Foundation Seismic Weight in Base Shear for Sliding? 1

[RDR89]

Hi friends, I have searched this forum and found some good information but nothing definitive from what I've seen, so I'm really asking for opinions and /or best practice at this point. I have an equipment foundation bearing 1'-0" below grade (t/slab = grade). I am wondering if I need to include the weight of the foundation in the calculation of horizontal seismic force for the sliding check. I've seen an engineer at my company include it, but he references ASCE 7-10 15.4-5. I don't think foundations were the intent of Chapter 15. Additionally, per ASCE 7-10, the definition of W per 12.7.2 is everything above the base, which leads me to believe that the foundation weight does not need to be included in the calculation of base shear for the system. If we do include it, I would need to include some sort of soil anchor as sliding will be an issue. Would love to know your thoughts!

 

[RDR89]

Follow up question: I'm using the LRFD combos in ASCE 7-22... so long as the 1.0E/(0.9*friction coefficient*D) < 1.0, sliding should not be an issue. I don't need to meet an additional safety factor since I believe the safety factors are already inherent in the load combination. Is this understanding correct?

 

[phamENG]

That is my understanding as well, yes. The 'old fashioned' safety factors that we think about for retaining walls and the like are based on full service level loading, which doesn't apply to modern LRFD and ASD load combinations.

 

[Jinal Doshi]

Understand the load path:

1. Equipment transfers the sliding shear to foundation.
2. Foundation resists sliding at the soil structural interface.

- If the equipment is anchored to foundation. The equipment anchorage transfers the shear to foundation and anchorage is designed to resist the equipment shear. The foundation carries this shear and resists the sliding at the soil structural interface via shear friction. In this scenario the weight of the equipment + Foundation is to be used for sliding shear resistance but the foundation's mass need not be used in the sliding shear demand calculations.

- If the equipment is not anchored to foundation, then only equipment's weight shall be used in sliding shear check.

 

[1503-44]

Jinal, what exactly are you calling "sliding shear demand"? What is that?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Jinal Doshi]

Sliding shear demand is the effective seismic force of the component per ASCE 7. So in short it is Cs*W where W is the weight of the equipment.

 

[1503-44]

Thanks.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."