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Two Stage Seismic Approach - Building Period

Two Stage Seismic Approach - Building Period

Two Stage Seismic Approach - Building Period

(OP)
When using a two stage analysis procedure for podium buildings, what do you use to calculate the period of the lower level.

It seems if you used the two stage approach and designed the lower level using the period based only on the number of lower floors (using the approximate period equation) you would get excessively high shears at the lower levels. This is because your period would decrease which increases your Cs. Then you are taking the reactions of the upper floors and scaling them based on R/p. This would give you much higher shears than if you considered the entire building as one unit and took the worst case R/p.

Any thoughts here?

Thanks!

EIT
www.HowToEngineer.com

RE: Two Stage Seismic Approach - Building Period

Quote (OP)

When using a two stage analysis procedure for podium buildings, what do you use to calculate the period of the lower level.

Something fancier than than the simplified equations based on building height. Software, Rayleigh method...

Quote (OP)

you would get excessively high shears at the lower levels.

Higher, lower level diaphragm contributions to storey shear. Not necessarily super high storey shears as much of that load is aggregated over the whole height of the building including those more flexible levels. And it's right that your diaphragm contributions below grade will be high. Usually the below podium structures are pretty damn rigid. Often you'll be on the constant acceleration plateau and relatively insensitive to precise period determination.

But yeah, R/p hurts.

I don't have access to ASCE7 this week but is your choice not between two stage and RSA? I'd expect both methods to be about equally punishing which, of course, is the very point of the thing.

Quote (OP)

Any thoughts here?

In high level terms, I'm a two stage method fan boy. I think that's it's a rational method well suited to the expedidient design of modest podium structures.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Two Stage Seismic Approach - Building Period

I have never had to deal with your particular issue, however the Woodworks 5 over 1 Design Example pdf has been my go to example for podium structures.

RE: Two Stage Seismic Approach - Building Period

My experience has also been with the kind of structure that Wallache describes. I'm surprised that this would even be an option here. I wouldn't expect a concrete moment frame to be 10x stiffer than strap braced CFM shear walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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