These units offer a watertight system, protecting the structure from dirt and polluted water, and can accommodate longitudinal movements of up to 1,200mm as well as movement and rotational capacities around all three axes. The initial investment is regarded as lower than the original roller shutter joint proposal, and modular expansion joints also have a low maintenance cost throughout their lifetime. They can be easily adapted to the existing bridge design.

The modular expansion joints supplied for the Kwang Ahn Bridge belong to the so-called 'fourth generation' of modular expansion joints. The main characteristic of the first modular expansion joint, developed in 1963 by Swiss specialist Mageba, was the multiple support bar system, supporting each lamella with one single support bar.

Twenty years later, the multiple support bar system was further developed by placing a single support bar between two multiple support bar packages, creating a system named the hybrid modular expansion joint.

Around 1989, the multiple support bar system was completely abandoned and the first modular expansion joint with a pure single support bar system was introduced. This breakthrough simplified the whole system by allowing a single bar to support all the lamella beams at once, but it meant that several existing elements had to be redesigned and some new ones developed. For instance the original rigidly welded connection between the support bar and the lamella beam was now replaced with an elastic sliding system.

As bridges became larger, designers began asking for expansion joints with greater movement capacities, and in the 1990s Mageba developed the single support bar system further. This time the main concept remained the same but new materials and detailed design changes were introduced for all critical components - highly durable steering springs, low friction sliding springs and spherical sliding bearings for example. This is what is today referred to as the fourth generation modular expansion joint, and it has been used on many large bridges such as the Tsing Ma Bridge, the Second Nanjing Bridge and now the Kwang Ahn Bridge.

These systems offer a number of benefits for bridge owners. The multiple support bar system means that constraint forces can almost be eliminated, thanks to the expansion joint’s elastic steering and damping system, resulting in an extended lifetime for the joint.

Such expansion joints can be designed with movement capacities of more than 2m in the longitudinal direction and +/-500mm in the transverse direction. Durability is also improved - with the introduction of sliding and elastomeric elements the resistance to fatigue as well as general wear and tear resistance has been increased considerably.

Installation is made easier due to the fact that very little space is required beneath the support bar; placement of reinforcement is very easy and the danger of heaving concrete air pockets underneath the support bars can be practically eliminated. Finally, the fact that the joint has only a single support bar means that the space below the lamellas is easily accessible for inspection and maintenance.

Installation of the expansion joints for the Kwang Ahn Bridge started in June and was due to be completed as Bd&e went to press. One of the main requirements by the client was that the joints be delivered in single units of 18m length. This not only created many obstacles during production and transport but also meant that special installation equipment was required on site.

For example, sliding plates with PTFE and a series of hydraulic bottle-jacks are needed to ensure that the joint is properly levelled and connected to the bridge while the bridge goes through its daily thermal movement cycle. The biggest expansion joint has a movement capacity of 1,000mm and weighs over 30t.

Mageba is currently in the process of further development of a modular expansion joint which is expected to be the world’s largest, in response to a contract for the Run Yang Bridge being built in China. The suspension bridge will have a main span of 1,490m; Mageba is supplying a modular expansion joint with 27 gaps, allowing a total longitudinal movement of 2,160mm.

Gianni Moor is deputy general manager of Mageba (Switzerland)

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