This example demonstrates how non-linear cable members can be modelled in STAAD. In this example, members 1 and 3 are cables.

STAAD PLANE

Every input has to start with the word STAAD. The word PLANE signifies that the structure is a plane frame (2-D) structure.

SET NL 4

This structure has to be analysed for 3 primary load cases. Consequently, the modeling of our problem requires us to define 3 sets of data, with each set containing a load case and an associated analysis command. To accommodate these requirements, it is necessary to have 2 commands, one called SET NL and the other called CHANGE. The SET NL command is used above to indicate the total number of primary load cases that the file contains. The CHANGE command will come in later (after the PERFORM ANALYSIS command).

UNIT KIP FT

Units for the data to follow are specified above.

JOINT COORD

1 0 0 0 ; 2 10 0 0 ; 3 20 0 0 ; 4 10 15 0

Joint coordinates of nodes 1 to 4 are specified. As the structure is a plane frame, the Z coordinates are all the same, namely, 0.0.

MEMB INCI

1 1 4 ; 2 2 4 ; 3 3 4

The connectivity of the members (the 2 nodes between which each member is connected) is specified above.

UNIT INCH

MEMB PROP AMERICAN

1 3 PRIS YD 1.0 ZD 1.0

2 TA ST W8x18

The cable members are defined as having a rectangular cross section (1.0 inches X 1.0 inches). The vertical member is assigned a W8X18 section from the American steel table.

MEMBER CABLE

1 3 TENSION 50.0

The cables are subjected to an initial tension of 50.0 kips.

CONST

E STEEL ALL

POISSON STEEL ALL

The material constants, namely, Modulus of Elasticity, and Poisson's ratio, are specified. The value assigned happens to be the built-in default value for steel.

SUPPORT

1 3 PINNED

2 FIXED

Nodes 1 and 3 are Pinned supported. Node 2 is fully restrained.

LOAD 1

JOINT LOAD

4 FX 0.01

Load case 1 happens to be a 10 pound force along the global X direction at node 4.

PERF CABLE ANALY SAGMIN 0.0 PRINT STATIC CHECK

In order to instruct STAAD to analyse structures having cable members using the non-linear analysis approach, the command required is PERFORM CABLE ANALYSIS. The SAGMIN parameter is specified as having a value of 0.0 to account for full sag E reduction. The static equilibrium report is sought using the keywords PRINT STATICS CHECK.

CHANGE

The CHANGE command restores the original structure to prepare it for the analysis for the next primary load case.

LOAD 2

JOINT LOAD

4 FX 40.0

Load case 2 consists of a 40 kip force along the global X direction at node 4.

PERF CABLE ANALY SAGMIN 0.0 PRINT STATIC CHECK

CHANGE

The ANALYSIS instruction is specified for the 2nd load case.

LOAD 3

JOINT LOAD

4 FX 46.3

Load case 3 consists of a 46.3 kip force along the global X direction at node 4.

PERF CABLE ANALY SAGMIN 0.0 PRINT STATIC CHECK

CHANGE

The ANALYSIS instruction is specified for the 3rd load case.

LOAD LIST ALL

In a multiple analysis sequence like the one above, by default, only the last specified load case is "active" at any given stage. The above command restores all load cases to the "active" state for further processing.

PRINT JOINT DISP

PRINT MEMB FORCES

PRINT SUPPORT REACT

Analysis results, namely, Joint Displacements, Support Reactions, and Member Forces are requested.

FINISH