Notes for Small or Large Delta (Option 1)

1. This command directs the program to perform the analysis that includes:
   1. Checking whether all information is provided for the analysis;
   2. Forming the joint stiffness matrix
   3. Checking the stability of the structure;
   4. Solving simultaneous equations, and
   5. Computing the member forces and displacements.
   6. For P-Delta analysis, forces and displacements are recalculated, taking into consideration the chosen P-Delta effect.
   7. If a RESPONSE SPECTRUM, TIME LOAD, or GROUND MOTION is specified within a load case or the MODAL CALCULATION command is used, a dynamic analysis is performed.
      Note: Computing P-Delta effects for dynamic load cases is not recommended since such effects are not considered.
   8. In each of the iterations of the PDELTA ANALYSIS, the load vector will be modified to include the secondary effect generated by the displacements caused by the previous iterations.
2. The default procedure of option 1 is based on "P-small & large Delta" effects.(sometimes referred to as P-δ & P-Δ). Enter the LargeDelta parameter to only include the "PlargeDelta" effects (P-Δ only). SmallDelta is recommended.
3. This PDELTA (n) ANALYSIS command (option 1) should specify 3 to 30 iterations to properly incorporate the P-Delta effect. With this many iterations, the PDELTA (n) ANALYSIS SMALLDELTA command results are as good as or better than the PDELTA KG (option 2) command results for static analysis. The advantage of this PDELTA (n) ANALYSIS command comes from not having to re-form and then triangular factorize the stiffness matrix for every iteration within every case. Also this command allows tension/compression.
4. Be aware that global buckling can occur in P-Delta analysis, resulting in large or infinite or NaN values for displacement. Do not use the results of such a case. Sometimes the loads from Repeat Load combination cases are too large; sometimes partial moment releases rather than the full release is needed, sometimes connectivity needs to be corrected. Always check the maximum displacements for P-Delta analyses.

Notes for Stress Stiffening Matrix (Option 2)

The P-Delta analysis also provides the option of including the stress stiffening effect of the Kg matrix into the member / plate stiffness.

1. A regular STAAD P-Delta Analysis (option 1) performs a first order linear analysis and obtains a set of joint forces from member/plates based on both large and small P-Delta effects.  In contrast, the P-Delta KG Analysis (that is, with the Kg option selected) includes the effect of the axial stress after the first analysis is used to modify the stiffness of the member/plates. A second analysis is then performed using the original load vector. Large & small P-Delta effects are always included in this KG option.
2. This command directs the program to perform the analysis that includes:
   1. Solving the static case.
   2. Reforming the global joint stiffness matrix to include the Kg matrix terms which are based on the computed tensile/compressive axial member forces.
   3. Solving simultaneous equations for displacements;
   4. If a RESPONSE SPECTRUM, TIME LOAD, or GROUND MOTION is specified within a load case or the MODAL CALCULATION command is used, a dynamic analysis is performed. The static cases solved for a PDELTA KG analysis command will be solved first then the dynamic analysis cases.
      Note: The stiffness matrix used in the dynamic analysis will be the K+Kg matrix used in the last iteration for the last static case. This is a stress stiffened dynamic analysis, sometimes known as a PDELTA Dynamic analysis.
3. A PDELTA KG ANALYSIS will correctly reflect the secondary effects of a combination of load cases only if they are defined using the REPEAT LOAD specification (TR.32.11 Repeat Load Specification). Secondary effects will not be evaluated correctly for load combinations since only final results are combined.
4. P-Delta KG effects are computed for frame members and plate elements only. They are not calculated for solid elements. The results are based on "P-large & small Delta" effects.
5. For static analysis, the other P-Delta command [ PDELTA (n) ANALYSIS (SMALL) ] with 20 or more iterations (option 1) is preferred.
6. Tension/compression only members are not allowed with this PDELTA KG command. You must use the PDELTA (n) ANALYSIS (SMALL) command instead.
7. Be aware that global buckling can occur in a PDELTA KG ANALYSIS. This condition is usually detected by STAAD.Pro. A message is issued and the results for that case are set to zero. STAAD.Pro will continue with the next load case.   
8. Global buckling may not be detected which could result in a solution with large or infinite or NaN values for displacement or negative L-matrix diagonals or stability errors. Do not use the results of such cases. This condition may require a nonlinear analysis. Sometimes the loads from Repeat Load combination cases are too large; sometimes partial moment releases rather than the full release is needed, sometimes connectivity needs to be corrected. Always check the maximum displacements for P-Delta analyses.