The Design of Modern Steel Bridges - TEDI

7.3.2 Ropes
causing lateral compression; this, and the inclination of the constituent wires
with the axis of the strand reduce the tensile strength of the strand by 15–25%
compared to single wires. The axial stiffness or the nominal Young’s modulus
of the strand is also 10–15% less than that of a wire. Spiral strands have been
popular in the UK; the Erskine Bridge in Scotland had four stays, each consisting
of 24 strands in 6 4 rectangular formation, each strand being 76 mm in
diameter. The cable-stays of Dartford Bridge, which is a multi-stay bridge, are
spiral strands of 137 mm in diameter.
Several strands can be spirally wound around one core strand to form a cable;
in USA these are called ‘ropes’ (see Fig. 7.5). These are more flexible to bend
round saddles, etc. But the strength and stiffness of a rope is less than that of an
equivalent strand, and its outer surface is more difficult to protect against
Figure 7.4 Large spiral strand.
Figure 7.5 Multi-strand rope (USA).
Cable-stayed Bridges 191