Equipment Seismic Load vs Base shear 3

[Lily_Linton]

Hello fellow engineers,

I have an amateur question regarding base shear.

The building is an industrial structure with a lot of equipment on the side of the level 2. My question is do I have to add the seismic load computed from ASCE 7-10 Chapter 13 to the calculated over all base shear of the building in my STAAD model?

Because base shear calculation also includes the weight of the equipment and then the equipment seismic calculation also based from its weight. I'm afraid we are having a double dip on it if we provide these both on my STAAD model

Is it the Fp from chapter 13 for local checking only?

Thanks to all who will respond this question

 

[JoshPlumSE]

Yes an no.

For the "main lateral force resisting system" you do NOT need to include your chapter 13 loads. But, for the elements that directly support the equipment, you would need to analyze them for the Chapter 13 loads.

Generally speaking I design the entire load path from the equipment for the chapter 13 loads....until I get to the main moment frames.

 

[Lily_Linton]

Hi Josh

Thanks for replying
Do you mean we need to load the Fp on STAAD model on the equipment location along with the Base shear automatic loading?
thanks

 

[WARose]

Do you mean we need to load the Fp on STAAD model on the equipment location along with the Base shear automatic loading?

I use to do a lot of process buildings so I struggled with this one. At the end of the day what I did was I had 2 models: one checking the main Lateral Force Resisting System (LFRS)....and another with the loads coming out of Chapter 13/15. For the latter, it was just a matter of getting it to the columns without overstressing the framing in that bay.

Of course, that is a building where the equipment weight is more than the framing itself. For situations where the equipment load is nothing, I'll have the Chapter 13/15 seismic loads in with the rest of the seismic.....yeah, it's double dipping....but not by much.

 

[Lily_Linton]

Ok I get your point

Thank you very much

 

[Lily_Linton]

O I saw statement in section 15.3.1 about the requirements if its under 25% of combined weight that supporting structure shall be designed in accordance with chapter 12.

 

[JoshPlumSE]

Yeah, pretty much agree with WARose.

I'll keep a different model (or just different load cases) for situations where the equipment loads are large. If they're small compared to the story forces, I just double dip for simplicity.

 

[Agent666]

Keep in mind as well that often the maximum equipment seismic loads are not necessarily going to be in phase with the point in time where maximum loads in the lateral force resisting system occur. So while yes in a real sense some of it is additive to the buildings base shear at a certain point in time for a real building responding to earthquake shaking, its not fully additive in the sense that it dominates the dynamics of the base building.

Think of it similar to doing a modal analysis, you don't directly add the effects of all modes, its more of an average when you consider many modes and have combined them. The effects only become directly additive if the period is the same for most modal combination techniques. If the periods are even slightly different the additive effect drops of very rapidly. In most practical structures there is probably quite a low risk of the equipment period of vibration matching the building exactly.

The 25% limit is intended to capture cases where the mass of the item and its response should be considered for the overall base shear coefficient as the response associated with the equipment may dominate/impact on the buildings response.

The larger forces derived for equipment are in part a function of the equipment getting 'whipped' around and responding in its own way to the accelerations and higher mode effects imparted on it by the level at which it is attached.

There is always the question, how far do you chase these larger loads, like others have noted I simply take it back to something that can obviously take the load (this might simply be 'by inspection' or it might require some calculations to justify on a case by case basis). Sometimes this might just mean taking the load as far as the boils holding it onto the structure, or simply checking the beams supporting it for the resultant vertical seismic loads in dealing with any overturning moment.