Mat foundation (22ft x 16 ft) 1

[rahuldby]

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.

 

[DaveAtkins]

Isn't vc for punching shear equal to 4 times the square root of f'c? That would be 219 psi for 3000 psi concrete.

DaveAtkins

 

[UcfSE]

You compare vu to phi*vc. If phi = 0.75 then 0.75*219=164, so the program is probably working right.

You can add shear reinforcement if you need using the ACI, but do not count the flexural reinforcement. It doesn't work that way. You can also build a base around the column to increase b0 to a suitable length, or use a stronger concrete f'c. Though you said you need 30 inches max, perhaps you can get away with increasing the thickness local to the problem columns only. I would go with something other than adding shear reinforcement.

 

[rahuldby]

Thanks for your responses.

UcfSE,

I would probably go with increasing the thickness of the mat locally at the problem column. And I would like to know if there is any specific reason for not considering the shear reinforcement. Is it going to be expensive? Just curious

 

[UcfSE]

It just isn't common to have shear reinforcement in a footing. That means it's likely to cost more than simply digging the hole deeper or using stronger concrete.

 

[DaveAtkins]

Since when is phi equal to 0.75? Isn't it 0.85 for shear? If so, just increase f'c to 3,500 psi. You won't get f'c = 3,000 psi anyway; more likely it will be 4,000 psi no matter how you specify the mix design.

DaveAtkins

 

[UcfSE]

Since ACI 318-02, phi for shear is equal to 0.75. [9.3.2.3]

You have the option of using the old load combinations and resistance factors if you follow ACI 318-02 Appendix C - "Alternative Load and Strength Reduction Factors."

 

[DRC1]

In most area 4,000 psi concrete is availble for little increase in cost over 3,000. That may be the easiest solution