Two Stage Seismic Analysis?

[SteelPE]

I am in the process of designing a steel/wood framed building. I have designed wood framed buildings and steel framed buildings but never the two of them combined together. The structure is 2 stories of wood over a single story steel frame with approximate dimensions of 70’ square (not large). The building is in a somewhat high wind zone (Vult = 130mph).

I am currently running the lateral loads analysis on the building. I have never done a 2 stage analysis before but from what I can tell, it seems like a chicken or the egg scenario (in order to get a proper period you need to know deflections of the building so you can run a more accurate period analysis…. But you need lateral loads to run this analysis).

Regardless, I have run an analysis utilizing R=3 for the entire structure. Based upon this analysis I am seeing less demand in the structure due to seismic than I am for wind (wind demand is 47 kips for the top two stories vs 30 kips for seismic in the top two stories using R=3 for the entire structure).

So the question is, in this instance, is there any benefit of using a 2 stage analysis?

 

[KootK]

So the question is, in this instance, is there any benefit of using a 2 stage analysis?

As you know, many light framed wood buildings are of the residential or hospitality variety where you've basically got a bunch of units feeding into a long-ish corridor. These tend to make for buildings that are long in one direction and short in the other. Where this is the case it is quite a common occurrence for seismic to govern in the long direction while wind governs in the short. This, particularly where floors are topped with gypsum or concrete which can add a fair bit to the seismic weight. Obviously, where seismic governs, the two stage procedure offers the potential to reduce seismic demand in the upper structure.

Another thing that the two stage procedure tends to do lower the seismic load in the upper structure at the same time as it may increase it in the lower. This can benefit the design of the wood structure even if it doesn't improve overall base shear at the bottom of podium which is often better suited to resisting lateral load.

All that said, if these things are not true of your building then it is entirely plausible that your structure would not benefit from the use of the two stage procedure.