Seismic foundation capacity-based design

[ugandabob]

Question regarding foundation design for earthquakes. A clause in the Canadian code requires foundations to resist the overturning capacity of the SFRS, not the applied loads only. (The idea is to avoid having the foundation be the yielding element in the event of higher-than-expected seismic forces.)

I am looking for guidance for tension-compression braced frames.

Should the overturning capacity of the SFRS be based on the load at which the compression brace buckles, or should it be based on the capacity of the single brace in tension (often higher)? For Conventional Construction (Rd=1.5; Ro=1.3), the steel design code does not require redistribution of the post-buckling compression forces for the design of the steel structure. But the overturning capacity may be much larger when considered as tension-only.

If the overturning capacity is based on the buckling load, it seems that foundation could have issues if the seismic forces are larger than expected.

Thank you

 

[TLHS]

I'll try to come back to this, but this clause is an adventure, in my experience. For something that has a significant impact on foundation design forces and concept, there is little direction from the code writers. From discussion, the formulation of the clause seems to have been conceived for shear wall concrete systems and hasn't really been documented well for other situations. There is a paper out there discussing this for steel frame systems that I'll try to track down.

 

[skeletron]

I think A23.3 forces you to consider if Rd<2.5 (need nominal capacity of brace bay) or if Rd>2.5 (use probable moment capacity of brace bay with Cu, Tu, Cu'). So it depends then if you are MD CBF (Rd=3, Ro=1.3) or LD CBF (Rd=2.0, Ro=1.3).

Then calculate the factored overturning (based on your loading), the nominal/probable moment (based on Rd), and then your cutoffs for the moment at RdRo=2.0 and 75% of the nominal moment capacity. These four values are used in your comparison for the design moment on the footing (probably some more explanation is usable here). And I think that CL 27.1.2 (S16) allows for bail-out corresponding to RdRo=1.3, but must consider the redistribution of forces due to brace buckling.

Clear as mudd...

 

[ugandabob]

Thank you TLHS.


Skeletron:

But what about Conventional Construction (CC)? I understand that for LD or MD braced frames that buckling and redistribution is considered in the steel design, so the SFRS capacity would be based on yielding of the tension member (plus a little post-buckling compression strength). But steel design of CC does not require redistribution. So, is the capacity of the SFRS based on the lateral for that buckles the compression member or that yields the tension member?

Thank you.

 

[atrizzy]

ugandabob, have you calculated your shear and overturning capacity at the 2 stages you define, namely 1) at point of compression brace buckling, both braces still contributing or 2) after compression brace buckles and only the tension brace contributes?

 

[ugandabob]

atrizzy, I have done some preliminary calculations. The length of some braces is long, so the tension capacity is much larger than compression in some cases. For tension-only, the horizontal force can be much larger than that associated with buckling, which results in a large footing. It is not a particularly large building, so the large foundations look a bit odd to me. Just want to make sure I am not overthinking / overdoing it.

 

[skeletron]

From my understanding, I would design for the nominal compression capacity at RdRo = 1.5x1.3, and then look at if the cutoffs as they would apply for your case.

 

[ugandabob]

Thanks Skeletron; I will go with that approach