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Section 1
1-51
K
KK
comb
sag elastic
=
+
1
11//
K
comb
= (EA/L) / [1+w
2
L
2
EA(cos
2
α)/12T
3
]
Note: When T = infinity, K
comb
= EA/L
When T = 0, K
comb
= 0
It may be noticed that as the tension increases (sag decreases) the
combined stiffness approaches that of the pure elastic situation.
The following points need to be considered when using the linear
cable member in STAAD :
1) The linear cable member is only a truss member whose
properties accommodate the sag factor and initial tension. The
behavior of this cable member is identical to that of the truss
member. It can carry axial loads only. As a result, the
fundamental rules involved in modeling truss members have to
be followed when modeling cable members. For example,
when two cable members meet at a common joint, if there isn't
a support or a 3rd member connected to that joint, it is a point
of potential instability.
2) Due to the reasons specified in 1) above, applying a transverse
load on a cable member is not advisable. The load will be
converted to two concentrated loads at the 2 ends of the cable
and the true deflection pattern of the cable will never be
realized.
3) A tension only cable member offers no resistance to a
compressive force applied at its ends. When the end joints of
the member are subjected to a compressive force, they "give
in" thereby causing the cable to sag. Under these
circumstances, the cable member has zero stiffness and this
situation has to be accounted for in the stiffness matrix and the
displacements have to be recalculated. But in STAAD, merely
declaring the member to be a cable member does not guarantee
that this behavior will be accounted for. It is also important