Form: Set Load Cases to Run, Choose Tables
The Analyze menu > Set Load Cases to Run command opens the Set Load Cases to Run form. Use that form to check the analysis status of the cases, delete results for cases that have already been run, set which cases are to be run, and run the analysis or just save the settings.
The table on the form lists all defined load cases and shows the following:
Case: The name defined for a load case.
Type: The type of load case.
Status: Shows the status of the case following its most recent analysis. Values are:
Not Run: The case has never been run or its results have been deleted.
Could Not Start: An attempt was made to run the case, but a prerequisite case was not finished.
Not Finished: The analysis was unable to reach its requested final state.
Finished: The analysis completed successfully.
Action: A value set by the user indicating what should happen the next time an analysis is run:
Run: Indicates that the program should run the load case.
Do Not Run: Indicates that the program should not run the load case.
Run/Do Not Run Case button. Highlight a load case and click the Run/Do Not Run Case button to toggle analysis on and off. When Run displays in the Action List, the analysis will begin and all cases set to run and their prerequisite cases will be run.
Delete Results for Case button. Highlight a Case Name and click the Delete Results for Case button to delete results for the selected case. The status of those cases will be set to Not Run.
Note: To delete a load case, click the Define menu > Load Cases command to access the Define Load Cases form. Highlight the case to be deleted and click the Delete Load Case button.
Run/Do Not Run All Cases button. Use the Run/Do Not Run All Cases button to toggle all load cases on or off simultaneously.
Delete All Results button. Use the Delete All Results button to delete all cases simultaneously. If results for one case are deleted, the results for all cases that depend on that case also are deleted.
Important: When the user deletes results, the user will be asked for confirmation. After results have been deleted, these cannot be recovered except by rerunning the analysis. Clicking the Cancel button will not undelete them.
Show Load Case Tree button. Click this button to display the Load Case Tree form. As the name suggests, the form displays a tree of the load cases defined for the model. Right click an item in the tree to display a drop-down list of commands, including Expand All, Collapse All, and Edit Load Case {Type}. Use the Expand All and Collapse All subcommands to show all cases or limit the display of cases, respectively. Use the Edit Load Case {Type} subcommand to display the form needed to edit that load case.
Note: The Show Load Case Tree button also appears on the Define Load Cases form.
Run Now button. Click the Run Now button and the cases set to Run will be run.
OK and Cancel buttons. Click the OK button and the action settings will be saved but the analysis will not run. Click the Cancel button and the action settings will be left unchanged.
Analysis Monitor Options. Use these options to specify when the Analysis window displays. Scrolling through the information provides the opportunity to check for any warnings or errors that might invalidate your analysis. Also note that this information is saved, in text form, in the .LOG file. Access the .LOG file using the File menu > Show Input/Output Text Files command, or any text editor or word processing program.
Always Show option. When this option is selected, the Analysis window will display during the entire analysis processing period.
Never Show option. When this option is selected, the analysis will run, but the Analysis window will not display, and thus, the results will not be shown. To review the results, click the Analyze menu > Last Analysis Run Log command to display the most recent analysis results in the Analysis window.
Show After ___ seconds option. When this option is selected, specify the number of seconds to elapse before the Analysis window displays.
Note: As the analysis is running, a Show Details button will be added to the Status bar. Clicking this button will display the results in the analysis window if they are not already displayed. Also, after an analysis has been run, the Analyze menu > Last Analysis Run Log command can be used to display the most recent analysis results in the analysis window. The Less/More buttons on the Analysis window can be used to decrease the size of the form, or restore it to full size, respectively.
Automatically save tables to Microsoft Access after run completes check box. As the name suggests, when this check box is checked, ETABS will automatically save results of the analysis run in Microsoft Access compatible format.
Use the Filename edit box to specify the path and filename for saving the file.
Use the Table Set drop-down list to choose a Named Set of tables to be included in the Access file.
The program can accommodate more than 700 data table types. Click the Add New button to display the Choose Tables form. Use that form to name/define a Named Table Set and limit the database tables to be included in the Access file.
Diaphragm Centers of Rigidity and Calculate Diaphragm Centers of Rigidity check box. As the name suggests, when this check box is checked, ETABS will calculate the diaphragm centers of rigidity during the analysis. The original concept of center of rigidity dates back to manual rigidity analysis techniques associated with the lateral analysis of single-story shear wall buildings. The center of rigidity was defined as the location of the centroid of the stiffnesses of single-story lateral resisting elements (typically planar) arbitrarily located in plan. For single-story structures the definition worked well because the stiffness for each wall frame was a 1 by 1 matrix with no interstory coupling or compatibility factors to complicate the problem. The analysis technique was extrapolated for multistory lateral analysis whereby multistory buildings were analyzed as a series of single-story buildings stacked over one another with no interstory displacement compatibility. Needless to say, for complex three dimensional structures this assumption was approximate at best.
Modern computer techniques do not require the explicit evaluation of the center of rigidity. However, the center of rigidity still needs to be evaluated because some building codes refer to it as a reference point to define design eccentricity requirements in multistory buildings.
In the general three-dimensional analysis of a building, where the behavior is coupled in plan as well as through the height of the structure, the center of rigidity requires a broader definition. In this broader definition when translational lateral loads are applied at the center of rigidity of a particular floor diaphragm, with no loads applied to any of the other floor diaphragms, the displacements of that diaphragm will have only translational components with no rotations. it should be noted that the resulting displacements of the diaphragms at other levels in general will contain translational as well as rotational components.
To evaluate the center of rigidity at a particular diaphragm, the structure is analyzed for three load cases. The loads are applied at the center of mass (or any arbitrary point). Load case 1 has a unit load applied in the global X direction and results in a diaphragm rotation of Rzx. Load case 2 has a unit load applied in the global Y direction and results in a diaphragm rotation of Rzy. Load case 3 has a unit moment applied about the global Z-axis, giving a diaphragm rotation of Rzz.
The center of rigidity relative to the center of mass (or the arbitrary point) is then given in by the coordinates (X, Y), where
X = -Rzy / Rzz and Y = Rzx / Rzz. This point is a function of the structural properties and is independent of any loading.
As described above, the determination of the center of rigidity can be numerically intensive as it involves a static analysis of the whole structure for three independent load cases for each floor diaphragm. This procedure is implemented in ETABS so that the centers of rigidity for every diaphragm are automatically calculated as part of the solution process.
Three Load Cases used to determine the center of rigidity.
To view the results of the calculation, click the Tables tab in the Model Explorer, then click Analysis > Results > Displacements to display the various data types available, including Diaphragm Center of Mass Displacements. Click on a data type to display the table of results in the lower portion of the ETABS window.
To print results of the calculation, click the File menu > Create Report > Add New User Report command to display the User Report form. Use the Output tab on that form to select the data to be printed.