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ASCE 7-05: RSA Procedure per 12.9

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Okay, I'm trying to slog my way through this, and I'm corn-fused.

12.9.4 says that I must scale the "design values of combined response," and proceeds to talk to me about calculating a base shear V according to 12.8 (which I thought I was escaping by using the RSA in the first place), and then comparing it to Vt, the base shear from the RSA. This is supposed to done in each orthogonal direction.

Now, my first question is, What IS the Vt in a given orthogonal direction? For instance, when I run the RSA for my current problem, using the first twenty modes, and then look at the summation of the reactions for the RSA in the X-direction and then in the Z-direction (the plan directions, since positive Y is "up"), I actually get forces in X AND Z (and Y, for that matter), in each case. They are close (about 3% difference) so I guess I'm doing something right.

But is the Vt ONLY the summation of reactions in the particular direction of interest, or is it, perhaps, the vector sum of the reactions in both X and Z in each case?

I hope I've made myself clear - I'm easily confused and assume others are as well.

I just cannot seem to find anyone around here, in the flesh, who knows diddly-squat about any method other than "equivalent static force."