Form: Joint Load Assignment - Ground Displacement
Assigning a displacement to a joint object is meaningful only if structural objects are connected to the joint object. Otherwise, the joint object displacement will have no effect on the structure. Thus, the joint object must be connected directly to a structural object, or it must be on top of a floor-type shell object that ETABS can automatically mesh. Joint objects are connected to the ground through one of the following:
Restraints - When ground displacements are assigned to a joint object that is restrained, the displacement takes place at the joint object.
Springs or grounded link elements assigned to a single joint object - When ground displacements are assigned to joint objects that have springs or grounded link elements assigned to them, the displacement takes place at the grounded end of the spring or link, not at the joint object. This is a subtle but very important distinction.
If a ground displacement is applied to a joint object that is not connected to the ground through a restraint, spring or grounded link element, that displacement is ignored by ETABS when the analysis is run.
Joint object ground displacements are meaningful only when they are applied to joint objects that are connected to the ground in the direction that the displacement is applied. Note that the ground displacements are specified in global coordinate system directions.
Assign ground displacement load to a joint object as follows:
Select the joint object(s) to which the ground displacement is to be assigned.
Click the Assign menu > Joint Loads > Ground Displacement command to access the Joint Load Assignment - Ground Displacement form.
Use the following drop-down lists, edit boxes, and options to specify the parameters for the displacement load:
Load Pattern Name drop-down list. Select the name of a defined load pattern to which the specified displacements are to be assigned. Note that the Define menu > Load Pattern command is used to define load patterns.
Displacements edit boxes. Input the displacements in the global coordinate system directions in this area. Positive directions of rotations (shown in the sketch below) are based on the right-hand rule.
Options
Add to Existing Loads: Adds the specified displacements to the joint object. If one or more displacement assignments have already been made, this option increases the total displacement assigned to the joint object, assuming a positive displacement is being specified.
Replace Existing Load: Replaces the currently specified displacement, if any, with the new displacement assignment. If no assignment exists, the new assignment is still made. This is the default option. Note that only the loads in the load pattern that is specified above are replaced.
Delete Existing Loads: Deletes any and all displacement assignments made to the selected joint object(s). When this option is selected, the items in the Displacements area of the form are ignored when the Apply button is clicked. Note that only the loads in the load pattern that is specified above are deleted.
Note that the default option is Replace and that the program defaults to this every time the form is opened.
Click the Apply button to complete the assignment.
When the Apply button is used, the Joint Loads Assignment - Ground Displacement form will remain open until it is closed by clicking the Close button. This allows selection of another joint object(s), to which another assignments can be made.<="">
If only one assignment is being made to only one set of selected objects, the OK button can be used to both apply the assignment and close the form.
See Also