Koppigen Bridge

Location
Switzerland, Koppigen
Date of completion
2025
Owner
Canton of Bern
Specialized company
VSL

Constructing one of the world’s first post‑tensioned timber bridges

VSL Switzerland contributed to one of the world’s first post‑tensioned timber bridges, built as a wildlife crossing over the main highway linking Zurich and Bern. The timber superstructure consists of two spans, including a 37‑metre main span that is fully post‑tensioned. On top of the timber beams, a concrete slab was cast, followed by one metre of soil and vegetation. This significant additional weight made post‑tensioning essential to ensure the structure could safely carry long‑term loads.

One of the key challenges in post‑tensioned timber construction is the safe introduction of high forces into the wood. A customised solution was developed: a large steel plate fitted with multiple small anchorages, each carrying two strands, to distribute forces evenly. Protective caps were added to cover the anchorage zone and improve durability.

The post‑tensioning system was integrated during production at the carpentry workshop. Once completed, the beams were transported to a pre‑assembly yard, where initial stressing was carried out before final lifting and installation on site. This staged approach ensured stability and precise alignment.

The project builds on extensive research with l’Ecole du Bois in Biel and ETH Zurich. Laboratory and field testing confirmed that post‑tensioning improves timber performance both in strength—enabling higher load capacity—and serviceability—reducing deformation under live loads.