• State Suggested Answer
  • Replies 4 replies
  • Answers 2 answers
  • Subscribers 110 subscribers
  • Views 6407 views
  • Users 0 members are here

Truss Design Instability Issue At Joint

mohd rithauddin
Offline

Hello Expert,

I had design a simple trusses. After ran the analysis there are an error said this:-

***WARNING - INSTABILITY AT JOINT 7 DIRECTION = FY
PROBABLE CAUSE SINGULAR-ADDING WEAK SPRING
K-MATRIX DIAG= 2.9787494E+03 L-MATRIX DIAG= 4.5474735E-13 EQN NO 12
***NOTE - VERY WEAK SPRING ADDED FOR STABILITY

***WARNING - INSTABILITY AT JOINT 4 DIRECTION = FX
PROBABLE CAUSE SINGULAR-ADDING WEAK SPRING
K-MATRIX DIAG= 4.2165520E+04 L-MATRIX DIAG= 3.4560799E-10 EQN NO 5
***NOTE - VERY WEAK SPRING ADDED FOR STABILITY

***WARNING - INSTABILITY AT JOINT 5 DIRECTION = FX
PROBABLE CAUSE SINGULAR-ADDING WEAK SPRING
K-MATRIX DIAG= 4.2165520E+04 L-MATRIX DIAG= 3.6743586E-10 EQN NO 7
***NOTE - VERY WEAK SPRING ADDED FOR STABILITY

I had refer some explanation in this forum regarding the instability  issue which is they proposed to put some code MP0.99 at the Staad Input but Its seem still has an error. Somebody please help me on this. Below is my input file. Thanks in advance.

STAAD TRUSS DXF IMPORT OF DRAWING1.DXF
START JOB INFORMATION
ENGINEER DATE 05-May-15
JOB NAME Gantry 2
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 1315.25 88.5891 0; 2 1315.25 89.1991 0; 3 1315.5 80.7391 0;
4 1315.5 88.5891 0; 5 1315.5 89.1991 0; 6 1316.25 88.5891 0;
7 1316.25 89.1991 0; 8 1317.47 88.5891 0; 9 1317.47 89.1991 0;
10 1318.69 88.5891 0; 11 1318.69 89.1991 0; 12 1319.91 88.5891 0;
13 1319.91 89.1991 0; 14 1321.13 88.5891 0; 15 1321.13 89.1991 0;
16 1322.35 88.5891 0; 17 1322.35 89.1991 0; 18 1323.57 88.5891 0;
19 1323.57 89.1991 0; 20 1324.79 88.5891 0; 21 1324.79 89.1991 0;
22 1326.01 88.5891 0; 23 1326.01 89.1991 0; 24 1327.23 88.5891 0;
25 1327.23 89.1991 0; 26 1328.45 88.5891 0; 27 1328.45 89.1991 0;
28 1329.67 88.5891 0; 29 1329.67 89.1991 0; 30 1330.89 88.5891 0;
31 1330.89 89.1991 0; 32 1332.11 88.5891 0; 33 1332.11 89.1991 0;
34 1333.33 88.5891 0; 35 1333.33 89.1991 0; 36 1334.08 80.7391 0;
37 1334.08 88.5891 0; 38 1334.08 89.1991 0; 39 1334.33 88.5891 0;
40 1334.33 89.1991 0;
MEMBER INCIDENCES
1 1 4; 2 2 5; 3 3 4; 4 4 5; 5 4 6; 6 5 7; 7 6 7; 8 6 8; 9 6 9; 10 7 9; 11 9 8;
12 8 10; 13 8 11; 14 9 11; 15 11 10; 16 10 12; 17 10 13; 18 11 13; 19 13 12;
20 12 14; 21 12 15; 22 13 15; 23 15 14; 24 14 16; 25 14 17; 26 15 17; 27 17 16;
28 16 18; 29 16 19; 30 17 19; 31 19 18; 32 18 20; 33 18 21; 34 19 21; 35 21 20;
36 20 22; 37 20 23; 38 21 23; 39 23 22; 40 22 24; 41 22 25; 42 23 25; 43 25 24;
44 24 26; 45 24 27; 46 25 27; 47 27 26; 48 26 28; 49 26 29; 50 27 29; 51 29 28;
52 28 30; 53 28 31; 54 29 31; 55 31 30; 56 30 32; 57 30 33; 58 31 33; 59 33 32;
60 32 34; 61 32 35; 62 33 35; 63 34 35; 64 34 37; 65 35 38; 66 36 37; 67 37 38;
68 37 39; 69 38 40;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
END DEFINE MATERIAL
MEMBER PROPERTY BRITISH
1 2 5 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 -
54 56 58 60 62 64 65 68 69 TABLE ST PIPE OD 0.3185 ID 0.3047
7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 -
59 61 63 TABLE ST PIPE OD 0.1143 ID 0.1053
3 4 66 67 TABLE ST PIPE OD 0.4064 ID 0.3906
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
3 36 FIXED
MEMBER RELEASE
4 5 6 START MP 0.99
4 5 6 END MP 0.99
LOAD 1 LOADTYPE None TITLE GK
SELFWEIGHT Y -1
JOINT LOAD
15 27 FY -10.73
MEMBER LOAD
2 6 10 14 18 22 26 30 34 38 42 46 50 54 58 62 65 69 UNI GY -0.95
LOAD 2 LOADTYPE None TITLE QK
MEMBER LOAD
2 6 10 14 18 22 26 30 34 38 42 46 50 54 58 62 65 69 UNI GY -0.5
LOAD 3 LOADTYPE None TITLE QK (POINT LOAD)
JOINT LOAD
21 FY -1.5
LOAD 4 LOADTYPE None TITLE WK
JOINT LOAD
15 27 FZ 15.636
LOAD COMB 5 CASE 1
1 1.4 2 1.6
LOAD COMB 6 CASE 2
1 1.4 3 1.6
LOAD COMB 7 CASE 3
1 1.0 4 1.4
LOAD COMB 8 CASE 4
1 1.2 2 1.2 4 1.2
PERFORM ANALYSIS
PRINT ANALYSIS RESULTS
PARAMETER 1
CODE BS5950
CHECK CODE ALL
FINISH

