ETABS – Deleting Reference Planes

  1. Click the Edit menu > Edit Stories and Grid Systems command to access the Edit Story and Grid System Data form.

  1. Highlight a Grid System definition in the Grid Systems display area.

  1. Click the Modify/Show Grid System button to display the Grid System Data form.

  1. Click the Reference Planes button in the Click to Modify/Show area of the form.
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Zero Compression Element

When we want to model tension cable rods or any member with zero compression  and want to use finite element model for this purpose then follow the procedure below. This procedure is written considering CSI products (SAP or ETABS) but the concept should be similar for most of the softwares.

 

  1. Select the frame elements which should not take any compression.
    FRAME_ELVN

 

  1. Click Assign > Frame/Line > Tension/Compression Limits.

TENSIONCOMPRESSIONMENU

  1. Check the box for Compression and define a limit of zero in the text box (default=0)

TENSIONCOMPRESSIONBOX

 

In order to run the analysis for tension-only members, we have to run Non-Liner Static analysis which is very simple.

  1. Go to Define > Load Cases and select the case you want to change to non-linear static (usually lateral load cases).
    LateralLoadBox
  2. Select Non-Linear Static from the pull down menu.
    LateralLoadBoxNL

 

  1. Input other load cases in the box with appropriate load factors.
  2. Run the analysis and check the members with no compression and confirm that members are only taking tension.
    ZeroCompression
  3. Perform the design.

 

Alternate option for defining non-linear load cases:

  1. Click Define > Load Combination
  2. Select Load combinations needs to convert to Non Linear combos

LoadCombo

 

  1. Click button ” Convert Combos to Nonlinear Cases
  2. Run Analysis and check the forces in the members.
  3. Perform the design.

 

The Solution Lost Error in ETABS

What does the warning message “lost 6.4 digits of accuracy” mean?

ETABS equation solutions are performed in double precision arithmetic, which allows for 15 significant figures. If you have global or local instability (e.g., a node that is not restrained), the solution will suffer a loss of accuracy. The program issues a warning if the loss is estimated to be above 6 digits, and will cease processing if the detected loss is above 11 digits. If you have an analysis where the number of digits lost is less than 8 for example, you should verify the results by checking where the errors are coming from, and also the *.OUT file to make sure the applied load and the reaction are in balance within 1% of each other. Loss messages can be a result of having very stiff elements together with very flexible elements. The rigid diaphragm assumption together with flexible elements is one example of this discrepancy.

Source: www.csiberkeley.com