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AASHTO concrete-filled steel pipe beam-column flexural strength

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Hello all,

I'm re-sending this with AASHTO in the subject header, to help 
clarify the question and (I hope) grab the attention of you bridge 
types...

I have a question regarding the calculation of the flexural strength of 
a concrete-filled steel pipe beam-column using the 2002 LRFD 
AASHTO Bridge Design Specs.  We're looking at a 72" diameter 
pipe.  Axial loads are relatively small; flexure controls.  

For D/t ratios larger than a certain amount, the nominal flexural 
strength is equal to the composite section yield strength, Myc, 
which is a function of the short-term and long-term composite 
moduli of elasticity, among other factors.  

Are the transformed moments of inertia of the concrete portion 
based on the neutral axis at the geometric center of the pipe, 
taking the entire concrete area (pi*d^4/64), divided by "n" and "3n"? 

This doesn't really make sense since Myc is the strength at first 
yield of the pipe...so the concrete on the tension side will be 
cracked, and the neutral axis will be offset from the center.  On the 
other hand, if cracking is to be considered, it is not crystal clear to 
me what level of service(?) moment to use to calculate the short- 
and long-term elastic moduli.  Could be a nasty iterative procedure 
for the short-term moduli.  The provisions seem most applicable to 
girders with concrete decks, not filled pipe.  Does anyone have 
references with worked examples with filled pipe? Does AISC have 
similar provisions for flexural strength at large D/t?  

Thanks,
Dave Evans, P.E.
TNH, Inc.


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