ASD factor on seismic

[JT-1995]

I am using ASD to design a support structure for heavy equipment. The equipment and support structure will be independent of other framing in the building. I have calculated the dynamic machine loads, machine gravity loads, and the machine's seismic load and translated them to the support feet of the machine. I was starting to move to the step of the support framing design and I have started to go down a rabbit hole with the load combinations from ASCE.

For this isolated case (no wind, Lr, Rain, etc) the loading reduces to DL (of framing), LL (including the dynamic loads of the machine), and EQ. Using the ASCE load combinations would end up applying factors to the L and E loads. Certainly I would not apply a factor to the L of the equipment (it is what it is...always). But I started to consider the 0.7 in front of the calculated EQ load. Nowhere in the ASCE ASD load combinations is E factored as 1.0, which made me think its derivation using the Ss/S1 (and so on) is all developed planning on factoring the resulting load by a factor in the load combinations.

My structure will have a full Live Load present when an earthquake happens. My question is, are the E forces calculated using ASCE actual inertial forces that are expected from an oscillating mass or are they not actual until they are factored down for design load cases?

Or, are the load case factors a function of applying reducing factors to certain load types to account for probabilities that the full loads will not happen a the same time? This is what I have always thought, so in my case I can be pretty sure that the full load will exist when a full calculated earthquake happens.

Any thoughts or sections of the code that confirms correct assumptions will be greatly appreciated.

 

[SJBombero]

'E' as derived using Ch. 11, 12, 13, 15is calibrated to LRFD load combinations which is why those combinations do not have a factor in front of the 'E'.

 

[JT-1995]

Yes, thank you.
It dawned on me a bit ago that there is an ASD combination D+0.7E. So the 0.7 is not reducing the possibility that it will occur with the other load, since all of the DL will always be there. So the 0.7 is translating E from LFRD to ASD. Does that seem correct thought?

 

[SJBombero]

Correct. But the LC your are using is oversimplified. You need to check 2.4.5 for the full load combination for seismic effects. You are only allowed to use 0.6 DL to counteract seismic and you have to subtract vertical effects from that. So in significant seismic zones, you only really get to use 0.4 DL.

 

[JT-1995]

Thank you