Override Diaphragm Eccentricities Form

When defining automatic seismic lateral loads as part of a load pattern definition, an option exists to specify that the load is eccentric to the center of mass of the diaphragms. An eccentricity ratio may be specified that applies to all diaphragms. In addition, the eccentricity may be overwritten individually for one or more diaphragms.

Eccentric loading is applied to diaphragm constraints only; it is applied as a torsional moment. The non-eccentric portion of the load is applied to the whole model based on mass, and does not require diaphragm constraints.

Use the following buttons on the Override Diaphragm Eccentricities form to override the eccentricity ratio for one or more diaphragms.

Add button
1. From the Diaphragm drop-down list, select the diaphragm name for which eccentricity values are to be specified.
2. Enter a value in the Eccentricity, X-Dir (Eccen Y) or Y-Dir (Eccen X) edit box
3. Click the Add button.

Note: Eccentricities are specified as lengths, not ratios. If you are defining a load in the X direction, the eccentricity is measured in the Y direction, and vice-versa.

Modify button
1. In the Diaphragm/Eccentricities display areas, select the diaphragm name for which eccentricity values are to be modified.
2. Enter a revised value in the Eccentricity, X-Dir (Eccen Y) or Y-Dir (Eccen X) edit box.
3. Click the Modify button.
Delete button
1. In the Diaphragm/Eccentricities display areas, select the definition to be deleted.
2. Click the Delete button.

Access the Override Diaphragm Eccentricities form as follows:

Diaphragm constraints must be assigned to joint objects in the model for the auto lateral load to apply eccentric loads.

Note: Eccentricity overrides apply to diaphragm constraints whose associated coordinate system is Global, and whose associated axis is Z. Thus, this type of diaphragm must be included in the model for this feature to be available.

Method 1

Click the Define menu > Load Patterns command to display the Define Load Patterns form.
Select a Quake Load Type.
Specify an Auto Lateral Load Pattern other than None and click the Add New Load Pattern button.
Highlight the newly defined load case and click the Modify Lateral Load Pattern button to display the {Code} Seismic Loading form.
Click the Override button to display the Override Diaphragm Eccentricities form.

Method 2

Click the Define menu > Load Cases command to access the Define Load Cases form.
Click the Add New Load Case button, or highlight a previously defined case and click the Add Copy of Load Case or Modify/Show Load Case button, to display the Load Cases Data form.
Select Response Spectrum from the Load Case Type drop-down list to display the Load Case Data - Response Spectrum form.
Click the Overrides button in the Diaphragm Eccentricity area of the form to display the Override Diaphragm Eccentricities form.