Moxon's sleek curving bridge supported on two giant glulam beams provides a valuable lesson in how to create elegant but sustainable public infrastructure
A graceful, sustainable pedestrian and cycle bridge in the town of Balingen in southern Germany is supported on two massive engineered timber beams that span a continuous 40m stretch over the River Eyach with no intermediate pillars.
The competition-winning design is based on a so-called trough construction, whereby the glulam beams form two parapet sides up to railing level, separated by a steel U-frame cross-beam structure that prevents the beams from tipping outwards.
A 3m-wide fibre-reinforced plastic deck sits on top and the structure widens in plan to 5m at each end of the bridge to enable easy access by bikes and pedestrians.
Using glulam for the superstructure was estimated to cut carbon emissions in production and construction by a factor of 2.5-3, compared to Moxon’s previous Somers Town Bridge, which is of a similar size.
‘Considering the bridges have the same intended design life, the timber option is at least 2.5 times more sustainable,’ said Ezra Groskin, director at Moxon Architects, who added that steel was considered the only option for the cross-beam portion of the bridge due to the high structural forces involved.
The town is located in the foothills of the Alps and snow melt in the spring causes the river to surge, so the ark of the bridge had to be high enough to avoid contact with the water.
‘We knew the beams needed to be higher in the middle, so the path ramps up towards the middle, in elevation,’ said Groskin. ‘The beams also change in plan so you get something like a three dimensional curve going on. It was a chance to do something a little bit unusual and exciting; the way you experience it is quite dynamic.’
The organic structure is deceptive, the two beams are in fact more or less identical and ‘rotationally symmetrical’. Although each was bespoke, the same jig could be used for both, rotated 180 degrees.
A key challenge was to prevent the timber from getting wet and rotting over time. The inner, walkway-facing surfaces of the glulams are covered with a waterproof membrane and clad in removable timber slats designed to be replaced every 15-20 years. Lighting and handrails are integrated into the slats.
The exterior-facing surfaces of the beams are exposed, stepped and set at an incline of 22.5 degrees, which combined with an overhang above ensures that most wind-driven rain doesn’t land.
The use of engineered timber is commonplace in German infrastructure, but misconceptions around its use in the UK prevent it from playing a more prominent role, said Groskin. ‘In Germany it’s a normal way of working and not seen as an exotic one off ... but in the UK we have this chicken and egg situation, where people aren’t comfortable with it [due to concerns over maintenance] because there are no examples of successful projects more than 20 or 30 years old.’
He added: ‘Timber is everywhere in buildings now, it's just the question of when people are going to stick their necks out and start doing it in infrastructure.’
Moxon Architects’ Quarry Studios in Scotland won the RIBA National Award in 2022