foundation loab comb

[SHBH]

Need to know, should i ise the load comb no 5 , 6 & 8 (using omega factors with seismic)as given in cl 14.4.3.2 of asce 7-05 for the overturning/uplift/sliding of the foundations.

 

[JKStruct]

We've argued about this before in our office. I don't believe the building code is very clear (someone correct me if I'm wrong)... I believe the intent of the code is to design the connection of the superstructure to the foundation using omega level forces, but it doesn't explicity say to apply omega to the foundation design. That said, we've been asked by OSHPD "What's the point of designing to forces to be transfered to the foundation if the foundations can't take it?" Makes sense I guess. I'd design for omega forces, and no one can question you as being unconservative. Although depending on the building, it may lend itself to some pretty large foundations.

 

[JAE]

The point is that the connection may be a brittle, abrupt failure mechanism (thus requiring higher levels of capacity) so that other failure mechanism (which may be more ductile) can occur first and absorb the energy.

 

[SHBH]

JKStruct is very right in the sense that what is the use of designing the anchors for these amplified forces when you foundation can't take it but as mentioned by JAE, we actually don't want to design our anchor bolts for these amplified forces but do so to keep some margin in the capacity of anchor bolts.
Any other thoughts.
But JAE if you look that way then what about the section 8.5 of seismic design provisions which states that for high seismic zones don't reduce your anchor bolt capacity by 25% against the clause mentioned in ACI318 app D .