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Concrete-filled steel pipe beam-column flexural strength

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Hello all,

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 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 moduli.  Could be a nasty iterative procedure for the 
short-term moduli.  Does anyone have references with worked 
examples?  Does AISC have similar provisions for flexural strength 
at large D/t?

Dave Evans, P.E.
TNH, Inc.

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