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 drop-down lists, edit box, and buttons on the Overwrite Eccentricities form or the Eccentricities - Response Spectrum Analysis form to overwrite the eccentricity ratio for one or more diaphragms.
Note: When the form is displayed by clicking the Diaphragm Eccentricity Modify/Show button on the Load Case Data form for a Response Spectrum (see the methods to access the form, below), the form includes a Default Eccentricity for Response Spectrum Analysis area and an edit box to specify an Eccentricity Ratio that applies to all diaphragms except those overwritten in the lower portion of the form.
Use the Overwrite Eccentricities form or the Eccentricities - Response Spectrum Analysis form to change the eccentricity on a diaphragm-by-diaphragm basis.
Story drop-down list. Use this drop-down list to select the story where the diaphragm is located.
Diaphragm drop-down list. Use this drop-down list to select the name of the diaphragm assigned to point or area object(s).
Ecc Length or Eccentricity (ft) edit box. Enter a value in this edit box for the eccentricity. Note that the eccentricity is input as an absolute length.
Add button. With selections made in the Story and Diaphragm drop-down lists and a value in the Ecc Length edit box, click this button to apply the overwrite to the specified diaphragm.
Modify button (only on the Overwrite Eccentricities form). Highlight an existing diaphragm overwrite in the Story, Diaphragm and Ecc Length display lists. Note that the data for the selection will appear in the edit boxes at the top of the display lists. Make the necessary modification and then click the Modify button.
Delete button. Highlight an existing diaphragm overwrite in the Story, Diaphragm and Ecc Length display lists. Click the Delete button to remove the overwrite from the specified diaphragm.
Sort button (only on the Eccentricities - Response Spectrum Analysis form). Click this button to arrange the rows of data based on the Story ID.
Access the Overwrite Eccentricities form or the Eccentricities - Response Spectrum form as follows:
Overwrite Eccentricities form:
Click the Define menu > Load Patterns command to access the Define Load Patterns form.
Type a unique name for the load case in the Load edit box, select Seismic from the Type drop-down list, and select an option other than None or User Loads from the Auto Lateral Load drop-down list.
Click the Add New Load button.
Highlight the newly define load case in the display area and click the Modify Lateral Load button to display the {Code} or User Defined Seismic Loading form. Choose an option that includes specification of an eccentricity in the Direction and Eccentricity area.
Click the Overwrite button.
Eccentricities - Response Spectrum form:
Click the Define menu > Load Cases command to access the Load Cases form.
Click the Add New Case button, or highlight a previously defined case and click the Add Copy of Case or Modify/Show Case button, to display the Load Case Data form.
Ensure that Response Spectrum has been selected for the Load Case Type.
Click the Diaphragm Eccentricity Modify/Show button.