slab on grade design for rack loads 1

[cfarmer]

Designing slab on grade to support significant rack loads (15000# per post) and using PCA charts to design it appears as though a plain slab is off the chart (when considering 2 adjacent racks back to back will give 2-15000# post loads only 8" apart). Using finite element analysis software (ROBOT) we have determined the max moment to be 40kN-m. Which lends to a 10" slab with 15M @ 12" o.c. E.W top and bottom or 20M @ 10" o.c. ea. way at centre of slab. This seems like a very significant slab, especially to the ears of the client, but it is what it is!
Does anyone have experience with this type of design with respect to this reinf. and thickness?
Secondly, if I go with 20M @ 10" oc. I will sawcut at about 12'-6" o.c. ea way across the slab, but if we elect to use 15M top and bottom, can I elminate the sawcutting all together? (can't sawcut with only 2" cover). Any experience on what option would be best with respect to cost and performanc?

Thanks in advance for any thoughts.

 

[Ron]

It would appear that you are analyzing the slab as a beam with a finite width. In this case, for a fully supported slab on grade, punching shear will likely control the design. Check the shear and see what thickness values you'll get. Generally no reinforcement is needed for this and I would expect a slab thickness in the 8-inch range. Check it by an elastic layer analysis (such as for a pavement). If shear value is marginal, use a baseplate to spread the contact area and thus increase the shear area.

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[cfarmer]

I agree punching shear is important and should be checked, but there will also bending stresses within the slab, especially on a subgrade with k=100pci and significant post loads close together. As a result with the maximum bending stresses created I am needing a slab of this depth and at this depth punching shear is ok.

With reference to my second question, placement of control joints and construction joints with respect to rack supports is difficult as owners want the flexibility to move racking as desired. Bars through contraction joints should be kept to a minimum but at the same time by reducing the bars across a joint will reduce the moment capacity near the joints. As a result I would like to keep the jointing to a minimum. Joint spacings for plain slabs on grade is readily available, but what about with a reinforced slab?