Direction of placing flexural outer reinforcement in two-way slabs

[AhmedJ.J.]

Hello dear engineers,

Can someone comment on what is written in ACI MNL 17(21) and also in the ACI SP-17(14) - Clause 6.9.1 regarding the direction of placing flexural outer reinforcement in two-way slabs. The clause says: "If the panel geometry is rectangular rather than square, the outer layer is usually placed parallel to the longer span". Wouldn't it be more efficient if the outer layer is placed parallel to the SHORT span and not the longer one?

Thanks,
Ahmed

 

[ggcdn]

In a 2 way slab, where I'm starting with a grid of minimum bottom steel, I'm going to put the lowest layer in the direction with larger spans. It is the direction that is working harder, and doing so will require fewer extra bottom bars to be called up.



-JA
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[rowingengineer]

Interesting we have no direction in our codes but i have generally done shortest span first layer.

 

[le99]

I guess it is due to the concern of shrinkage.

 

[KootK]

Wouldn't it be more efficient if the outer layer is placed parallel to the SHORT span and not the longer one?

I'm going to take a guess at what I think might be throwing you off.

If we were talking about a single, rectangular panel that was supported on walls on all sides, you probably would want your rebar running the short direction to have the greatest lever arm. That's because, in this situation, the shorter span will tend to absorb more of the load and the longer span will absorb less. For aspect ratios in excess of 2.0, we don't even bother with the long direction steel being structurally active.

With a two way slab -- at least one designed to conventional assumptions -- it's a different animal. The slab in each direction is designed for the full load, regardless of the panel aspect ratio. Because of this, it becomes advantageous to assign the greater rebar lever arm to the longer span because, loads being the equal, the longer span will develop greater moment.

 

[rapt]

I think it is, as Kootk says, a matter of terminology

2way slab - normally a slab continuously supported on 4 sides - the short direction would normally be the one I would have the reinforcement with the lesser cover.

Flat slab - supported on discreet columns - the long direction would normally be the one I would have the reinforcement with the lesser cover.

 

[AhmedJ.J.]

Thanks everyone. Appreciated very much.