Area Springs

Form: Shell Assignment - Area Springs

Area springs may be linear or nonlinear; that is, they can support both tension and compression, or compression only, or tension only. Area springs can be assigned in any of the local axes directions of the shell object.

ETABS distributes the springs associated with the shell object to all of the nodes associated with the internal-to-ETABS (analysis model) representation of the shell object. Note that in some cases internally ETABS may mesh (break up) a shell object into several elements with associated points between each element.

If a slab or mat on an elastic foundation using area springs is being model, meshing the shell object yourself will ensure that, internally in ETABS, a sufficient number of springs are used in the analysis model. ETABS will automatically determine the required stiffness for each spring. This saves a considerable amount of time when the joints where the springs actually occur are not uniformly spaced.

Assign area springs to shell objects as follows:

Select the shell object to which area springs are to be assigned.
Click the Assign menu > Shell > Area Springs command to access the Shell Assignment - Area Springs form.
Select a spring property from the display list and click the Apply button to make the assignment. Any previously assigned spring properties will be replaced by this assignment.

To remove all assignments, select the shell object(s), highlight the None option, and click the Apply button.

Note that clicking the Modify/Show Definitions button will display the Area Spring Properties form, which can be used to define a new spring property.

When the Apply button is used, the Shell Assignment - Area Springs form will remain open until it is closed by clicking the Close button. This allows selection of another shell object(s), to which a different 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.

Important Note: It is possible to assign negative spring stiffness to an area object as long as the total stiffness at any point still remains positive (or zero). If some negative spring stiffness is assigned to an area object, do it with great care because it can cause trouble. If negative spring stiffness occurs at any point in a model during the analysis, ETABS terminates the analysis and provides an error message that there is an instability. ETABS does not check for negative spring stiffness before running the analysis.