Pre-cambering Steel Beams
Pre-cambering Steel Beams
(OP)
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?
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?






RE: Pre-cambering Steel Beams
If your camber is less than the minimum they can provide it's not worth the bother.
RE: Pre-cambering Steel Beams
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RE: Pre-cambering Steel 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
RE: Pre-cambering Steel Beams
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.
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
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...
RE: Pre-cambering Steel Beams
(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.
RE: Pre-cambering Steel Beams
You have a different approach to us - we check total deflection or imposed load deflection only, depending on the circumstance. Clearly total load deflecton IS important in lots of cases but imposed load deflection is usually more important as this is the load that could be removed and replaced leading to actual deflections occuring in the building. Dead load deflection is just a curve in the frame. Once it's happened it's permanent. It does not crack walls or cause doors to jam.
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
RE: Pre-cambering Steel Beams
Precamber will not reduce the deflection experienced by anything supported by the beam. So vibration checks done based on LL deflection and deflections experienced by brittle finishes such as block wills will not be reduced. The deflection is simply measured from a different starting point.
RE: Pre-cambering Steel Beams
Be careful with those AISC tables that give camber info. They're MILL CAMBER, not camber put in by the fabricator. The upcoming Manual will have an improved discussion of this. Look in the 3rd Ed. LRFD Manual Code of Standard Practice. More useful information is given there.
I did an experiment a few years ago to test camber accuracy, although admittedly in a statistically-insignificant way. I had 6 W16 and W21, unshored composite beams that were cambered 0.75" using 85% of the DL deflection. They were randomly scattered over a large hospital floor. I talked the inspector into measuring the camber at the following points:
1. before cambering (the natural, or mill camber)
2. after camber
3. just before transport
4. after deliver to the jobsite (a several hundred mile trip)
5. in the air
6. with wet concrete on there
I was amazed at the accuracy and precision for the 6 beams in my little experiment. None were off by more than a 1/16", and the small errors were all under-cambered, which is good IMO. None of the camber came out during transport. They were just about perfectly flat (measured, not eye-balled, although I can't remember the numbers) with the wet concrete on there.
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