Use the Seismic Load Pattern form or the {Code} Seismic Loading form to review and modify the parameters for the specified seismic load. The name of the form and the options on it depend on the code selected in the Auto Lateral Load drop-down list on the Define Load Patterns form.
Note: If User Defined is selected for the Auto Lateral Load, the User {Seismic, Wind] Loads on Diaphragms form will display.
Direction and Eccentricity options. Choose to specify the X or Y direction of the seismic loading, or to specify the direction with a percentage of eccentricity that is applicable to all diaphragms. Use the % Eccentricity edit box to specify a value for eccentricity. Five percent is the default and is entered as 0.05. The eccentricity options have meaning only when diaphragms have been assigned to joint or shell objects. The program ignores eccentricities where diaphragms are not present.
Where diaphragms are present, the program calculates a maximum width of the diaphragm perpendicular to the direction of the seismic loading. This width is calculated by finding the maximum and minimum X or Y coordinates (depending on direction of load considered) of the joints that are part of the diaphragm constraint and determining the distance between these maximum and minimum values.
After the appropriate diaphragm width has been determined, the program applies a moment that is equal to the specified percent eccentricity times the maximum width of the diaphragm perpendicular to the direction of the seismic loading times the total lateral force applied to the diaphragm. This moment is applied about the diaphragm center of mass to account for the eccentricity.
Overwrite button. Click this button to access the Overwrite Eccentricities form and modify eccentricities on a diaphragm-by-diaphragm basis.
Time Period options. Choose an option for specifying the time period for the seismic loading. When the User Defined option is selected, enter a value in the T = edit box. When the Method A, Approximate Period, or Program Calculated option is selected, no entry is required in the associated edit box, although the value can be changed by the user. Method A and the Approximate Period are as specified in the code. The value used by the Program Calculated option is as recommended in the code.
Note: Additional code-specific information concerning seismic loading is available in the Automated Lateral Loads manual (Help menu > Documentation > Manuals > Lateral Loads Manual command).
Story Range options. Choose the Top Story and Bottom Story to specify the elevation range over which the automatic static lateral loads are to be calculated. By default the bottom story is the base of the building and the top story is the uppermost level of the building.
In most instances, specify the top story as the upper-most level in the building, typically the roof. However, in some cases, a lower level may be chosen. For example if a penthouse is included in the model, it may be best to calculate the automatic lateral load based on the roof level, excluding the penthouse roof level, as the top story, and then add in additional user-defined load to the load case to account for the penthouse.
The bottom level would typically be the base level. However, if, for example, a building has several below-grade levels, and the seismic loads are assumed to be transferred to the ground at ground level, it may be best to specify the bottom story to be above the base of the building.
Note: No seismic loads are calculated for the bottom story. They are calculated for the first story above the bottom story and for all stories up to and including the top story.
Factors edit boxes. If available, use the default code-specific factors related to seismic loading or use the edit boxes to specify your own values.
Seismic Group drop-down list. If available, choose the applicable Seismic Group, as specified in IBC2003 or NEHPF97, from the drop-down list.
Seismic Coefficients options, drop-down lists and edit boxes. The parameters vary by code. For more information about the specific input required, please refer to the code specific manuals (Help menu > Documentation) or the codes themselves.
Other Factors edit box. When Other Factors need to be specified, use the default or enter your own values in the edit boxes.
Structure Type options. For 1995 NBCC, choose Moment Frame or Other. When the Other option is chosen, also specify a value for Ds.
See also
Seismic Load Pattern - User Defined Form
Access the Seismic Load Pattern - {Code} form or {Code} Seismic Loading form as follows:
Click the Define menu > Load Patterns command to access the Define Load Patterns form.
Type a unique name for the load pattern in the Load edit box, select Seismic from the Type drop-down list, and select an option other than None, User Defined, or User Coefficient 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.
After a Quake load case has been defined as described above, highlighting the name of the previously defined load and clicking the Modify Lateral Load button will also display the form.