Suspended from a stainless steel wire net, rather than traditional suspension cables
Jakob Rope Systems, a local manufacturer of stainless steel ropes, rods and mesh systems, developed an innovative solution for the new Himmelhaus Bridge, its design seeing the bridge suspended on a stainless steel wire rope net instead of traditional suspension ropes.
Although the Webnet system is proven in projects worldwide, this project marks the first use of the Webnet stainless steel mesh as a replacement for bridge suspension cables.
The bridge features a braced, tensioned construction, made up of supporting cables and bridge deck beams, as well as the stainless steel net. In combination, these materials increase the stiffness of the structure and reduce the susceptibility to movement, something that is sometimes seen on suspended structures.
The use of Webnet also provides protection against falls for users of the bridge.
The Himmelhaus bridge over the river Trub in Trubschachen
Innovative design delivers strength and rigidity
With a slight incline as it crosses the river, the bridge spans a total of 25.8 metres, with the suspended section of the bridge stretching 21.5 metres between the supporting pylons. The bridge joins the road to the neighboring district, with its 2.2 metre width enabling the municipality’s 5 tonne snow-clearing tractor to cross in the winter. The bridge is designed for a payload of 4.0 kN / m2.
Manufactured from steel gratings, the bridge deck is supported on four HEA 140 longitudinal steel beams. These in turn are supported by seven transverse yoke beams across the suspended section. The structure is then braced by Webnet, which creates a superelevation sag of 280mm from the horizontal in the bridge. The design, including the inclination due to the superelevation sag, was developed to allow easy access for people with impaired mobility.
The beams feature angled twin arms which taper towards the outside edges of the bridge, which with the vertical forces from the longitudinal steel beams and the laterally arranged steel tubes (CHS 88.9 × 5.0). By design, the cross members not only allow reduced distances to sit the longitudinal beams on, but also achieve a ‘Vierendeel effect’ to achieve the necessary transverse strength and rigidity. The longitudinal tubes that distribute the load on the cross beams also contain the required electrical cables.
The bridge’s lower Webnet anchoring rail is located above the longitudinal tube (CHS 42.4 × 3.2) and is coupled with clevises with external threads which enabled the Webnet to be connected.
Suspended stainless steel net
The Webnet specified for this project consists of 3mm stainless steel ropes, with a construction of 6 × 19 + WC, which with sleeves, forms a mesh width of 80 mm. The resistance of the sleeved joints, as well as the behaviour to the forces and resistance to expansion, were all thoroughly tested prior to installation.
The net was manufactured in one piece precisely to the final project dimensions, taking into account and compensating for all necessary structural deductions.
As a result the Webnet system was delivered to site without any need for further on-site adjustments, enabling fast and efficient installation.
The Webnet is connected to 26 mm diameter suspension cables, which are 1 x 37 open spiral strand stainless steel cables, with Jakob Forte turnbuckles fitted at both ends.
The suspension cable has a 2.4 m sag over its 23.7 m length, which gives an f / l ratio of 1/10. These cables are attached to the pylon heads, together with the Jakob Forte M36 wire ropes and the tie-back cable.
The 5.3 m high articulated pylons are manufactured from welded hollow sections and support the longitudinal profiles of the bridge girder on their crossbars, these being connected to the pylons by means of bolted joints.
In total, the bridge structure, including its gratings, weighs about 12 tonnes and is anchored into a heavy weight concrete solution.
Construction programmed
After completion of the foundations, the articulated pylons were positioned in place together with the temporary suspension cables which had already been attached.
The bridge deck was manufactured off-site and transported to Trubschachen in one-piece, where it was lifted in position by pneumatic crane and secured to temporary cables.
Temporary chain hoists were then arranged at regular intervals to align the bridge deck with the suspension cables.
By accurately manufacturing each element of the bridge off-site, its subsequent construction and assembly were quick and efficient – and avoided costly and time-consuming adjustments having to be made. Compared to conventional approaches, the design has proved to be cost-effective, with this solution also removing the need for additional guardrails and the cable supports they would require.
Following the opening of the bridge, regular measurements are being taken of the cable forces to collect more data for future projects.
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Project team
Client: Trubschachen community
Bridge design: Jakob Rope Systems, Trubschachen
Foundations: Wüthrich Ingenieur und Planungs AG, Langnau
Steel / metal construction: Niederhäuser AG, Erlenbach
Construction: Thuner Bau AG, Trubschachen
Rope and net construction: Jakob Rope Systems, Trubschachen
Pension fund company: Kambly Thuner Bau AG
Outstanding community project
For the inhabitants of Trubschachen, the new bridge provides more direct access to the station and a safe cycle path, taking cyclists away from a busy main road.
As well as its unique construction, the new bridge is also distinguished by an excellent partnership between the public and private sectors. A true community project, it has involved a number of local companies, including Thuner Bau AG, the Kambly SA Pension Fund and Jakob Rope Systems, all working together with the municipality of Trubschachen. The bridge was officially opened in November 2020 by a billy goat, who was the first to cross it to the sound of Alpine horns.
The project demonstrates the innovation that is a feature of Jakob’s systems and solutions, with a completely new method of construction having been successfully introduced, as well as the breadth of the company’s capability – including engineering, design, planning, manufacture and installation.
