When defining automatic seismic lateral loads as part of a load pattern definition (Method 1 above), 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
From the Diaphragm drop-down list, select the diaphragm name for which eccentricity values are to be specified.
Enter a value in the Eccentricity, X-Dir (Eccen Y) or Y-Dir (Eccen X) edit box
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
In the Diaphragm/Eccentricities display areas, select the diaphragm name for which eccentricity values are to be modified.
Enter a revised value in the Eccentricity, X-Dir (Eccen Y) or Y-Dir (Eccen X) edit box.
Click the Modify button.
Delete button
In the Diaphragm/Eccentricities display areas, select the definition to be deleted.
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
Method 2
|