Can someone please correct stiffness issues in this space truss model?

Code is as follows -

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 28-Aug-18
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET POUND
JOINT COORDINATES
1 -3.375 0 0; 2 -2.958 0 0; 3 -1.125 0 0; 4 -0.875 -1.574 0;
5 -0.29167 -1.574 0; 6 0 0 0; 7 1.667 -1.574 0; 8 1.667 0 0; 9 3.334 0 0;
10 5.001 -1.574 0; 11 5.001 0 0; 12 6.668 0 0; 13 8.335 -1.574 0; 14 8.335 0 0;
15 10.002 0 0; 16 11.669 -1.574 0; 17 11.669 0 0; 18 13.336 0 0;
19 15.003 -1.574 0; 20 15.003 0 0; 21 16.67 0 0; 22 18.337 -1.574 0;
23 18.337 0 0; 24 20.004 0 0; 25 20.2957 -1.574 0; 26 20.879 -1.574 0;
27 21.129 0 0; 28 22.962 0 0; 29 23.046 0 0;
MEMBER INCIDENCES
1 1 2; 2 2 3; 3 2 5; 4 3 5; 5 4 5; 6 3 6; 7 5 6; 8 5 7; 9 6 7; 10 6 8; 11 7 8;
12 7 9; 13 8 9; 14 7 10; 15 9 10; 16 9 11; 17 10 11; 18 10 12; 19 11 12;
20 10 13; 21 12 13; 22 12 14; 23 13 14; 24 13 15; 25 14 15; 26 13 16; 27 15 16;
28 15 17; 29 16 17; 30 16 18; 31 17 18; 32 16 19; 33 18 19; 34 18 20; 35 19 20;
36 19 21; 37 20 21; 38 19 22; 39 21 22; 40 21 23; 41 22 23; 42 22 24; 43 23 24;
44 22 25; 45 24 25; 46 24 27; 47 25 26; 48 25 27; 49 25 28; 50 27 28; 51 28 29;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+009
POISSON 0.3
DENSITY 489.024
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 7.2e+006 FU 9.36e+006 RY 1.5 RT 1.2
G 1.60615e+009
END DEFINE MATERIAL
MEMBER PROPERTY SJIJOIST
1 TO 51 TABLE ST 24LH09
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
2 28 FIXED BUT FX MY MZ
MEMBER TRUSS
3 4 7 9 11 12 15 17 18 21 23 24 27 29 30 33 35 36 39 41 42 45 48 49
LOAD 1 LOADTYPE Dead TITLE DEAD
MEMBER LOAD
1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51 UNI GY -137.5
1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51 UNI GY -10
LOAD 2 LOADTYPE Snow TITLE SNOW BALANCED
MEMBER LOAD
1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51 UNI GY -137.5
LOAD 3 LOADTYPE Snow TITLE SNOW DRIFT
MEMBER LOAD
22 TRAP GY -8.48224 0 0 0.624
19 TRAP GY -31.1423 -8.48224 0 1.667
16 TRAP GY -53.8024 -31.1423 0 1.667
13 TRAP GY -76.4625 -53.8024 0 1.667
10 TRAP GY -99.1225 -76.4625 0 1.667
6 TRAP GY -114.415 -99.1225 0 1.125
2 TRAP GY -139.332 -114.415 0 1.833
1 TRAP GY -145 -139.332 0 0.417
51 UNI GY -185
50 UNI GY -185 1.75 1.833
LOAD COMB 4 D+S
1 1.0 2 1.0 3 1.0
PERFORM ANALYSIS
*PARAMETER 1
*CODE AISC UNIFIED 2010
*METHOD ASD
*DJ1 4 MEMB 5 8 14 20 26 32 38 44 47
*DJ2 26 MEMB 5 8 14 20 26 32 38 44 47
*DFF 0.09064 MEMB 5 8 14 20 26 32 38 44 47
*DJ1 1 MEMB 1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51
*DJ2 29 MEMB 1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51
*DFF 0.11 MEMB 1 2 6 10 13 16 19 22 25 28 31 34 37 40 43 46 50 51
FINISH

  • There are a few issues in your model:

    1) You should not be using a Joist properties for this truss. Instead, use some standard section, i.e. channel, angle or any other from which the truss will be made.

    2) Do not release FX direction for both supports.

    3) Change MEMBER TRUSS specification to Partial Moment release with a factor 0.99 at both ends of the truss members. This will allow the truss member to experience or carry a very little moments even though in usual way we analyze the truss to carry axial force only, but will eliminate instabilities. You should change the input from:

    MEMBER TRUSS
    3 4 7 9 11 12 15 17 18 21 23 24 27 29 30 33 35 36 39 41 42 45 48 49

    to:

    MEMBER RELEASE
    3 4 7 9 11 12 15 17 18 21 23 24 27 29 30 33 35 36 39 41 42 45 48 -
    49 BOTH MPY 0.99 MPZ 0.99

    With these modifications all warnings should disappear.



