Restoration and strengthening strategies for 19th century iron pedestrian suspension bridges

The history of mainland Europe's oldest suspension bridges dates back to the beginning of the first half of the 19th century. The two World Wars as well as continued renovations to these bridges make that the earliest suspension bridges in central Europe (Belgium, France and Germany) can only be physically documented by decorative private suspension bridges. Mainland Europe's second oldest surviving suspension bridge (1824, Wissekerke, Belgium) is used to illustrate a particular restoration and strengthening approach used to make these bridges comply with current concepts of safety. The initially private pedestrian bridge spanning 20.5 m across the castle's moot, is at present part of a public park. Due to ill-maintenance and deterioration over decades as well as a function shift (from private to public), the bridge does not fore fill the current European norm for pedestrian bridge design. Two options offer themselves: the bridge is closed to the general public (with limited private access), restored and kept as an "architectural" object or the bridge is strengthened ensuring maximum public utility with minimum loss of authenticity. Six different strengthening strategies are briefly presented. These strategies include the addition of extra support to reduce the span, the addition of a structure under the deck (e.g. beams, truss, arch), the addition of a structure in the deck (e.g. pre-stressing cables, girder), the addition of a structure above the deck (e.g. truss along hand rail), strengthening of the suspension cable and finally strengthening through cable stays. The architectural and engineering advantages and disadvantages of these strategies are highlighted. One option is selected on the basis of maximum public use, (and compliance with current European Norms), optimal preservation, least visual impact and easy future maintenance. This strategy preserves and restores most of the authentic elements (being the cast iron mast, the wrought iron suspension chain, the back stay and the railing) and replaces the structurally inadequate non authentic deck with a closed steel box girder with timber deck. In this solution the suspension structure thus solely carries its own weight and temperature influences. The new girder designed for strength, stiffness, dynamics and carries dead loads and live loads laid down in the European Norms. Based upon this representative case study, general conclusions are given for restoration and strengthening strategies for 19th century iron pedestrian bridges.