The Design of Modern Steel Bridges - TEDI

192 The Design of Modern Steel Bridges
corrosion. In Britain the term rope usually means a strand with only one layer
of wires wound helically around a core wire.
The first tensioning of a helical strand causes an irreversible elongation due
to radial compaction of the constituent layers. To avoid this non-elastic elongation
in first service loading, the strand is prestretched with a tension 10–20%
above the maximum tension that the strand will be subjected to during service.
7.3.3 Locked-coil strands
In Germany ‘locked-coil spiral strands’ have been developed to produce more
compact cables with a smoother outer surface for more effective corrosion
protection. The longitudinal stiffness of locked-coil spiral strands are about
mid-way between those of straight wires and ordinary spiral strands. Lockedcoil
strands experience less lateral pressure at saddles, sockets and anchorages.
Wires of different cross-sectional shapes are used in different layers to achieve
surface, rather than point, contact between wires. For example, a normal strand
with round wires is wound with one or more layers of wedge-shaped wires,
with the outermost one or two layers made up of wires of S-shaped crosssection
(see Fig 7.6). These specially shaped wires fit tightly together to form a
compact cable with about 90% material density, as against about 70% in spiral
strands with round wires. The tensile strength of shaped wires is slightly less
than that of round wires. In Germany the wires of locked coil strands were
usually not galvanised, in order to avoid possible hydrogen embrittlement. The
first modern cable-stayed bridge, the Strömsund in Sweden, had 16 stays each
made up of four locked-coil strands of 66 or 88 mm in diameter. Each stay
cable of the multi-stay Chao Paya Bridge in Bangkok is 167 mm diameter
locked-coil strand.
Figure 7.6 Locked-coil spiral strand.