Large point load slab-on-grade

[spieng89]

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.

 

[JAE]

I would think with 50 kips cracking would definitely occur. Of course this assumes you would have some "typical" soils beneath the slab.

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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[JLNJ]

If I recall correctly, the Shentu method was developed in concert with the storage rack industry. It yields capacity results much higher than other methods.

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.

 

[clarke1973]

Sorry to hijack the thread but I have a question on the Shentu method. The forumula Pn = 1.72((KsR1/Ec)10^4 + 3.6)Fcd^2 seems to be a simple calculation which determines the load carrying capacity of the slab.
Is this supposed to account for punching shear aswell as bending. Anyone?

 

[XR250]

Why take the risk? Cut some footings and be done with it.

 

[Ron]

Agree with XR250. Not worth the risk. 50 kips is a relatively high concentrated load for a slab on grade,and even though the Shentu Method shows it to be acceptable, I would use an isolated footing for such a load.

 

[msquared48]

Me too. You are looking at a 6 foot sore footing at 1500 psf. Slab on soil WILL fail.

Mike McCann, PE, SE (WA)

 

[spieng89]

Thanks for the helpful feedback! I agree as it's been said a few times, it's not worth the risk. I'm going to design separate footings for this load and not have to rack my brain about issues I can't confidently forecast.

 

[peal724]

I have run into the same thing.

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

 

[txeng91]

I have debated this numerous times before, but usually with loads around 10-15 kips. I've done numerous house remodels where I have required footings under new columns, typically 18" deep, assuming a bearing capacity of 1500 psf. I usually design on expansive soils with waffle slabs so it can be a concern if they hit a grade beam. If you are bearing on just on a 6" thick slab I would have them cut the slab concrete & reinforcement, dig a footing, and drill dowels into the slab and cast them into the footing w/ a chamfer at the edge of the footing and fill with sealant to act as a control joint. Honestly with 50 kip loads you should probably have a geotechnical study to determine soil bearing capacity.