Defining Load Cases

Lateral loads are defined and assigned as story\diaphragm forces. These forces, or the data used to generate these forces, and their load cases are created using the Loads - Load Cases command.

There are five types of story\diaphragm force load cases:

Wind
Seismic
Dynamic
Notional
Other

Wind, Seismic, and Notional story forces may be user-specified or they may be generated based on the requirements of a variety of building codes. "Other" story forces are user-specified only. Dynamic load cases are not story forces in the same sense as the other types; they include a response spectra analysis and an Eigen solution. Note that if there is more than one diaphragm at a story, generated seismic forces at the story are distributed among the diaphragms based on their relative diaphragm masses and generated Notional loads at the story are distributed among the diaphragms based on their relative gravity loads. Similarly for wind load cases, diaphragm extents are considered in calculating wind loads at each diaphragm.

The load cases are created by specifying a load case label and a load case type, then selecting the particular load option (e.g., ASCE7-98/IBC 2000). In the case of the generated loads, certain selections result in multiple related load cases. For example, selecting both X and Y Axis directions and plus and minus eccentricity when inputting the load case data causes four load cases to be created. Each has the same load case label but the type label will vary to identify each one individually.

User defined story forces (also virtual load case story forces) are defined per rigid diaphragms. Thus, the user may enter forces for each diaphragm.

The mass data must be defined before the automatic seismic load generator can be invoked. The exposure data must be defined before the automatic wind load generator can be invoked. The gravity loads for Notional loads data must be defined before the automatic Notional load generator can be invoked.

There are two additional Load Case types:

Virtual
Centers of Rigidity

Virtual Load cases are for use with the Drift Control module. The Centers of Rigidity load case can be invoked if explicit calculation and output on the center of rigidity of each floor is desired. Note that center of rigidities are calculated for each diaphragm.