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(OP)
Hello all,
How do I calculate the load bearing capacity (in kg/m2) of a slab on grade that is 25cm thick?

Math...

(OP)

Missing: soil properties below; concrete strength and elastic properties; allowable settlement

(OP)
thanks oldestguy.....lets assume soil bearing capacity of 25KN/m2 and C25 concrete. Slab has been existing for about 5 years, let's neglect settlement.

Also missing - the loading footprint and reinforcement. Without that, the number of possible answers is infinite.

(OP)
Hotrod. It's more of a concrete road pavement with 6mm wire mesh. Just wanna convert it to a storage area and wanna know the load it can withstand. Trucks have been moving on it for years.

Are you expecting to store anything that will impose a load greater than 9000kg on an area 30cm by 50cm? If not, then I would say you have nothing to worry about.

(OP)
Hi hotrod, maybe. Can't say for sure. I just wanna know how to calculate permissible load for the scenario described, worst case. It doesn't have to be precise.

As I said, there are a million different answers depending on the footprint and magnitude of the load and the stiffness of the soil the slab is bearing on. There is no simple equation to determine what the slab can support. The other aspect that adds another layer of complexity is the question of what constitutes failure - cracking? Punching through? The capacity changes by a factor 5 to 10 depending on the answer.

All I can tell you with any degree of confidence, is that if this is an in-place concrete pavement slab that has successfully supported heavy truck traffic, it will support the load of a heavy truck. It's hard to imagine loading as a storage area that would be greater than that.

sholah - agree with HotRod10.

If you have a perfectly uniform loading (kg/m^2 everywhere) the slab can take essentially its material strength which is huge.
What happens is that the soil would settle under load.
If the load is uniform everywhere, and the soil is uniform everywhere, then the soil will settle uniformly and not bend/stress the slab at all.

There are numerous books and documents "out there" that you can reference such as the few I posted above.

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