Hi,

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time.

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up.

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties. I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived. Do the equations account for all this in some way, or is that what the Kg factor is doing?

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know).

Thanks,

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Daniel Foltz EIT, A.M.ASCE

Engineer

Williamsburg VA

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