Kieldrecht Lock – Why bascule bridges are the preferred mean to cross a lock
Road and railway traffic often need to cross a lock when it is located in urban regions. A couple of different moveable bridges can be used but in most cases a bascule bridge over the lock is the preferred solution. A bascule bridge is supported on an axis which is perpendicular to the bridge’s longitudinal centreline axis. They place no restriction on the height of the vessels passing through and the time required to pass vessels is shorter than that of a vertical lifting bridge or a swing bridge. This blog posts briefly discusses the design of the bridges at the Kieldrecht lock as well as the most important technical challenges encountered during the design.
The Kieldrecht lock (formerly known as the Deurganckdok lock) measures 500 x 68 x 17.8m and is currently the largest lock in the world. At each lockhead, a mixed rail and road bridge already ensures that traffic can continuously travel over the lock. Cyclists and pedestrians can also cross the lock in this way at any time. However, the 2 existing bridges did not offer the required capacity regarding road traffic. For this reason, two additional road bridges are placed next to the existing bridges. SBE engineered and optimised the design of the road bridges.
The bridges are each 15m wide and 76,6m long. For each bridge, two large steel trusses are connected by crossbeams and an orthotropic steel deck. This type of deck is widely used in moveable bridges because of its light weight when compared to their load carrying capacity.
A few technical challenges were encountered during the design. The geometry of the bridges had to fit in the existing concrete basements which had complications for the position and design of the counterweight. A combination of heavyweight concrete and steel slabs was used for the counterweight in order to have the centre of gravity in the desired position after taking the position of the support points and the geometry of the concrete basement into account.
Another challenge for this type of bridges is the fatigue design of the steel structure. The cycling loads on the bridge due to the traffic and the opening and closing of the bridge induce stress changes in the steel that can result in progressive and localised structural damage and the formation of cracks. For the trusses and the counterweight, detailed finite element models (FEM) were made and analysed in order to calculate the stresses accurately and to verify fatigue resistance.
For the orthotropic deck, SBE could rely on the expertise of its own engineers. Orthotropic decks have been prone to fatigue problems and to delamination of the wearing surface. During a concurrent project, SBE investigated the fatigue resistance of an orthotropic steel deck in an extensive FEM analysis. In these FEM analyses various parameters were considered such as increased traffic volume and accompanying axle loads, historical positions of the heavy lanes, historical road pavements and their temperature-dependent load spreading effects. In conclusion, accurate fatigue damage was determined for all fatigue details.
You are invited to read more on this topic in the paper below, which is an article presented and published by SBE during the IABSE Future of Design NYC conference in New York.