In the design of timber bridges durability is the crucial issue. In the Scandinavian countries, wood is often treated with chemicals in order to expose it to the elements. In the Alpine countries on the contrary there is a long and distinguished tradition of protecting timber work thanks to specific design features. These devices are not limited to the construction of roofs and side protective elements. In some cases for instance it is reasonable to use a central beam that can be easily protected against the weather.
This solution is based on a certain hierarchy of space and importance: the more central a structural element is section wise the better it is protected from sun, rain and snow, and the greater its life expectancy. Railings and banisters are ancillary parts and must be constructed so as to facilitate their replacement. If built of chestnut and larch they can last up to about forty years, the same goes for all the structural components that are weather exposed. If it is necessary to have such components, they must be replaceable and the structural conception of the work must be designed in such a way that the transitional absence of one such element does not affect the soundness of the overall structure during the replacement process. In such cases, a bridge will be closed during replacement work and therefore there will be a smaller operating load during construction.
Central truss bridges are sensitive to asymmetric loads. Eccentric effects due to wind force or accidental loads stress the bridge with torsional moments. These can be resisted by bulky sections resistant to torsion or with paired forces located on distant parts of the structure that are then transferred to the supporting points.
Our board presents the sections of four wooden bridges whose structural concept is based on a central girder. Apparently so different, they share a common structural setting. For the understanding of a structure it is necessary to know the underlining concept of the project. An analysis based solely on form or image leads to a superficial understanding.
Steiermark (A) 1995 Architekten Marcel Meili und Markus Peter, Zürich
The bridge is a pedestrian walkway and bicycle path which spans over the Mur River for 47.20 m. The covering and the deck have each a single central girder made of nailed laminated wood which is connected with strong steel plates to the vertical supports.
Plates and girders form a massive longitudinal single span framework. This framework solves the problem of four different kind of access to the bridge but it also offers in its central part unobstructed views of the river in all directions. The torsional stresses are compensated primarily by the lateral “flanges” of the top and bottom part of the T shaped structural frame.
Constructional reasons have forced to build the long girders in two parts. The scarfing joint surface is made with a fused epoxy resin. The bottom chord is longitudinally post-tensioned in such a way as to avoid any traction in the joint that constructively does not present any weakening due to the incision as it would have been inevitable in the case of a mechanical kind of fastening. The main girder is made of fir and red fir while the more weather exposed parts are made of solid larch.
Erster Traversiner Steg
Viamalaschlucht bei Thusis (CH) 1996
Collapsed in 1999 due to a rock fall.
The footbridge was used by the pilgrims of the “Via Spluga”. Due to the steep and inaccessible nature of the site, the 47 m long substructure was shaped like a threechord truss girder weighting only 4.2 t in order to be delivered by helicopter. The space-truss system element working in compression was made by larch laminated boards that were waterproofed under the deck surface. Two parabolic cables acted as lower longitudinal chords.
Each of the struts was made up of four larch boards, the nodes were designed to allow easy replacement of all the connecting elements. The diagonal tie-bars were made of galvanized-steel.
After the air delivery of the main substructure, the bridge deck and side rails were also transported by helicopter and then assembled on site. Rigid balustrades were used to compensate the torsional forces and were individually connected to the bridge sides.
After its collapse, the bridge was rebuilt in a safer place and with a different design.
Peiden Bad, Val Lumnezia (CH) 2002
The bridge replaces a previous steel truss structure that was very corroded. The concrete driveway is “balanced” on a thin wooden structure placed at the intrados. The span between the two masonry abutments of the old bridge is 24.60 m. The municipality wished that local raw red fir wood would be utilized, indeed it was used to build the multiple braced framework. The longest timbers are 13 m long and work in compression, while the longest struts are all linked with threaded rods and wooden blocks to resist buckling and are then connected to the upper nodes . The nodes are big concrete blocks cast on site.
A layer of solid wood beams almost a roof has been realized while above it there is a 25 cm thick concrete slab. The thick edges of the slab protect the wooden structure from weathering and resist torsional stress thanks to a pair of vertical forces. The waterproofing and edge finishing have been completed respecting the Canton Grisons’ building codes for concrete bridges.
Trutg dil Flem, Flims (CH) 2013
The bridge serves a mountain trail, it is 18 m long and it offers spectacular views of the glacial flounces of the Flem stream. The horizontal continuous girder is located on a trestle of 12.5 m span. To protect the girder larch boards from weathering there is a waterproof cover and on top there are the U shaped parapets. The girder is supported by four struts in solid larch. These redundant supports are designed so they can be replaced individually. Decking and railings are made of solid pine wood and they have been realized starting from the elevation points measured on site after building the bridge supports. The truss structure was transported on site by helicopter along with temporary struts. The weak moment due to eccentric forces is resisted by the junction points of the struts as well as the end supports. The V shaped struts fixed to the side of the girder bear part of the torsion force, however, they also produce an additional horizontal flexion for the girder.