Net load for ground bearing slab 1

[KJJ]

Dear All

I am designing an RC slab to support a silo which bears on to the slab via 4 No. legs (square on plan). Each leg produces a point load of 140kN and the self weight of the slab produces a UDL of 12kN/m2. The slab is ground bearing. Working on a safe bearing pressure of 75 kN/m2 how do I work out the net load, as a UDL, carried by the slab across the diagonals in order to determine the tension reinforcement. I picture as an inverted simply supported beam.

Many Thanks

KJJ

 

[whymrg]

The easiest thing is design this slab as mat footing using PCA-Mat software. You may use RISA-3D or STAAD to create a grid of plates with spring supports at corners. This will give you moment distribution and you will be able to find reinforcement. Spring constant or modulus of soil "k" shall be taken from geotechnical report.

Good luck.

 

[JAE]

What whymrg said...exactly.

Also, if you have no geotechnical report, you can use the following approach to get a conservative result:

Assume a soil subgrade modulus, K, of between 75 pci and 200 pci. (20,400 kN/m3 and 54,300 kN/m3 ...I think).

The idea behind the modulus is that 20,400 kN/m2 pressure would deflect the soil 1m.

So if you set up a grid of plate elements in RISA, each joint would represent a certain area of slab, say As.

So your spring stiffness would be (As) x K = ks
ks is in units of kN/m

You can first do your analysis with the low end springs and this will maximize your bending forces in your slab to give you a conservative ceiling on the reinforcing. Switching to the higher springs, you would then get more direct load transfer to the soil and maximize your soil pressures beneath the slab for another conservative check.

 

[cyak]

I would suggest you use shell elements to model the slab if the legs of your silo are not connected at their base. The shell elements would allow you to include reinforcement for possible tie-action that the slab will provide for the legs...

Concerning what JAE said I would recommend for a start, using a subgrade modulus of:

ks = 40 (SF) qa

where qa is the allowable bearing capacity of the soil and SF, the Safety Factor against this qa. ks is in kN/m^3 and qa should be in kN/m^2.

Your slab seems to be 0.5 m in thickness so a punching shear check would also be at hand.

I would also note that your analysis should involve compression only springs if after a first analysis via elastic mat foundation you find that there is tension in some springs.

You could also model the situation by equivalent beams on elastic foundations if you do not have a software that can do these kind of analysis. Solutions for various types of loading exist in tables...