# EX. US-25 Analysis of a Structure with Compression-Only Members

This example demonstrates the usage of compression-only members. Since the structural condition is load dependent, the PERFORM ANALYSIS command is specified once for each primary load case.

This problem is installed with the program by default to C:\Users\Public\Public Documents\STAAD.Pro CONNECT Edition\Samples\Sample Models\US\US-25 Analysis of a Structure with Compression-Only Members.STD when you install the program.

### Example Problem No. 25

This example has been created to illustrate the command specification for a structure with certain members capable of carrying compressive force only. It is important to note that the analysis can be done for only 1 load case at a time. This is because, the set of "active" members (and hence the stiffness matrix) is load case dependent.

where:
• L = 15 ft, H = 10 ft
• Load case 1: P1 = 10 kips & P2 = 15 kips
• Load case 2: P3 = -10 kips & P4 = -15 kips
```    STAAD PLANE
* EXAMPLE FOR COMPRESSION-ONLY MEMBERS```

The input data is initiated with the word STAAD. This structure is a PLANE frame. The second line is an optional comment line.

`    UNIT FEET KIP`

Units for the commands to follow are specified above.

```    JOINT COORDINATES
1 0 0 ; 2 0 10 ; 3 0 20 ; 4 15 20 ; 5 15 10 ; 6 15 0```

Joint coordinates of joints 1 to 6 are defined above.

```    MEMBER INCIDENCES
1 1 2 5 ; 6 1 5 ; 7 2 6 ; 8 2 4 ; 9 3 5 ; 10 2 5```

The members 1 to 10 are defined along with the joints to which they are connected.

```    MEMBER COMPRESSION
6 TO 9```

Members 6 to 9 are defined as COMPRESSION-only members. Hence for each load case, if during the analysis, any of the members 6 to 9 is found to be carrying a tensile force, it is disabled from the structure and the analysis is carried out again with the modified structure.

```    MEMBER PROPERTY AMERICAN
1 TO 10 TA ST W12X26```

Properties for members 1 to 10 are defined as the STandard W12X26 section from the American AISC steel table.

```    MEMBER PROPERTY BRITISH
1 TO 10 TA ST UC152X152X30```
```    DEFINE MATERIAL START
ISOTROPIC STEEL
E 29000
POISSON 0.3
DENSITY 283e-006
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 36 FU 58 RY 1.5 RT 1.2
END DEFINE MATERIAL
CONSTANT
MATERIAL STEEL ALL```

The DEFINE MATERIAL command is used to specify material properties and the CONSTANT is used to assign the material to all members.

```    SUPPORT
1 6 PINNED```

Joints 1 and 6 are declared as pinned-supported.

```    LOAD 1
2 FX 15
3 FX 10```

Load 1 is defined above and consists of joint loads in the global X direction at joints 2 and 3.

`    PERFORM ANALYSIS`

The above structure is analyzed for load case 1.

```    CHANGE
MEMBER COMPRESSION
6 TO 9```

One or more among the members 6 to 9 may have been in-activated in the previous analysis. The CHANGE command restores the original structure to prepare it for the analysis for the next primary load case. The members with the compression-only attribute are specified again.

```    LOAD 2
4 FX -10
5 FX -15```

In load case 2, joint loads are applied in the negative global X direction at joints 4 and 5.

```    PERFORM ANALYSIS
CHANGE```

The instruction to analyze the structure is specified again. Next, any compression-only members that were inactivated during the second analysis (due to the fact that they were subjected to tensile axial forces) are re-activated with the CHANGE command. Without the re-activation, these members cannot be accessed for further processing.

```    LOAD 3
1 1.0 2 1.0```

Load case 3 illustrates the technique employed to instruct STAAD to create a load case which consists of data to be assembled from other load cases already specified earlier. We would like the program to analyze the structure for loads from cases 1 and 2 acting simultaneously. In other words, the above instruction is the same as the following:

```    LOAD 3
2 FX 15
3 FX 10
4 FX -15
5 FX –10
PERFORM ANALYSIS```

The analysis is carried out for load case 3.

`    CHANGE`

The members inactivated during the analysis of load case 3 are re-activated for further processing.

