On Rigi Fruttli, the dilapidated concrete bridge over the Rotenfluebach had to be replaced. The world's first timber-UHPC composite bridge for 40-ton loads was built.
Project description
On Rigi Fruttli, the dilapidated concrete bridge over the Rotenfluebach stream had to be replaced. An innovative 40-ton bridge made of wood and the concrete-like high-performance material UHFB (ultra-high-strength fiber composite building material) was built. The Fruttli Bridge is the first timber-UHFB composite bridge for 40-ton loads in Switzerland. It thus has pioneering character.
The Fruttli Bridge is located on the road from Arth in the direction of Rigi-Klösterli, which is used not only for alpine transport but also for regional timber transport. A load of 40 tons is therefore to be expected. Last year, during the periodic inspection, it was determined that repairing the concrete bridge, which is over 50 years old, was no longer technically feasible and the bridge had to be replaced. Together with the Office for Forests and Natural Hazards Schwyz, the Lower Allmeind Korporation Arth had two variants for the replacement of the 10 m long bridge prepared.
The choice was between a conventional concrete bridge and a composite bridge made of wood and UHFB. Due to the lower costs and the shorter construction time, the decision was made in favor of the composite timber bridge. After preparatory work on the existing abutments, the entire bridge superstructure was erected within one week. The bridge was open to traffic again just four days later. This meant that an emergency bridge could be dispensed with.
The design principle of the composite bridge allows timber and UHFB to work together to master the large forces of 40 tons acting on the supports. When the bridge is loaded, mainly compressive forces act in the UHFB above and tensile forces in the wood below. This means that the materials used are cleverly deployed according to their natural strengths. In order for the materials to interact as described, the UHFB plate had to be statically connected to the wooden girders. Thanks to its hardness and impermeability, the UHFB slab can be driven over directly. This meant that additional waterproofing and a road surface could be dispensed with. The four 52 cm high wooden girders made of high-quality Swiss glulam sit on the bridge bearings. The timber girders, manufactured by neue Holzbau AG, are structurally connected to the UHFB using the very efficient and rigid GSA technology. GSA (threaded rod anchor) is a connection system for timber construction developed by neue Holzbau AG in collaboration with Professor Ernst Gehri (ETH). In this system, steel rods are glued into the wood with an epoxy resin. The three components - wood, steel and resin - are optimally coordinated to form a highly efficient connection system. The UHFB was produced on site in the same way as concrete and poured in liquid form into a formwork where it was allowed to cure. As formwork, two superimposed 27 mm thick wooden panels were placed on the girders, which remain in the bridge. The formwork for the lateral projection was already pre-assembled on the girders and removed after curing. The UHFB slab has a thickness of 8.6 cm to 14 cm and a slope of 5%. It can be driven over directly, serves as a waterproofing and, thanks to the lateral projection, also acts as weather protection for the wooden beams. To ensure slip resistance, grooves are milled into the surface. The steel fibers that protruded more as a result of the milling were then insulated with a gasflamm. This was a first, which led to the desired, flawless result. Due to the absence of cracks and the frost and de-icing salt resistance of the UHFB, maintenance costs are expected to be significantly lower than for a conventional concrete bridge.
Special features
Durable and ecological novelty
Compared to a conventional concrete bridge, CO2 emissions have been halved. In addition, the amount of CO2 permanently stored in the wood used is about the same as that emitted during UHFB production. The Fruttli Bridge is the first wooden UHFB bridge in Switzerland for 40-ton loads and with a scarfed surface. It shows that this construction method can compete with a conventional concrete structure in terms of price. And at the same time it offers major advantages in terms of construction time, durability and ecology.
