Beam cambering
Beam cambering
(OP)
I have a WF beam (W40X324)cambering issues for help. We have a 60-ft span beam and will deflect about 1.5" under dead load. It will be expensive to camber this beam to 1.5". The beam will be under L/240" deflection under total load. Do I need to camber the beam? Any suggestions? Thanks.






RE: Beam cambering
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Another option, check the beam and other members for the "ponding" concrete and have the contractor pour the floor flat (not sure how they would do this).
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Excellent thread reference, it explains most of the questions.
RE: Beam cambering
Another approach is to build cambered plate box girders. Overhead crane box girders are commonly built with camber. This might invoke other problems - but may be worth considering.
Lastly, do not count on mill camber to be consistant! Beams come in with a huge variety of sweeps & cambers. We have not used W40's, but we see a lot of W36's that are dead straight - and have seen a lot that have 2" in 60 ft as well
RE: Beam cambering
But make sure you will never get smooth cambered beam. They will camber in the middle third part of the beam and u might end up with an odd shape at the centre. If it is for roof beam, then it might be fine.
RE: Beam cambering
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You don't say whether it is a roof beam or a floor beam and if floor beam, what is the occupancy of the floor?
You don't say what the beam supports. Is it a slab or open web steel joists spaced at six foot centers?
If the beam supports joists, you could provide variable depth shims under each joist to achieve the same effect as camber.
In any case, 1.5" dead load deflection in a sixty foot span is pretty trivial. If you can achieve fifty percent of that by natural camber, I suspect nobody is going to notice.
RE: Beam cambering
1. The rotational restraint of the beams is never accounted for. For a W40x234, I am sure you have plenty of bolts at the ends, which offer a considerable restraint, which is never accounted for when you calculate your deflections in analysis. A true pin was probably assumed.
2. On the contrary, cambering maybe important. If the beam only had mill camber, then there will be some deflection on the beam with only wet concrete. Sometimes this deflection might increase the amount of concrete needed to finish the pour (more concrete that was not accounted for). Generally, a 5 psf addition is used for extra concrete.
3. Camber is not a science and it is never precise. Typically during cambering they let the beam yield a little and then let it relax. Plus, camber is typically required to be measured in the fabricator's shop and not in site. There is variability that needs to be accounted for.
Out of curiosity, is the beam W40x234 from strength requirements or camber requirements? If it is from camber requirements, why don't you use a smaller beam and specify camber?
It am surprised that a W40x34 is deflecting 1.5" only under the weight of concrete. What is the spacing on your beams?
RE: Beam cambering
Thus Dead Load = Live Load = 330#/'. I don't think there will be much concrete on that. It must be a roof beam spaced at about eight or ten feet on center.
Wouldn't it be more economical to use open web steel joists for this roof? Then you can get camber at no extra cost.
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Please clarify the roof dead load, live load and beam spacing.
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Every time I see camber guidelines, there is a dire warning to not camber a spandrel beam. I don't buy this for a second. Here's my reasoning and I'm interested to know why I'm wrong.
Option 1: Say I camber a spandrel beam to 85% of the deflection due to the deck, concrete, and beam weight. WHEN THEY START PUTTING ON THE CLADDING, the slab will be more or less flat at the spandrel.
Option 2: Say I don't camber the spandrel beam and the spandrel has deflected 1.5"-2" and THEN they start to install the cladding.
Can somebody explain to me why Option 2 (what all the "experts" say to do) is better?
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This may be a good application for a shored composite beam. You could reduce the weight of steel substantially and achieve the desired camber by adjusting the height of the shores under the beam.
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I agree with you, but if beams are resting on spandrel, is it easy to camber it.
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Spandrel beams should not be cambered because spandrel beams should be supported so that they do not deflect significantly. It is not a particularly dire warning. You may do whatever you like, but a spandrel beam which deflects 1.5" to 2" is too flexible, in my mind. It should be supported by an additional column half way between the existing ones so that the deflection is kept to a minimum.
Large deflections are simply not acceptable on an exterior wall because there can be windows and doors subject to damage from excessive deflections.
An additional column provides the opportunity of using a two span continuous beam in the exterior wall, decreasing deflection even more and making camber totally unnecessary.
In the final analysis, it will be dependent on the judgment of the engineer doing the design.
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Deflections that affect cladding are usually computed using the transformed moment of inertia, which can be 2-4x larger than the bare steel Ix depending on the situation. The uniform load from cladding is often very small compared to the load from wet concrete + deck + steel beam, also.
It is common to select a spandrel beam, using camber to take care of the bare steel/wet concrete deflection and then have the deflection due to superimposed loads be tiny. What's the problem if this is the case?
Also, I could probably count on one hand the cases in which I've been able to talk an architect into giving another column along a spandrel. If only I could get so lucky!
I still don't see it and am thinking the "experts" just throw that out there with no real justification.