Truss member of STAAD and the instability problems

Hello everyone,

I am modeling a steel frame stand for holding a heavy steam casing. This stand will be used inside a factory, therefore only dead load and seismic load are analyzed. The profile of the stand and its post design are shown below: the pinned connections for the cross bracings only have one row and are not designed to transfer moments. 

I had some difficulties with the long cross bracing members retaining the shape of the stand in the longitudinal direction. The two cross bracings are connected to the posts of the stand by non-slip critical bolted connected. Therefore, they are not intended to take any moments and the moments at these connections are released. Since the bracings are longer, their mid-points were pinned together so that their slenderness ratio are within the acceptable range. 

We modeled the two long bracings as continuous members with a pin in the mid points. But our client wanted us to release the moments in the pin connection because the pin is not a rigid type of connections that could transfer moments. I tried to set them as truss members and it ended up with huge displacements in the mid points.

My question is:

1. What type of connection should I use for the pin at their mid points? Should I combine the two mid points or use separate mid points and then compare their displacements? Why there are huge displacements if I used "TRUSS" member?

2. What are the differences between TRUSS members and the beam members with X-, Y-, and Z- directions released? For this analysis, both the long cross bracings and short cross bracings are connected to the posts by non-slip critical connections. So they are not taking the moments. Which type of elements should I use?

I am attaching the codes from STAAD below. If our STAAD expert could have a look at it, I will really appreciate. Thanks a lot, :)

Fiona

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 03-Apr-17
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 0 0.3683 0; 3 0 0.6477 0; 4 0 0.8001 0; 5 0 2.2479 0; 6 0 3.3782 0;
7 0.229108 0 2.04457; 8 0.229108 0.3683 2.04457; 9 0.229108 0.6477 2.04457;
10 0.229108 2.2479 2.04457; 11 0.229108 3.3782 2.04457;
12 2.48054 3.3782 1.05979; 13 2.59715 1.4478 2.04457;
14 2.59715 1.4478 2.04457; 15 4.96519 0 2.04457; 16 4.96519 0.3683 2.04457;
17 4.96519 0.6477 2.04457; 18 4.96519 2.2479 2.04457;
19 4.96519 3.3782 2.04457; 20 5.1943 0 0; 21 5.1943 0.3683 0;
22 5.1943 0.6477 0; 23 5.1943 0.8001 0; 24 5.1943 2.2479 0; 25 5.1943 3.3782 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 3 4; 4 4 5; 5 5 6; 6 2 8; 7 5 9; 8 10 3; 9 7 8; 10 8 9;
11 9 10; 12 10 11; 13 12 6; 14 12 11; 15 13 9; 16 10 14; 17 16 8; 18 4 23;
19 12 19; 20 14 17; 21 18 13; 22 12 25; 23 15 16; 24 16 17; 25 17 18; 26 18 19;
27 21 16; 28 17 24; 29 22 18; 30 20 21; 31 21 22; 32 22 23; 33 23 24; 34 24 25;
START USER TABLE
TABLE 1
UNIT INCHES KIP
TUBE
STRAIGHTBRACING
7.6875 8 3 0.375 56.5479 11.1182 29.314 6 2.25
END
UNIT INCHES KN
DEFINE MATERIAL START
ISOTROPIC STEEL
E 132258
POISSON 0.3
DENSITY 0.00125885
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 163.355 FU 263.096 RY 1.5 RT 1.2
ISOTROPIC CONCRETE
E 14011.9
POISSON 0.17
DENSITY 0.000386106
ALPHA 1e-005
DAMP 0.05
TYPE CONCRETE
STRENGTH FCU 17.7929
ISOTROPIC RIGID_STEEL
E 1.32258e+010
POISSON 0.3
DENSITY 1.25885e-006
ALPHA 1.2e-005
DAMP 0.03
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 TO 5 9 TO 12 23 TO 26 30 TO 34 TABLE ST PIPE OD 8.625 ID 7.625
UNIT INCHES KIP
MEMBER PROPERTY AMERICAN
6 TO 8 27 TO 29 TABLE ST C5X9
MEMBER PROPERTY AMERICAN
18 UPTABLE 1 STRAIGHTBRACING
17 TABLE ST W4X13
MEMBER PROPERTY AMERICAN
13 14 19 22 PRIS YD 2.5
15 16 20 21 TABLE ST C6X8
UNIT INCHES KN
CONSTANTS
MATERIAL STEEL MEMB 1 TO 12 15 TO 18 20 21 23 TO 34
MATERIAL RIGID_STEEL MEMB 13 14 19 22
UNIT INCHES KIP
SUPPORTS
1 7 PINNED
15 20 FIXED BUT FX FZ MX MY MZ
UNIT INCHES POUND
MEMBER OFFSET
7 START 0 0 4.3125
28 END 0 0 4.3125
8 START 0 0 -4.3125
29 END 0 0 -4.3125
21 START -4.3125 0 0
16 START 4.3125 0 0
7 END 0 0 -4.3125
28 START 0 0 -4.3125
8 END 0 0 4.3125
20 END -4.3125 0 0
18 START 4.3125 0 0
18 END -4.3125 0 0
17 START -4.3125 0 0
15 17 END 4.3125 0 0
29 START 0 0 4.3125
6 27 START 0 0 4.3125
6 27 END 0 0 -4.3125
16 END 0 0 0
20 START 0 0 0
UNIT INCHES KIP
MEMBER RELEASE
6 TO 8 17 27 TO 29 START MX MY MZ
6 TO 8 17 27 TO 29 END MX MY MZ
13 14 19 22 END MX MY MZ
UNIT MMS NEWTON
MEMBER TRUSS 
15 16 20 21
UNIT INCHES POUND
LOAD 1 LOADTYPE None  TITLE WEIGHT OF STEAM CASING
UNIT INCHES KIP
SELFWEIGHT Y -1.41 
JOINT LOAD
12 FY -60.7
UNIT MMS NEWTON
LOAD 2 LOADTYPE None  TITLE SEISMIC LOAD
SELFWEIGHT X -0.21 
JOINT LOAD
12 FX -56714.8
LOAD 3 LOADTYPE None  TITLE 1.4 X DEAD LOAD
REPEAT LOAD
1 1.4 
LOAD 4 LOADTYPE None  TITLE 1.0 X DEAD LOAD + 1.0 X SEISMIC LOAD
REPEAT LOAD
1 1.0 2 1.0 
UNIT INCHES KIP
PERFORM ANALYSIS PRINT ALL
CHANGE
PARAMETER 1
CODE CANADIAN
FYLD 50 MEMB 1 TO 5 9 TO 12 18 23 TO 26 30 TO 34
UNIT MMS NEWTON
PARAMETER 4
CODE CANADIAN
FU 364.754 MEMB 1 TO 8 11 TO 32 34
PARAMETER 5
CODE CANADIAN
FYLD 248.211 MEMB 6 TO 8 15 TO 17 20 21 27 TO 29
CHECK CODE ALL
FINISH