Question regarding seismic component requirements. 1

[3doorsdwn]

Equation 13.3-1 in ASCE7-05 (p.144) gives the horizontal seismic load for a component. But something I have never been quite clear on is just where this is applicable (Or: how far does it go?). Chapter 13 says that this applies for nonstructural components “for their supports and attachments”. So does this apply to the supporting steel? Just the primary support members? Or should you take the overturning force (et al) all the way to the girders?

And if you do take this force all the way to the girders, aren't you kind of "double dipping" [that is if you (of course) included the dead load of the equipment in your seismic dead load for the equivalent lateral force procedure for the whole building]?

 

[JAE]

You use the component force from that equation and ensure that the component can be successfully supported both vertically and laterally under the Fp load and the vertical force indicated in the paragraphs toward the end of 13.3.1.

This includes the full load path into the main body of the building frame. Once in the main building frame, you don't have to necessarily check every beam, column, brace, etc. through to the ground.

 

[mrpid]

I am not an authority on the subject as until recently I have been designing structures in FL which seismic is not an issue. But my interpretation there will be two cases to consider: 1) the lateral resisting system of the structure shall be designed to include all effective seimic weight which will include these components and 2) Each element of the struture i.e a beam supporting a roof top unit or a spandrel beam supporting a precast panel shall be designed to resist the controlling load combo which may include the seismic force indicated in Chapter 13. In my opion these loads would need to be included in the beams, girders, and columns otherwise you may have a localized failure were an earthquake to occur and be the controlling factor. There is my 2 cents.

 

[3doorsdwn]

Thanks JAE. What are your thoughts on the “double dip” scenario? Assuming you have a model, I would think if you included this force; it would then not be necessary to include the component force AND have the equipment weight still in there for the seismic dead load (for the whole building; i.e. contributing to the base shear).

 

[JAE]

No double dipping in my view.

The component design is strictly for the attachment of the component as an individual load combination. This "set" of calculations uses the Fp and verifies that the component is adequately attached to the structure (i.e. the beams and girders that support it).

Once this has been accomplished, it is essentially put aside and not used in any fashion with the overall lateral force resisting system design for seismic. The calculation of base shear and the application of the main frame seismic forces WILL include the component weight in the mass of the whole building, but not the Fp as that is a separate design effort.