How do I model Tension-Only Bracing in ETABS by WillisV
The following instructions were created based on trial and error and discussions with staff at CSI as these features go largely undocumented.
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Setup model including braces and applied lateral loads.
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Select all braces that are to be tension-only braces and make sure that they DO NOT have any section properties assigned to them.
a. Go to ASSIGN: FRAME LINE: FRAME SECTION: NONE.
b. This is an important step because otherwise the properties of a shape would override the non-linear, tension-only element and allow the brace to take compression. -
Define a non-linear ôNLINKö element to act as the tension-only brace.
a. Go to DEFINE: LINK PROPERTIES
b. Click ADD NEW PROPERTY
c. Type in a name û maybe ôTensionOnly1ö
d. Select the type as HOOK (this is the tension-only property)
e. The mass and weight section may typically be left as 0s
f. Activate the U1 Directional Property for an axial-only brace
g. Make sure that you have the Nonlinear box clicked ON
h. Click ôModify/Show for U1ö box
i. Enter in BOTH the effective stiffness under linear properties AND the stiffness under nonlinear properties as k=AE/L of the member which the NLINK element is supposed to represent. For example if your current units are kips and inches and you have a 15Æ long 1ö diameter round steel rod as a tension element, the stiffness value would be{ [A=(pi*1^2/4)] * [E=29,000ksi] } / [L= 15Æ * 12in/ft] = 126.47 k/in.
j. Damping values may be left as 0 if all you are looking for are tension-only braces for an elastic type analysis. -
Repeat step 3 for as many different tension elements sizes that you have.
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Define Non-Linear Load cases for analysis
a. Go to DEFINE: STATIC/NONLINEAR PUSHOVER CASES
b. Click ADD NEW CASE
c. Name the new non-linear load case something easy to remember and that is associated with the load case it represents. For instance if you are creating a non-linear load case based on WX1, I would typically name it WX1NL.
d. Under options select ôLoad to Level Defined by Patternö [li]Member unloading method should be ôUnload Entire Structureö[li]
e. The defaults are fine for all of the minimum steps etc.
f. Choose the load pattern that you want this non-lin case to be associated with (e.g. choose a lateral load case)
g. The active group should be ALL
h. Choose type of Geometric Non-Linear Effects (Typically None or P-Delta) that you would like to include.
i. Click OK to enter the load case. -
Repeat step 5 for as many lateral load cases that you need to analyze for tension-only members.
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Run a regular analysis
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Run a non-linear analysis by clicking on ANALYZE: RUN STATIC NONLINEAR ANALYSIS
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To recover the ôtension-onlyö forces in the NLINK elements click on DISPLAY: SHOW MEMBER FORCES/STRESS DIAGRAM: LINK FORCES.
a. Note that ETABS will also ask for the step number û typically these type analyses converge in only a few steps. All you would really be interested in is the last step so if you just enter in a high number ETABS will automatically display the last step that it saved.
[Note: ETABS in version 8.5 has a feature under ASSIGN: FRAME LINE: TENSION/COMPRESSION LIMITS. This feature is NOT active in version 8.5 and does not do ANYTHING. CSI has an annoying habit of including undocumented or incomplete features in their ETABS builds and this is one of them. It will supposedly be activated in version 9.0 which is already well behind their intended release schedule.]