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AASHTO concrete-filled steel pipe beam-column flexural strength

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It sounds like you’re on the right track w.r.t. the nasty iterative procedure.  I would second the advice given to reference the CRSI manual for a very clear example of how to do this (mine is 1992).  I thought that the “n versus 3n” business got canned.  Does the AASHTO LRFD code still have it?  If so, you want to use short-term loads (e.g. live and wind loads) with the “n” value, and long-term loads (dead load which acts on the composite section, and long-term live load, e.g. load which lasts at least several months).  This is intended to account for creep in the concrete under long-term load; it has the effect of decreasing the effect of the concrete on the composite section.  I don’t know how realistic this is; I don’t think any other code in the world considers it (which is moot if the code you’re bound to does).  I believe the loads should be factored loads if you’re doing a load factor design or analysis.


I was tracking through your problem pretty well until I came to the part about the 72 inch diameter.  I haven’t researched either AISC or AASHTO on the subject of size considerations, and if they address it, great.  But the area of concrete goes up as the diameter squared, and thus so does the required shear transfer into the shell to qualify as composite.  If the shell is just a concrete-filled pipe with no explicit means of shear transfer beyond bond, the perimeter only goes up as the diameter.  At some diameter the shear flow increases beyond the ability of the perimeter bond to carry it.  I don’t know what that magic diameter is, but gee that’s a big pipe you have, grandma.  At least where I’m from.


Like you needed something else to wonder about.


Mike Hemstad, P.E.


St. Paul, Minnesota