Seismic Load Case

If a diaphragm defined as semirigid, mass associated with the diaphragm (including mass from surface, line and point loads, which are defined in the Modeler, and member self mass including decks) is distributed to diaphragm nodes as explained in the previous sections. The Center of Mass command on the toolbar menu shows lumped mass for rigid diaphragms as well as nodal mass assigned to nodes on semirigid diaphragms. Also, the "Criteria, Mass & Exposure" report shows nodal mass quantities for semirigid diaphragms.

Based on chosen building code, the program calculates building base shear and then it distributes it over the height of the building as a lateral load to be applied to each diaphragm. Finally, the loads are distributed to nodes on the semirigid diaphragm, which are in proportion with nodal masses at each node. If t here exist any eccentricity, these loads are modified as explained in Eccentric Loading . The "Loads and Applied Forces" report tabulates loads applied to each node for semirigid diaphragms.

Response Spectra Load Case

Analysis is conducted based on distributed nodal masses over semirigid diaphragms. Thus, analysis results reflect the effect of spatial distribution of mass related to semirigid diaphragms. If there exist any eccentricity, nodal masses are modified as explained in Eccentric Loading .

Wind Load Case

The wind loads on semirigid diaphragms are automatically computed for both the windward and leeward side. The wind loads are applied on the diaphragm edges as line loads. The edges on a diaphragm are internally marked as the windward edge or the leeward edge depending upon the direction of the wind i.e. +X, -Y etc. The computations of windward and leeward forces are based on the respective pressures and the normal areas on which the loads are applied. When computing the line load on a diaphragm edge it is assumed that the edge is covered both above and below the diaphragm. The program applies forces to the diaphragm edges even if they are partially or fully covered by some other edges (e.g., see orange edge in the following figure).

Some building codes specify a certain minimum wind loading on the structures which acts as a minimum threshold. These codes, however, don't specify the windward and leeward pressures for such cases. In these situations program lumps the entire wind load on the windward edges of the diaphragm.

If there exist any eccentricity, wind line loads are modified as explained in Eccentric Loading .