Form: Frame Assignment - Line Springs
Line springs can be assigned in any of the local axes directions of the frame object. Line springs are linear; that is, they support both tension and compression. Tension-only or compression-only line springs are allowed for vertical frame objects only.
ETABS distributes the springs associated with the frame object to all of the nodes associated with the internal-to-ETABS (analysis model) representation of the frame object. Note that internally ETABS may mesh (break up) a frame object into several elements with associated joints between each element. When modeling a beam on elastic foundation using a line spring, it may be advantageous for the user to specify the frame object meshing to ensure that internally in ETABS a sufficient number of springs is 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 line springs to frame objects as follows:
Select the frame object to which the spring assignment is to be made.
Click the Assign menu > Frame > Line Springs command to access the Frame Assignment - Line 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 frame object(s), highlight the None option, and click the Apply button.
Note that clicking the Modify/Show Definitions button will display the Line Spring Properties form, which can be used to define a new spring property.
When the Apply button is used, the Frame Assignment - Line Springs form will remain open until it is closed by clicking the Close button. This allows selection of another frame 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 a frame object as long as the total stiffness at any joint remains positive (or zero). If some negative spring stiffness is assigned to a frame 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.
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