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# AASHTO concrete-filled steel pipe beam-column flexural strength

• To: seaint(--nospam--at)seaint.org
• Subject: AASHTO concrete-filled steel pipe beam-column flexural strength
• From: "Dave Evans" <DEvans(--nospam--at)tnh-inc.com>
• Date: Fri, 25 Oct 2002 11:20:17 -0900
• Priority: normal

```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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