Structural Separation as menthioned in ASCE 7-16-CLAUSE 12.12.3 1

[Kirby2]

I did an Analysis for 4 multi-story building with a dynamic analysis -elastic type. This building is separated into 3 parts with a seismic Joint and I want to calculate the seismic joint width between each part. As per my understanding of this clause in ASCE, I have to consider inelastic displacement from each part for the max displacement and consider the Square root of summation of both displacements.

My question is if Both buildings will deflect in the same direction, so what the meaning of Summsion both values?

 

[JoshPlumSE]

They may NOT be deflecting in the same direction. That's the point of it. If the two building have very different periods, then they could be moving in different directions. The SRSS is a statistical method of combining two unrelated terms.

This is the same reason behind using the SRSS method to account for different modes in a response spectra analysis. Or, for combining the effects of an earthquake force applied separately in to orthogonal directions.

It provides similar to the 100% + 30% method as well.

Now, if your basic premise is that this SRSS could potentially be an over conservative method based on modal interaction.... I'd have to think about it more.

In response spectra, we use CQC for interaction instead of SRSS because it accounts for modal interaction effects of closely spaced modes. But, it results in a GREATER response than the SRSS method. So, what you'd be talking about is the reverse.... Like a subtraction of the interaction term instead of an addition. That seems to make some sense to me.

In reality I think the SRSS method is conservative and using ONLY the maximum displacement of a single structure is unconservative. So, this is an interesting concept.