huangyhg

precomposite dead load deflection limits
there are no hard and fast rules for the deflection of a composite steel beam under the weight of wet concrete (talking about unshored construction). just wondering what engineers have come across and apply in their designs.
do you limit the deflection at all, even if the beam is to be cambered? in other words, if you camber the beam for 80% of the dead load, do any problems develop if all of that camber (say 1-1/2" to 2" plus of deflection) all comes out when the wet concrete is placed. (we can tell the contractor to pour the slab to a set thickness, so having either ponding or a thin slab wouldn't be an issue for my question).
thanks,
one company i worked for had a policy of limiting camber to l/300 to make sure that the beam was not too bouncy. that was before the aisc vibration design guide came out, so maybe this rule-of-thumb can be exceeded if serviceability is explicitly checked.
another issue is that you may want to provide short slotted holes in deep beams to accomodate the rotation that occurs as the camber comes out under load.
i usually camber the beam to the dead load deflection. of course there are a lot of judgement calls to be made and many conditions where i dont camber a beam but for a typical floor filler beam i camber the beam to try to get it to be flat when construction is done and before it is loaded.
i typically camber out 3/4 of precomposite dl if this value is greater than 3/4". given the slight rotational stiffness of simple connections, i get a little concerned that if i camber for the full dl, my studs may start showing at midspan if the camber doesn't come out as i expect
i camber for dl deflection only. as jjeng said, one should use his judgment call based on full spectrum of project circumstance.
regards,
lutfi
are you using propped or un-propped permanent shuttering?
if propped, it may be easier to prop the beams also to eliminate deflection of the beams during your pour.
just a thought.
vb

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