Large point load slab-on-grade
Large point load slab-on-grade
(OP)
I currently have a situation where column loads of up to 50 kips will be applied to a slab-on-grade (6in thick reinforced). Using the Shentu Method for minimum base plate area and slab capacity, I can get a reasonably sized base plate to distribute the load effectively and avoid punching shear.
My question is should I be worried about differential settlement around this column inducing radial cracks? Would I be better off designing a separate shallow foundation for these columns? The reason they bear on the slab now is the locations are not known, but I was thinking if cracking is an issue I might design a strip footing with isolation joint.
My question is should I be worried about differential settlement around this column inducing radial cracks? Would I be better off designing a separate shallow foundation for these columns? The reason they bear on the slab now is the locations are not known, but I was thinking if cracking is an issue I might design a strip footing with isolation joint.






RE: Large point load slab-on-grade
If the slab had infinitely rigid rock...or something close to that, there'd be very little stress and deflection in the slab and no cracking.
Also - sometimes 6" thick slabs only get constructed at 3 1/2 inches. Depends on how desperate the contractor is.
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RE: Large point load slab-on-grade
In addition to concerns to questions surrounding the inevitable yet unknown proximity to existing joints and cracks, the two things which concern me are: overestimating the subgrade modulus and overestimating the stiffness of the base plate. The subgrade (of course) plays a huge role in capacity. As for the base plate, you need to take care that the entire area of the plate acts like a stamp (does not flex) to evenly distribute the delivery of the load to the soil. My sneaking suspicion is that most column loads are driven right through the contact area for the column and a typical base plate does little to distribute the load.
RE: Large point load slab-on-grade
Is this supposed to account for punching shear aswell as bending. Anyone?
RE: Large point load slab-on-grade
RE: Large point load slab-on-grade
RE: Large point load slab-on-grade
Mike McCann, PE, SE (WA)
RE: Large point load slab-on-grade
RE: Large point load slab-on-grade
I had a project where the suggested was made (by GC) to support bldg columns on the slab-on-grade and not have individual footings. Columns supported 3 floors plus roof. Trying to not be close-minded, I entertained the idea and looked into it. I was given a set of calcs where other engineers had applied yield line methods with steel fibers. When I got the example calcs, they had neglected punching shear, actually it was addressed but simply said "does not control." Really? My punching shear checks gave a slab up to 50% thicker than their design. Also, the calcs were based on the storage rack type (closely spaced with aisles) of methods whereas the column /spacing loadings were not similar. If a slab-on-grade supports storage racks, then ok use slab-on-grade ACI 360 methods or other to design by. But if a column supports multi elevated floors and roof, then ACI 318 is the applicable code.
In summary, I put individual spread footings (like always) and called it a day. I could not agree with what I viewed as too liberal usage of a misapplied design method
RE: Large point load slab-on-grade