`    LOAD LIST ALL`

At the end of any analysis, only those load cases for which the analysis was done most recently, are recognized as the "active" load cases. The LOAD LIST ALL command enables all the load cases in the structure to be made active for further processing.

`    PRINT ANALYSIS RESULTS`

The program is instructed to write the joint displacements, support reactions and member forces to the output file.

`    FINISH`

## Input File

``````STAAD PLANE EXAMPLE FOR COMPRESSION-ONLY MEMBERS
UNIT FEET KIP
SET NL 3
JOINT COORDINATES
1 0 0 ; 2 0 10 ; 3 0 20 ; 4 15 20 ; 5 15 10 ; 6 15 0
MEMBER INCIDENCES
1 1 2 5
6 1 5 ; 7 2 6 ; 8 2 4 ; 9 3 5 ; 10 2 5
MEMBER COMPRESSION
6 TO 9
MEMBER PROPERTY AMERICAN
1 TO 10 TA ST W12X26
UNIT INCH
DEFINE MATERIAL START
ISOTROPIC STEEL
E 29000
POISSON 0.3
DENSITY 283e-006
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 36 FU 58 RY 1.5 RT 1.2
END DEFINE MATERIAL
CONSTANT
MATERIAL STEEL ALL
SUPPORT
1 6 PINNED
2 FX 15
3 FX 10
PERFORM ANALYSIS
CHANGE
MEMBER COMPRESSION
6 TO 9
4 FX -10
5 FX -15
PERFORM ANALYSIS
CHANGE
MEMBER COMPRESSION
6 TO 9
1 1.0 2 1.0
PERFORM ANALYSIS
CHANGE
PRINT ANALYSIS RESULTS
FINISH
``````

```                                                                  PAGE NO.    1
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*                                                  *
*           Version  22.08.00.***                  *
*           Proprietary Program of                 *
*           Bentley Systems, Inc.                  *
*           Date=    OCT 19, 2021                  *
*           Time=    17:55:24                      *
*                                                  *
*  Licensed to: Bentley Systems Inc                *
****************************************************
1. STAAD PLANE EXAMPLE FOR COMPRESSION-ONLY MEMBERS
INPUT FILE: US-25 Analysis of a Structure with Compression-Only Members.STD
2. UNIT FEET KIP
3. SET NL 3
4. JOINT COORDINATES
5. 1 0 0 ; 2 0 10 ; 3 0 20 ; 4 15 20 ; 5 15 10 ; 6 15 0
6. MEMBER INCIDENCES
7. 1 1 2 5
8. 6 1 5 ; 7 2 6 ; 8 2 4 ; 9 3 5 ; 10 2 5
9. MEMBER COMPRESSION
10. 6 TO 9
11. MEMBER PROPERTY AMERICAN
12. 1 TO 10 TA ST W12X26
13. UNIT INCH
14. DEFINE MATERIAL START
15. ISOTROPIC STEEL
16. E 29000
17. POISSON 0.3
18. DENSITY 283E-006
19. ALPHA 6E-006
20. DAMP 0.03
21. TYPE STEEL
22. STRENGTH FY 36 FU 58 RY 1.5 RT 1.2
23. END DEFINE MATERIAL
24. CONSTANT
25. MATERIAL STEEL ALL
26. SUPPORT
27. 1 6 PINNED
30. 2 FX 15
31. 3 FX 10
32. PERFORM ANALYSIS
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    2
P R O B L E M   S T A T I S T I C S
-----------------------------------
NUMBER OF JOINTS          6  NUMBER OF MEMBERS      10
NUMBER OF PLATES          0  NUMBER OF SOLIDS        0
NUMBER OF SURFACES        0  NUMBER OF SUPPORTS      2
Using 64-bit analysis engine.
SOLVER USED IS THE IN-CORE ADVANCED MATH SOLVER
TOTAL      PRIMARY LOAD CASES =     1, TOTAL DEGREES OF FREEDOM =      14
TOTAL LOAD COMBINATION  CASES =     0  SO FAR.
*** LOAD CASE     1 -- START ITERATION NO.   2
**NOTE-Tension/Compression converged after  2 iterations, Case=    1
33. CHANGE
34. MEMBER COMPRESSION
35. 6 TO 9
38. 4 FX -10
39. 5 FX -15
40. PERFORM ANALYSIS
*** LOAD CASE     2 -- START ITERATION NO.   2
**NOTE-Tension/Compression converged after  2 iterations, Case=    2
41. CHANGE
42. MEMBER COMPRESSION
43. 6 TO 9
46. 1 1.0 2 1.0
47. PERFORM ANALYSIS
