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Bridges: Winning design

A collaboration between British and German designers has resulted in the construction of two iconic footbridges over busy rail lines, as Max Thompson reports.

Opladen bridge

Opening up: The new bridges are intended to stimulate regeneration

Defined by its ­crisscross network of rail tracks, Opladen, on the ­northern edge of Leverkusen in west Germany, is the Teutonic equivalent of Crewe: an industrial staging post most people glimpse through a train window on their way elsewhere.

But maybe that is about to change, for Opladen, which sits on the main rail artery between Cologne and Düsseldorf, will soon boast two new pedestrian bridges, each straddling a dozen or so tracks and opening up a swathe of brownfield land, becoming a beacon for new investment.

Won in competition in 2008, the two 100m long bridges were designed by High Wycombe-based bridge specialist Knight Architects, in partnership with Stuttgart engineering firm ­Knippers Helbrig.

The first to be completed, the €1.5M (£1.2M) Campusbrücke, or Campus Bridge, opened in May 2013, and is already providing a much used east-west link to a sprawling 16ha site that was once the home of a vast maintenance depot for German rail operator Deutsche Bahn.

Over the next decade the site will be converted into a major mixed use development that will include homes, retail and commercial units and a new campus for Cologne University, hence the bridge’s name.

And 300m north of the Campus Bridge, work is currently at full tilt on the second of the two structures, the €4.5M (£3.6M) Bahnhofsbrücke, or Station Bridge. Due to open next year, it will replace a grotty underpass proving access to Opladen station’s platforms and will also include new elevators, escalators and ramps.

“We didn’t have the budget for a huge arch bridge, but Deutsche Bahn still needed something iconic; a landmark, but in a simple way”

Roman Schieber, Knippers Helbrig

“We didn’t have the budget for a huge arch bridge,” ­explains Knippers Helbrig project engineer Roman Schieber, “but the client [Deutsche Bahn] still needed something iconic; a landmark, but in a simple way.”

Opladen bridge

Elegant: Angular timber fins, made from Siberian larch, are fixed to the rear edge of every stiffener

The tight budget and the need for a design that could be erected with minimum disruption to the rail network rapidly led the team to opt for a simple series of single spans resting on concrete supports between the tracks.

With a single span solution agreed, one of the biggest considerations for Knight Architects project architect Bart Halaczek was how the design would respond to the site’s industrial heritage.

“The city’s character has been shaped over the last 100 years by Deutsche Bahn,” he explains. “There are red rusty rails everywhere, and also a lot of very beautiful shed buildings all around, some of which are being listed.”

With this backdrop in mind, the team rapidly hit on the idea of using weathering steel, as the material’s rusty external patina would blend in seamlessly with the industrial hinterland.

But it wasn’t just the material’s industrial chic that ticked boxes. “From an engineering point of view weathering steel is low maintenance and very durable,” explains Schieber.

Opladen bridge

Simple solution: The Campus Bridge is made up of single spans with piers between the railway tracks

“The costs are 10% to 20% more compared with normal steel, but there is very little maintenance.

“Normal steel would need painting every 12 to 15 years, which would be extra difficult over the lines, but with weathering steel maybe only the bearing and connections to the abutments will need attention,” Schieber adds.

Halaczek says drainage details were key, as any rust-coloured drips could stain the concrete piers, but from an engineering point of view the biggest consideration was the fact that weathering steel, well, weathers.

“Structurally, the steel plates are 8mm thick, but over a lifetime of, say, 120 years, 1mm will disappear on all sides; it rusts,” explains Schieber. “So we have oversized it on both sides, giving us 10mm plates.”

“From an engineering point of view weathering steel is low maintenance and very durable”

Roman Schieber, Knippers Helbrig

Weathering steel is essentially a flat pack system, with no I-beams or any rectangular profiles in either bridge.

On the Campus Bridge the steel is welded into five, single span troughs that vary between 12m and 24m in length, with a deck width of 3m. The Station Bridge has six spans varying between 15.5m and 24.3m, with widths between 5m and 7m.

The webs of each trough also vary in depth, from 550mm where they abut the concrete support piers, up to 1.1m deep between spans; in elevation the varying depths produce the bridges’ distinctive sinusoidal shape.

The support piers sitting between the tracks are made with C30/37 concrete and are engineered to withstand a freight train crashing into them, but other than that, like the single span troughs, their basic design is, says Schieber, “simple”.

Simple they may be; but the overall designs are miles away from your average pedestrian railway bridge. This is partly down to the bridges’ rusty patina, but mainly thanks to the dramatic effect of regularly spaced weathering steel ­stiffeners.

Opladen bridge

Industrial: Weathering steel was chosen to reflect the area’s rail heritage

Placed every 600mm, these stiffeners are key to the bridges’ structural integrity, but they also have a significant architectural impact. Viewed externally they look like a row of undulating steel ribs, giving the bridges a no-nonsense robustness, but, as Halaczek explains, the outside edges of the stiffeners have angular kinks, each at a different height:

“We varied the geometry of every single stiffener, so the kinks in the stiffeners run up and down counter to the trough’s top flange.”

The effect enhances the sinusoidal ebb and flow of the bridges, but the stiffeners have another function in that they also act as fixing posts for what Halaczek describes as the ­”architectural lamellae”.

These are angular timber fins made from Siberian larch that are fixed to the rear edge of every stiffener. From the pedestrian or cyclists’ point of view they soften the bridge and, at a height of 1.8m from deck level, also act as a safety barrier.

Like the stiffeners, each timber section also has an angular kink in it at a height that mirrors the sinusoidal curve of the steel trough below.
In combination the architectural and engineering lamellae have a striking visual impact: “In long view they seem opaque, so users feel enclosed and protected,” says Halaczek.

“From a distance, or from the cab of a train, the bridge[s] appear very transparent”

Bart Halaczek, Knight Architects

“But from a distance, or from the cab of a train, the bridge[s] appear very transparent.”

On the subject of transparency, the scheme was an exemplar of collaborative working. The designs were drawn up using the Rhino 3D modelling program and batted back and forth between High Wycombe and Stuttgart in a painless process that Schieber says worked because each firm had confidence in one another.

“Knight Architects is very focused on structures, so it was a really smooth collaboration,” he explains. “There were no boundaries like you maybe get on other projects, with other firms.”

Schieber won’t reveal what projects, but adds: “When we work with famous architects they often have crazy ideas that they want us to realise; these guys [Knight Architects] just have a completely different understanding.”

As part of the Campus Bridge project, a single span has been designed so in a few years’ time it can be swung 180o from its current position, allowing redundant tracks to be replaced while providing access to a new development currently in planning.

But for now the bridge is already providing a new level of access, the importance of which is best exemplified by something Schieber noticed at the opening ceremony.

“I saw a guy in his wheelchair, and he had tears in his eyes because it was the first time he had been able to cross the tracks,” he says. “It shows what an important link it is; a huge cut through to the city.”

Opladen bridge

Flat pack: The bridges were made in a factory and positioned on site

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