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ANSWER: To retrieve membrane forces which occur in-plane to semi-rigid diaphragms when user-defined seismic loads are applied to a structure, loads should loads may be directly applied either to floors as distributed loads, or at discrete points according to seismic mass distribution. ETABS (v9 and v13) automatically distribute the auto wind forces by redistributing wind load to 10 points along the Wind direction for a semi-rigid diaphragm model. For auto seismic load cases, seismic loads are spread over to every point which has mass so stress concentration can be avoided in a semi-rigid diaphragm model.
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