Omega factor when designing foundations?? 1

[schau]

Hi, I would like to ask you a general question about the designig of a foundation. I am from germany and i am doing for the first time a statik calculation accordnig to AISC, that is why i need a little help.

My job is to give the foundation loads to another eng. firm which will do the calculating of the concrete foundations. But i don't know if i have to use the security factor for Earthquake (Omega) in the load cases. Normally for the designing of connections we should use it. But i am not sure if it is right for foundations too. Or perhaps i should use it just for the dimensioning of the anchoring rods?
I'll be very gratefull if you can help me. Some links could be helpfull too.
Thanks in advance! Bye

 

[Taro]

This is a "gray area" that is not addressed well in the code. In general, frame connections are designed for either the capacity of the members or amplified (omega) forces so that the connections are not the weak link in the system. Philosophically, it makes sense to also design the connections to the foundation for these forces.

But it is debatable whether the foundation itself should also be designed for amplified forces. If you have shallow foundations, it entire system might have an overturning instability before these force levels are ever reached. You often need ridiculously large foundations to resist the amplified uplift. But we have never found it to be a big problem that entire buildings would tip over in an earthquake (in the absence of soil liquefaction). On the other hand, for deep foundations it probably makes sense to use amplified forces for foundation design.