6 | Bridgehead.

7 | In-situ butt-jointing of a main beam by finger-jointing and gluing.

for further stabilisation. In the case of wood there can be hardly any build-up of natural oscillation. The construction was optimized by means of wind tunnel tests. To prevent twisting of the slim girder row by lateral wind loads the system has been given a box-like cross-section and braced; on top it has a double layer of diagonal boarding, on the underneath side diagonal bracing of squared timbers.

9 | Overall view of bridge, scale 1:1500.

Structure | The overall length of the wooden bridge structure between the massive bridgeheads is approx. 195 m,the clear width of the footway 3.20 m.The structure is designed to carry a traffic load of 500 kg/m2 and will thus take light cleaning vehicles and ambulances. Nine 220 x 600 mm shaped glulam beams extend from abutment to abutment in a single band over three trestle piers. The system of beams thus covers four spans of 30, 32,73 and 35 m.The diagonal bracing on the underneath side of the glulam beam system, built using 60 x 160 mm timbers, displays a continuous lattice grid that determines the coordinates of the bridge beams. Jointing of the lattice bracing has been performed in such a way that the superimposed diagonal timbers display common points of intersection with those beneath them. Through these points of intersection runs the line of coordinates which determine the positions and dimensions of all the other parts of the structure and the bridge parapet latticework. Grid length and thus the parapet post spacing is 4.60 m.

The intermediate wooden bridge supports are in the form of trestles that open out fanlike towards the top. They are carried by concrete piers. The timbers of the trestle-type intermediate supports, hinged to the bridge beams, carry the vertical forces from each line of beams separately into the concrete supports. Horizontal wind forces are absorbed by diagonal steel ties within the supports.

The connecting elements are in the form of cast iron hinges, which transmit the forces acting at various angles smoothly and free from strain into the bracing. The hinge pins perform merely a holding function, serving also to counteract slight lifting forces at individual points; the main pressure forces are transmitted directly by the hinge straps and cups. The hinges are the same for all the points of intersection.

At the bridgeheads the main forces of the structure are transmitted as tensile forces of a magnitude of some 4000 kN. These bridgeheads are massive reinforced concrete abutments carried by large cast-in-situ concrete piles and held secure by ground anchors. The pull of each line of beams is absorbed by the concrete abutments via steel fork-end fasteners and stretching steel anchors pre-stressed to 500 kN. In order to prevent inadmissible strain at the sensitive pre-stressed bearings caused by deformation of the bridge girder system all the forces are led into the supports by way of a common pivotal point. The vertical and lateral forces are led away via shafts on raised brackets, at the side of and between the grouped beams, to the supports. The brackets remain stable only for vertical and lateral forces; in the pretensioned direction they are more or less flexible. All the important structural parts are left visible and accessible for inspection purposes.

The parapet is a lightweight, braced timber construction. At a coordinate spacing of 4.60 m there are two beams perpendicular to the roadway that cantilever out at footway grid height. These carry the outlying diagonally strutted parapet

9 | Overall view of bridge, scale 1:1500.


10 | Detail of top covering, scale i:io. On the girders double 25 mm diagonal boarding, 50 x 150 mm canted diagonal planks, double welded bitumen sheeting, over this a titanium-zinc covering, on Neoprene intermediate layers and 80 x 140 mm timbers the footway grid with 50 x 100 mm beams of bongossi.The cantilevered 100 x 130 mm timbers for the parapet posts of larch.

111 Section through girders at bridgehead, scale 1:50. The common stay bar of 100 mm diameter is held vertically at four points. The tensile forces of each main girder are transmitted to the concrete supports via fork-end connectors, which are fastened to the girders by way of nailed plates.

posts by means of gripper-type connectors. These components are the same throughout the whole of the bridge and were completely prefabricated at the factory. Fixed between the parapet posts are two 100 x 100 mm parapet beams, the upper one of which is braced in the middle by outlying diagonal timbers. The space between the top and bottom parapet beam is filled by a framework of angle steels holding a welded steel wire lattice. At all the intersections of the parapet framework are identical fasteners consisting of head plates with hinge sleeves and two connectors designed to adapt to any angle. Between the parapet connectors and independent of the suporting structure there is a wooden grid-type footway covering designed to take loads of up to 10 kN as required by vehicles. 50 x 100 mm bongossi planks are screwed to 80 x 140 mm supporting beams, also of bongossi, with a space in between. These grids are fixed only to the parapet connectors, so as to avoid any penetration of the bridge girder system covering. Convex bending of the bridge girder system causes downward movement of the grids; when concave movement occurs they adapt to the change. The footway grids are renewable.

Because of the high loads involved all the connections are of steel castings produced by a state-of-the-art technique. Nailed connections have also been used since high forces can be led effectively to the wood via nailed plates. Apart from that, nailed plates can be precision-fitted at the works and are not in the way during transportation. On the site, projecting castings and other connectors can also be bolted to the nailed plates.

All the parts of the main structure are of Class I glued laminated fir. The double diagonal boarding on top of the bridge girders is of Class II fir, the squared timbers of the underneath diagonal bracing of solid Class I fir. To withstand weathering the bridge parapet is of larch, the footway covering of bongossi. All the steel parts are of stainless steel, the steel castings of hot-dip galvanised GS 52.

As weather protection for the bridge girder system the top diagonal boarding is covered with a double layer of welded bitumen sheeting and, on top of that, with zinc sheet. Rainwater is led away alongside the diagonal planks, except over the highroad, where it is collected in gutters and disposed of to the side of the road via waterspouts. All the parts of bongossi are protected by an air-permeable varnish applied in three layers over a primary waterproof coating.

Construction | It was decided to erect the bridge without the use of scaffolding, by utilising the inherent stability of the girder band, prefabricated in sections at the works. For on-site assembly only trestles were thus used for point to point support. As far as possible the various components were prefabricated. A field workshop was set up at the site for the pre-assembly of larger units. The shaped and glued girder sections in lengths of 30 to 43 m were prefabricated at the factory and finger-jointed and glued (butt-jointed) with such precision that, after tensioning at the abutments, there were no noticeable differences in length or height between the individual strings of girders.

The next step was to fit the underneath diagonal bracing with the necessary precision using a suspended mobile scaffold. Accumulation of the unavoidable tolerances had to be prevented at all costs by compensating for them from one point of intersection to the next. The diagonal members, prefabricated and complete with their nailed plates, were accurately marked and drilled on the spot with hole cutters.

After that the top double diagonal boarding was laid with the slanted planks, the sealing layer and the metal sheeting. Finally the parapet lattice and the footway grids were fitted using a lightweight, mobile suspended scaffold. Before the bridge was opened to the public it was subjected over the whole of its length to a loading test with water-filled containers, which confirmed the structure's computed stability.

12 | Cast iron hinges on concrete supports: the cups for pressure forces and the forks for the ties of the wind bracing.

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