When to use Omega for Foundation Design per ASCE 7-05 1

[kmc23]

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!

 

[JoshPlumSE]

A continuous braced frame? I don't believe there is a requirement like this for that type of system. Perhaps if it can be considered a storage rack, then you'd be looking at 15.5.3.2 (of ASCE 7: 2010). But, that is really for the anchorage of the rack, not the design of the footing itself.

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?

 

[SethGuthrie]

This is the kind of thing that DSA or OSHPD might require in California. My reading of those requirements indicates that the "strength" of the foundation needs to resist the amplified forces, but it never really addresses stability of the foundation under amplified forces. Since this presents a dilemma for a continuous footing there are different opinions out there. Some engineers have told me that they calculate the bending and shear in the foundation under service combos and then factor those results by some overstrength factor which avoids the equilibrium problem and gives a conservative result for the steel. Other engineers are only comfortable enlarging the footing to insure stability even under the amplified combinations.

 

[DST148]

@ kmc23: Ref: 2000 IBC Seismic Design Manual - Vol 3, Ex 1A, Section 9 - The anchorage to the foundation must be designed for the the amplified seismic load(include omega). The foundation itself need not be designed to preclude uplift. (exclude omega)

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

 

[sandman21]

DSA does not require the overstrength factor on stability or bearing pressure. It is only applied to the strength of the members.

 

[Teguci]

I thought the purpose of the Omega factor was to eliminate sudden failures under dynamic loads for significantly stiff elements (ex. battered pile punching through a pile cap)? That being the case, I don't care if the footing exhibits a well defined plastic hinge zone as the whole structure tips over (what I would consider a sudden failure under dynamic loads).

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.

 

[sandman21]

Over strength is meant to keep elements in the elastic range. The forces in a building during an earthquake are much higher than code design equations. The force is dissipated through inelastic behavior of the structure certain elements are key to this so the code applies higher loads to try to keep them in the elastic range. Overturning and soil bearing are two areas that are already conservative.

 

[JennyNakamura]

I agree with JoshPlum. I don’t think there is a requirement to use the special seismic load combinations with system overstrength factors for a continuous braced fame – including the foundation.

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.

 

[sandman21]

You have to decide whether you consider the foundation a collector element, I do. If you decide that the foundation is a collector the special seismic provisions apply and omega is required for the design of the strength of the member, it does not apply to stability or bearing pressure. DSA/OSHPD specifically amends the CBC to reference the ways to design the foundation, CBC 2010 1615A.1.10