desing of mat foundation

huangyhg

desing of mat foundation
i have to design a mat foundation (22ft x 16ft). due to some environmental issues the maximum thickness of the foundation is limited to 30 in. here are the details.
fc = 3ksi. i have used pca mat to design the foundation. the thickness seems to be ok for flexure, punching shear and i am getting good results. the only problem where the punching shear is not satisfied is at one of the circular columns (dia = 2ft). here are the punching shear check details.
vu = 199 psi and vc = 164 psi.
vu - factored shear stress at the critical section
vc - factored shear resistance of concrete (without considering shear reinforcement)
is there any solution for this problem. can the bars provided for flexure reinforcement account for any shear resistance. or can i provide any extra shear reinforcement to get this 30 inch mat work. any suggestions regarding this will be greatly appreciated. thanks
on solution is to consider higher strength concrete, you only have 33 cu. yds. the differential cost should be relatively small.
if this is not possible, there are many products manufactured for your situation. here is a link to a summary of thes approaches
notice this thread is also posted in the "foundation engineering" forum.
thankyou all for your suggestions.
you should be able to add shear reinforcement in a 30" slab if required.  at a localized area, cost would be minimal.
be sure you're figuring the critical section at the right radius.