crane loads - slab on grade

huangyhg

crane loads - slab on grade
i have a 5" thick 3.5ksi concrete slab.  the contractor wishes to load the slab with a crane used for lifting and placing panels.  the maximum outrigger applied load is 115k!! the crane sub insists this is ok if using 6ft x 6ft steel plates to spread out the load.  i figure this reduces the stress to about 22 psi if i assume a rigid plate.  at first glance this looks as though it will work, am i missing anything, should i be concerned about sub-grade reaction?  any tips or insite would help.
thanks,
not knowing what other details you do not have about the concrete slab which appears rather thin and what may lay underneath this concrete pad, i would lay pantoons of at least 18"x18" by 12'long bolted side by side on which i then would place the suggested steel plates for the crane outrigger pads.  also suggest you read shapiro's "crane and derricks"; there is good material on outrigger pad support.  
the plate will bend with 22 psi.  thicken the dunnage to spread the load out to the perimeter of the plate.
you need to consider shear and bending in the slab on grade--this is not just an issue of bearing stress.  can the plate punch through the slab?  will the reverse curvature of the slab outside of the plate cause cracking in the top of the slab?
daveatkins
check for tunnels, raceways, basements, etc under the concrete.  i have seen lightly loaded cranes punch through these unknown "holes"