AASHTO Bridge Design_Steel Beams

[Wanna_be_SE]

Good evening,

I'm new to bridge design and have begun my first calcs for "somewhat" doing a load rating for some bridge repairs. My short career thus far has been just over 3 yrs in steel design, but primarily doing stress analysis for various structures (in a shipyard), and so I wasn't using industry standards until this past May when I decided to change career paths (within engineering) and go a civil-structural route. So now I'm doing deep dives into various code requirements trying to understand why some requirements are the way they are. AISC requirements are very similar to what I was doing at previous employer, but seems AASHTO has put their spice on many different requirements (at least the ones I've come across so far). Many similarities between AISC and AASHTO, but nonetheless definitely different.

So, 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 (section 6). 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 (at least in the section 6 I was navigating) 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 (even though it's not exactly how the parallel axis theorem works, right?)?

Any insight would be greatly appreciated.

 

[ajw771983]

Hi VA_Structural:

When you say "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." these are my thoughts:

While I agree that the distance to the outer fiber goes up, the positive moment capacity does not go down because the composite moment of inertia increases quite a bit.

Kg is a parameter used to calculate live load distribution factors and it's a longitudinal stiffness parameter - for calculations related to strength of a girder composite section there are other considerations. For instance, non-composite behavior is assumed during deck pouring and then composite behavior is assumed for live loads.

 

[Wanna_be_SE]

ajw771983 (Structural),

I shot myself in the foot on that one, totally agree - I guess I was only considering the section of steel, not composite section.

The calcs I'm doing are for steel repairs and I realize there's certain reasons for only considering steel, not composite section, and these calcs are not designing the beam, but focused on repairs so I guess they are different and i was confused with why things were being used for calcs.

Thanks for input!

- Dan, EIT (who cares)