Such demands influence the design and manufacture of numerous parts of the bridge. For example, expansion joints and bearings are unique to each bridge – these mechanical devices must allow for large movements in all directions and are subject to high vertical and horizontal loading from increased dead loads, earthquake loads and accidental traffic loads caused by braking or skidding.

Watertightness of expansion joints is a basic requirement, in order to prevent damage from salt water. If joints are watertight they also prevent rainwater damaging the bearings. Modular expansion joints can fulfil these requirements providing there is a reliable sealing system.

Expansion joints have to allow unrestrained movements and rotations in all directions and around all axes due to a number of reasons such as temperature changes, shrinkage, creep, earthquakes and settlements. Modular expansion joints with single support beams – such as those manufactured by Mageba - can accommodate all these movements, the company claims.

Bridges are often communication lifelines; if an earthquake causes major damage, it is important that such bridges remain useable. If an earthquake destroys or damages an expansion joint, the bridge may become impassable so it is vital that expansion joints are capable of resisting seismic forces.

Modular expansion joints with single support beams allow big movements in all three directions and they are built in modules which can be adapted to the needs of the bridge designer. Trapezoidal joist boxes with special spherical sliding bearings for transverse beams are available to deal with the large transverse and vertical movements typically caused by earthquakes or as a result of ship impact. Mageba has already produced lamella joints with longitudinal movement capacities of ±1000mm and transverse movement capacities of ±250mm, but this is not the limit, the company claims.

In addition to allowing bridge movements, large modular expansion joints for extremely long span bridge structures must allow all kinds of traffic to cross easily, while also ensuring safe transmission of braking forces and preventing skidding. Each fully opened expansion joint at Tsing Ma Bridge or China’s Jiangyin Bridge forms a 4m wide metallic structure consisting of about 50% gaps and 50% metallic lamellas.

While smaller expansion joints only require corrosion protection coatings, on very large modular expansion joints the metallic lamella surfaces require special treatment. A suitable friction coefficient must be assured for a service lifetime under all weather conditions, and the special anti-skid coating should be resistant to radiation from sunlight and pollution. Mageba’s coating is a special surface treatment which guarantees a friction coefficient of at least m =0.5 for service lifetime under the most adverse traffic and weather conditions.

The horizontal load transmission system for braking forces caused by traffic should be investigated and specially designed for large modular expansion joints. As well as other gap control and steering systems, the Mageba flexible controlling device has a reinforced watertight sealing strip as a special gap-limiting feature.

The horizontal braking forces are transferred to the edge profiles and damped by the combined action of the sealing strip and the elastic steering system. This sealing strip was developed and tested on various occasions and has proven to play a key role in the safety and design of the joint. The reinforced water sealing strip profile is strong enough to transfer horizontal forces to the adjacent structures.

But despite its capacity for transferring large horizontal forces, it is not bolted to the lamellas in order to allow easy replacement. The combination of these characteristics can ensure safe driving on very large expansion joints in any weather.

It is also vital to take care in developing good fatigue load design to deal with the heavy dynamic wheel loads caused by traffic. Hence only expansion joints which have been tested for fatigue and had dynamic analysis carried out should be used. For the same reasons, load-carrying welds should be avoided as they have a reduced fatigue resistance.

Large expansion joints may be so big that they require special measures for transporting them to the bridge site, as they form oversize loads. They may even have to be assembled on site, which requires extreme care as site conditions are very different to those in the manufacturing shop. A good installation may even extend the lifetime of an expansion joint by reducing the dynamic impact of wheel loads.

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