Can we Drift limit and stability index only by RSA? 1

[Kovan A.]

Hi

Is it enough to check drift limit, stability index only by RSA or we need to check those for Equivalent lateral force too, when both method permitted in table 12.6-1 for analyzing the building?
what about when only RSA method permitted for analyzing the building (ELF not permiited) do we need only check those by RSA is it acceptable according to ASCE 7?

 

[JoshPlumSE]

Kovan -

If both methods are permitted then you can do the checks using whichever method you like. You can do some checks with one method (if it's easier) and other checks with another method.

If RSA is required, you can still do the ELF method.... in ADDITION to using RSA. In fact, I would very much encourage this. Why? There are times when running a response spectra analysis where it becomes more difficult to have a good physical understanding of how your structure is behaving.

 

[Kovan A.]

JpshPulm

Thanks
As you know some buildings doesn't permitted to be analyzing by Equivalent lateral force method, that is why the only two methods for linear analyzing are Modal Response Spectrum analysis and Linear Response History Analysis , I prefer to check drift limit,stability index and other building limit with MRSA which the easier one.

 

[JoshPlumSE]

Kovan -

There is a slight misunderstanding. It's not that the ELF procedure is not permitted. It's that one of the other procedures is REQUIRED. The RSA procedure is generally required when there is some kind of mass or stiffness irregularity. This type irregularity means that the ELF procedure (which is based on a single mode dominating the dynamic behavior) may be unconservative.... Hence you are required to use one of the dynamic analysis procedures. But, that doesn't prevent you from supplementing the RSA analysis results with a set of ELF results as well.

The dynamic analysis procedures can be a bit complex and confusing. I will always suggest using BOTH ELF and the dynamic analysis procedure. Why? Because if you start with ELF and get most of your design done, then you will better understand the load path of your structure. It's just easier to track loads in a static procedure. This includes using accidental eccentricities and such.

Now, your final design must be done using RSA. Which is fine. But, you want to check to see where the two methods differ substantially. Maybe you have to increase the size of some members or the stiffness of other members slightly. But, if you're getting hugely different results then you probably want to understand why. Maybe you did something incorrect in the RSA analysis to produce the dramatic difference of results.

 

[Kovan A.]

Thanks Josh

Agree with you, i always do both , which usually i need ELF for scaling my RSA and finding size of my structural elements. however for final limit check and design my building not permitted for ELF method according to the table below. So i i think checking limit only with RSA should be enough.

 

[JoshPlumSE]

My point was only that it doesn't hurt anything to do ELF as well. Even if it's only for your own benefit. Which is often true.

At times, I have included the ELF results in my final submital to the plan check department. So, that they understand the results. They may no understand the RSA results very well. So, if I include both (and demonstrate the building satisfies both), it can be easier to get through plan check.