Engineering and Construction

Wire rope cables are spun from high tensile wire. For structural work the cables  are multi-strand, typically 6 x 19 or 6 x37 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 corrosion-resistance, filled strand or locked-coil strand can be used to which a shrunk-on polyurethane or polypropylene sleeve can be fitted. Stainless steel, although appar- ently 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 50mm (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-

sive for small numbers and so for smaller structures machined aluminium components may be preferred. Double cables can have a swaged aluminium extrusion  prefixed to each pair of cables, which can then be connected with a single bolt. For the attachment of net cables to edge cables, forged steel clamps are generally used (Fig. 5.28(e)).Lower cost alternatives are bent plate or machined aluminium clamps.

Cable life is reduced by corrosion and fatigue. Galvanized cables under cover suffer very little corrosion; external cables properly protected should have a life of 50 years. Plastic sheathing has the great disadvantage of making inspection of the cable impossible.

Fatigue investigations have shown that it is wise to limit the maximum tension in a cable to 40% of its ultimate strength for long-life structures. For structures with  a design life of up to ten years a limit of 50% is acceptable. Flexing of the cables at clamps or end termination will cause rapid fatigue damage.

Fig. 5.28 Cable fittings: (a) swaged eye, (b) swaged terminator, (c) hot-poured white metal eye, (d) cross clamp, (e) net and boundary connection