Plane frame stability analysis considering pdelta effect

[Shu Jiang PEng SE]

I am trying to verify the example 3.1 given in "AISC Steel design guide 28-Stability of steel buildings". The member sizes, basic load cases and load combinations in my model are same as those in the example. The member internal forces derived from the analysis are very similar as those given in the example except load combination 4 and 6. The program reminds that " load case 12, 14 has infinite or nan values for displacements, structure is either unstable or has buckled or an iterative solution has diverged". Attached is the staad.pro (ver20.07.11.45) input file. What is going wrong?


Please note "tension only" and "change" commands are not repeatedly used for it seems that the repeat of these command are not required.

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STAAD PLANE
START JOB INFORMATION
ENGINEER DATE 13-Oct-16
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 20 0; 3 0 40 0; 4 40 0 0; 5 40 20 0; 6 40 40 0; 7 80 0 0;
8 80 20 0; 9 80 40 0; 10 120 0 0; 11 120 20 0; 12 120 40 0; 13 160 0 0;
14 160 20 0; 15 160 40 0; 16 200 0 0; 17 200 20 0; 18 200 40 0; 19 80 5 0;
20 80 10 0; 21 80 15 0; 22 80 25 0; 23 80 30 0; 24 80 35 0; 25 120 5 0;
26 120 10 0; 27 120 15 0; 28 120 25 0; 29 120 30 0; 30 120 35 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 4 5; 4 5 6; 5 2 5; 6 3 6; 7 7 19; 8 8 22; 9 5 8; 10 6 9;
11 10 25; 12 11 28; 13 8 11; 14 9 12; 15 13 14; 16 14 15; 17 11 14; 18 12 15;
19 16 17; 20 17 18; 21 14 17; 22 15 18; 23 7 11; 24 10 8; 25 8 12; 26 11 9;
27 19 20; 28 20 21; 29 21 8; 30 22 23; 31 23 24; 32 24 9; 33 25 26; 34 26 27;
35 27 11; 36 28 29; 37 29 30; 38 30 12;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
ISOTROPIC STEELRE
E 3.341e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
G 1.60615e+006
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 2 19 20 TABLE ST W8X48
3 4 7 8 11 12 15 16 27 TO 38 TABLE ST W12X65
5 9 13 17 21 TABLE ST W40X149
6 10 14 18 22 TABLE ST W24X76
MEMBER PROPERTY AMERICAN
23 TO 26 PRIS YD 1.5
CONSTANTS
MATERIAL STEELRE ALL
MEMBER RELEASE
5 6 9 10 13 14 17 18 21 22 START MY MZ
5 6 9 10 13 14 17 18 21 22 END MY MZ
MEMBER TENSION
23 TO 26
SUPPORTS
1 4 7 10 13 16 PINNED
LOAD 1 LOADTYPE None TITLE DEAD LOAD
MEMBER LOAD
6 10 14 18 22 UNI GY -1.11
5 9 13 17 21 UNI GY -2.585
LOAD 2 LOADTYPE None TITLE LIVE LOAD
MEMBER LOAD
5 9 13 17 21 UNI GY -4
LOAD 3 LOADTYPE None TITLE ROOF LOAD
MEMBER LOAD
6 10 14 18 22 UNI GY -1.2
LOAD 4 LOADTYPE None TITLE WIND LOAD
JOINT LOAD
3 FX 10
2 FX 20
LOAD 5 LOADTYPE None TITLE N 1 LOAD
JOINT LOAD
3 FX 0.71
2 FX 1.65
LOAD 6 LOADTYPE None TITLE N2 LOAD
JOINT LOAD
3 FX 0.71
2 FX 4.21
LOAD 7 LOADTYPE None TITLE N3 LOAD
JOINT LOAD
3 FX 1.48
2 FX 1.65
LOAD 8 LOADTYPE None TITLE N4 LOAD
JOINT LOAD
3 FX 1.29
2 FX 3.57
LOAD 9 LOADTYPE None TITLE COMB1 1.6D+N1
REPEAT LOAD
1 1.6 5 1.0
LOAD 10 LOADTYPE None TITLE COMB2 1.6D+1.6L+N2
REPEAT LOAD
1 1.6 2 1.6 6 1.0
LOAD 11 LOADTYPE None TITLE COMB3 1.6D+1.6LR+N3
REPEAT LOAD
1 1.6 3 1.6 7 1.0
LOAD 12 LOADTYPE None TITLE COMB4 1.6D+1.2L+1.2LR+N4
REPEAT LOAD
1 1.6 2 1.2 3 1.2 8 1.0
***********************************************************
LOAD 13 LOADTYPE None TITLE COMB5 1.6D+1.6W
REPEAT LOAD
1 1.6 4 1.6
LOAD 14 LOADTYPE None TITLE COMB6 1.6D+1.2W+1.2L+1.2LR
REPEAT LOAD
1 1.6 2 1.2 3 1.2 4 1.2
LOAD 15 LOADTYPE None TITLE COMB7 0.96D+1.6W
REPEAT LOAD
1 0.96 4 1.6
PDELTA 20 ANALYSIS SMALLDELTA
FINISH

 

[Shu Jiang PEng SE]

Got reply from Bentley technical support. To do pdelata analysis is better to partially (99%) release the beam connections even the connection is pin in theory.