How to model foundation for overstrength when unstable? 1

[AaronMcD]

For seismic stability, foundations need not consider overstrength. But for bending, we need to use overstrength. If I model the foundation with compression springs, I can't check the foundation because the model fails stability. What do you do in this situation?

Typically if I have a moment frame and am doing overstrength check on the foundation I just put a point load and assume worst case scenario the foundation is held down at the ends - no need for a model. But it's pretty easy to model a foundation on springs, so I'm doing that for a current project, but running into the stability issue when using omega = 2.5.

http://www.hellodwell.design

 

[AaronMcD]

Also, it's nice to have the soil stiffness modeled to check drift, especially where I have a big window in a moment frame.

Perhaps I can run the analysis with a factor of 1 and multiply maximum bending stress by 2.5 and do a hand check for bending. Any other ideas?

http://www.hellodwell.design

 

[driftLimiter]

What part of the code specifically is leading you to use overstrength on the foundation element? This whole idea that the foundation needs to be designed for a load that is beyond the stability limit is a bit precarious if you ask me. Perhaps, if indeed you need to design this element for overstrength, you could use a capacity limited approach and say the stability of the footing is the capacity limit.

Also don't neglect ASCE 7-16 section 12.13.4 which allows for reduction at the foundation interface of loads due to EQ overturning.

 

[AaronMcD]

@driftLimiter
I was wondering if I can limit the load by the overturning capacity. The overstrength is for a beam element supporting a seismic load. The foundation acts as a grade beam.

I could also just add extra dead load at the ends where the perpendicular walls come in for the beam check. I'm not sure how much of this to add so I usually just use 5 feet conservatively.

http://www.hellodwell.design

 

[driftLimiter]

To me it seems like still a misunderstanding of the overstrength factor seismic irregularities and trigger for application of the overstrength factor. I can't reason why a grade beam would need to be designed for overstrength? I have always stopped overstrength at the anchorage because ACI requires anchorage to be ductile or designed for overstrength or another capacity limit. ASCE 7-16 12.13.9.1 Says foundations designed for basic load combinations. A beam supporting a discontinuous vertical element of the SFRS would need to be designed for overstrength, but the foundation has a continuous load path to the ground.

I guess I'm still not clear why this element needs to be analyzed this way.

 

[le99]

In short, the over-strength factor is intended as a safeguard for the connections (links) of the seismic resisting elements. It has nothing to do with global stability concerns. See the paragraph regarding "Does ASCE 7 require foundations to be designed for the overstrength factor?" in this article, and item 6) in this article

 

[driftLimiter]

@le99 the overstrength factor is applied in different contexts not only connections but also framing elements and columns might see this load. In the context of something like a Braced frame or moment frame one could understand the overstrength factor as protecting connections. In the context of irregularities the overstrength factor may be applied to entire elements design forces to protect that element from additional overturning demand beyond the ductile limit corresponding with the R factor of the MSFRS.

 

[AaronMcD]

@driftLimiter
The grade beam is a beam supporting a discontinuous element. If there is uplift on the center of the grade beam, and it is held down primarily at the ends where most of the building weight is attached, the load path is not directly to the ground.

http://www.hellodwell.design

 

[driftLimiter]

@AaronMcD, . In my practice I am not seeing that done. But have at it if you feel that this is required, at least it will be conservative.

[URL unfurl="true"]https://www.skghoshassociates.com/blog/foundation-overstrength-factor/[/url]

 

[AaronMcD]

Do you know of anywhere in the code that provides an exception to beams supporting discontinuous seismic loads when those beams are in contact with soil? The sources above are (1) not code, and (2) claim the code does not explicitly demand that foundations be designed for overstrength, but don't point to any exception in the code for a beam supporting discontinuous seismic load, when that beam also happens to be in contact with earth, and supported by earth on one end.

I would love to not have to design for overstrength but I can't see any reason why a concrete element in contact with earth would be exempt.

http://www.hellodwell.design

 

[driftLimiter]

ASCE makes no specific distinction between 'beam' and 'grade beam' to apply to this question. Also 'continuous' is not well defined. However there are specific areas of the building code that do require grade beams or foundation elements to be designed for overstrength, these are stated explicitly as such. One might also note that the code refers to beams, and grade beams each individually throughout.

SO the grey area is left up to our interpretation, and standard of care.

Although those resources are not code, they do well to establish a standard of care and they are industry acknowledged sources. Not to mention that SK Gosh is the CHAIR of the of the ASCE 7 Seismic Task Committee on concrete and also sits on the board for the Subcommittee on Seismic Loads.

Also I will again reference ASCE 7 16 Section 12.13.9.1 which plainly states the loading requirements for foundation design.

So lets answer your question then and not try to bother ourselves with the question of omega.

If the grade beam is unstable under the loading you have determined you can:
- Increase the mass
- Increase the width of soil bearing (or length)
- You can include contribution from soil weight, shear
- You can limit the overstrength forces by a capacity limited approach as I mentioned earlier.
- Increase the bending stiffness of the grade beam by adjusting its section properties.

 

[AaronMcD]

The thing is I don't want to make it stable under omega loading. I just want to check bending capacity under this unstable load condition. I think I will just add more mass to each end for omega load combinations to force the model to be stable in order to check maximum bending. I can remove the load for stability load combinations.

http://www.hellodwell.design

 

[driftLimiter]

Should be okay, might want to check ACI 318-14 sections 13.2.3.1, 13.2.3.2, 18.2.2.3, and 18.13.3, and 18.6. Specifically the maximum reinforcement ratio of 18.6.3.1. I think ACI offers the clearest distinctions for grade beams, compared to CBC and ASCE.

 

[jittles]

I'm unsure of your particular condition, but aren't there usually other seismic system exceptions for "maximum load that can be delivered to the system" i.e. you design just up to the load that causes overturning in the foundation? If omega = 2.5 is an unstable condition, then that load input can be reduced until it's just barely stable.

 

[structSU10]

Does the grade beam have soil bearing pressure on its full length or does it span between footings? I could see two approaches - one for each case.

Design the grade beam as simple span between footings with the amplified seismic loads - ignore the 'instability' of reactions at each end and add the needed bar the the grade beam.

Alternatively you can take the bearing pressures from the unfactored seismic loads and just amplify those by 2.5. It is ignoring the extra overturning in a sense but maybe it bulks things up in an acceptable way?

 

[bones206]

I might consider increasing the allowable bearing pressure by the omega factor in your analysis software. That way the analysis does not result in a stability failure and produces the actual internal foundation forces that would occur if those higher contact pressures were achieved in a seismic event. I would think of it as a dynamic strength increase factor for the soil, or a reduction in the geotechnical safety factor.