I have a question about what is almost the first step in the response spectrum analysis procedure. I live in Canada, so everything I do is based on the National Building Code of Canada (although I would think an RSA would be similar everywhere in the world). In the building code commentary, it outlines a 9 step procedure for calculating the scale factor for use in an RSA. I am unclear about a step at the very beginning. One of the first things it tells you to do is to "restrain all displacements other than those in the direction of earthquake forces, then carry out a linear dynamic analysis to determine the fundamental lateral period, Ta, and the base shear, Ve". What I am unclear about is this: is the base shear, Ve, that they are asking for at this point suppose to be based off of only the fundamental mode which we got our fundamental period, Ta, from? Or are we suppose to base the Ve off of all of the possible modes using a "square root of the sum of squares" type of method? I'm not sure what is the right way to do it. Obviously if we are suppose to base it off of all the modes together, the Ve will generally end up being higher. Any insight would be appreciated. Thanks.
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