Parents
  • 0 Offline

    Hi,

    You have specified all members in your model as truss member using the command STAAD TRUSS. When you declare all members connecting at specific nodes to be truss members, the alignment of the members must be such that the axial force from each member must be able to make its way through the common node to the other members. As the truss members are able to carry and transmit only the axial forces, so the load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through. For that reason there are several instability warning messages. An instability is a condition where a load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through, and may result in a lack of equilibrium between the applied load and the support reaction.

    You can use the following command- 

    STAAD SPACE

    You can define the members as truss members by partially releasing the moments at the start and the end nodes (1% fixity) as--

    MEMBER RELEASE

    <member number> START MP 0.995

    <member number>  END MP 0.995

    You can also go through the following post--

    http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/staad-pro-instability-and-zero-stiffness-faq



  • 0 Offline in reply to Surojit Ghosh
    Thanks Suro spending your time to answer my question. From your response I have a further question regarding that matter:-

    1. From your explanation above, did you mean the right method to design a trusses is to set input file to STAAD SPACE and put START OR END MP0.995 for each member. If that a right way so what purpose of the command for STAAD TRUSS and MEMBER TRUSS assign in PROPERTIES. May i know where to use this command.

    2. In trusses design is that a wrong design if we analysis trusses with input file STAAD SPACE and DID NOT put the START OR END MP0.995. Because the analysis and design result still pass but node have a Z value which is to cater moment. In trusses basic design node is design for axial load with no moment. Is that a correct analysis for trusses? Can we design trusses with have a moment at node?

    Thanks in advance.

    Rithauddin
  • 0 Offline in reply to mohd rithauddin

    I would think that your top and bottom chord members would be continuous members and would be subjected to bending. So you should not declare these or the vertical columns as TRUSS members. Instead you should declare the verticals and inclined braces, connecting the top and the bottom chords, as TRUSS members.

    Partial releases are usually applied to stabilize the models but with the changes mentioned above, I did not see instabilities in the model anymore. So you do not have to use partial releases.

    A modified file is attached for your reference

    7433.Structure1.std



Reply
  • 0 Offline in reply to mohd rithauddin

    I would think that your top and bottom chord members would be continuous members and would be subjected to bending. So you should not declare these or the vertical columns as TRUSS members. Instead you should declare the verticals and inclined braces, connecting the top and the bottom chords, as TRUSS members.

    Partial releases are usually applied to stabilize the models but with the changes mentioned above, I did not see instabilities in the model anymore. So you do not have to use partial releases.

    A modified file is attached for your reference

    7433.Structure1.std



Children
Communities
Support and Services
Training and Learning
Social Media