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Steel Designers' Manual - 6th Edition (2003)
196 Other structural applications of steel
(1) Formfinding: In this stage the cables are treated as constant tension elements
and the geometry is allowed to move into its equilibrium position. On completion
of the formfinding stage the cable net geometry under the prestress forces
will be defined.
(2) Load analysis:The prestress model is converted into an elastic model.To do this
the slack lengths, l, of all elements must be set so that
T
t0 = l-
EA
The model is then analysed for the defined dead, wind and snow loads.
(3) Cutting pattern definitions: Cable net structures are fully prefabricated with
exact cable lengths so that the prestressed form can be realized. In this stage
the form model will be refined so that the mesh lengths are exactly equal, etc.
The offsets for the boundary clamps must be allowed for. The stressed cable
lengths can then be defined.
During prefabrication of the cables they must be prestretched to eliminate construction
struction stretch; they must then be marked at the specified tensions.
Tolerances in fabrication are of the order of 0.02%.
5.3.3 Detailing and construction
5.3.3.1 Cables and fittings
Wire rope cables are spun from high tensile wire. For structural work the cables
are multi-strand, typically 6 ¥ 19 or 6 ¥ 37 with independent wire rope core and
galvanized to Class A. For increased corrosion-resistance, the largest diameter
wire should be used, and cables can be filled with zinc powder in a slow-setting
polyurethane varnish during the spinning process. For even greater corrosionresistance,
filled strand or locked-coil strand can be used to which a shrunk-on
polyurethane or polypropylene sleeve can be fitted. Stainless steel, although apparently
highly corrosion-resistant, is affected by some aggressive atmospheres if air is
excluded; the resulting corrosion can be more severe than with mild steel.
The simplest and cheapest type of termination is a swaged Talurit Eye made
round a thimble (Fig. 5.28(a)) and connected into a clevis type connection or on to
the pin of a shackle. The neatest and most streamlined fitting is a swaged eye or jaw
end termination (Fig. 5.28(b)). Hot-poured zinc terminations have to be used for
very heavy cables of greater diameter than 50 mm (Fig. 5.28(c)). Epoxy resin with
steel balls can be used as a filler in place of zinc, offering an improvement in fatigue
life at the termination.
On-site connections can be made with bulldog clips but they are ugly and damage
the rope. For cable net construction the standard detail is a three-part forged steel
clamp, of which the two outer parts are identical (see Fig. 5.28(d)). Forging is expen