sections for the newer transition. Further, the lateral placement of the curb must be as detailed in the original report (with the back of the curb adjacent to the face of the post).

Although not specifically detailed in the consulting summary, what we had concluded was that any curbs that accompany a given thrie beam transition design should remain part of the "new" system.  However, the curb should be terminated ( via sloped or flared end) without extending into the w-to-thrie transition element.  Our concerns were that the addition of a curb could (1) lead to further snagging of small cars between the curb and the downward slope of the bottom of the w-to-thrie transition element, and (2) cause vehicle instabilities due compressing the suspension and creating vehicle climb.  If the curb used in your existing system already terminates prior to the downstream end of the w-to-thrie-transition piece, then use it as previously designed.  If not, the upstream end of the curb should be altered to meet this specification.