huangyhg

incease capacity of composite beams
i need to increase the load carrying capacity of some of the composite beams in a typical steel framed structure. all the steel is in place and the floor is poured. i am thinking of adding a steel plate to the bottom flange of all the beams that are under consideration. can anyone point me to specific references that might be useful for this job. thanks in advance.
-strucguy

if the beams were designed to be partially composite you could place additional shear studs in non-shrink grout to increase the capacity of the system. as you suggested, plates can be added as well (just be careful with the minimum 25% composite requirements). the system can be analyzed as you would for any other composite beam. just be mindful of how much load is on the beam in its current state.
as far as references go, any textbook on steel design should show you the process of composite beam design. i typically reference the salmon and johnson textbook.

bottom plates are the way to go. i have done this many times. some of the old steel manuals have various beams with bottom reinforcing plates to get you in the right ballpark. if i recall correctly, enercalc had a program where you could add bottom plates.
i agree with steelpe with the following exception - the loading sequence is inconsequential. aisc says as much in the spec where it talks about the precomposite loading not affecting the composite strength.
structuraleit,
what if the floor is already fully loaded?
thank you all for your prompt responses.
structuraleit -
do you mean to say there would be no difference in plate being added to the beam with and without relieving the stresses caused due to dead load? could you be a little bit more specific about where in aisc this has been addressed.
thank you.
-strucguy
steelpe-
if the full dl is applied before composite action, then it doesn't matter. the spec makes no distinction between shored or unshored construction for composite, positive flexural moment strength. see page 3-6 in aisc 360-05 under "shored and unshored construction". it says, "the available flexural strength is identical for both shored and unshored construction......". the only difference is that for unshored construction you must ensure the bare steel can take the construction loads.
strucguy-
that's right, it doesn't make a difference if the dl stresses are relieved or not. i'm making this statement strictly from a strength standpoint. there may be serviceability issues to consider.
another resource is aisc design guide 5 - they have a discussion on strengthening in-place composite beams including an example in the appendix.
structuraleit is referring to the ultimate strength of the composite section after significant stress redistribution has occurred. the sequence would matter to serviceability issues such as deflection and service load stresses. if you attached the plate without jacking the beam up then you are building in the current stresses and curvature at the current loading. jacking the beam up before attaching the plate would allow you to reduce the stresses and deflection somewhat if it matters. if you're only looking for additional strength then there's no need to jack.

struturaleit
all i was saying is to be mindful of the existing loading conditions. if the beams have some post-composite loads on them then that needs to be considered in the design.

shored and unshored has an impact on deflection also.
dik

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