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Load Generation Process
Building masses for walls, floors, ceilings and roofs are either input directly or generated by the program using self-weights that you specify. The masses are accumulated into a total mass on each story and on the structure as a whole. The masses are then converted to seismic loads based upon their proportion of the building mass and the design code provisions relating building mass to base shear and story shear.
The ASCE 7 Equivalent Lateral Force Procedure from 12.8 is used. Provisions are also taken from Chapter 11 and the rest of Chapter 12.
Generation of Seismic Loads
Using the design code provisions, the total mass is converted to a base shear and then distributed to each level as a story shear. Each story shear is then distributed to the building elements according to their relative contribution to the total mass at that level. Floor, ceiling and roof elements will generate line loads with the same profile as the building mass. Walls parallel to the direction of the applied ground acceleration will generate point loads and perpendicular walls will generate uniform line loads.
Wall Mass Distribution
Before the load generation takes place, the building masses created for the walls are apportioned to the appropriate levels. The mass of a wall is distributed evenly (half up and half down) to the diaphragms it connects. For this reason, the seismic loads generated for a particular level do not correspond exactly to the building masses for that level. Also, the bottom halves of the walls on the first level are not considered among the building masses used to generate total mass or individual loads.
Level-by-Level Generation
It is possible to specify different self-weights for the elements on different building levels, and generate the masses level-by-level, so that the resulting loads are different for the same sized elements on different levels.
Note that the seismic loads that are generated for a particular level after the entire structure is complete will be different than those at an intermediate stage, because of the difference in total building mass. The seismic loads generated at the intermediate stages should not be used for design, that is, wait until loads and masses are generated for all levels before proceeding to the design stage.
The program automatically deals with the fact that diaphragm loads on a particular level come from masses on the level above and below. However, to be certain that the seismic loads generated correspond to the final set of building masses, when you are finished generating these masses, you can select all levels and generate the seismic loads with Generate Loads Only selected.
Generate Seismic Loads Only
Via a checkbox in Load Genertion view, it is possible to generate seismic loads without generating building masses. The reasons for doing so are:
Story Elevation
The story elevation h is used in the equations to distribute base shear to the stories. It is the distance from the top of the foundation to the floor surface or top of the ceiling. The Structure Input Form permits you to specify the foundation elevation, diaphragm depths and wall heights so that the proper elevation of each story can be determined. Note that the foundation elevation is significant only for wind load generation.