ZPF Ingenieure

Bridging the river Birs at its mouth is a comparatively minor matter. The recreational area upstream of the bridge Schwarzwaldbrücke draws its identity from the Rhine and the Birsfelden power plant. For this reason, the new footbridge (replacing Switzerland's first cable-stayed bridge from 1963, which was no longer serviceable) is not a suspension bridge or a voluminous load-bearing structure, but little more than a stretch of path, a few metres of hovering asphalt.

The surface seems to detach itself from the ground and guides the rambler, jogger or cyclist over the Birs in a gentle arc. With its language of forms (slim load-bearing framework and picket railings) and its materials (metal and asphalt), this stretch of path also integrates into the system of the park. The sophisticated picket railing runs into the mastic asphalt surface. LED strips are cast into the handrail, so that the walkway is optimally lit without any glare.

The two abutments are new: reinforced concrete bars, each on six micropiles. The bearing on the Basel side is fixed, while the one on the Birsfelden side is flexible. These also hold the bridge horizontally. The two-span beam, with a length of 75.5 m and span widths of 50.5 m and 25 m, is very thin, with a construction height of just 68 cm.

Two mass dampers reduce the vibrations. Thus, with a technical approach, the load-bearing framework concept results in a compellingly slim appearance. The walkway's light but rigid orthotropic steel plate is what made this extreme thinness possible. It is part of a hermetically sealed load-bearing steel framework, 4.9 m wide. Despite the walkway being twice as wide as that of the previous bridge, the new weight is less, so the central column can use the existing foundation.