Foundation Overturning for SCBF 1

[amendale]

Hello all,

I have a question concerning foundation design for SCBF systems as per AISC 341. I understand that the whole design philosophy behind SCBF is that all members should remain elastic while your braces undergoe yielding and buckling. Therefore we design our connections for the tensile capacity of the braces, as well as beams and columns for capacity/unbalanced load. My question is about how we design the foundation to account for this behaviour. If the brace tensile capacity is taken as the uplift force then the required foundation to resist overturning is very large.

Do we use RyFyAg of the brace as the uplift force to design for overturning?
Do we use the amplified seismic load to design for overturning?
Do we use the basic seismic load to design for overturning?

The code is very unclear about this.

 

[JoshPlumSE]

Connections in an SCBF may experience locally very higher forces (based on actual rather than minimum yield) in the tension braces. That's why they are designed to the RyFyAg. That is a local requirement for that brace connection. You do not have to carry those connection forces through the rest of your structure and into the foundation.

Critical elements in a foundation could certainly be designed to the amplified seismic loads. Elements where you are trying to avoid brittle failure. Maybe tension bar pullout in a pile cap, or anchor rod concrete cones or such. But, I would not normally design a typical footing or mat foundation for the amplified loads.

 

[JoshPlumSE]

I forgot to point out that AISC uses Foundation Overturning / Uplift as a valid means for reducing the design forces in the SCBF columns. See example 5.3.7 of the AISC Seismic Design Manual 2nd edition. Meaning you can take a look at the column force that would result in foundation uplift / overturning and use that as the maximum force for which you need to design your column.... reducing it from the force you would otherwise have gotten.

For that reason, I would say that you are not required to inclue the RyFyAg or the amplified forces in your uplift or overturning checks for the foundation.

 

[chrislaope]

Hi, JoshPlum:

I have a similar question to ask, do you mean if it is other than SCBF system, the anchor bolt and foundation uplift and overturn (i.e. structure weight+concrete+soil weight)design still have to use overstrength factor to calculate the seismic uplift?

Thank you very much for your clarification.

 

[JoshPlumSE]

I'm not sure the code is entirely clear on this. I think the use of overstrength loads on foundation design is really up to the engineer. I just mentioned some cases where you might want to use it to prevent non-ductile failure of a foundation element. This would be more important for an SCBF where the R value used is based on the assumption of highly ductile structure behavior. But, it could apply to a lesser degree to an OCBF.

Even if you decide to use the amplified seismic forces for some aspects of foundation design, I would NOT use them when I check for overturning or uplift.... which is what I think the original post was about.

 

[RareBugTX]

I had the same question, did research on the web and found nothing. Even the worked design examples from the ICC publications omit it.

 

[msquared48]

Evrything was so much simpler with WSD...

Mike McCann
MMC Engineering

 

[amendale]

"For that reason, I would say that you are not required to inclue the RyFyAg or the amplified forces in your uplift or overturning checks for the foundation"

Ok so we design the bracing system with R = 6, so that the braces undergo plastic deformation under real earthquake loads, and design everything to handle these forces. But when it comes to the foundation design, we dont need to consider this? Seems inconsistent and contradictory to the whole approach of ductile design. I'm not saying you're wrong, just that the design philosophy seems flawed.

 

[amendale]

"Connections in an SCBF may experience locally very higher forces (based on actual rather than minimum yield) in the tension braces. That's why they are designed to the RyFyAg. That is a local requirement for that brace connection"

How can this be possible, the forces need to flow into the system, they cant just stop at the connection just because.

 

[wannabeSE]

ASCE 7-05 allows you to reduce the overturning forces for foundations design. See section 12.13.4. I don't think any major failures occur if the foundation lifts off the ground a little. If I get a chance, I will look for a reference that explains this reasoning.

 

[wannabeSE]

Here are a couple references:

FEMA 451 Commentary to section 5.2.5
"In the design of the foundation, the overturning moment calculated at the foundation-soil interface may be reduced to 75 percent of the calculated value using Eq. 5.2-14. This is appropriate because a slight uplifting of one edge of the foundation during vibration leads to reduction in the overturning moment and because such behavior does not normally cause structural distress."

FEMA 751 Commentary to ASCE 7-05
"C12.13.4 Reduction of Foundation Overturning.Since the vertical distribution of forces prescribed for use with the equivalent lateral force procedure is intended to envelope story shears, overturning moments are exaggerated. (See Section C12.13.3.) Such moments will be lower where multiple modes respond, so a 25 percent reduction is permitted for design (strength and stability) of the foundation using this procedure. This reduction is not permitted for inverted pendulum or cantilevered column type structures, which typically have a single mode of response. Since the modal response spectrum analysis procedure more accurately reflects the actual distribution of shears and overturning moments, the permitted reduction is only 10 percent."

 

[JoshPlumSE]

The idea with those locally higher forces is that you want to prevent non-ductile behavior such as rupture of the gusset plate. The intent is you want ductile deformation of the structure to absorb more energy (i.e. increasing the area under the the hysteresis loop).

Does partial foundation uplift or overturning improve the energy absorbtion of the structure? If so, then it is desirable behavior and there isn't a need to go to extreme measures to prevent it. That's what the FEMA commentary seems to be saying.

Now, there are certainly cases where you can argue that this would not be appropriate where you could get brittle failure of the foundation which results in overall structural failure. I mentioned some earlier (anchor rod or pile pullout), but you could add "cantilever column" to the list. In a moment frame structure, if one column footing lifts up or overturns slightly that does NOT result in overall failure. But, it would in a cantilever column system.

 

[SAIL3]

IMO, to me the seismic requirements are a patchwork of redo's based on the results of the last significant earthquake and are not consistently logical or continous....it's a complex problem and perhaps this is the most practical way to approach it...I typically design the col base locally for the amplified loads..meaning, base pl, AB's and it's conn to the conc and all other critical connections in the structure, ie col splices, etc...
when all is said and done, no one really knows the exact magnitude of these forces and they can vary from structure to structure depending on the inherent variabilty of stiffeness and it's location in each structure...when it comes to carrying the amplified loads to the fdn as awhole, my gut feeling/engingineering judgements says this guessing game has gone far enough.....