huangyhg

deflection of beam supporting a concrete slab.....
i'm looking for a "rule-of-thumb" for steel beam deflections. i have looked in lrfd and found the l/360 for live load (ll) deflections but i can't seem to find anything for total load (tl) deflections. i've also searched the archives and nothing came up that applied to what i'm looking for.
i have a 20 ft span and i'm getting a (service) tl deflection of about 0.8 inches.
i'm trying to determine if this amount of deflection will crack the concrete floor that is supported by the steel beam?? if it will crack, to what extent will it crack (severly or just hairline cracks)??
i have no feel for acceptable deflections so i, again, must turn to you guys for advice. any comments appreciated.
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andrew
wouldn't the dead load deflection be taken out from the pre-cambering of the beam?
no, there is a dead load being added to the beam that increases the deflection of the beam. furthermore, i can't find anything in the construction plans that states the beams are cambered. they could be cambered but i'm figuring it as if they are not.
thanks for the reply.
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andrew
as per the steel spanish code, at 6 m length, the total deflection (service level under live, dead and superimposed dead loads) must not exceed l/400 if not supporting masonry partitions and l/500 if supporting masonry partitions or bearing walls. roofs have less stringent limits.
you at l/300 are not meeting these limits, so it would be out of the spanish code compliance and some problems it warns off could develop.
dmwwengr,
the purpose to limit the deflection of steel beams is to avoid damage to other items supported on or from the beam, as concrete slabs, plastered ceilings, etc.
i would limit the deflection of the beam under the live load plus any superimposed dead load added after the concrete slab is poured to the 1/360 limit given by aisc.
aef
for a given slab section, you can find the moment that will cause the rupture stress (fr)of the concrete, then calculate the deflection of the slab at rupture. limit your steel beam deflection by this value to avoid cracks. see aci 318 for fr.
accrding to ibc 2000 the live load deflection should not be more than l/360 and total load deflection l/240. 0.8 in deflection you have shown looks like ok and may not give much probles(this is said, assuming your floor is of concrete metal deck). but in practice we normally make sure the total deflection not to exceed l/360.
i would say span/250. the span/depth ratio in bs8110 for slabs are based on limiting the slab deflection to below l/250, whereby deflection calculations are not necessary. otherwise additional reinforcement is provided to reduce the deflection.
the deflection of the steel beam can be therefore be limited to span/250, unless brittle finishes appear above the beam (on floor or wall) where span/360 would be adviseable.
the pre-cambering of the beam is generally only done for "large" spans, due to the associated costs.
l/360 is the one.
also, if your tl deflection is .8, i would assume your dl deflection is about .4? its tough to camber a 20' beam so little. it probably has/had a natural camber of that when it rolled off the mill.
to: haynewp,
using the procedure you suggested (in aci 9.5.2.3) i'm getting a ridiculouly low moment and allowable deflection. this seems like a great idea but it's not working in this case (or i've made, yet another, calculation error ). i'm getting such a low moment that the slab would be cracked in its currently loaded state...which it is not. have you used this procedure sucessfully before with steel construction??
to: pylko,
there is not an even split in dl to ll deflection. the dl deflection makes up most of the deflection. the deflection due to the ll is around 0.20". fyi.
thanks to everyone for the great responses.
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andrew
the deflection of the steel beam only needs to be limited to l/360 if there are brittle finishes which may crack. (ielaster etc) i would normally limit the delfection of a steel beam carrying a concrete floor to l/360 for imposed loads only and l/250 for dead + imposed.
a quick way to size the beam would be to let the moment of inertia =18ml where m=moment and l=span if you are working with ultimate moments just divide by 1.5

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