Manchester Bridges: 9. Corporation Street Footbridge

On 15 June 1996, an IRA bomb went off in Manchester, on Corporation Street, between the Arndale shopping centre and a branch of Marks & Spencer. It was the largest peacetime bomb to be exploded in the UK, and even though the neighbourhood had been evacuated beforehand, over 200 people were injured, and about £700m of damage caused.

Extensive redevelopment followed, including the construction of new Arndale and M&S buildings. The old buildings had been linked by a covered footbridge, and a £600,000 new bridge was put back in its place in November 1998. The new structure was designed by Arup, with Hodder and Partners as architect, and built by Severfield Rowen for main contractor Bovis.

The new bridge is, as far as I know, unique in conception and form. Its glazed enclosure takes the form of a hyperboloid of revolution, a geometry best known to structural engineers for its use in thin shell power station cooling towers. I'm not aware of any examples (other than the Corporation Street Bridge), where it has been turned on its side.

Link bridges between buildings are often visually awkward. There are few structures which can bridge between flat building facades without looking at odds with the environment. Not far from Corporation Street, I walked under the three bridges pictured on the right, typical examples of an often shabby genre. I've discussed Edinburgh's Greenside Place Link Bridge here previously, but other recent examples which try to get away from cliche include the Paradise Street bridge in Liverpool, and Cabot Circus bridge in Bristol.

The Corporation Street footbridge is essentially a giant tubular truss, 18m long and 6.2m in diameter, consisting of eighteen 114mm diameter steel tubes alternating with 28mm diameter prestressed tie rods. Each main member is straight (a feature of hyperboloid geometry which makes it easier to describe and set out), and these members are interlinked with a series of circular hoop frames at intervals.

The hoop frame support a grid of steel beams which forms the sloping walkway, which is finished with oak plank flooring. The whole steel assembly weighs 28 tonnes. It's supported from the buildings via circular end frames, which have differing geometry to take account of the fact that the crossing isn't perpendicular to the buildings, nor are the building facades parallel.

The glazing consists of triangular panels, with ten different sizes and shapes of triangle required in total. These are fixed to the structural frame with cast stainless steel nodes, each of which clamps together six glazing panels. It's nice to see the numbering of the glazing panels still there a decade after it was built.

The glass is now somewhat grimy, and this isn't the only maintenance problem for the bridge. Between the walkway and the glazing, there is a small gap, and dirt, debris and litter has passed through, coming to lie on the glass panels forming the bridge's underside. I've read that the lower panels are designed for higher loads so that maintenance workers can access the space below the walkway, but there's little sign that anyone has done so for some time. That's a shame, because the impression of neglect definitely detracts from the bridge.

However, it still retains some capacity to enchant. The geometry of the bridge, which as seen on the right creates a star-shaped pattern when viewed end-on, creates a sense of occasion and an experience quite unlike any other shopping centre bridge that I've used. It has a nicely crystalline quality which would only be enhanced if it were to be cleaned up. I hope it will remain unique: it would be cheapened if it were to be repeated elsewhere.

Further information:

• Structurae
• Google maps
• Hodder and Partners (PDF)
• Arup (PDF)
• Severfield-Rowen
• Rebuilding Manchester
• Bridge Builders

Manchester Bridges: 8. Trinity Footbridge

From Castlefield I headed into Manchester city centre. First stop, Trinity Footbridge.

This is Santiago Calatrava's only bridge in the UK (his only structure of any type, I think). Manchester (or to be precise, it's close neighbour Salford) should be proud to have attracted the world's most prominent bridge designer, and to get one of his better designs.

The £1.9m footbridge, named for the nearby Trinity Church, was opened in 1995. Like several of the bridges I've already covered, it was part of a regeneration initiative, in this case intended by Salford City Council to draw development funding out of Manchester City Centre and across the River Irwell into Salford. The structure is therefore required to act both very much as a landmark ("here be development") and as a gateway ("enter our City here").

