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
2. For all other load cases keep the zero initial condition
3. Run the analysis, it will include the P-delta effects.
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)
I found this problem when I was importing HSS sections from AISC profile file, that the default material for HSS is not 46ksi and all the tube were imported with Fy=50ksi. It was tedious to go and change every HSS material one by one to Fy=46ksi.
Here is the procedure which should be followed in order to avoid this issue:
1. Click Section Properties > Frame Sections
2. Click “Import New Property” button
3. Click “Tube” button
4. Select file to import from computers and structures > SAP2000 folder. File name will be AISC13.pro for AISC 13th Edition in English units and or AISC13M.pro for metric units. (Also sizes per 14th edition is also included in new version)
5. The box will appear with sizes to import. Change the material to A500GrB46 or any material which is required.
Select sizes to import and click ok.
6. The material for all HSS tube will be A500GrB46.