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Seismic loads are generated based upon the distribution of the dead load building mass on each story of the structure. The masses are represented as line, area, or point loads similar to the dead loads that Shearwalls uses for overturning resistance, however the dead loads input for that purpose do not contribute to building mass for seismic load generation, separate building masses are generated by the program for that purpose using self-weights of building elements that you specify. You can also enter your own masses, modify generated masses, or enter a negative mass to model an opening in the diaphragm.
Building masses are generated by for walls, floors, ceilings and flat roofs, and sloped roofs or the projected area in the horizontal plane of roofs. The program automatically divides irregularly shaped floor areas into rectangular elements for the purpose of generating loads.