During the construction of the bridge, each half arch was built out from each shore supported on each of the main bearings. As the main bearings are designed to allow a small amount of rotation (to take up changes in loads, and expansion and contraction due to temperature change) each half-arch had to be supported in some way to prevent it from falling into the harbour. This support was provided using cables mounted to 'link plates' near the top of the half arch and passing through 'U' shaped tunnels underground.
One hundred and twenty-eight cables were fixed to the south-eastern top chord of the arch. They passed through the tunnel and terminated on the south-western top chord. A similar arrangement was used on the north shore.
To be able to control the position of the arch, the lengths of the cables had to be adjustable. This was particularly necessary when the two half-arches were lowered to enable them to be joined together.
To allow for length adjustment (to raise and/or lower each half arch), each cable was fixed into a steel socket. The following images show how this was done at both ends of each cable.
The process was somewhat like a soldering process.
Each cable was threaded through a prepared steel socket (there were 512 sockets in total). The individual strands of each cable (217 in each cable) were splayed out, cleaned and folded back onto themselves. The cable was pulled back until the wires were tightly fixed into the tapered hole in the socket.
After this process the socket was heated. Molten ‘white metal’ was poured into the socket. (White metal is a general name for low melting point alloys, typically of lead and tin.)
The result is that the molten white metal reacted with the hot steel wires and socket to form an intermediate alloy in the same way that electrical solder alloys with the electrical wire to form strong joints.
When finished, the steel socket was ready for attachment to the link plates.