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TR.23.2 Member Cable Specification

This command may be used to model a specified set of members as cable members.

Cable members, in addition to elastic axial deformation, are also capable of accommodating the stiffness effect of initial tension and tension due to static loads. When used in a nonlinear cable analysis, cable members are capable of accommodating large displacements. See G.8.2 Cable Members for a theoretical discussion of cable members.

General Format

member-list cable-spec 
cable-spec  = { TENSION f1 ( { START | END } )| LENGTH f2 } *{ (FWX f3) | (FWY f4) | (FWZ f5) }


TENSION f1 Initial tension in cable member (in current units), when the TENSION option is specified.
LENGTH f2 Unstressed cable length (in current units), when the initial LENGTH option is specified.
FWX f3, FWY f4, FWZ f5 Multiplying factors on self weight components applied in the global X, Y, and Z directions, respectively.
START, END The initial tension is measured at the cable start node or cable end node, respectively. This is used for advanced cable analysis (refer to Note 5 in Nodes for use with Advanced Cable Analysis).

Notes for use with Standard Cable Analysis

  1. The tension specified in the cable member is applied on the structure as an external load as well as is used to modify the stiffness of the member. The tension value must be positive to be treated as a cable otherwise it is a truss (See G.8.2 Cable Members). If the TENSION parameter or the value is omitted, a minimum tension will be used.

    The end at which initial tension is measured is not used for standard cable analysis (i.e., START or END is ignored).

  2. This is a truss member but not a tension-only member unless you also include this member in a MEMBER TENSION input (See TR.23.3 Member Tension/Compression Specification). Note also that Member Releases are not allowed.
  3. The tension is a preload and will not be the final tension in the cable after the deformation due to this preload.
  4. The tension is used to determine the unstressed length. That length will be shorter than the distance between the joints by the distance that the tension will stretch the cable.
  5. No weight (other than the assumed self weight) is used with standard cable analysis.
  6. Cables should not be used in analysis involving frequency extraction or for dynamic loading conditions such as response spectrum, time history, or steady state.

Notes for use with Advanced Cable Analysis

  1. A cable member is a truss member that has three translational degrees of freedom only. Note also that member releases are not allowed for a cable member.
  2. FWY is used to add any additional weight that may be acting on the cable along with self weight before the application of external applied load. This additional weight will be added to cable self-weight in global Y direction. These are used to find the initial cable profile before the application of the external load.

  3. If no self weight is included in the external applied load and also FWY parameter is not defined, the program will calculate self weight of the cable member and include it in the analysis.

    Warning message will be issued in the output file.

  4. If self weight is included in the external applied load, during calculation cable self weight will not be considered in the external applied load vector. The reason is that it will be considered separately while finding the initial cable configuration under self weight and thus cannot be considered as external applied load.

  5. If START or END is not specified with the initial TENSION parameter in the cable member, then the average tension is assumed. The average option is suitable only for a taut cable. This is because in a taut cable, the undeformed length is shorter than the chord length. due to this, the taut cable carries significant tension with little sag. The undeformed legth, Lu, is calculated as:
    L u = L c ( 1 + T a v g / E A )
    chord length
    initial average tension in the taut cable
    Young's modulus
    area of the cross-section

    However, for a slack cable, this does not hold true because the undeformed length is longer that the chord length and the cable member has significant sag. The catenary curve effect is required to be included. Thus, for a slack cable, initial TENSION should be defined either at the START or END node of the cable member. For a cable member which is considered to have significant sag, defining the average tension is not a suitable option.

  6. Cables should not be used in analysis involving frequency extraction or for dynamic loading conditions such as response spectrum, time history, or steady state.


A series of cable members with a tension of 15.5 specified for use with either standard or advanced cable analysis:

20 TO 25 TENSION 15.5

A series of cable members with a tension of 15.5 at the cable start node for use with advanced cable analysis. An additional weight on the cable of 20% of the cable self weight which acts in the same direction of the cable's self weight is used.

20 TO 25 TENSION 15.5 START FWY -0.2