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Now let's appear at why we need to recognize how flexible the sub-grade is. To start with it's vital to understand that a slab on ground is designed as "plain" concrete.
That way we do not count on the reinforcing steel to carry any of the loads. But hang around, you say, there's steel in the slab—mesh and re-bars. Yes, but that steel is merely there for crack control—to hold any cracks tightly together.
It usually does not extend through the joints—at joints we only want to transfer shear forces, not bending moments and certainly not lateral restraint. That's what the joint is there for in the first position, to allow lateral shrinkage in the slab.
BASE SLAB REINFORCEMENT
We
go to all this effort to get the appropriate soil support system and what we
end up with is a lone input value for the slab design. The most normally used
value is the modulus of sub grade reaction, k. This value is not directly linked
to bearing capacity and k does not tell the designer if there is compressible
or expansive soil. What it does is specify how firm the sub-base / sub-grade is
over small deflections (about 0.05 inches).
Now let's appear at why we need to recognize how flexible the sub-grade is. To start with it's vital to understand that a slab on ground is designed as "plain" concrete.
That way we do not count on the reinforcing steel to carry any of the loads. But hang around, you say, there's steel in the slab—mesh and re-bars. Yes, but that steel is merely there for crack control—to hold any cracks tightly together.
It usually does not extend through the joints—at joints we only want to transfer shear forces, not bending moments and certainly not lateral restraint. That's what the joint is there for in the first position, to allow lateral shrinkage in the slab.
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