The Design of Modern Steel Bridges - TEDI

190 The Design of Modern Steel Bridges
the chemical composition, and heat-treatment of the rod to get optimum
pearlitic microstructure. The percentage elongation of these wires is only up to
4%, against at least 15% for structural steel. The limits of the percentages of
the main alloying elements in the chemical composition are:
Carbon – 0.81 to 0.85.
Silicon – 0.18 to 0.32.
Manganese – 0.50 to 0.70.
Phosphorus under 0.030.
Sulphur under 0.030.
The main engineering properties of the wires, typically, are:
Stress at 0.7% elongation – minimum 1030 N/mm 2 .
Tensile strength – minimum 1520 N/mm 2 .
Elongation on 250 mm – minimum 3.0%.
The wires are usually galvanised for corrosion protection, though galvanisation
is suspected to cause hydrogen embrittlement and some reduction in
strength.
Instead of round cross-section, other shapes of wires are used in locked coil
strands, as described later; but the strength of non-circular wires tends to be
slightly lower.
7.3.1 Spiral strands
Several wires are bundled together spirally to form a spiral strand. The
simplest spiral strand is with one core wire round which six wires are wound in
one layer helically in identical pitch and direction (see Fig. 7.3). Larger spiral
strands are made by winding several layers of wires, the successive layers
having opposite directions of helix in order to balance the torque on the strand
(see Fig. 7.4). When tensioned, the wires in the strand bear on each other
Figure 7.3 Spiral seven-wire strand.