Calatrava's design is a 79m long asymmetric cable-stayed bridge, in his trademark white-painted steel, with a main span of 54m and a 41m tall inclined pylon. The 210 tonnes of structural steel were assembled by main contractor Dew Group, having been fabricated by Spanish steel specialist Urssa (who had experience on other Calatrava bridges).

The bridge's key feature is the cable geometry with a conventional arrangement on the main span paired with two hyperbolic crossed fans on the twin back spans, each of which curves away from the main bridge deck (see aerial photo on Google maps linked below). It's not the most efficient cable arrangement, but pure efficiency is of less relevance at this span, and the result is undoubtedly spectacular. The asymmetry is justified both by the general spatial constraints of the site, and by the marked level difference between the two river banks, which requires the bridge to rise up from west to east.

The pylon is tilted at 60°, and is attractively cigar-shaped (varying from 0.55m to 1.22m in diameter, and hence efficient at resisting buckling), and the 4m wide triangular box-girder deck profiled to appear more shallow than it really is. The little triangular fins where the main span cables attach to the deck are nicely done (although the attachments of the back-span cables are less visually successful). The pier below the pylon is also carefully shaped, and looks good in profile, although a bit more peculiar from close-at-hand.

The cables are anchored in recesses in the pylon, presumably to hide the cable anchorages and give a cleaner visual effect. I think it looks very good, although the downside is that maintenance of the anchorage has been made much harder.

Judging by the bridges already visited in this series, maintenance seems to be a big problem for Manchester's bridge authorities. It seems easy to find the funding to erect a monument, but harder to find the cash to look after it.

According to a report in the New Civil Engineer magazine, Salford City Council didn't even carry out a full Principal Inspection of the bridge until 2006, eleven years after construction (the normal interval would be 6 years), although admittedly the bridge was refurbished in 2002 (re-tensioning the cables and replacing end joints). The inspection in 2006 was prompted by corrosion appearing at end tie-downs and cable connections, although it's unclear if these relate to poor design detailing or a poorly applied protective paint system.

The NCE has used this as evidence that clients are using design competitions to select "complex, high maintenance" designs, but there's little about Trinity Footbridge which should make it much harder to maintain than any other cable-stay footbridge. Indeed, an extensive article they published on the subject in 2006 offers a set of more balanced points of view (including one from a designer opposed to "the Calatrava route" which makes for particularly ironic reading).

The bridge is now in reasonable condition, but there's still plenty of rust and grime visible at the back-span tie-downs. My photo of these also shows the awkward bearing arrangement, with its mixture of tie-downs, vertical and horizontal bearings indicating that the bridge relies on substantial restraint here.

Since the bridge was built, there seems to have been a limited spread of development radiating out from it into Salford, consisting of flats, offices and the Lowry Hotel, which overlooks the bridge. However, the development doesn't extend very far into Salford, presumably because unlike the Manchester side of the river, there's nothing else in the immediate vicinity to attract investment, no obvious hinterland.

The result has compromised the setting of the bridge, which seems caged in by tall surrounding buildings, also restricting what can be done to make the most of the open space around it. There's a walkway along the river to the north, but not to the south, and the area seems barely half-finished.

This detracts from the bridge, restricting views of it and making its scale seem more overly intrusive than may originally have been intended. Nonetheless, it's a spectacular structure, and in my one view, one of Calatrava's best designs.

Further information:

• Structurae
• Google maps
• galinsky.com
• Unofficial Calatrava website
• Calatrava Bridges (Frampton et al)
• Calatrava Bridges (Tzonis / Caso Donadei)

Manchester Bridges: 7. More Castlefield bridges

To round off the Castlefield segment of my Manchester trip, here are a few more bridges spotted in the vicinity, with indications as to further information where I can find it.

