2-Way Slab Systems

[todd2ny]

I am designing a two-way slab and want to use the direct design method outlined in ACI 318. The question I have has to do with the distribution of moments in the slab. I was thinking of just calculating the statical moment in each direction and basing my flexural steel design on that. This woulb be conservative but it shortens the design time. I would simply use the same steel on both the top and bottom each direction. Would this be correct?

T.

 

[JAE]

It's late and I'm very tired...but my first thought was to be sure to check that you don't overstress the slab (i.e. where the first failure mode is crushing of the concrete).

Also - this may be a bit too conservative and costly.

 

[asixth]

todd2ny,

I don't understand the approach you are taking here. If you are designing your negative support moments and postive midspan moment as the static moment then this approach would be quite conservative.

Generally for a typical internal span, I would distribute 65% of the static moment to the negative support moments and 35% to the positive midspan moment.

Of the negative moment, I would distribute 75% to the column strip and 25% to the two middle strips.

That's if I were to use the simplified approach as set out in the design code. It would totally be worth your while to investigate patterned loading that can often cause your slab to hog midspan.

In Australia, we are going towards using full live loading on alternate spans, this can often give you a moment envelope that will exceed the static moment.