When to use Omega for Foundation Design per ASCE 7-05
When to use Omega for Foundation Design per ASCE 7-05
(OP)
I've been directed to use the ASD load combo(.6-.14 Sds)D + .7 rho Qe to design for overturning and soil bearing on a continuous braced frame footing. And then to use the Strength Design load combo (.9-.2 Sds)D + Omega Qe to design the reinforcing steel in the foundation.
My 4' wide foundation is ok for overturning and soil bearing with the ASD load combos. If I use omega when calculating e = Mu ot/Pu the foundation becomes unstable, e > L/2. Should I neglect omega when calculating e and then multiply my bearing pressure by omega? When that's the case I need a ridiculous amount of steel for my footing. Do I even need to include omega when calculating the reinforcing?
Can someone please direct me to the section in the ASCE 7-05 that explicitly states that omega needs to be applied to the foundation?
Thanks!
My 4' wide foundation is ok for overturning and soil bearing with the ASD load combos. If I use omega when calculating e = Mu ot/Pu the foundation becomes unstable, e > L/2. Should I neglect omega when calculating e and then multiply my bearing pressure by omega? When that's the case I need a ridiculous amount of steel for my footing. Do I even need to include omega when calculating the reinforcing?
Can someone please direct me to the section in the ASCE 7-05 that explicitly states that omega needs to be applied to the foundation?
Thanks!






RE: When to use Omega for Foundation Design per ASCE 7-05
I can certainly see designing to the overstrength forces for anything considered a critical connection. Maybe the base of a cantilever column, or the connection between pile and pile cap. Is there any reasoning given for why this braced frame footing should be considered so critical?
RE: When to use Omega for Foundation Design per ASCE 7-05
RE: When to use Omega for Foundation Design per ASCE 7-05
I did not find anything in ASCE 7-05 which explicitly points to include omega for the foundation design.
You may also find the following provisions of ASCE 7-05 helpful:
Sect. 12.4.2.2 Ev = 0.2 Sds D = 0 @ soil-structure interface i.e. 0.2 Sds need not be applied to the weight of the footing and the overburden.
Sect 12.13.4 Reduction of Foundation Overturning
RE: When to use Omega for Foundation Design per ASCE 7-05
RE: When to use Omega for Foundation Design per ASCE 7-05
I suppose for overturning failure there is a cyclic return force/righting force. And, if the frame is properly proportioned, the frame will yield and absorb much of the force keeping the footing from attracting loads in the Omega factor ballpark.
ACI 318, chapter 21 specifies where to use the Omega factor for concrete design. Expect to use it for short, axially loaded elements. For noodles with well defined bending, Omega is not needed.
RE: When to use Omega for Foundation Design per ASCE 7-05
RE: When to use Omega for Foundation Design per ASCE 7-05
Even so, 1605.4 of IBC 2006 where the special seismic load combinations are required per section 1605.1, "...elements and components shall be designed to resist the forces..." The foundation does not count as part of the "elements and components."
Furthermore, 1605.1 states that structures shall be designed to resist the special seismic load combinations of 1605.4 where required by ASCE 7 sections 12.3.3.3 or 12.10.2.1. ASCE 7-12.3.3.3 pertains to "Elements Supporting Discontinuous Walls or Frames" and ASCE 7-12.10.2.1 pertains to "Collector Elements Requiring Load Combinations with Overstrength Factor in SDCs C through F."
In summary, I don't think the overstrength factor is required as you describe it, unless there is some local code amendment that requires it – I'm not familiar with OSHPD or DSA, I'm just going straight off the IBC/ASCE 7. It might just be a misunderstanding of the code by whomever is directing you to do so – similar to how with light framed structures, a lot of folks think you have to design your hurricane ties at the roof rafters for C&C just because everything else from the roofing down to (and including) the rafters are – but you don't really have to do so.
RE: When to use Omega for Foundation Design per ASCE 7-05