Set Load Cases to Run

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:

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.

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.

Note:  The Show Load Case Tree button also appears on the Define Load Cases form.

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.

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.

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.