SUSPNDRS: a numerical simulation tool for the nonlinear transient analysis of cable support bridge structures, part 1: theoretical development Page: 120 of 138

                                
12.6 Slacking/tensioning of a cable segment

To investigate the effect of slackening and tensioning of a cable segment, the problem
shown in Figure 59 was analyzed with the SUSPNDRS program. The problem consisted
of a 5" x 5" steel column with a 1/4" diameter steel cable attached to the upper end. In this
analysis, the structure was first subjected to gravity loading to initialize the cable geometry
and, subsequent to the gravity initialization, a transverse tip load was applied at the top of
the cantilever. The tip load was first applied in the direction of the cable, and then in a
direction opposite the cable. During the first part of the loading the cable continues to sag
and does not resist transverse loading (See "B" in Figure 59), after the load is reversed and
the magnitude of the load is increased, the slack is finally removed from the cable (see "D"
in Figure 59) and a sudden increase in stiffness is observed as the stiffness of the cable is
then enaged. During the initial loading the system stiffness is essentially the stiffness of
the steel column and after the cable sag is removed the effective system stiffness is the
stiffness of the column plus the axial stiffness of the steel cable. A comparison of the finite
element model computed stiffnesses with the analytical stiffnesses is shown in Table 2,.
TABLE 2. Computational and analytical stiffnesses of the steel column/cable
system
Portion of       Stiffness from      Stiffness from
loading regime  computational model  analytical expressions
Cable sagging K1      562 lb/in           566 lb/in
Cable tensioned K2    13,333 lb/in        13,300 lb/in

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