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Brunel's Paddington Bridge

BRUNEL’S FIRST IRON BRIDGE  

The discovery and saving of the Paddington Canal Bridge  Steven Brindle MA, D.Phil, FSA.  Inspector of Ancient Monuments, London Region, English Heritage.  

Isambard Kingdom Brunel (1806-59), the most celebrated engineer in history, is so famous that many people would find it surprising that there could be anything significant left to discover about him.  The recent discovery of his canal bridge at Paddington did not just disprove this, it has saved the bridge from certain destruction: the story has aroused a remarkable level of public interest, ranging from the national press, to the Institution of Civil Engineers, to the BBC children’s programme ‘Blue Peter’.   

The discovery arose out of research undertaken for a new history of Paddington Station, just published by English Heritage.  At a very late stage of the work, I found another group of Brunel documents in the National Archives.   Of the 50-odd volumes, 12 turned out to be important:  six volumes of letter-books, and six of his private research notebooks, which with nicely Brunellian directness are simply entitled ‘Facts.’  They have notes on subjects ranging from calculating brick arches, to sources of lime, to metallurgical tests for iron, to ship design, and seem (like much of IKB’s vast archive) to have been barely touched by scholars.

One reason, perhaps, is that they have neither indexes nor tables of contents.  The only way to ascertain if there was any Paddington material was to look at every page, and on page 96 of the second volume a remarkable thing turned up: designs for iron beams for a bridge over the canal at Paddington, and for load-testing them, with the results of the load-test, dated December 1838. 

Brunel was being remarkably thorough, even for him:  all 22 beams were tested using a hydraulic press to 20, 25 and 30 tons; with the amount of deflection measured (one could tell it was a real test, for one of the beams had broken at 28 tons).   Why?  The answer, as it transpired, is that not only had Brunel produced a novel, indeed unique design, this was the first time he had actually built an iron bridge. 

Back in 1828 he had won the Clifton Bridge competition for a suspension bridge, but that employed a quite different technology (using wrought iron), and hadn’t actually been built (and wasn’t until the 1860s, after his untimely death).  Since 1836, Brunel had designed numerous bridges for the Great Western Railway’s (GWR) main line from London to Bristol, but they were all of brick or timber.  He had designed three large cast-iron bridges to carry his main railway line, all in West London, and in the summer of 1838 one of the main girders of the biggest one, over the Uxbridge Road in Hanwell, broke. 

It was while addressing this crisis, that Brunel began work on designing his fourth iron bridge, close by Paddington.  There was a specific reason why Brunel was obliged to do so at Paddington.  When the GWR obtained the Act of Parliament for their extension to Paddington in July 1837, one of the conditions attached to it required them to build a number of bridges.  The easternmost of these, the one actually at Paddington, was to carry a new road over the broad cutting in which the new station was located.  Brunel spanned the train-tracks with a straightforward brick viaduct, which became known as the Bishops Road Bridge.  However, the bridge also had to cross the Paddington branch of the Grand Junction canal.   The canal was at higher level than the railway, and this gave him a problem.  His bridge would have to span the canal and towpath, about 60 feet wide, leaving a clearance of at least 12 feet over the water, but it would then have to drop down to the Harrow Road.  To keep the road at a safe gradient, Brunel had to keep the canal bridge as shallow (in terms of its structural depth between the top of the arch and the roadway) as possible.  A brick arch wide enough to span the canal would be far too deep.  So he was pushed towards using cast-iron beam, as the only way of achieving the necessary shallowness of structure.  

