Two way slab 1

[jeffatGE]

I've got a two-way slab system w/ bearing walls. I am approaching it with the direct design method (wall strip, middle strip, moment coefficients, etc.). My question is, how should I treat the values of the negative moment right at the walls?

 

[craigory28]

Jeff:

There is a few things you need to look at. First off is what kind of bearing wall are you using. If using masonry I would not count on any negative moment at the wall. It would be very difficult to develop the bar into the block. If the wall is concrete you will still have to develop the bar into the wall and design the wall for the moment the slab will apply on it. If you have a simple support no moment will develop however if it is very stiff you could say that the exterior edge is fully restrained. (See ACI commentary 13.6.3.3.)

Personally I tend to be conservative and design for no restraint for my positive moment and fully restrained for negative. I just hoping to avoid cracks.

 

[jeffatGE]

Good advice, would this mean re-distributing the moment (co-efficient derived) that was intended to go into the walls--into the slab?
ex.- A.C.I prescribes that 0.7 of Mo goes to the interior negative moment. A PCA design guide then breaks down that 0.7 into percentages that go into either the wall, the edge strip, or middle strip.
If I suddenly assume none of the moment is going into the wall, wouldn't the slab be required to resist much more negative moment?
Thanks.

 

[craigory28]

Jeff:

You have never told me if it is a concrete or CMU bearing wall you will be using. If it is CMU I would design the interior using 0.75 Mo. If it is concrete use 0.70 and a minimum of 0.30 for negative at the exterior. (This matches 4 in the tables. We assume the wall is similar to a stiff beam.) Please review 13.6.4 in ACI especially 13.6.4.3. I think this is what you are looking for.

Take care at the corners where two walls intersect at 90 degrees. This area tends to have lot of shrinkage cracks.