**NOTE-Tension/Compression converged after  1 iterations, Case=    3
48. CHANGE
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    3
50. PRINT ANALYSIS RESULTS
ANALYSIS RESULTS
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    4
JOINT DISPLACEMENT (INCH RADIANS)    STRUCTURE TYPE = PLANE
------------------
JOINT  LOAD   X-TRANS   Y-TRANS   Z-TRANS   X-ROTAN   Y-ROTAN   Z-ROTAN
1    1    0.00000   0.00000   0.00000   0.00000   0.00000  -0.00041
2    0.00000   0.00000   0.00000   0.00000   0.00000   0.00050
3    0.00000   0.00000   0.00000   0.00000   0.00000  -0.00004
2    1    0.04314   0.01262   0.00000   0.00000   0.00000  -0.00025
2   -0.05088  -0.00373   0.00000   0.00000   0.00000   0.00024
3    0.00364   0.00080   0.00000   0.00000   0.00000  -0.00001
3    1    0.07775   0.01618   0.00000   0.00000   0.00000  -0.00022
2   -0.07766  -0.00381   0.00000   0.00000   0.00000   0.00015
3    0.00255   0.00215   0.00000   0.00000   0.00000   0.00001
4    1    0.07766  -0.00381   0.00000   0.00000   0.00000  -0.00015
2   -0.07775   0.01618   0.00000   0.00000   0.00000   0.00022
3   -0.00255   0.00215   0.00000   0.00000   0.00000  -0.00001
5    1    0.05088  -0.00373   0.00000   0.00000   0.00000  -0.00024
2   -0.04314   0.01262   0.00000   0.00000   0.00000   0.00025
3   -0.00364   0.00080   0.00000   0.00000   0.00000   0.00001
6    1    0.00000   0.00000   0.00000   0.00000   0.00000  -0.00050
2    0.00000   0.00000   0.00000   0.00000   0.00000   0.00041
3    0.00000   0.00000   0.00000   0.00000   0.00000   0.00004
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    5
SUPPORT REACTIONS -UNIT KIP  INCH    STRUCTURE TYPE = PLANE
-----------------
JOINT  LOAD   FORCE-X   FORCE-Y   FORCE-Z     MOM-X     MOM-Y     MOM Z
1    1     -0.13    -23.33      0.00      0.00      0.00      0.00
2     24.87     23.33      0.00      0.00      0.00      0.00
3      2.18      0.00      0.00      0.00      0.00      0.00
6    1    -24.87     23.33      0.00      0.00      0.00      0.00
2      0.13    -23.33      0.00      0.00      0.00      0.00
3     -2.18      0.00      0.00      0.00      0.00      0.00
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    6
MEMBER END FORCES    STRUCTURE TYPE = PLANE
-----------------
ALL UNITS ARE -- KIP  INCH     (LOCAL )
MEMBER  LOAD  JT     AXIAL   SHEAR-Y  SHEAR-Z   TORSION     MOM-Y      MOM-Z
1    1     1    -23.33      0.13     0.00      0.00      0.00       0.00
2     23.33     -0.13     0.00      0.00      0.00      15.87
2     1      6.90     -0.22     0.00      0.00      0.00      -0.00
2     -6.90      0.22     0.00      0.00      0.00     -25.90
3     1     -1.47      0.03     0.00      0.00      0.00       0.00
2      1.47     -0.03     0.00      0.00      0.00       3.24
2    1     2     -6.58      0.24     0.00      0.00      0.00      12.72
3      6.58     -0.24     0.00      0.00      0.00      15.70
2     2      0.15     -0.11     0.00      0.00      0.00      -2.32
3     -0.15      0.11     0.00      0.00      0.00     -11.30
3     2     -2.50     -0.03     0.00      0.00      0.00      -2.74
3      2.50      0.03     0.00      0.00      0.00      -0.79
3    1     3      0.11     -0.15     0.00      0.00      0.00     -15.70
4     -0.11      0.15     0.00      0.00      0.00     -11.30
2     3      0.11      0.15     0.00      0.00      0.00      11.30
4     -0.11     -0.15     0.00      0.00      0.00      15.70