Engineers had been developing the use of cast-iron for bridges ever since Abraham Darby III built the famous bridge at Ironbridge in 1779, but there were still limitations as to how it could be used, and this seems to have been the first time Brunel had done it.   On 18 May 1838, he wrote to R.C. Sale, secretary of the Grand Junction Canal Company, explaining his design (and displaying his customary impatience with punctuation):  

“My Dear Sir, I forward you an elevation of the Bridge we propose to erect over the basin at Paddington in lieu of the present foot bridge by the provision of our Act it is to be a carriage road bridge – 40 feet wide and to communicate with the Harrow road which limits our height.   The bridge will therefore be of Cast Iron.  I have made the principal opening as large as I possibly could consistent with safety, and have provided a second Arch which is also large enough for barges to pass freely.  And the whole is as wide as the present basin.  I propose to get in the foundations during the stoppage of your canal next month and shall therefore feel obliged by an early reply. If Mr Holland wishes to see me on the subject, I shall be happy to meet him, but I must ask him to call on me, as I am still somewhat of an invalid. Yours very truly, I. K. Brunel.”

 

The canal company, though, were not happy. They evidently replied asking for a single-span bridge over the canal, prompting the following slightly tetchy reply from Brunel dated 25 May:  

“My Dear Sir, I think Mr Holland does not bear in mind that the basin and Towing path at the point of crossing is upwards of 60 ft – with the head way (i.e. headroom) we have it would be impossible to build such in one opening, perhaps a larger opening than the one I have proposed might be made although really I should not like to try it, but there would not be room to navigate the small arch – by the present proportion.  The large opening is very ample probably the largest on your canal while the side arch is still quite large enough for all barges – moving about without a tow rope.  With respect to the loss of water, that difficulty also may be removed as it is proposed to drive a stank or dam across the basin and thus save the water south of it, and to drain only that part of it between the stank and the present bridge.  I trust these explanations will be quite satisfactory, and that so far as the sanction of your company is necessary we may proceed.  A plan shall be sent. I am my dear Sir, Yours very truly, I.K. Brunel.”

 

The letters, of course, provide further and conclusive proof that the bridge is Brunel’s own design.   

The canal company accepted Brunel’s design on 30 May, and he seems to have got his foundations in soon after.  On 27 October 1838 the GWR let a contract to build the canal bridge to Messrs Sherwood, who were probably still at work on the main brick-arched section of the Bishops Road Bridge immediately to the south. 

The subcontract for the ironwork was let to Gordon’s of Deptford, and as we have seen, in December 1838 Brunel had every one of the 22 iron beams for his bridge load-tested, just to be absolutely sure.   The bridge presumably went up in the first half of 1839.  For all the difficulties of the brief, Brunel had produced an elegant and well-proportioned design.  The main 35-foot opening, with its very shallow arch, was flanked by subsidiary 16-foot openings.  That to the NE, also over the canal, was spanned with a second iron arch, while the one to the SW, over the towpath, was spanned with a simple brick arch; nevertheless, the design reads as being essentially symmetrical.  Brunel capped it with a Portland stone cornice and handsome iron railings.  Where his bridge was really distinctive, though, was in the design of the two iron arches themselves. 

In the 18th and early 19th centuries, a few engineers worked out how to build cast-iron bridges that would stand up (indeed, some of them are still standing), but they couldn’t have explained why in any scientific way.  In the 1820s and 1830s theory started to catch up with practice, thanks to two scientific thinkers, Thomas Tredgold and Eaton Hodgkinson.  Tredgold’s view was that the best form of a beam was with top and bottom flanges of equal size, and this was the form adopted by Brunel for his first three iron bridges.  By the time he was building them, though, Hodgkinson had looked more deeply into the matter, and realised that as cast iron was much stronger in compression than in tension, that it made sense to make beams with a much larger lower flange, as it is this which is put into tension when the beam is loaded.  

For reasons we do not understand, Brunel stuck with the more out-of-date Tredgold view, rather than Hodkinson’s thinking, even though at the same time, c. 1837-8, his great rival Robert Stephenson was building several cast-iron bridges on the Euston extension of the London & Birmingham Railway, using beams designed according to Hodgkinson’s principles (sadly, none of these bridges survive today).  

