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A509.4.3 Lateral Load Patterns
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Two different vertical distributions of loads shall be used. The first load pattern, termed as the uniform pattern, shall be based on lateral forces proportional to the mass at each story level. The second pattern, called the modal pattern, shall be selected from one of the following:
- A lateral load pattern represented by Cvx, if more than 75 percent of mass participates in the fundamental mode in the direction under consideration. Cvx is given by the following expression:
wi = Portion of the total building weight, W, located on or assigned to floor level i. hi = Height in feet from base to floor level i. wx = Portion of the total building weight, W, located on or assigned to floor level x. hx = Height in feet from base to floor level x. k = 1.0 for Te ≤ 0.5 sec. = 2.0 for Te ≥ 2.5 sec.
Linear interpolation shall be used to estimate k for intermediate values of Te.
- A lateral load pattern proportional to the story inertia forces consistent with the story shear distribution computed by combination of modal responses using response spectrum analysis of the building, including a sufficient number of modes to capture 90 percent of the total seismic mass and the appropriate ground motion spectrum.