Concentrated load - SLAB on GRADE
Concentrated load - SLAB on GRADE
(OP)
Occasionally I get questions from the manufacturing industry about how large of a concentrated load that their existing concrete slab can sustain without cracking. Normally they cannot tell me anything about quality of concrete, size of reinforcement or quality of subsoil. I would like to assume we have a 4" slab, no reinforcement and 2000 psf soil. This seems to be a conservative approach. How do I estimate the capacity. I have looked at the slab as a 4" thick footing with no reinforcement. The concrete would spread out the load at a 45 degree angle and apply the load to the soil. Does this make sense? Am I crazy? I have looked in several books and have not found any documentation on a rule of thumb for concrete slab capacities for concentrated loads. I would appreciate any time you guys could spend on this. Thank you in advance.






RE: Concentrated load - SLAB on GRADE
The PCA has a program that can reasonably estimate the tensile stress in the concrete based on the above criteria and it's a judgement call from that point on...
There is some argument that 4" is too thin for a significant slab and your dialog with the owner should be clear that he assumes whatever risks are involved (assuming no catastrophic stuff).
RE: Concentrated load - SLAB on GRADE
Reduce the plate to 4x4” and the limit load is around 65 kip.
Note however that this describes strictly static local punching, for perfectly sound concrete, and this may not be the case for at least any slab on the ground. Other aspects like deflections under the loads etc can have a say, and better recommend the thing be suitably reinforced. More, in any case, punching failure and shear as it is, in the absence of reinforcement the failure is deemed to be fragile. If you had reinforcement enough beyond the working stresses your structure will impart, you'll have further capacity and the failure will be ductile at the end of the moment capacity of the plate, even if the limit punching strength has already been attained. The question is that surely moment capacity of your plate even supported by the ground and reinforced is likely to be the controlling phenomenon.
In the absence of any rebar, I would recommend a bigger safety factor that say, 1.5.
Search in Mathsoft site Collaboratory for Mathcad 2000 , Civil Engineering Folder the sheets
Effect of Wheel Loads on Slabs on Grade
Punching of a RC plate according to Menétrey
RE: Concentrated load - SLAB on GRADE
RE: Concentrated load - SLAB on GRADE
http://www.mathcad.com/Library/DisplayCategory.asp?c=40&p=10&s=0
RE: Concentrated load - SLAB on GRADE
Use a big base plate and be very conservative!
RE: Concentrated load - SLAB on GRADE
Punching shear gives yet another limit; as noted, 125K or 65K depending on the base plate size.
Another limit is the flexural strength of the concrete; this is determined by the size of the base plate and the soil below. The mathcad solution noted by PEinc yields a load of about 5K based on f'c = 3.5ksi and a 8x10 BPL and a SF of 2 with a modulus of subgade reaction of 200 pci... This would likely be a better approximation of the load capacity of the slab... less with a lesser concrete strength using the square root of the ratios. This could be diminished by existing cracks, reduced thickness, sawcuts, etc. I'm not familiar with the source referenced in the mathcad program, but the thickness required is of a magnitude that I would suspect. The stress can be augmented depending on the placement of the legs; due to the elastic behaviour of the slab, another point load in proximity can increase the concrete stress.
I'll try to run this through the airport slab program from the PCA and see what the concrete stress is...
RE: Concentrated load - SLAB on GRADE