Eccentically Load Pad Footing Design

[LTTANG]

In many design examples from the ACI318 Design Handbook (for footing strength design for Vu, Mu), the values of W or E is given in terms of a concentrically load P and subsequent q is derived (with proper load combinations) yielding a simple q=P/A. This q is then used to evaluate Vu and Mu and subsequent As and Av.

My question is how can I derive the q in these terms when I have an overturning moment at the footing and essentially an eccentric P? In some cases, applying 0.9D+E or 0.9D+1.6W, yields 'e' larger than the width of the footing (far beyond B/2) and yet q < Q allowable when evaluating stability of footing.

Should I just give up trying to figure out what q is and use whatever allowable Q given by the soil engineer or by code since it’s a sin to go beyond the Q.

I’d greatly appreciate your thoughts.

 

[Brad805]

You should move your Q to the Structural section if you want anyone to respond. The BIM section is rarely ever visited.

Brad

 

[englahmadi]

Lttang
for calculating q you have to derive stress equation as following:
q=P/A+MyX/Iy+MxY/Ix. by using this equation you can calculate q in any point and can draw shear and moment diagrams. now you can design footing by calculated stresses. recently i have deisgned a footing which belongs to very big elevated tank and i had the same problem. please let me know if you could not solve your poblem.