Camber - need clarification 1

Cambering a beam is helpful in long span conditions especially unshored conditions prior to the onset of composite action. Typically beams are recommended to be cambered no more than L/360.

There are 2 conditions: composite construction and non-composite construction.

Non-Composite:
If your deflection due to total load is say 3", then cambering the beam 2" will cause a final deflection of 1" (L/480). Cambering does not override the deflection checks, it only helps in keeping it within the code requirements.

Composite:
Your camber is controlled mostly by the pre-composite deflection (self weight of beam + construction load + wet weight of concrete). The idea is to camber the beam enough such that it does not deflect (sag) due to the wet weight of concrete. It is important because if the beam sags under the wet weight of concrete, then you will need to add more concrete to keep the floor level, and it is a vicious cycle. Typically, beams are cambered to around L/360 before bumping up the member size.

Cambering a 40' beam to equate the dead load deflection does not void the L/240 requirement. It only means that you don't have to include the dead load deflection (which will now be ZERO) in computing total deflection. The total deflection will now be only due to the live and superimposed loads.

I would also be careful to check for MINIMUM and maximum cambers. It varies based on beam depth and length. There is a good table in the green book, but I don't know of the top of my head where it's located. I'll check it out on Tuesday and let you know, unless someone posts it before then.

The page SEIT is referring is attached.

Part of the question was wrt cladding deflection calcs. For typical composite construction, the deck and slab are installed before the cladding, so the camber is gone before the cladding is installed. The slab should be pretty flat when they start to put the cladding on. Whatever deflection happens after that (cladding + superimposed DL + live load) is what you compare to the limit, so the amount of camber plays no role.

All this depends on the order of installation, so use your judgment.

That being typed, the usual recommendation is to not camber spandrel beams supporting cladding. I do not understand this recommendation and have violated it many times without a problem. Still, some people who've been around a long time recommend against it.

The masonary limit of L/600 is not affected by precamber. This is a limit on the amount of deflection the masonary on the beam will experience. This is not changed by precambering, it simply starts from a different deflected position (unless you actually were arle to cause the precamber after the m,asonary is built which is not practical but theoretically could be done by prestressing the steel member after the masoinary is in position).


A total deflection limit is a visual deflection limit and it is affected by precamber.

If there is also a live load deflection limit, it would not be affected.