pre-cambering steel beams

huangyhg

pre-cambering steel beams
hi guys.  has anyone got any advice on pre-cambering steel beams?
i was going to pre-camber an amount equal to the total dead load for a beam supporting precast concrete slabs and blockwork walls.  then all my beam should deflect is the live load deflection, which i've calculated as being within specified limits.
am i on the correct train of logic?
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not a bad idea for longer   
concur with jedclampett. for more background, here is link to a classic article in the septermber 2004 issue of modern steel construction
in the mid 70s fears structural lab did some testing of composit sections constructed by inverting the slab and beam.  the inverted beam was supported at the end points and the slab was poured into a frame attached to the beams.
when the slab reached 7-day strength, the beam was inverted.  this preloaded the beam such that when the beam was inverted, you had cancellation of the dead loads (almost).  saw the installation of several short span structures using this method.
best, tincan
quote:
then all my beam should deflect is the live load deflection, which i've calculated as being within specified limits.
no.
your camber+ll deflection should be less than allowable deflection.
rule#1- never pre-camber steel beam until absolutely necessary as driven by service requirement.
rule#2 never ever pre-camber steel beam for any structural reason.
ciao.
flamby, are you saying that you subtract your camber from your ll deflection to compare to the allowable deflection?  ...because it doesn't make sense to add the absolute camber deflection quantity with the absolute live load deflection quantity-- they are deflections in opposite directions.  i disagree.  or, were you talking about total deflection (service dl + service ll)?
something else to think about in cambering is the economics involved.  as summarized from a recent modern steel artical it costs about $50-65 dollars per beam to camber a reasonable (0.75"-2.5") amount.  with the current cost of steel running about $0.30 per lb this equates to a weight "cost" of 167lb to 217lb to camber a beam.  considering a 30 foot span this would equate to around 5.5-7.5lbs per foot.  so to camber a w18x35 verus using an uncambered w21x44 is that would mean the cost differential would not be large (not considering other considerations such as studs etc) to go with the deaper non-cambered beam provided head room etc. was available.  
willis, you're picking the shortest beam possible to work your numbers.  it would be unusual to camber a 30 ft. long w18 beam.  if you're talking about a 50 ft. long beam, you're only increasing the cost by an equivalent 4 lbs. per foot.  plus, even though you would get a reduction in deflection, using the w21 vs. the w18, you would still have a deflection that might require a camber.
agreed just wanted to make sure people thought about $$$.
flamby,
i'm not sure i agree with you. live load deflection is surely the criteria as this is (potentially) cyclic. deflection due to slab weight is a one way only load - once it's there it can't be removed.
i'd be careful with significant wall loads in setting the pre-camber value. it might be better to treat these as live load...
pba,
(dl+ll)deflection should be less than allowable deflection. providing camber will not change the total deflection. i don't think, i can take advantage of camber in how i show the deflection is within limits. i don't think the deflection limits of beams, as specified, care whether the load is deal or live or self.
ciao.