3     3      6.28      0.00     0.00      0.00      0.00       0.79
4     -6.28     -0.00     0.00      0.00      0.00      -0.79
4    1     4      0.15      0.11     0.00      0.00      0.00      11.30
5     -0.15     -0.11     0.00      0.00      0.00       2.32
2     4     -6.58     -0.24     0.00      0.00      0.00     -15.70
5      6.58      0.24     0.00      0.00      0.00     -12.72
3     4     -2.50      0.03     0.00      0.00      0.00       0.79
5      2.50     -0.03     0.00      0.00      0.00       2.74
5    1     5      6.90      0.22     0.00      0.00      0.00      25.90
6     -6.90     -0.22     0.00      0.00      0.00       0.00
2     5    -23.33     -0.13     0.00      0.00      0.00     -15.87
6     23.33      0.13     0.00      0.00      0.00       0.00
3     5     -1.47     -0.03     0.00      0.00      0.00      -3.24
6      1.47      0.03     0.00      0.00      0.00       0.00
6    1     1      0.00      0.00     0.00      0.00      0.00       0.00
5      0.00      0.00     0.00      0.00      0.00       0.00
2     1     29.63      0.00     0.00      0.00      0.00       0.00
5    -29.63      0.00     0.00      0.00      0.00       0.00
3     1      2.66      0.00     0.00      0.00      0.00       0.00
5     -2.66      0.00     0.00      0.00      0.00       0.00
7    1     2     29.63      0.00     0.00      0.00      0.00       0.00
6    -29.63      0.00     0.00      0.00      0.00       0.00
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    7
MEMBER END FORCES    STRUCTURE TYPE = PLANE
-----------------
ALL UNITS ARE -- KIP  INCH     (LOCAL )
MEMBER  LOAD  JT     AXIAL   SHEAR-Y  SHEAR-Z   TORSION     MOM-Y      MOM-Z
2     2      0.00      0.00     0.00      0.00      0.00       0.00
6      0.00      0.00     0.00      0.00      0.00       0.00
3     2      2.66      0.00     0.00      0.00      0.00       0.00
6     -2.66      0.00     0.00      0.00      0.00       0.00
8    1     2      0.00      0.00     0.00      0.00      0.00       0.00
4      0.00      0.00     0.00      0.00      0.00       0.00
2     2     11.60      0.00     0.00      0.00      0.00       0.00
4    -11.60      0.00     0.00      0.00      0.00       0.00
3     2      4.51      0.00     0.00      0.00      0.00       0.00
4     -4.51      0.00     0.00      0.00      0.00       0.00
9    1     3     11.60      0.00     0.00      0.00      0.00       0.00
5    -11.60      0.00     0.00      0.00      0.00       0.00
2     3      0.00      0.00     0.00      0.00      0.00       0.00
5      0.00      0.00     0.00      0.00      0.00       0.00
3     3      4.51      0.00     0.00      0.00      0.00       0.00
5     -4.51      0.00     0.00      0.00      0.00       0.00
10    1     2     -9.55     -0.32     0.00      0.00      0.00     -28.60
5      9.55      0.32     0.00      0.00      0.00     -28.21
2     2     -9.55      0.32     0.00      0.00      0.00      28.21
5      9.55     -0.32     0.00      0.00      0.00      28.60
3     2      8.98     -0.00     0.00      0.00      0.00      -0.50
5     -8.98      0.00     0.00      0.00      0.00       0.50
************** END OF LATEST ANALYSIS RESULT **************
51. FINISH
EXAMPLE FOR COMPRESSION-ONLY MEMBERS                     -- PAGE NO.    8
*********** END OF THE STAAD.Pro RUN ***********
**** DATE= OCT 19,2021   TIME= 17:55:25 ****
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