  • First and second point is very stupid I did. Thank You. I have changed that. I tried using that third point you have mentioned but then the thing is then every member of the top chord will fail for the defined loading. You can see that in this code I have modified -

    STAAD SPACE
    START JOB INFORMATION
    ENGINEER DATE 28-Aug-18
    END JOB INFORMATION
    INPUT WIDTH 79
    UNIT FEET POUND
    JOINT COORDINATES
    1 -3.375 0 0; 2 -2.958 0 0; 3 -1.125 0 0; 4 -0.875 -1.574 0;
    5 -0.29167 -1.574 0; 6 0 0 0; 7 1.667 -1.574 0; 8 1.667 0 0; 9 3.334 0 0;
    10 5.001 -1.574 0; 11 5.001 0 0; 12 6.668 -1.574 0; 13 6.668 0 0;
    14 8.335 -1.574 0; 15 8.335 0 0; 16 10.002 0 0; 17 11.669 -1.574 0;
    18 11.669 0 0; 19 13.336 -1.574 0; 20 13.336 0 0; 21 15.003 -1.574 0;
    22 15.003 0 0; 23 16.67 0 0; 24 18.337 -1.574 0; 25 18.337 0 0; 26 20.004 0 0;
    27 20.2957 -1.574 0; 28 20.879 -1.574 0; 29 21.129 0 0; 30 22.962 0 0;
    31 23.046 0 0;
    MEMBER INCIDENCES
    1 1 2; 2 2 3; 3 2 5; 4 3 5; 5 4 5; 6 3 6; 7 5 6; 8 5 7; 9 6 7; 10 6 8; 11 7 8;
    12 7 9; 13 8 9; 14 7 10; 15 9 10; 16 9 11; 17 10 11; 18 10 12; 19 10 13;
    20 11 13; 21 12 14; 22 13 14; 23 13 15; 24 14 15; 25 14 16; 26 15 16; 27 14 17;
    28 16 17; 29 16 18; 30 17 18; 31 17 19; 32 17 20; 33 18 20; 34 19 21; 35 20 21;
    36 20 22; 37 21 22; 38 21 23; 39 22 23; 40 21 24; 41 23 24; 42 23 25; 43 24 25;
    44 24 26; 45 25 26; 46 24 27; 47 26 27; 48 26 29; 49 27 28; 50 27 29; 51 27 30;
    52 29 30; 53 30 31;
    START USER TABLE
    TABLE 1
    UNIT FEET POUND
    GENERAL
    L1X1X0.109
    0.00142805 0.083 0.083 0.0833 0.083 9.23571e-007 9.32e-007 3.88736e-008 -
    1.56679e-005 1.57707e-005 0.000627538 0.000630195 3.97331e-005 2.00648e-005 -
    1.97952e-011 0.083
    PROFILE_POINTS
    -0.024203 -0.024053 0.059097 -0.024053 0.059097 -0.01497 -0.01512 -0.01497 -0.01512 0.058947 -0.024203 0.058947
    L1.5X1.5X0.138
    0.00274275 0.125 0.125 0.125 0.125 4.0893e-006 4.0893e-006 1.20199e-007 -
    4.5687e-005 4.5687e-005 0.00119379 0.00119379 0.000115727 5.8267e-005 -
    1.41145e-010 0.125
    PROFILE_POINTS
    -0.0354932 -0.0354932 0.0895068 -0.0354932 0.0895068 -0.0239932 -0.0239932 -0.0239932 -0.0239932 0.0895068 -0.0354932 0.0895068
    L1.5X1.5X0.113
    0.0022655 0.125 0.125 0.125 0.125 3.42943e-006 3.42943e-006 6.74622e-008 -
    3.7993e-005 3.7993e-005 0.000976568 0.000976568 9.63999e-005 4.84214e-005 -
    7.99326e-011 0.125
    PROFILE_POINTS
    -0.0347352 -0.0347352 0.0902648 -0.0347352 0.0902648 -0.0253185 -0.0253185 -0.0253185 -0.0253185 0.0902648 -0.0347352 0.0902648
    L1.25X1.25X0.109
    0.00180985 0.104167 0.104167 0.104167 0.104167 1.88205e-006 1.88205e-006 -
    4.96043e-008 2.51709e-005 2.51709e-005 0.000785407 0.000785407 6.37924e-005 -
    3.20949e-005 4.06177e-011 0.104167
    PROFILE_POINTS
    -0.0293962 -0.0293962 0.0747708 -0.0293962 0.0747708 -0.0203129 -0.0203129 -0.0203129 -0.0203129 0.0747708 -0.0293962 0.0747708
    2L2X2X0.163
    0.00642248 0.166666 0.166666 0.319749 0.166666 1.38921e-005 3.70354e-005 -
    3.99687e-007 0.000104535 0.000231653 0.00156886 0.00368668 0.00035424 -
    0.000184644 8.46334e-010 0.166666
    PROFILE_POINTS
    -0.159874 0.0201882 -0.0067915 0.0201882 -0.0067915 -0.132895 0.0067915 -0.132895 0.0067915 0.0201882 0.159874 0.0201882 0.159874 0.0337712 -0.159874 0.0337712
    2L2X2X0.137
