Can someone tell me the error in the following staad code. For background - smaller 3.5 ft WT member are supposed to be cantilevers that's why only My released.

For the cantilever W10X33 sections (joint instabilities at nodes 1 and 4), is there a need to release those short cantilevers such that they won't carry moment about their local Y? Would those not be continuous from the back span?

Similarly, you have released the MY for the WT (here prismatic T's) members, but will they be connected in such a way as to fix bending about Z but not Y? Are those member end releases necessary? That may resolve the joint instabilities at nodes 16 and 17 as well.

Hello Jason.

Thanks for the reply. I have corrected the mistake of My for small cantilevers. They should not be included. But for WT sections I included My as it is a cantilever. But still, I am getting instabilities in 2 nodes 20 and 21. Should I release Mz (would this not be wrong as Mz is major axis bending)?

New Code - 

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 04-Apr-18
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 1.67 0 0; 3 19.66 0 0; 4 21.33 0 0; 5 1.67 -6.5 0; 6 19.66 -6.5 0;
7 0.29 0 0; 8 5.33 0 0; 9 10.67 0 0; 10 16 0 0; 11 21.04 0 0; 12 0.29 0 3.67;
13 0.29 0 -3.67; 14 5.33 0 3.67; 15 5.33 0 -3.67; 16 10.67 0 3.67;
17 10.67 0 -3.67; 18 16 0 3.67; 19 16 0 -3.67; 20 21.04 0 3.67;
21 21.04 0 -3.67;
MEMBER INCIDENCES
1 1 7; 2 2 5; 3 3 6; 4 7 12; 5 8 14; 6 9 16; 7 10 18; 8 11 20; 9 7 13; 10 8 15;
11 9 17; 12 10 19; 13 11 21; 14 2 8; 15 3 11; 16 7 2; 17 8 9; 18 9 10; 19 10 3;
20 11 4; 21 12 14; 22 14 16; 23 16 18; 24 18 20; 25 13 15; 26 15 17; 27 17 19;
28 19 21;
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
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 14 TO 20 TABLE ST W10X33
2 3 TABLE ST W8X35
MEMBER PROPERTY AMERICAN
4 TO 13 PRIS YD 0.6675 ZD 0.5833 YB 0.6254 ZB 0.0254
MEMBER PROPERTY AMERICAN
21 TO 28 TABLE ST HSST4X2X0.375
CONSTANTS
BETA 90 MEMB 2 3
MATERIAL STEEL ALL
SUPPORTS
5 6 FIXED
MEMBER RELEASE
4 TO 13 START MY
21 TO 28 START MY MZ
21 TO 28 END MY MZ
LOAD 1 LOADTYPE Dead TITLE DL
SELFWEIGHT Y -1
MEMBER LOAD
*Load of Channel
21 TO 28 UNI GY -0.02
*10psf Load of deck plus misc on trib 5.34
5 TO 7 10 TO 12 UNI GY -0.0534
*10psf Load of deck plus misc on trib 2.7
4 8 9 13 UNI GY -0.027
PERFORM ANALYSIS
FINISH

Instability warnings do not cause any harm in your model as static check are ok and displacements are reasonable. However, to avoid these warnings, you can change the moment releases with partial moment releases (these release 99.5% of the moment at a connection - 0.5% is retained to help with the instability). This is a common practice and is a reasonable assumption as no pinned connection truly releases all the moment in reality. So input lines should be changed from 4 TO 13 START MY to 4 TO 13 START MPY 0.995.

Instability warnings do not cause any harm in your model as static check are ok and displacements are reasonable. However, to avoid these warnings, you can change the moment releases with partial moment releases (these release 99.5% of the moment at a connection - 0.5% is retained to help with the instability). This is a common practice and is a reasonable assumption as no pinned connection truly releases all the moment in reality. So input lines should be changed from 4 TO 13 START MY to 4 TO 13 START MPY 0.995.