Picking up where I left off, at the Staffordshire Basin footbridges, the photo shows Castlefield's four railway viaducts in the background. From front to back, the brick arch is part of the Manchester South Junction & Altrincham (MSJ&A) Railway Viaduct, built 1848-9 as part of a line out to Salford. Behind that is the 1894 Great Northern Railway Viaduct, with its steel lattice trusses. The 1877 wrought iron Cornbrook Viaduct, built for the Cheshire Lines Committee, is behind that, although you can't really see it in this image. Finally, the 1848-9 cast iron arch belongs to the MSJ&A's Altrincham branch viaduct.

The next photo is from below the viaducts. To the left, the Great Northern, now disused. To the right, Cornbrook Viaduct, which now carries trams. On the far right are the MSJ&A's brick arches, and below them all a pretty cast iron footbridge, of a type typical to the canal network. Lettering on the side declares it to be by Marsh Bros Engineers, Bakewell 1990. Sadly, it is a modern fake - a steel structure with cast iron cladding.

The Merchants' Bridge is on the other side of the viaduct, and looking back through it, you can see some of the same bridges in reverse. Behind Merchants' Bridge is the cast-iron MSJ&A span, abutted by its brick arch approaches, which reportedly use more than 50 million bricks. The Metrolink Cornbrook Viaduct is visible behind and above.

Turning around and facing in the opposite direction, the photo shows the spectacular Middle Warehouse, built around 1830, and now converted into flats, offices, shops and radio studios. This Grade II Listed Building is a bridge in its own right, with twin arches which originally allowed two canal arms to pass within, much like the Staffordshire Warehouse mentioned in the last post. In front of it is a peculiarly large timber bascule bridge (presumably sized so the counterweight remains above head height when operating). I haven't found any information online for this structure.

Further round the canal basin is another of the Marsh Bros fakes. I've read that a bridge of this type was considered for the site of the Architect's Footbridge, and rejected, which is a blessing. Castlefield's strength as a heritage site is the conjunction of several different historic developments, and rebuilding entirely arbitrary elements just misses the opportunity to add further to the layering. Behind it is the Bridgewater Viaduct, which carries the main A56 highway into central Manchester.

The twin-leaf bascule bridge in this photo is found just across from the Marsh Bros bridge, and allows foot access across the mouth of two tunnels into the sandstone cliff. Canal barges would dock in these, and coal and goods could be hoisted up to street level above. Although mechanisms remain inside, it doesn't look to me that barges would ever enter again, and it's not very clear why opening footbridges were preferred over a fixed link walkway.

Finally, there are many historic bridges within five minutes walk of Castlefield, but I didn't have time to stop and photograph them. I did stop briefly to take a picture of Exhibition Footbridge, which is also nearby. This was erected in 1985 and connects Deansgate Rail Station to the G-Mex tramstop, spanning across Whitworth Street. There are photos of the interior online which show a very grubby interior, a semi-transparent plastic tube which is broken, graffitied and filthy. It's easy to see how it might have once looked a strong, modern statement on paper (or shortly after completion), but the lack of maintenance seemingly endemic to Manchester's bridges has combined with the now somewhat dated visual design to render it quite unattractive.

Further information:

• Wikipedia
• Google Maps
• Castlefield Heritage Trail (includes details on several of the bridges, PDF)
• Killing Castlefield
• Civil Engineering Heritage: Northern England

Manchester Bridges: 6. Staffordshire Basin Footbridges

Time for a couple of mysteries. Walking from Merchants' Bridge under the railway viaducts (more about them later) brings you to Staffordshire Basin and the Castlefield Arena.

The arena, with its attractive fabric canopy, was designed by DEGW with Arup and while much touted as a redevelopment success story, apparently sits empty most of the time. Perhaps there's more life here in the summer.

