Two-Way Slab definition

[Guest090822]

Does anyone know where in the ACI code the specific definition of a two-way slab is? Dr. Google says you assume two-way action when the ratio of the long span to the short span is less than 2.0, but my textbook says 1.5. I can't find the definition in ACI318-14 which seems strange.

 

[phamENG]

Those are good rules of thumb, but I don't think ACI defines it. That's because nearly every slab can be a 2 way slab. The question is how we detail it and define the load path. A large aspect ratio will make 2 way action impractical, but woe to the engineer who ignores its effects at the ends if there is a spandrel beam or some similar stiffening element - you'll end up with some unpleasant cracking if you don't deal with it properly. Similarly, a square slab can be designed and detailed as a one way slab, but you have to consider the stiffness impacts and detail it carefully to allow rotations where they need to occur to satisfy your design assumptions.

Confusing enough?

 

[phamENG]

ACI 318-14 allows 2 way analysis and design procedures as long as all edges are supported, or for flat plates. No limit on aspect ratio. In fact, if you have an aspect ratio greater than 2 the commentary directs you to minimum slab thickness requirements for one way slabs, but you're still allowed to analyze and design it as two way.

 

[dauwerda]

I don't know that ACI specifically limits the span ratio for 2-way slabs, Section 8.2.1 states the following:

8.2.1 A slab system shall be permitted to be designed
by any procedure satisfying equilibrium and geometric
compatibility, provided that design strength at every section
is at least equal to required strength, and all serviceability
requirements are satisfied. The direct design method of 8.10
or the equivalent frame method of 8.11 is permitted for
design where applicable.

So, if you use the design procedure provided in 8.10, you are limited as follows:

8.10.2.3 Panels shall be rectangular, with the ratio of
longer to shorter panel dimensions, measured center-tocenter
of supports, not to exceed 2.

But, if you use some other design procedure, you may not have this limitation.

 

[JAE]

The first section of Chapter 8 - it states in 8.1.1 it refers to "slabs reinforced for flexure in two directions" as though it acknowledges that if you reinforce for two way action - it IS a two way action slab.

Section 8.3.1.2 shows a Table and the commentary refers to "panels having a ratio of long-to-short span greater than 2".
So this implies that >2:1 is applicable.

For use of the the Direct Design Method - there is a limitation in 8.10.2.3 of a 2:1 ratio.
The commentary for this section implies that if you have a ratio >2:1 then "the slab resists the moment in the shorter span essentially as a one-way slab."

For the equivalent Frame Method I can't find anything specific.

Definitions in Chapter 2 are no help either.

 

[r13]

Rick,

Instead of define two way action vs one way action by a rigid ratio, I suggest to look at the plate behavior under different aspect ratios - construct a plate model using FEM, with uniform load and all sides supported, then plot the flexural stress diagrams for varies aspect ratios. From this simple exercise you will have a better feel on when one way action is dominant, and when it isn't. I believe, beyond 1:4, one way action previals.

 

[minorchord2000]

There are some nice tables with different aspect ratios in "Design of Concrete Structures" 12th Edition by Arthur Nilson, pages 408-411. The tables give you coefficents to calculate the moments in either direction.

 

[Tomfh]

One way slab is a slab with insignificant bending forces in one direction. Two way slab is every other slab.