14 March 2010

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:


Marchie said...

Just to let you know, there is now a footpath heading south from the bridge, as well as north, which heads to New Bridge Street.

Anonymous said...

Planning permission has now been granted for a 14 storey hotel next to the new footpath. It will be sited forward of existing buildings and partially block existing views from and towards the Calatrava bridge.

stone cutting said...

Gosh, that "bearing". So the horizontal restraint is below the NA and creates anti-clockwise moment but the bigger link is on the left so also producing anti-clockwise moment. The two links, different lengths and different centres so generating different geometric changes with movement. Hope the movements are small!