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# Concrete slabs on grade-theory of length to width ratio limit?3

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## Concrete slabs on grade-theory of length to width ratio limit?

(OP)
Hi to all,

I'm wondering what's the theory behind limiting the length to width ratio between joints in a slab on grade?
It's widely known that the ratio of panel length to width shouldn't exceed 1.5. Panels with excessive length to width ratios are likely to crack at the mid-panel point or at some other location between joints. But from where this number came from?

Thank you for all comments in advance

Take care

### RE: Concrete slabs on grade-theory of length to width ratio limit?

As with most things about slabs on grade, it is mostly experience rather than theory.

### RE: Concrete slabs on grade-theory of length to width ratio limit?

There have been some attempts to quantify it and use it to define reinforcement requirements. They take into account typical drying shrinkage stresses and strains in the panel, coefficient of friction between the concrete and vapor barrier or soil, etc. Even with all that, there are plenty of unpredictable variables and it still falls back on the empirical experience that hokie is talking about.

### RE: Concrete slabs on grade-theory of length to width ratio limit?

I normally use about 1:1.5 as a maximum.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

### RE: Concrete slabs on grade-theory of length to width ratio limit?

3

#### Quote (op)

I'm wondering what's the theory behind limiting the length to width ratio between joints in a slab on grade?

That theory is basically just the theory of elasticity in the context of shrinkage induced tensile strain. When panel aspect ratios drift too far from unity, the ability of the the jointing to absorb strain around the perimeter loses it's two way character and effectively become a less efficient one way system.

I like to imagine three equal area trampolines where the mat is a bit old and crusty and I'm trying to prevent a tear. You have to imagine stiff diaphragm shear restraint around the perimeter to make the analogy work. No doubt the 1.50 value that we use is very approximate and experience based rather than calculated.

### RE: Concrete slabs on grade-theory of length to width ratio limit?

It’s an empirical rule of thumb.

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