LRFD A13.4.3.1 - Overhang Design 1

[rebarstud]

thread607-239942

I have a question about the referenced thread, in the 5th edition of LRFD bridge design, equation A13.4.3.1-1 distributes the plastic moment resistance of a single post over the width of the base plate plus the distance from the outer edge of base plate to the inner most row of bolts. This produces a very high moment. Does anyone have any experience with the current code and the size of the additional overhang rebar required based on this design moment? The additonal required rebar is coming out to be #7 @ 8" with a standard isotropic reinforcement of #4 @ 8" top and bottom, each way. Following the NYSDOT bridge manual and bridge detail sheets. If I could distribute the force over the b distance as shown in Figure A13.4.3.1-1, the design moment should be smaller, and thus reducing the addtional rebar. Any thoughts would be much appreciated.

 

[bridgebuster]

BD-SS1E calls for #6 @ 8" alternating with #4 @ 8". NYSDOT doesn't like #7 bars in the top mat. Recheck what you've done and if it's still coming up #7 talk to someone in your Regional office or MO Structures.

 

[rebarstud]

My supervising engineer has checked the calculations and does not see a fault in the methodology. He does however agree that this moment is high but can assume this is due to the newer LRFD code requirement. The old AASHTO Standard specs used a much larger distance to distribute this moment over. Do you have a specific example or source stating that #7 additional bars are not preferred by NYSDOT? I am familiar with that call out on BD-SS1E but I believe that is a starting point with isotropic reinforcement, consisting of the primary reinforcement, #4 @ 8" plus the additional #6 @ 8" which provides the minimum 0.96 sq.in./ft reinforcement, as stated on BD-SS6E in the notes to designer. In my opinion, it is a minimum starting point for an overhang with or without a barrier (steel or concrete).

 

[bridgebuster]

There's nothing in writing about the #7 bars; it's just something you learn from working with them.