Sizing a bearing plate for point load on concrete slab

[markk1pe]

We use air pallets to move heavy loads, but when we set the load down it causes a point load on the concrete slab. The desire is to dissipate the point load by providing a metal plate to set the point load down on that would spread the load out over the concrete. The problem is calculating the size of the plate to use. We are considering using 1/8 inch thick steel plates. The point load is about 6000 pounds. The concrete is older 3000 psi compressive strength, no rebar.

I searched for a similar question, found a few, but none that was exactly like this. I also look at the formulas in X Calcs, but couldn't find a match.

 

[dimjim]

You have 6000 pounds on each point or
6000 pounds total?
If the concrete is rated at 3000psi
then 1 inch square will withstand
3000psi. 2 inch square 6000psi etc.
You pick the safety factor.

 

[markk1pe]

Thanks DimJim, seems a little oversimplified though. So what you are saying is if we put a sheet of .001 steel down that is 1 inch by 1 inch, we get the same result as 1/4 inch thick plate? I guess the question was not clearly stated. How do we determine the proper size and thickness of the metal bearing plate?

 

[DaveAtkins]

You really need to check the slab for bending and punching shear--dimjim is only considering bearing pressure, which won't control.

I use the method in PCA's slab-on-grade design book.

DaveAtkins

 

[SteveGregory]

To check bending in the slab, check out "Designing Floor Slabs On Grade" 2nd Edition by Ringo & Anderson, p. 52.

Steve

 

[CJJS]

Once you have checked the slab and assuming it works, then you can design your bearing plate per AISC Specifications. The AISC Manual of Steel Construction has design examples for this.

 

[cdi12]

markk1pe:

I would check this bearing plate as base plate per AISC Manual.

 

[UcfSE]

Typically we simply the calculation by assuming the bearing cantilevers past the column and is loaded by the bearing pressure. This results in a simple cantilever model where your moment is wL²/2. You can then calculate the required section modulus and required thickness, if you know the allowable bending stress for your base plate material. It's not absolutely that simple but it's close. Generally you find that as you increase the dimensions of the plate, the bearing stress goes down but the plate thickness goes up.

 

[rapt]

To assume that you have even pressure under the plate, the plate has to be thick enough that it acts as a rigid plate, not as a flexural plate, otherwise the bearing stress is not constant over the plate. If you assume equal pressure and base the design on stress then your starting assumption is wrong and the pressure near the centre will be much higher, especially if you use a sheet of .001 steel.

It is not sufficient to simply limit the stresses to normal stress levels, you need to limit the rigidity.

I would think that Timoshenko would cover this.

 

[dik]

Is the problem the design of the base plate or the tensile stresses in the slab... I would typically use a plate 3/8" min. If it's the design of the slab, then the thickness of the slab, the subgrade modulus, the load and the spacing of the loads come into play. If you can provide spacing information and slab thickness, I can try to model it and give you an indication of what the concrete stresses would be.

Dik

 

[COEngineeer]

Mark, you can size the pad however big you want depending how much pressure you want. The big question is, what is holding the pad? Lets say you have 6"x6" pad, this pad is attached to what? 3x3 pipe? I dont know the pallete looks like. Once you give me this information, I can give you the formula from AISC. So this is the info you need

1. Size of pad you want
2. Whats holdidng the pad (centered in the pad)

t min = l x SQRT(2Pu/0.9 Fy.B.N)

l= the cantilevered lenght, for example if you have 3x3 column on 6x6 pad, then your cantilevered length is 1.5"
Pu=Ultimal axial load
B= Usually I use the symetrical column so the pad is symetric also. SO B = N = dimension of pad
Fy= steel strength

So for example I have 60kips on 3x3 column with 6x6 pad then my t min should be .373".

Let me know if I did this correctly guys!

 

[COEngineeer]

Oh.. I forgot to put where I got this from, Page 14-5 AISC manual of steel contstruction