huangyhg

adding studs to existing composite beam
i am adding a large stair to a steel composite building. i am having difficulty getting the existing beams to work. the girder that the beams frame into was overly designed while the beams were designed just below failure at 100 psf ll. with the additional load even adding a wt section under the beam does not keep the stresses below both 0.66fy and 0.9fy checks. has anyone added studs to an existing beam? i cannot find any information about this and was wondering if it had been done before. thanks.
rc
all that is necessary for the triumph of evil is that good men do nothing.
edmund burke
how would you add studs to an existing concrete slab/composite steel beam? isn't the slab already poured, the studs already encased with concrete?
i think i've heard about this with coring out the concrete deck, welding the studs, and grouting. but i could have dreamed it.
hg
the thought is, if you core to the top of the flange, and weld new studs on, and repour non-shrink grout, it would act as one. thus adding composite action to the existing beam.
rc
all that is necessary for the triumph of evil is that good men do nothing.
edmund burke
it depends what % composite you are at. if you are already at 100% composite, then i can't imagine that adding studs will help.
if you aren't a terrible amount of load you should be able to get a wt to work. i had some existing (non-composite) wf beams where the space was being changed from 40 psf ll to 100 psf ll and i needed a relatively large wt, but i was able to get it to work. what kind of additional loads are you looking at? can you get creative with adding a wt with a channel on the bottom flange of the wt?
the problem is i'm at 40% composite existing. when i add the new load, i drop to 25% composite. the more steel i add, the steel starts to control and requires more studs, which then causes me to drop below the minimum of 25%. this is why i'm asking if adding more studs is conceivable. i can only add 5.2 in^2 of steel before i drop below 25% and i am at 44ksi<0.9*50 ksi and at 34 < 33 ksi. (elastic check). this case is for the more lightly loaded beam.
rc
all that is necessary for the triumph of evil is that good men do nothing.
edmund burke
i went thru my collection of papers on strengthening existing steel
if you have a partial composite beam, and apparently you do, there is absolutely no problem coring and adding steel studs to increase the percent composite action. we have done exactly this on several occasions.
just core the holes large enough for the stud welder, and at least four times the diameter of the head of the stud for grouting purposes, and be certain that you use a non-shrink grout (at least equal in compressive strength to the concrete slab) to fill the hole. stud size should be as normally designed for new composite beams.
also, if you have excessive deflection under the proposed load (or even under the existing loads if the beam was mistakenly underdesigned), jack up the beam slightly before adding the additional studs to transfer the dead load to the composite section.
strengthening by adding cover plates to the undersdie of the beam is also a relatively simple procedure, which we have done in numerous existing buildings. for economy in field welding, try to use a plate wider than the flange so that the welding is performed in a comfortable position, instead of overhead. also, design the cover plate and the welding to suit the loading conditions, do not be overly conservative by specifying a continuous weld, and cut-off the ends where approriate. a full length cover plate is never required.
note that jacking of the girder before welding the cover plate will also transfer the dead loads to the composite section. (place the plate in position, then jack against the plate, clamping it against the underside of the beam.)
ok its saturday - i'm a bit slow today - but how does adding load to a composite beam change its percent composite value? that is a static value based on geometric properties is it not?
i would second epr's comments. been done before. but i would first try to add steel to the bottom of the beam as its probably much more cost effective.
jae - adding load doesn't but adding a wt does - percent composite is calculated by the force that can be transferred into the concrete by studs divided by asfy of the steel section - increasing asfy by adding cover plates or wts therefore reduces the composite percentage.
that (and that fact that the ductility required of the studs to perform efficiently at 25% composite is extremely high) is the reason why i rarely go much less than 40-50% composite. $15 worth of shear studs saves a lot of headaches later.
willisv-
percent composite being controlled by asfy is only true if the pna is at or above the top of the top flange, correct? if the pna is in the steel section then the percent composite is related to ac(f'c)?

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