This was also the former site of the Staffordshire Warehouse, which apparently stood eight storeys high, with two large arched entrances covering canal arms which ran into the building. A similar structure can be seen in my last post, viewed behind Merchants' Bridge (the second photo), and I'll show it again in the next post. The Warehouse is no longer there, but the two canal arms are still present, taking the form of a tuning fork in plan.

The entrances to each canal arm are spanned by identical swing footbridges. Well, I say "spanned", but when I was there, both bridges were padlocked in the open position and I suspect they generally stay that way.

I've been unable to find any information at all for these bridges online, not even their name, let alone dates or who designed or built them. Hence the "mystery" alluded to at the start of this post.

Both bridges have clearly been designed to be ultra-lightweight, and swung by hand using prominent bobbled lever arms. The parapets are nearly insubstantial, with horizontal cable railings and even a cable handrail on top. They could almost be omitted entirely or reduced to a top rail: the risk of falling in the water is surely greater from the canal banks or off a canal boat.

I like the detailing on these bridges, the way the parapet wires are brought to ground at the tip of the bridge; the curved and ballasted plate that sits above the pivot; and the curved recess in the paving for the ballast plate to swing into.

Further information:

• Google Maps

Manchester Bridges: 5. Merchants' Bridge

Merchants' Bridge is one of the best known "iconic" bridges to have been built in Manchester. It's a mere minute's walk from Architect's Footbridge within the Castlefield area, and spans across the main canal basin, providing a key connection for the area's residents and trendy diners.

The designers Whitby Bird (now part of Ramboll) wouldn't hesitate to accept that its tilted white arch was inspired by the designs of Santiago Calatrava, and the bridge most commonly cited as its inspiration is Calatrava's 1991 La Devesa footbridge in Ripoll, Spain. The whole ensemble of a deck curved in plan hung from and counterbalanced by an inclined arch seems something of a cliche now, so it may be a surprise to see how few bridges of this precise type there actually are, even in Calatrava's oeuvre.

Indeed, the Ripoll bridge has a straight rather than curved deck, an arrangement with far less logic to it than the Whitby Bird design, which could be considered an improvement. The vast majority of Calatrava's inclined-arch bridges have a straight or near-straight deck, a typology that Whitby Bird later turned to for the York Millennium Bridge. On the comparison between Merchants' Bridge and Calatrava, Mark Whitby wrote:

"I am uneasy with his extremes but a reinterpretation of his La Devesa Footbridge in Ripoll, Spain was the genesis for our Merchants' Bridge in Manchester, which at the time we instinctively felt was a more honest structure. In fact, it is the opposite, albeit possibly more sculptural."

As with many of the bridges I'm covering in this series of posts, Merchants' Bridge was the winner of a design competition seeking a regeneration centrepiece for its locale. Built in 1995 by P Casey & Co Ltd (Watson Steel as steel fabricator), it cost £416,000. Like the Architect's Footbridge, it was funded by the Central Manchester Development Corporation.

The bridge deck is 67m long, with a 38m main span supported from the arch. The deck is a very slender 375mm deep (or 475mm, depending which website you read), and appears to consist of edge tubes and cross-beams connected into a torsion box by steel panels, in the form of a stressed-skin structure. The high torsional stiffness is vital, as the arch provides minimal restraint against the twisting inherent in a deck supported on only one edge. The extreme slenderness is an effort to satisfy the canal headroom constraint while not leading to unduly long approach ramps, and in that the design succeeds admirably, flowing smoothly into its south approach ramp without looking at all awkward.

The arch and deck also serve to brace each other, such that the stiff deck helps prevent the arch from buckling in its plane, while the cantilever arms connecting the two prevent it from buckling out-of-plane (pictured right, which also shows in the background the Castle Street bridge, Architect's Footbridge, and Beetham Tower, Manchester's tallest building). As such bridges go, this is an arrangement which combines structural efficiency with visual attractiveness.

