SAP2000 P-DELTA

Before running P-Delta analysis, always check the model for simple modal analysis and gravity loads to make sure that analysis model has no stability issues or other warnings/errors.
I have tried two methods for running P-Delta, both are explained below, hope this will be helpful.

Method 1: Initial P-Delta

1. Define load case for e.g “P-Delta” with

2. Include all dead load cases in that P-Delta case

3. For all load cases click on ” the Stiffness at end of nonlinear case” to activate it and select “P-Delta” load case in the drop down menu.

4. Run the analysis, SAP will include the P-delta effects.

Method 2: Non-Linear Load combos

1. Convert all the load combos into Non-Linear P-Delta case

CONVERT COMBO TO NON-LINER

NON-LINEAR LOAD COMBO

2. For all other load cases keep the zero initial condition

3. Run the analysis, it will include the P-delta effects.

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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.

 

SAP2000-Accidental Torsion in Response-Spectrum Case

NOTE: This discussion applies only to SAP2000. ETABS automatically includes the accidental eccentricity specified in the response-spectrum load case.

To consider accidental torsion in a response-spectrum case, eccentricity should be added as a separate static load case. The response-spectrum (SPEC) and eccentric (ECC) load cases should then be combined as SPEC + ECC and SPEC – ECC load combinations. Each level should have a rigid diaphragm, otherwise accidental eccentricity is not of significance.

Accidental torsion is applied through the following process:

  • Define a load case named ECC (or other) of Type = Quake, Self-weight multiplier = 0, and Auto = None.
  • Select any point on the diaphragm at each level.
  • Select Assign > Joint/Point Loads > Force > Moment Global ZZ, then specify the appropriate value for torsional moment.
  • Evaluate SPEC + ECC and SPEC – ECC load combinations. Note that the software already considers SPEC in both directions.

Accidental torsion may be considered without rigid diaphragms, though loading should be more distributed. Torsional moment is calculated at joint locations within a flexible diaphragm by first resolving the total lateral force in a given direction at each story level. This is done by summing the auto-seismic loads applied to joints within each story level. This force is then multiplied by the eccentricity to generate torsional moment (T). This torsion is then distributed to all joints at the story level in proportion to the joint mass as follows:

  • Torsion at a given joint at a story level = T * (Mass at the joint / Total mass of all joints at the story level)

It may be convenient to use an Excel spreadsheet to calculate torsion at joint locations. Using interactive database editing, data may then be entered into SAP2000

Source: https://wiki.csiberkeley.com/display/kb/Accidental+torsion+in+response-spectrum+case