However with large scale bridges, their movement characteristic becomes more complex and more intensive. Cable-stayed and suspension bridges in particular, where expansion joints may only to be positioned at the ends of the span, need joints that can cope with extraordinarily large longitudinal movements. For instance, today’s suspension bridges have longitudinal movement requirements that are already exceeding the 2m mark.

It is not only the large longitudinal movement capacity that is necessary, but also the daily, monthly and yearly movement frequency that demands high performance solutions from expansion joint manufacturers.

For example, changes in the solar radiation intensity have a strong influence on the movement frequency of the bridge. For instance, if the sun is covered by a cloud, the large scale bridge, usually made out of steel, immediately cools down due to the reduced solar radiation intensity, and contracts, although the ambient temperature has not changed yet. A few minutes later, when the cloud moves further and the sky has opened again, the sun’s radiation immediately warms the steel structure up again and the bridge expands back to its original length.

This kind of movement may quite easily take place a hundred times in a day. Assuming a daily movement range of 300mm as well as many micro movements due to solar radiation variations, variable traffic load influences, and wind loads, and taking into account an expansion joint’s expected life of 40 years, this would result in a total movement requirement of several hundred kilometres; far higher than the generally applied durability test requirements of 5km to 20km.

Such ever-increasing wear and tear requirements mean that continuous development and testing of its individual components is necessary in order to guarantee an optimal performance during an expansion joint’s lifetime.

The final requirement, in order to protect the inside of the superstructure from dirt, polluted water and dangerous liquids - for instance petrol from a traffic accident - expansion joints must be made watertight at the bridge surface and not just be covered with a drainage channel underneath the expansion joints.

Apart from tailor-made designs, today there are basically only three different expansion joint types in the market that allow longitudinal movements of greater than 1m.

The supported cantilever finger-type joint is a development of the finger joint. Normally cantilever finger joints have a maximum movement capacity of about 400mm. But by supporting the cantilevered fingers with auxiliary devices from below, very large movement capacities can be achieved.

However, this joint has three main disadvantages. No transverse movements of the bridges can be tolerated, otherwise the fingers would immediately touch each other and large constraining forces would result. No vertical movements are acceptable either, as the fingers would stick up into the carriageway and would be dangerous to traffic.

Finally, the connection between the finger plates and the support is usually built as a hinged steel connection that under dynamic load impacts is very sensitive and can become a big source of problems, as past records show.

Roller shutter joints were introduced in the 1930s and over the course of several decades were installed in numerous bridges requiring high longitudinal movement capacity. Over the last twenty years, however, this type of joint has almost disappeared from the market as it has disadvantages such as limited movement capabilities - only possible in one direction – extremely high initial cost, the fact that it generates high noise levels, has limited adaptability to the bridge geometry and of course its lack of watertightness.

Watertight modular expansion joints, first introduced in the 1970s have been continuously developed since then. Today, several types are available in the markets, but their capabilities differ greatly. The fourth generation is the latest development of modular expansion joints, differing completely from the original design (Bd&e issue no 28). These joints have an extended lifetime, and constraint forces can almost be eliminated, thanks to the expansion joint’s elastic steering and damping system.

Such joints can be designed with movement capacities over 2m in the longitudinal direction and +/-500mm in the transverse direction. The movement system is durable: with the introduction of sliding and elastomeric elements the system’s resistance to fatigue and wear and tear has been increased considerably.

Installation is made easier - due to the minimal space required beneath the support bar, the placement of reinforcement becomes very easy and the danger of concrete air pockets creating heave beneath the support bars can be practically eliminated.

As the system has a single support bar, the space below the lamellas is easy to access for inspection and maintenance activities.

Engineers on what will be the world’s third largest suspension bridge, part of the Runyang Bridge (Bd&e issue no 30) had a tough challenge to decide what type of expansion joint to use. The requirements are enormous: the joint needs to be able to accommodate a movement capacity of 2160mm – a record-breaker.

After a comprehensive evaluation process, the client awarded the contract to Mageba, to supply fourth generation modular expansion joints not only to the 2.5km-long Bei Cha suspension bridge but also to the 1.4km-long Nan Cha cable-stayed bridge.

The contract was signed in August last year, and by last month, the first batch of expansion joints left Mageba‘s workshop and is currently on its way to China. They have a longitudinal movement capacity of 800mm, and are expected to be installed on the cable-stayed bridge by the end of this year.

Meanwhile the design for the largest modular expansion joints ever built, with 27 gaps totalling an overall longitudinal movement capacity of 2160mm, is under way. Manufacture of these units will start around July and the shipment of individual parts is due to take place in November.

Due to its large size, the expansion joint must be assembled in situ, a process that has already been carried out successfully on other projects, such as the Tsing Ma Bridge in Hong Kong. However assembly on site will take several months and will require precise workmanship, bearing in mind that each joint is composed of more than 40t of special steel and material components.

An expert team will be brought in from Switzerland to supervise the complete assembly process in order to guarantee the final product achieves the highest possible quality standards.

With the official Italian announcement to go ahead with the world’s largest suspension bridge with a main span of 3000m, connecting southern Italy with Sicily across the Messina Strait, new challenges are given to expansion joint manufacturers.

It is very likely that also for this bridge a modular expansion joint will be chosen as it best can fulfil the requirements in terms of movement capacity, durability and watertightness; and moreover to guarantee a proper functioning under such extreme conditions for a life time of 40 years and more.

Gianni Moor is deputy general manager at Mageba

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