AASHTO LFD Composite Deck / Negative Moment Region Question 1

[PKSo]

Hi all - I am designing a bridge where the concrete deck is composite with the steel stringers and am using LFD. Per AASHTO 10.50.2.3 in AASHTO 17th Edition, it mentions that the min longitudinal rebar must be 1% of the concrete area when the longitudinal stress in the concrete slab is greater than 0.9 x mod of rupture.

Can anyone provide guidance on how to calculate this slab stress? I always thought that in the negative moment region, the slab is assumed to not contribute any tensile resistance (tension = 0).

I figured perhaps to calculate the rebar tension but all the sources I see indicate to set the rebar tension to Fy x A rebar. Thanks kindly for your help.

 

[BridgeEI]

Fb = M/S you should have all the variables. If the stress exceeds the allowable concrete stress then provide additional longitudinal steel. At negative moment regions there will be a tensile stress in the concrete, there has to be. The rebar takes the tension instead of the concrete deck.

 

[PKSo]

Thanks BridgeEI! That is quite helpful and makes a lot of sense. Can't believe I didn't think of it!

 

[BridgeEI]

No problem, that's why we're here. A lot of the time the answer is right in front of you.