When either the Rigid, Semirigid, or Pseudo-Flexible diaphragm option is selected, lateral forces are applied as story/diaphragm forces. You may specify the story/diaphragm force values and coordinates. Story/diaphragm forces can also be generated using the automatic load generators. Story/diaphragm forces are input using the Loads > Load Cases command.

The seismic provisions of the IBC 2000 are based on the 1997 NEHRP provisions. There are two main requirements that have been implemented: the response spectra curve for dynamic analysis and the story forces based on the equivalent lateral force procedure

The applicable requirements of BOCA 93 and SBC 94 are identical to those of ASCE 7 - 93, so the same forces will be generated regardless of the selection.

For ASCE 7, BOCA, and SBC, the story\diaphragm forces can be generated using the provisions for Member forces or for Drift. For Drift, the actual building period is used rather than the limited value T ≤ C_aT_a. The resulting reported drifts are not factored by C_d.

For UBC, the story/diaphragm forces can be generated using the provisions for Member forces, Drift, or Foundation forces. For Drift, the actual building period is used rather than the limited value of T_B. For Foundation forces, F_t = 0.0.

The forces will be applied at the eccentricity if/as specified by the user. The default value of eccentricity is 5% of the building dimension as defined by the minimum and maximum dimensions of the slab edge polygon. This dimension can be over-ridden using the Loads > Masses command.

No story/diaphragm force will be applied to a level whose mass has been "Combined" to another level or whose mass is zero.

Note that for a structure consisting of two towers coming up off of a common base structure, the fundamental period used to calculate story forces will be the fundamental period of the structure overall (which will usually be the period of one of the towers or the other); the program does not calculate and use the periods of each tower independently. The program will use the same fundamental period for both towers when calculating seismic story forces. Thus if the period of one tower is substantially different than the other, the generated seismic story forces for one of the towers may be incorrect, in which case it will be necessary to create a user-specified story force load case rather than using the generated story forces.

Models with Semirigid diaphragms, calculated lateral seismic forces for the diaphragms are distributed over diaphragm nodes in such a way that these distributed point loads are in proportion with diaphragm mass distribution. In other words, calculated diaphragm forces are converted into nodal loads and applied at diaphragm nodes based on mass distribution over the diaphragms.