Having said that, it's worth noting that this is a pretty lively bridge, quite easy for a single pedestrian to excite into vibration. I noticed two circular patches in the deck near midspan which looked like they might be covers hiding pits for tuned mass dampers to be added, but if so, then it seems unlikely that dampers were ever installed.

The bridge is a fitting response to its site, to the geometrical constraints, the pedestrian desire lines, and to the need to create a structure which is prominent and quickly comprehensible from a wider variety of viewpoints.

There are two side spans to the south, and one to the north. These are where the limitations of the design begin to make themselves known. The north span is extremely short, to the extent that it is held down with tie bars (pictured above left) rather than supported on bridge bearings.

The south pier is also very peculiar effort. It's inclined so as to give the visual impression that the arch thrust continues into it, but in fact the only significant loads supported are vertical. The need to adapt the thrust pier to carry the torsion from the deck has led to a very odd cantilever with a slender prop/tie, which I find quite unsatisfying, but this is a detail which was never going to be straightforward to resolve, and the sort of area where I think it's fair to say that Calatrava does it better.

The bridge parapets are a series of horizontal flat plates (breaching the normal practice that parapets should be designed not to be climbable), with a stainless steel top plate.

The bridge has won several awards, including (slightly oddly) a Millennium Product Award - try sticking one of these in the back of your car with that new hi-def telly. But fame is no protection against grime, and like many of the bridges I saw on this trip, the bridge is encrusted with grime and parts in need of repainting. Surely the bridge bearing pictured on the left ought to be high on any maintenance list, what with its way off-centre loading and flurry of rusty shim plates.

But neither grime nor bounciness, nor the ghost-town surroundings, can drag this well-designed bridge down. It fits well into its context, and is a fine contemporary addition to the townscape.

Further information:

• Structurae
• Google Maps
• Whitby Bird
• "Light curve - architectural design of the Merchants Bridge in Manchester, England", from the Architectural Review
• Bridges/Puentes by Marth Torres Arcila
• Footbridges by Mike Schlaich and Ursula Baus
• Merchants Bridge at British Steel

Manchester Bridges: 4. Architect's Footbridge, Bridge 100a

I'm not really sure whether this bridge has a proper name. It's called "Architects Bridge" in one book I have, and simply "Castlefield Footbridge" in another. There's also a plate on the bridge reading "100a", which will be a number given to it by the canal authority, British Waterways.

This small footbridge was built as part of the extensive redevelopment of Manchester's Castlefield canal basin, and is of particular interest despite being one of the smallest and most self-effacing of the many bridges in the area. The next three posts will all be devoted to the bridges of Castlefield.

Buildings at Castlefield can be dated back to Roman times, but much of what can now be seen relates to its situation at the junction of three canals, Bridgewater, Ashton and Rochdale. Castlefield became a hugely important commercial site during much of Manchester's industrial development, an interchange between several different forms of transport, and home to several commercial warehouses. The industry is long gone, and the area is now home to a mixture of new and refurbished offices, flats and "trendy wine bars".

Following the redevelopment of the canalside Eastgate building, a new footbridge was required in 1996 (or possibly 1998 - sources vary) across the Rochdale Canal, a short distance to the north of the two hundred year old Castle Street highway bridge, which is numbered bridge 101 (pictured right). This footbridge connects into a passageway through the offices, and essentially seems to be there to provide quicker access between car parks. It was funded by the Manchester City Development Corporation.

As with many regeneration bridges, it's instructive to wonder whether taxpayer grant aid has produced value-for-money, as other than housing, development at Castlefield apparently stalled some years ago, with several offices and restaurants now closed and empty. When I visited, there were few people around, mainly dog walkers, and for such a spectacular and resonant site, it seems to have become something of a ghost town.

