Sliding of Foundation 1

[Surya77]

I am checking foundation for sliding. I have the base shear form the structure above.

Now my question do i have to consider lateral force due to foundation weight itself to add to the demand sliding force.

If that is case, I have hard time qualifying foundation for the sliding. Any thoughts

 

[JStephen]

Did run across this quote:
"As part of normal practice, all of the foundation weight and half of the first-story wall weight are commonly omitted in the analysis of seismic diaphragm loads (See Sec. 6.34). However, such a provision is not explicitly defined in the IBC for calculating base shear, nor is such a provision mentioned in the Blue Book commentary." Footnote, Page 6-24, "Seismic Design of Building Structures", 10th Edition, by Lindeburg and McMullin, parts of which are on Google.

 

[Surya77]

JStephen

Thank you for ur input. I have that lindberg book, but I wanted similar statement from IBC or CBC (both are silent).

Also that may be ok for a multi story building as the foundation weight is relatively small. If I am designing a foundation for equipment (large), then i cannot agree with that ideology.

I dont this will have any impact on soil-struction interaction as this foundation will be like a needle in haystack.

this looks like a interesting topic, whom ever i asked in person, first reaction is "hmmmmm"

 

[mike20793]

I think considering self weight of the foundation is too conservative if you do not include soil-structure interaction. If you do consider these effects, then I think it is reasonable to use the self weight. Also, what type of analysis was used? I would say this flies in the face of the Simplified and Equivalent Lateral Force Procedures without soil-structure interaction and I would not consider it for these. Those procedures are inherently conservative with their base shears (assumes fixed base, how can this assumption be violated?). Now if you were using a Response Spectra analysis? Yea, I would include it, but I would also include soil-structure interaction. Either way, considering the soil-structure interaction is going to reduce the base shear.

 

[Surya77]

@mike20793

I am not violating the Fixed base assumption. that fixed base assumption is between superstructure and foundation, not between foundation and soil.

The only reason, to neglect the wt of foundation in multi-story bldg is, its a negligible wt compared to total super structure wt, not any other complex phenomenon.

 

[mike20793]

How is a support that translates in a seismic event fixed? If your saying your foundation will have an inertial force then this is a sliding bearing type condition mathcadboy mentioned above and it is not truly "fixed" but rather acts like a spring hence the reduction in base shear. Yes, including it violates the fixed base assumption without including soil-structure interaction, which reduces base shear. Including soil-structure interaction would not violate it because the Equivalent Lateral Procedure in ASCE 7 Chapter 19 takes the effects of soil springs into account.

 

[mike20793]

Surya,

Sorry, I see your point now. I was misunderstanding what you were trying to say until I created a paper model and it "clicked."