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Generating seismic loads methods

Generating seismic loads methods

Generating seismic loads methods

Can somebody help me understand the difference betweent the Equivalent lateral force procedure / Response history procedur / modal response spectrum procedure to generate the seismic loads and when it is recommended to use each method ?

RE: Generating seismic loads methods

My answer is based on definitions given in Eurocode 8 (EN1998-1:2004)"Design of structures for earthquake resistance". You may check it for your reference.
Generally, analysis can be either Linear or Nonlinear. For the linear analysis, there are two methods: Equivalent Lateral Force and Response Spectrum Analysis. For the nonlinear part, it's either Pushover Analysis (PA), or Time History Analysis (THA). Let's look a bit deeper into the ones' in your question.
*The equivalent lateral procedure (also named seismic coefficient method) from its name, we basically use the static load equivalent to dynamic one from an earthquake. This method accounts for the total base shear for the building under seismic loading determined by using an empirically calculated Time period, and then distributed over the stories as lateral load proportional to an assumed 'mode shape'. Thus the EC8 limits a time period condition in order to analyze with this method. According to clause of EC8, I quote:
- "This type of analysis may be applied to buildings whose response is not significantly affected by contributions from modes of vibration higher than the fundamental mode in each principal direction."

This requirement is deemed to be achieved if the following two conditions are met:
1-the fundamental periods of vibration in the two main directions which are smaller than 2.0 sec or 4*Tc (Tc is the upper limit spectral parameter according to ground type, Table 3.2)
2-Elevation regularity criteria ( are met.
Otherwise, Response Spectrum Analysis (RSA) can be used.

*Response Spectrum Analysis (RSA): A response spectrum is simply a plot of the peak acceleration of each of a series of hypothetical oscillators of varying natural frequency. that are forced into vibration at its base by the same seismic ground motion record. The RSA calculates maximum response values in each mode of a structure from the spectrum curve and then combine these responses using modal superposition. This is cumbersome using hand calculations, thus softwares are often used.

For the second type you mention, did you mean Time History procedure? Coz I've not heard of method such as "response history". It's either Time History Analysis (THA), or Response Spectrum Analysis (RSA).

If you do not see it, you cannot design it!

RE: Generating seismic loads methods

Here is a basic rundown of the seismic methods you asked about. Please understand I am an east coast US engineer so the Response History Procedure is not something I do every day and it is something I have limited exposure to in Graduate School. I would love to learn more about it and do more with it, but none of my projects have the scope, schedule, or fee that would allow me to use the procedure.
  1. Response History Procedure: This is the most complex analysis method. You would need to gather a suite of recorded or generated ground motions (depending on the governing code), Then you have to scale them to meet the criteria given in the code. You then apply the resulting ground motion to the building and analyze the structure with either a modal analysis or with direct integration. You can have different flavors of analysis to deal with issues like geometric nonlinearity, and material nonlinearity. Including more realistic behavior is generally more time-intensive and costly. I there is an article available here that may help.
  2. Modal Response Spectrum Analysis: This analysis method uses the code specified seismic spectrum and the structure's fundamental periods to generate a response and relative contribution for each eigenvector of the structure. You have to combine the contributions via some method, generally SRSS or CQC. The combination causes the loss of sign on the value of interest. Up until the most recent code you were required to scale the MRSA base shear up to 85% of that predicted in the ELFA. The new code now mandates that you scale to 100%. Some people have commented that this may make the method die, but I still like using it to get a more realistic vertical force distribution. I use this for taller buildings that have bigger bottom floors (think podiums) and for buildings where the mass distribution isn't very uniform. This method keeps from "throwing" a lot of seismic force up to the top levels
  3. Equivalent Lateral Force Analysis: This is your most vanilla analysis method. You will need to do this first no matter what. It was once described to me as a modal response analysis where the first mode of the structure is the only one that contributes. There are limitations on the irregularities the structure can have in order for the analysis to be valid. The irregularities that preclude the ELFA are irregularities where higher mode effects could contribute. For most small buildings this is the way to go. You will have to perform the analysis for the other methods and if the impact is minimal then there is no reason to go to a more complex analysis. For the method, you will calculate a base shear and then use a code provided exponential equation to distribute the base shear to the diaphragms/levels

Robert Hale, PE

RE: Generating seismic loads methods

Dear Mr. Robert Hale and M.Honda, Thank you very much, really appreciate your help. That was very helpful and informative.

RE: Generating seismic loads methods


I suppose the following article too might help you in getting some basic idea of Equivalent Lateral Force Analysis, Modal Response Spectrum Analysis, Time History Analysis; and each’s corresponding Non-linear counterpart:


Rahul Leslie

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