After one of his Tredgold-type beams broke at the Uxbridge Road Bridge, the easy thing for Brunel to do would have been to follow Hodgkinson’s principles, too. Instead, he continued to reject them, and produced a completely different design of his own.  His beams have large bulb-shaped top flanges, about 40% larger in cross-section than the lower flanges.  Nothing like them been made before (and nothing like them has been made since) – and they directly contradicted Eaton Hodgkinson’s theory.  We do not know why, and for this reason the discovery has aroused great interest and discussion amongst engineers.  It may be that Brunel put the extra mass into the top flange to help his beams act like arches (and, indeed, we found some evidence that there were slight ‘arching forces’ in the structure, though perhaps not enough to make very much difference).    What we can say is that the design was essentially sound.  Indeed, the bridge was very ‘over-designed’ in relation to the loads that it had to take when it was new, and Brunel acted with the greatest thoroughness in testing every single beam. 

We can also say that it has stood for over 160 years with minimal maintenance, and until recently was coping with full modern vehicle-loadings. It has come apart in such good condition that every single piece of ironwork will be re-usable.     We can also say, by way of explanation, that Brunel was famously independent-minded: this was the nature of his genius.  He liked to think out problems for himself, and had a way of coming up with totally original solutions, such as his route for the GWR, his decision to use the seven-foot ‘broad gauge’, or his decision to power the SS Great Britain with a screw-propeller instead of paddle wheels.  The Paddington canal bridge is thus another instance of his celebrated and at times wilful originality, though we cannot claim that it represented the future.   

When it came to spanning the gaps between the beams, Brunel was almost equally original.  Where Stephenson’s iron bridges were covered with brick jack-arches, Brunel designed iron soffit plates for his.  Here too, there is a somewhat wilful quality to his originality: he designed two different systems of iron plates, one for the narrow arch, and more complex system for the wider 35-foot arch. He also used brick jack-arches for the outer edges of his bridge, perhaps to help provide a solid base for the stone cornices.  So the bridge is considerably more complex than, perhaps, it needed to be.  

Another striking point is the near-absence of bolted fixings – the whole structure was designed to be held together by gravity, and the only ‘positive fixings’ in the ironwork are the nuts on the tie-rods: this was a huge advantage when it came to dismantling the bridge.  The Paddington Bridge did not have any imitators or followers:  although Brunel designed many more cast iron bridges – most of which have long gone – he developed different beam designs, and he never seems to have used this design again. 

By 1847, in any case, cast-iron was on its way out as a bridging material, and wrought-iron was on its way in.  On the one hand, the collapse of Robert Stephenson’s Dee Bridge near Chester in 1846 dealt a death-blow to trust in the material.  On the other hand, advances in wrought-iron manufacturing in the 1840s, led to the building of masterpieces like Stephenson’s Britannia Bridge over the Menai Straits (1850), and Brunel’s own Saltash Bridge (1859). 

The Paddington canal bridge thus seems to be quite unique.  Nevertheless, it disappeared from recognition.  This was hastened by the removal of its original railings in 1906, and their replacement with high brick parapets.  One could no longer see the canal from the road; and very people had access to that stretch of the towpath, which was closed to the public.  

At this point, we must jump forward to the moment, in the spring of 2003, when I found the notebook, with its design for load-testing beams for a bridge over the canal at Paddington.  The immediate question was, did it still exist?   It seemed very unlikely.  Highway engineers have long hated cast-iron, and replaced it wherever possible.  Furthermore, most of the Bishops Road Bridge had been rebuilt in 1906, with a big steel girder crossing the tracks.  From road level, the canal and bridge were rendered invisible by the brick parapets added in 1906:  the towpath was totally inaccessible.  It looked as if the whole structure had probably been replaced with Edwardian steelwork. Checking this point got added to a long list of things to do to finish off the book. In April, an E-mail was fired off to a colleague in Westminster City Council:  I had made a very interesting archival find: the bridge seemed unlikely to survive; but if it did, it would be a major discovery. Could they help with access, to check this? The E-mail did rather more than ruffle feathers at Westminster, who had no idea that this was a forgotten work by Brunel. 

