The Iron Bridge - How was it Built?

Page 2 of 6

• 1. A construction mystery
• 2. Building begins
• 3. Arches and ribs
• 4. Deck bearers
• 5. Facts and figures
• 6. Find out more
• Print entire article

Building begins

In 2001, a half-scale model of the main section of the Bridge was reconstructed as part of an experiment. Armed with old ledger entries and 18th century lifting manuals, advice from a range of experts and a recently discovered painting of the Iron Bridge under construction, the engineer Jamie Hillier and a labour force provided by the Royal Engineers built the model using only 18th century techniques.

As a result of the reconstruction, it is now believed that the Bridge was built in the following stages:

Stage 1

Stone footings were built using local sandstone, and topped by iron base plates. The rest of the massive abutments were not built at this stage. A pair of 21m (70ft) wooden derrick poles were stood in the river bed, which acted as cranes. They were angled slightly towards the middle of the river and were stiffened near the top by a horizontal timber brace which provided further lifting points. The whole arrangement could be lent over in either direction, upstream or downstream, to reach different positions. Castings were brought to the site by boat, probably having been cast at Bedlam Furnaces located on the north bank of the Severn just 500m downstream.

Stage 2

The arch has five parallel iron frames, built starting with the upstream one and working back towards the centre. The first pair of Inner Verticals was slotted into the base plate, one on either bank. A Lower Rib was lifted from a barge until its bottom end sat on the southern base plate and rested against the Inner Vertical. The top end was raised to the correct height, and the same process was repeated from the other bank until the two halves lined up.

The two arcs were joined at the Crown by a sophisticated scarf joint, which was secured by three large nuts and bolts. Balancing on a slender timber brace, this was a job for men with steady nerves and no fear of heights. Ropes stopped the castings tipping over at this delicate stage. According to a newspaper report, the first arch spanned the River Severn on 2nd July 1779.

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Published: 2002-01-01