    0.00573063 0.166667 0.166667 0.324083 0.166667 1.32237e-005 3.24129e-005 -
    2.80594e-007 0.000100615 0.000199918 0.00147354 0.00314488 0.000306415 -
    0.000176607 6.06695e-010 0.166667
    PROFILE_POINTS
    -0.161952 0.0238211 -0.00670238 0.0238211 -0.00670238 -0.131429 0.00637762 -0.131429 0.00637762 0.0238211 0.162131 0.0238211 0.162131 0.0352378 -0.161952 0.0352378
    END
    DEFINE MATERIAL START
    ISOTROPIC STEEL
    E 4.176e+009
    POISSON 0.3
    DENSITY 489.024
    ALPHA 6e-006
    DAMP 0.03
    TYPE STEEL
    STRENGTH FY 7.2e+006 FU 9.36e+006 RY 1.5 RT 1.2
    G 1.60615e+009
    END DEFINE MATERIAL
    MEMBER PROPERTY AMERICAN
    4 11 17 22 24 30 32 37 43 50 UPTABLE 1 L1X1X0.109
    7 12 41 47 UPTABLE 1 L1.5X1.5X0.138
    9 44 UPTABLE 1 L1.5X1.5X0.113
    15 19 25 28 35 38 UPTABLE 1 L1.25X1.25X0.109
    1 TO 3 6 10 13 16 20 23 26 29 33 36 39 42 45 48 51 TO 53 UPTABLE 1 2L2X2X0.163
    5 8 14 18 21 27 31 34 40 46 49 UPTABLE 1 2L2X2X0.137
    CONSTANTS
    BETA 180 MEMB 5 8 14 18 21 27 31 34 40 46 49
    MATERIAL STEEL ALL
    SUPPORTS
    2 PINNED
    31 FIXED BUT FX MY MZ
    12 19 FIXED BUT FX FY MX MY MZ
    MEMBER TRUSS
    3 4 7 9 11 12 15 17 19 22 24 25 28 30 32 35 37 38 41 43 44 47 50 51
    *MEMBER RELEASE
    *2 6 8 10 13 14 16 18 20 23 26 27 29 31 33 36 39 40 42 45 46 48 49 -
    *52 START MP 0.99
    *2 6 8 10 13 14 16 20 21 23 26 27 29 33 34 36 39 40 42 45 46 48 52 END MP 0.99
    LOAD 1 LOADTYPE Dead TITLE DEAD
    MEMBER LOAD
    1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53 UNI GY -137.5
    1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53 UNI GY -10
    LOAD 2 LOADTYPE Snow TITLE SNOW BALANCED
    MEMBER LOAD
    1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53 UNI GY -137.5
    LOAD 3 LOADTYPE Snow TITLE SNOW DRIFT
    MEMBER LOAD
    23 TRAP GY -8.48224 0 0 0.624
    20 TRAP GY -31.1423 -8.48224 0 1.667
    16 TRAP GY -53.8024 -31.1423 0 1.667
    13 TRAP GY -76.4625 -53.8024 0 1.667
    10 TRAP GY -99.1225 -76.4625 0 1.667
    6 TRAP GY -114.415 -99.1225 0 1.125
    2 TRAP GY -139.332 -114.415 0 1.833
    1 TRAP GY -145 -139.332 0 0.417
    53 UNI GY -185
    52 UNI GY -185 1.75 1.833
    LOAD 5 LOADTYPE Dead TITLE Special
    MEMBER LOAD
    1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53 UNI GY -573
    LOAD COMB 4 D+S
    1 1.0 2 1.0 3 1.0
    PERFORM ANALYSIS
    PARAMETER 1
    CODE AISC UNIFIED 2010
    METHOD ASD
    **DJ1 4 MEMB 5 8 14 18 21 27 31 34 40 46 49
    **DJ2 28 MEMB 5 8 14 18 21 27 31 34 40 46 49
    **DFF 0.09064 MEMB 5 8 14 18 21 27 31 34 40 46 49
    **DJ1 1 MEMB 1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53
    **DJ2 31 MEMB 1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53
    **DFF 0.11 MEMB 1 2 6 10 13 16 20 23 26 29 33 36 39 42 45 48 52 53
    CHECK CODE ALL
    FINISH

  • It does not matter if you are using MEMBER TRUSS or PARTIAL RELEASE specification - the members in the top chord will be failing in any case. The suggestion to use PARTIAL RELEASE specification was just to get rid off the instability warnings.

    I see that you are not defining any design parameters for the design. Do you really want to perform the design with the default values of the design parameters? In such case, you probably need to increase the section sizes of the members which are failing.



  • I will give design values. Skipped it for now as I want to see the behavior of the model. Thanks