Anyway, the designers of the £140,000 Architect's Footbridge were Ian Simpson Architects and Modus structural engineers (now part of SKM). Their bridge only has to span 15m, and the solution adopted is suitable minimalist. Its skeleton is a stainless steel frame, over which are hung a series of slender blocks of red sandstone, each 1.2m square, sourced from Locharbriggs in Scotland (the quarry at Collyhurst which would have supplied much of the sandstone previously used in the area is closed - you can also see in the photos some of the sandstone outcrop onto which Castlefield has been built). The bridge deck consists of granite setts embedded in a red-coloured concrete slab.

Its rejection of bridge design convention is charming and highly appropriate, and I definitely enjoyed its modest minimalism. However, like the two bridges already visited (and note that this will become something of a theme), it has not been especially well maintained. For such a young bridge, it's disappointing to see the extent to which the sandstone balustrades are cracking, with no attempt at repair (although I'm far from sure how they would be repaired). I guess the sandstone has been cut too thin and that this has led to failure under relatively minimal stress. The sense of neglect is in keeping with Castlefield's ghost town status, of course.

Further information:

• Google Maps
• CUBE Castlefield tour (PDF, the bridge is on page 5)

Manchester Bridges: 3. Mancunian Way Footbridge

Now, here's the first of several bridges I visited on my Manchester trip which deserves to be better known than it is. I've certainly not seen it in any of the coffee-table books on modern bridge design, and it's not on Wikipedia or Structurae. It's a fairly short walk from the Hulme Arch, and the next bridges I'll feature are also within walking distance, so the possibility of a walking tour of Manchester's bridges presents itself ...

The Mancunian Way Footbridge was conceived as a replacement to an unattractive subway, connecting the Hulme Park area to the southern edge of Manchester city centre, across the very busy A57(M) highway. The £1.6m pedestrian and cycle bridge was promoted by Manchester City Council with the Moss Side and Hulme Partnership, and funded by a variety of partners, again as part of efforts to regenerate the area.

Following a design contest, Arca Architects and Kingfisher Consulting were awarded the project in July 1999. The bridge was opened to the public almost exactly three years later.

The overall layout of the bridge is governed by the constraints of the adjacent highway layout, and by the need to provide disability-accessable approach ramps. It has one long ramp on each side running parallel to the Mancunian Way, and these ramps extend further in order to achieve the length required. The 50m main span over the highway is curved in plan so that it continues to gain in height as much as possible before passing above the required highway headroom.

This layout isn't especially unusual, but the structural means of support certainly is. Classic ways of supporting a highly curved footbridge deck include the use of cable-stays from a counter-inclined pylon (as on the South Quay Footbridge), or a counter-inclined arch (as on Merchant's Bridge, which I'll be returning to later). In both cases, it's usual to cantilever the bridge deck from a circular or box-shaped hollow member providing the necessary torsional stiffness.

At Mancunian Way, a very different approach has been taken. The bridge deck is supported from a Pratt truss on its outer edge, 150m long in total, braced against an inner tubular ring beam. I assume that resistance of the truss to the consequent torsion is provided by its arching behaviour in plan (assisted by the plan bracing to the bottom chord). The only vaguely similar bridge which comes to mind is on Craigieburn Bypass in Australia.

The truss sits on two inclined concrete piers, which also serve to support the staircases. Small steel struts from the piers hold up the inner edge of the walkway, and seem a little out of place, one of the notable flaws in this otherwise bold and interesting bridge.

The bridge deck and parapet screens are both made from imported cumaru timber (except for the staircases where the parapets use stainless steel wires). The latticework screens are an unusual feature in a modern bridge, and the presence of so much timber is one of the things I found most attractive about the bridge. I liked it so much, I didn't really notice the extent to which it obscures the view off the bridge towards the east. It's also not obvious to me why such a tall screen has been adopted.

Nonetheless, it's the contrast between the steel truss and the timber elements that makes this bridge a success, and one of my favourites from my visit to Manchester.

Further information:

• Google Maps
• Project information from Arca
• Archisnaps #463 - review of the bridge originally published in City Life magazine
• Kingfisher Consulting