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The need for common sense, a.k.a. composite beam studs

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Mark,
not to quibble, but you're basing your argument on the
existence of perfectly uniform loading on the beam. 
In the improbable event of not-quite-uniform real
loads on the real beam, the stud at midpoint of the
beam works just fine.

Also, if you've got a beam (uniformly) loaded at third
points, there's no reason to put any studs between the
loads except, in theory, to prevent slab buckling,
since there is no shear in the middle third.

AISC's rules on composite design represent a gross
simplification of what might actually be happening at
the steel-concrete interface, justified by invoking
ductility to iron (no pun intended) out the
differences.  Design a composite beam by the AASHTO
specs and it gets more complicated, and at least in
theory, more accurate.  They require tighter spacing
at the ends, as intuition suggests we should have. 
The reason is that when you rely too heavily on
ductility in a member subject to fatigue loading, it
breaks.

To delve even deeper (I'm almost done--bear with me),
none of the codes deal with the fact that you first
have to compress all the shrinkage cracks closed
before you even have a composite beam.  This shouldn't
affect strength, but it might make us question any
deflection calculations.  This may not change your
life much in a simple-span beam, but it devalues the
assumptions we make in analyzing a continuous one, and
perhaps those we use in vibration analysis.  AASHTO
used to (and maybe still does) require the use of a
smaller concrete modulus of elasticity for long-term
loads (dead loads applied after composite action
began, e.g. rail and sidewalk selfweight) to account
for creep.

My point is, there are a lot of questions about
composite beams that rank right up there with an odd
number of studs.  And yes, I think RAM's response is a
little high-handed.  It's not how I would have written
the program.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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