Equivalent Static / Accidental Torsion Load

This dialog can be accessed from the “Model | Seismic | Equivalent Static / Accidental Torsion Load” menu command. This command creates a load case corresponding to either an equivalent static load or an accidental torsion load to a selected seismic code.

Definition

Load Type

The same dialog is used for both Lateral load (equivalent static load) and Accidental torsion.

Dynamic Response Analysis and Base Shear

A list of the dynamic response cases allows the particular case to be selected. The base shear calculated in this analysis will then be used.

Seismic Code Procedure

To calculate either equivalent static or accidental torsion loads requires a seismic code to be selected and a set of code related parameters (e.g. peak accelerations, importance factors, etc.). The required seismic parameters are defined in the response spectrum, so either an existing can be used or a new spectrum defined. The appropriate procedure from the code is followed. Both the fundamental period and base shear are code values by default, but can be overridden.

Where the static procedures are used the program has to calculate the weight (or mass) of the structure. In most cases the dead load should be included in the seismic weight along with a component of live load. This uses the normal load syntax e.g. 1.4L1 +0.25L2. Leave blank if none.

Details

The direction of loading will be generated for either the X or Y direction. Z is assumed to be vertical. The mass offset is required for accidental torsion calculations. This is the percentage of the building dimension used in the accidental torsion case. By default this is 5% to satisfy the codes, but can be adjusted if required. GSA determines the building dimension by looking at the nodes that comprise the storey. Both the base level and number of storeys can be controlled by the user.

Output

The output from this is a set of node loads. These are created for the load case specified. A title can be used to identify this case.