Thursday, 12 January 2017

A Salford story from John Casey

Now John’s pictures have been a regular feature of the blog so when he sent over a story to accompany some of the Salford Bridge it was too good an opportunity to miss.

As part of the Trinity Way road scheme a new bridge had to be provided to
cross the river Irwell.

Tuesday 9th September 1986 was the day the first section of a new
road-bridge, over the river Irwell,  was placed in position.

The bridge would be made up of twelve girders placed across the river; two
girders a day being placed in position, finishing on the following Sunday.

The bridge girders were brought to site in sections, three sections making
up one whole girder.

Ten of the girders were put together using nuts and bolts to hold the
joining plates.

Each joint required 340 nuts and bolts and six plates to
complete the joint.

When the three sections were assembled they made up a girder 50 ft / 15.25M long, weighing 65 tons with a built-in upward curvature calculated so when the weight of the road surface was added the girder would straighten to within an accuracy of 0.01 ft / 2 mm.

These ten girders were placed between two girders with welded, instead of plated, joints to give a better cosmetic appearance to the sides of the
bridge.

All 12 girders were allowed to rust to a level where the rust would act as a protective coating.

The outer two girders with the welded joints were  initially sandblasted clean so they would both rust to a nice even colour.

The crane used in placing the bridge girders was, at the time, one of the
largest mobile cranes in Europe.

The main lifting arm was delivered to site in sections on trailers, and the whole thing was assembled in a day.

It was built with the help of a smaller crane which could lift 110 tons. Assembly of the arm was done by using steel pins to fix the sections together. The cable, wound on drums, was lifted into position on the main crane and then pulled off and laced with the help of the smaller crane.

When completed the larger crane was capable of lifting 850 tons but was mainly required for the
long reach it had; this long reach enabled it to stretch out over the river
when placing the 65 ton bridge girders.

As the arm is lowered toward the horizontal its weight, plus the added
weight of the load, tries to tilt the whole structure; to offset this, floor
mounted counter-balance weights automatically come into play to counter this shift in weight.

As these weights are floor mounted, and attached by cables
to the rear of the crane, they have to be disconnected to allow the crane to
rotate and pick up the bridge sections.

These sections were previously placed and positioned by the smaller crane, ensuring the main arm was almost vertical at this point. The weights are only reconnected once the arm is
about to be lowered to place the girder.

In 1986 the cost of hiring this crane was £3,000 per day plus £20,000 for
delivery to site.

© John Casey 2017

Location; Salford

Pictures; the bridge over the Irwell, 1987 from the collection of John Casey

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