AD.2007-1001.2.4 Modal Analysis Including Stress Stiffening Effect of KG Matrix
STAAD.Pro can include the stress stiffening effect (geometric stiffness) based on the axial member forces/plate in-plane stresses from a selected load case when calculating the modes & frequencies of a structure.
Position the selected load case from which the axial stresses are to be used to modify the stiffness matrix, such that it is the last static case before the dynamic case which is in turn immediately followed by a PDELTA KG command.
The dynamic load case should contain mass data followed by one of the following:
- A MODAL CALCULATION REQUESTED command.
- A response spectrum definition, i.e., set of SPECTRUM command data.
- A reference to a time history definition, i.e., include TIME LOAD commands.
- Valid Steady State data.
Example
...
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
******* This is static loading case from which the “axial stress” is
******* used to compute the stress stiffening effects (P-Delta)
******* This case will be solved as a PDelta case with large & small
******* P-Delta effects
SELFWEIGHT Y -1.0
JOINT LOAD
2 3 6 7 9 TO 12 FY -3
LOAD 2 LOADTYPE None TITLE LOAD CASE 2
******* Enter masses in weight units
SELFWEIGHT X 1
SELFWEIGHT Y 1
SELFWEIGHT Z 1
JOINT LOAD
2 3 6 7 9 TO 12 FX 10 FY 10 FZ 10
******* Declare this to be a modes/freq analysis
******* Note that dynamic cases use the factored matrix from the last ******* load case; which is a (K+Kg) case
MODAL CALCULATION REQUESTED
PDELTA KG ANALYSIS