Camber for two way slab

[whobelike]

Does anyone have any guidelines for the determination of camber for a two way slab. I have seen it done but I have not personally done it. I cannot find it in a code or any other reference that I had. I would have thought that the long-term calculated deflection value would be suitable at mid-slab.

 

[rapt]

It depends on what you want the precamber to achieve.

Is there brickwork or other brittle finishes that will be damaged by deflection of the slab. In this case precamber will do nothing to reduce the deflection experienced by the brittle finishes.

If it is simply to reduce the visible long term deflection, normally you would not prrecamber by more than the short term dead load deflection.

 

[whobelike]

I guess, what you ask, is actually part of my question. Why did "they" precamber the slab? Most likely just to have a flat condition at the start of live load application. This would be a similar case to using cambered owsj. As you noted, the cambering will have nil effect of finishes etc. and I think that this was not the reason "they" cambered the slab.

So you think that the immediate dead load deflection would be enough? Is this stated in a code that you know of?..Or some other reference text?

thanks
Dave

 

[UcfSE]

If you camber the slab then your depth when the slab is cast will be different in the middle than at the ends, at least until you have enough concrete in place to make up for this. It seems like that would give them trouble in the field casting and finishing the slab and that you may lose some of your effective depth. I don't have experience with this, but these problems come to mind.

 

[rapt]

Dave

Camber is not limited by any codes I know of. It's use is a "design/engineering judgement" decision, not something that needs to be limited by a code. Theoretically you can use any amount of camber you want as long as you maintain the deflected shape within the code limits at all times.

Having a flat condition at the time of application of live load is not a code judgement and is probably not achieveable unless you know the time of application of the live load and calculate deflections at that time due to the dead loads and long term effects. There is software that can do this for you But why would you want this?

The main use of precamber is to limit the long term visible deflection and ponding deflection on exposed slabs.

re the comment on thinner slabs, when precambering, the contractor should ensure that the slab thickness is maintained. Both the top and bottom surface of the member are supposed to be precambered. Simply precambering the bottom and maintaining a flat soffit is not logical.