Unknown to me, they had intended to replace the whole bridge since 1990 (it was a notorious traffic bottleneck, and the Edwardian steelwork was decaying badly).  They had started active planning for this spectacularly complex project in 1999, and the most difficult thing had been lining up the £62 million budget. After five years of planning, they were, quite literally, due to sign the main contract the following week.  A couple of days later, I met colleagues from Westminster on site:  at which point we established that Brunel’s iron bridge survived complete (except for its railings), but also that it was due to be demolished in 10 months’ time.  A project office had already been set up, hard by the bridge, which was due to close to traffic on 10 January 2004.   

One has to give Westminster all possible credit for the way they reacted, then and subsequently.  It was clear from the outset that the canal bridge could not possibly be incorporated in the new, much wider Bishops Road Bridge.  Westminster were concerned that their contract, tied to an inflexible timetable, would be at serious risk if the canal bridge were to be listed.  Yet they entirely understood its importance.

From English Heritage’s side, it was clear that to obstruct a project of this importance would be contrary to the public interest.  Yet we hardly wished to contemplate losing something of this historic significance. The only way forward was by voluntary, confidential negotiation.   We arranged for Malcolm Tucker, a specialist in recording industrial structures, to make a very detailed drawn survey; and for English Heritage’s in-house team to carry out a full photogrammetric survey; and for our engineers to advise. The investigative work, planning, and negotiations lasted from May till December of 2003.  The contractors, Hochtief PLC, dug two test-pits in the road, so we could look at the ironwork from above as well as below.  Sliding the bridge sideways was assessed, costed, and rejected.  Specialists in engineering history were consulted.  More historic documents turned up, notably copies of the original designs for the bridge in the Institution of Civil Engineers’ collection.  A provisional site for reconstruction was identified. 

By late September we had an outline salvage scheme costed at £500,000.  The crucial point was passed in late October when Westminster, thankfully, were able to approve this.  By mid-December their engineers, Cass Hayward, had produced an excellent full method statement for dismantling, and we were ready to go.   

The Bishops Road Bridge closed, according to plan, in January 2004.  The road surface and the lime concrete fill beneath it were taken up, the brick facings were dismantled, and the iron soffit-plates lifted. The bridge has come apart beautifully: every single piece of ironwork will be re-usable, and we have also salvaged 15,000 facing bricks, as well as the stonework for the cornice.  The major lifting operations took place between 30 March and 15 April, and the components were taken to Fort Cumberland, near Portsmouth, where English Heritage is housing them as one of our contributions to the project.  The operations went remarkably smoothly, thanks to a superb job by the main contractors, Hochtief PLC, and the demolition subcontractors, Gilpins Demolition Ltd.  

As to reconstruction, Westminster and British Waterways have identified a site for rebuilding the bridge, for pedestrian use, about 200 yards up the canal from its original location, close by the Harrow Road.  So the bridge will still relate meaningfully to its original setting, and will serve a genuine need for a footbridge there.  The rebuilt bridge will also incorporate a shop and information centre for British Waterways, a café with a kitchen to serve it, and a facilities block for canal dwellers. 

English Heritage is working with Westminster, British Waterways and other project partners on the development of the project and fund-raising.  The planning for this is well advanced, and an application for Planning Permission went in on the bicentenary of Brunel’s birth in April 2006.  The estimated cost is over £2.5million, and the British Waterways Trust have embarked on fund-raising for this.   The salvage of Brunel’s first iron bridge has been a happy example of what one can achieve by constructive engagement: Westminster City Council, British Waterways, their contractors and consultants, have responded in the most generous and positive way possible to what must initially have seemed a very difficult situation.  The contract for the new Bishops Road Bridge was not been delayed by a single day, and the new Bishops Road Bridge has just opened.  Brunel’s first iron bridge is saved, and this unique piece of engineering history will be reconstructed close to its original site.  We hope that IKB would be proud.

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