How suspension bridge cables are made

Bridge cables are spun in midair, as a wheel shuttles back and forth laying down wires one at a time.

stations: 7
things upstream: 65
raw sources: 22
steps from the ground: 10

The short version

A main bridge cable is built in place from thousands of zinc-coated high-strength steel wires. A traveling wheel carries each wire across between anchorages, usually 400 to 500 wires to a strand and 30 to 90 strands to a cable. Hydraulic compactors squeeze the bundle round, soft galvanized wire wraps it, and clamped bands carry the suspenders.

Coat the steel wire

The cable starts as thin, high-strength steel wire with a zinc coating to keep off corrosion.

Goes in
Steel wire Zinc
Pull across a pilot line

With towers and anchorages built, and saddles on the tower tops, a light pilot rope crosses the span by boat or helicopter. It then hauls progressively heavier lines over.
Spin the wires

A traveling wheel runs back and forth between the anchorages, laying down one wire at a time. Each wire is tensioned individually, and instruments keep watch, since small length errors change how the whole span behaves.
Bundle into strands

Conventionally, 400 to 500 wires make one strand and 30 to 90 strands make a main cable. Each strand is anchored at both ends, in bedrock tunnels where rock allows or in huge concrete anchorages.

400 to 500 wires per strand
Squeeze the bundle round

Hydraulic compactors press the strands into a tight, circular cross-section.
Wrap and seal

Soft galvanized wire is wound around the compacted bundle and painted with elastomeric coating to shed weather. Some bridges also blow dry air through the voids to keep humidity below 40 percent.

Goes in
Steel wire Paint
Clamp on the suspenders

Bands are clamped around the main line at set intervals, and vertical suspender cables hang from them to carry the deck segments.

Goes in
Wire rope

End of the lineSuspension bridge cable

Worth repeating

The Spider Bridge at Falls of Schuylkill, from 1816, was the first of its kind hung on wire cable.
Marc Seguin and his brothers built a temporary wire-cable bridge at Annonay in 1822 that spanned just 18 m.
Some projects skip spinning altogether and lay prefabricated strands, shipped in coils and carrying hot-cast anchors on each end.