ELF vs Modal Response Analysis

[Bh.Burhan]

Hello,

In determining the seismic forces and design shears for a building, which method will give us a higher base shear and seismic forces (ELF or Modal Response Analysis)?

 

[Agent666]

Totally depends on your structure. Too open ended to give a definitive answer.

But what I will say based on experience is that highly irregular structures tend to result in a lower base shear under Modal analysis. This is why most codes require the results to be scaled up to the equivalent static loads as a baseline lower limit.

I've seen cases where ELF is higher, and Modal/response spectrum is higher. It really depends on the structure, and its response.

Rather than designing for whichever case gives the highest base shear if that was where you are heading, you are better following your code advice on what is the correct procedure to use for your given situation.

 

[JoshPlumSE]

One thing to point out is that the Response Spectra results will often be scaled up to equal the ELF base shears. Right?

In which case, it's a tie. So, what's the point? It's the distribution of dynamic forces that should be captured more accurately. Think of the ELF method as being similar to what the RSA method would give.... if there were a single dominant mode similar to a cantilever column. It's when you have mass or stiffness irregularities and such that RSA becomes more accurate.

 

[KootK]

In ASCE7 you scale up to 85% ELF so you get little bump if things go your way. I agree in principle, though, that the choice to go RSA / NLA is usually driven more by having identified the need to capture complex dynamic behavior than by a desire to save on construction costs.

 

[EulersBucklers]

ASCE 7-16 revised the scaling up of RSA base shear to 100% of ELF so JoshPlumSE hits the nail on the head.

 

[KootK]

Thanks for the correction EB.