A unique iron bridge outside Paddington Station in London sheds new light on its celebrated designer and builder, Isambard Kingdom Brunel. Robin Partridge investigates.
Next week a 300t crane is due to lift out the first pair of cast iron beams from a Brunel masterpiece hidden for years behind the brickwork facade of an inner-city road bridge. Only a chance discovery among archive material prompted a reprieve for the historic structure just weeks before it was to be demolished as part of the £62M Paddington Bridge Project (NCE last week).
An English Heritage researcher found the documents which confirmed the bridge's existence last summer, just in time to prevent its destruction.
'The bridge had not been recognised as historically significant, ' says Dr Steven Brindle, inspector of ancient monuments for English Heritage. 'Notes and records made by Brunel at the time show this was the first bridge he had made from cast iron. He was clearly nervous of the material and must have spent considerable time designing the details.'
These details are unique to bridge construction and are now being exposed as the bridge is carefully dismantled.
Dating from 1838, the crossing was completed just before the first broad gauge train steamed out of Paddington station and only five years after Brunel had been formally appointed as the engineer for the Great Western Railway (GWR). Paddington was not his first choice for the GWR terminus; he had failed to persuade his friend and great competitor, Robert Stevenson, to allow him to share in the use of Euston Station.
This forced Brunel to undertake the transformation of the area around the Grand Junction Canal Company's basin at Paddington. Here he replaced a wooden footbridge over the canal to give access to his new station (originally located north of Bishop's Bridge Road).
It was to be another 16 years before the present station building was opened.
Brindle believes he knows why Brunel choose to use cast iron for the first time.
'The canal crossing is the highest point on Bishop's Bridge Road', he explains.
'Brunel needed to respect the clearance required by the canal company as well as giving access to the adjacent Harrow Road which actually runs parallel to, but below the level of, the canal.'
Minimum construction depth for the bridge was needed if a reasonable gradient was to be offered to his potential passengers. An iron bridge gave Brunel the best solution with a road level significantly lower than that possible with the more normal (and cheaper) brick arch.
However, crossing the combined canal and towpath would have demanded a span of 16.4m - too long for iron beam technology. He negotiated for a bridge pier in the canal, reducing the longest span to 10.7m, and in the process minimised construction depth still further.
Nothing like this bridge has been built before or since. Cast iron was still an unreliable and unpredictable material.
The engineer now known for his daring feats and a trademark defiance of accepted engineering wisdom, seems to have treated it with great respect and caution. His banana shaped beams may tend to develop some arching action - and so reduce any bending tension - although it is not clear whether this was intended.
'We are prepared to cater for some horizontal forces, ' says Sally Sunderland, project manager of Paddington Basin lead contractor Hochtief, which is dismantling the bridge.
'A jacking system has been designed, but so far we have not noticed any signs of movement as the mass concrete infill is removed.'
One possibility is that Brunel used the possible arching effect as a fail-safe feature. If the cast iron beams showed any tendency to crack the resulting loss of profile would have mobilised a compressive force, keeping the arch intact and loading up the concrete around the beams. But the pleasing architectural arch was more probably to reassure his passengers that they were crossing a traditional bridge.
Beam cross sections also seem to be conservative. The massive bulb of iron that forms the upper compression flange is 40% larger than the tension flange. Conventional wisdom in the 1820s held that cast iron's tensile and compressive properties were similar, so for a while beam flanges were made symmetrical.
But, by the time the Paddington bridge was built, knowledge had moved on. Stevenson had been involved in research in the early 1830s that indicated a factor of six in favour of compression. Was Brunel out of date, or was he just worried?
In any case he adopted the precaution of load testing each of the beams to prove that they would be safe in his bridge. This testing destroyed one of the beams but a spare had already been cast, just in case.
Having agreed the general arrangement for the bridge, Brunel needed to tackle its construction. Here he used cast iron double curvature 'buckle' plates between the main beams to carry mass concrete and the road formation. The prefabrication of plates will have saved construction time.
These buckle plates span longitudinally between cast iron cross girders on the 10.7m span, but they arch transversely between main beams on the 4.9m side span. On both spans the transverse arching loads are taken in a series of tie bars between pairs of beams.
Even here there seems to be an element of fail-safe thinking.
If any soffit plates were to crack the concrete would take the loads in triaxial compression provided the tie bars remained intact. 'In fact, several of the soffit plates have major cracks but the tie bars are in excellent condition, ' says Brindle.
Sunderland continues, 'The plates and cross girders in the main span seem to have been placed from the ends of the beams working towards the centre of the arch - just as masonry arch bridges are constructed - with cast iron wedges holding everything in place longitudinally.
'Once the main beams were positioned, erection of the rest of the bridge must have been very quick and easy. There are no bolts, rivets or welding in this bridge apart from the tie rods. It is like a great big jigsaw filled up with concrete.'
The Paddington canal bridge shows Brunel to be a careful engineer, worried about the material he had chosen and obsessed by appearances.
Nevertheless, Brindle notes, 'he had wilful originality'.