Foundation Rocking - How to determine if it will occur

[river55]

In seismic zones, how do you determine if a concrete shearwall or a steel braced frame will rock or not? Does anybody have a reference for how to determine this effect?

 

[JAE]

Could you elaborate a bit on what you mean by "rock"?

Do you mean deflect? Yes - they always will deflect to some degree.

Are you concerned with overturning?

 

[msquared48]

Do you mean "rock", as in overturn, or "rack" as in longitudinal shear deformation of the wall?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[keenaseng]

If there is insufficient weight or lack of hold down in a structure to prevent overturning, the strucure will lift off its foundation to uplift the structure above. This cyclic movement will cause rocking of the structure.

This can be used as a form of energy dissapation in seismic analysis, however the redistribution of forces is beyond linear analysis. I am only familiar with the concept, not the detailed analysis.

 

[river55]

Yeah, "rocking" would be where the concrete shearwall or braced frame basically rocks like a rocking chair and disipates energy. It basically gets confused because with the building code you reduce your seismic loads by an R factor, and then you design your foundations for the reactions based on that R factor. But the true forces seen by the foundation may be larger, may cause the footing to uplift a little bit, but then rights itself on the next cycle of the earthquake.

I am in the same boat as keenaseng where I am marginally familiar with the concept but not familiar at the detailed analysis. Its possible that rocking of the foundations can cause redistribution of the forces, as well as increasing the building period and increasing the amount of drift a building undergoes.

That said, I would appreciate it if anybody has any design references they can point me to on this.

Thanks for any help you can provide.

 

[SteelPE]

I'm no seismic wiz, but I always thought that the R factor was based upon the ability of the framing system to dissipate the energy of the earthquake. The higher the R factor the more likely the system can dissipate the energy.

Are you purposely trying to let the foundation rock? Or are you trying to design a foundation system and are worried if the foundation rocks what will happen to the structure?

 

[river55]

Mostly it would be that I am worried if the foundation rocks what would happen to the structure. Its not that the normal way that foundation systems are designed lead to unsafe structures (they don't), its more to do with if the seismic separation is big enough.

 

[a2mfk]

Like keenaseng was saying, you need to supply enough dead load in your foundation so there is no net uplift due to overturning, using the proper factors for design per your codes. And I have never heard the term "rocking" used except by contractors, so you may want to avoid usage of that term...

 

[bridgebuster]

I think the concept of Rocking is more common in bridge seismic retrofit.

 

[river55]

Rocking is something that gets involved in the more intensive seismic analysis, especially non-linear analysis (which I am not qualified to perform). For instance, in ASCE 41-06, on page 88, there is a heading titled "Consideration of Foundation Rocking," and they give some references for the design professional to look at. But these references seem pretty complicated on a first glance. I was looking for something more straight forward or more cookbookish (since I don't need it for a non-linear analysis).

I just have a plan review comment that my seismic joint may not be big enough because of rocking, and I need to check the rocking aspect.

 

[ATSE]

For shear walls, foundation rocking is real, and will dissipate energy during seismic shaking, and will also increase lateral displacements (requiring a larger theoretical gap between buildings to prevent pounding).
It is an engineering term as well, and is used in the 2010 California Building Code to amend ASCE 7-05, chapter 17.
However, it will not be used in plain vanilla or cookbookish building code analysis.

 

[river55]

I guess my question on the rocking is if there is a way to determine if it isn't going to occur. If you set an R = 1.0 for instance (which is like the force from ASCE 41), are those the 'real' seismic forces that the frame/shearwall will be exposed to? Obviously, if you did a dynamic analysis, those forces may get marginally smaller. But can you simply check the overturning at this R=1 case, and have a reasonably good estimate that rocking will or will not occur?

Or is there anything else that will save you from having rocking occur (besides upsizing your footings). For instance, if you have a wood diaphragm, it would seem somewhat difficult for a braced frame with large footings to rock when the load is being transmitted to it by a bunch of nails in 1/2" plywood. If the diaphragm yields, is that one thing that might prevent rocking from occuring?

 

[csd72]

Are you sure it was rocking and not racking that they meant?

 

[wannabeSE]

River55, I am somewhat confused by your explainations and do not understand what you are trying to accomplish. However, ASCE 7-05 chapter 19 has an Equivelent Lateral Force proceedure that that takes soil stiffnes into consideration when calcualting base shear. The first paragraph of the commentary describes the outcome:
"The use of these provisions will decrease the design values of the base shear, lateral forces, and overturning moments, but may increase the computed values of the lateral displacements and the secondary forces associated with the P-delta effects"

 

[a2mfk]

I stand corrected on usage of the term "rocking", I learned something today.

Judging from what ATSE and you have said, this is rocking of the actual foundation, ie, a slight overturning or rotational movement of a foundation? I would think this would be more of a phenomenon in very stiff shear walls with relatively short foundations, like stairwells and similar structures. But I will stay tuned to see where this discussion goes.

This rocking action would be in addition to story drift of your braced frame or shear wall...

 

[msquared48]

If you really want to know if your foundation will rock, just turn up the volume to your subwoofers.

Mike McCann
MMC Engineering

 

[a2mfk]

Come on Mike, you know that will just cause vibrations not rocking