Large bridges require expansion joints which can accommodate correspondingly large movements of the deck relative to its abutments. With the continued development of the long-span bridge sector, the demands on expansion joints continue to increase. In recent decades the length of the bridges that are being built has increased dramatically, resulting in a greater demand for expansion joints which can facilitate extreme movements.
Modular expansion joints are often best suited to satisfy these demands, but face a number of specific challenges related to scale in the case of very large bridges.
Extreme loading and large movements are probably the most significant factors influencing the overall performance and life cycle cost of a large expansion joint. With traffic volumes constantly rising, expansion joints are exposed to large loads at an increasing frequency. Axle loads also continue to increase, especially in highly-populated countries.
Assuming a bridge carries 6,000 vehicles a day on each traffic lane, the total design traffic volume per lane during a working life of 40 years will be likely to exceed 200 million axle loads. This enormous figure indicates why fatigue is a major consideration for designing durable expansion joints and why many joints start to fail after only a few years.
Under normal circumstances, bridge decks move in a steady and predictable manner influenced by temperature changes, traffic loads and wind. However large bridges, which these days may have longitudinal movements exceeding 2m, usually have quite complex movement characteristics which are sometimes very unpredictable. In addition to the normal daily and seasonal movements, micro movements that occur every time the sun is blocked by a cloud, for example, can easily result in a total movement requirement of several hundred kilometres over the lifetime of an expansion joint.
Expansion joints which must accommodate these extreme movements may require particuar solutions. The sliding material normally used for the moving parts of a modular expansion joint may not be able to withstand such extreme movements, and a suitable alternative must be specified. One material which meets these demands is Robo Slide, a high-grade sliding material with excellent abrasion resistance and very low friction characteristics. Tests carried out on this material showed that over a sliding distance of 2.5km, the friction level is approximately five times lower than that of PTFE. It has also been shown to be 20 times more durable and two and a half times stronger in compression than PTFE.
Modular expansion joints generally include symmetrical control systems which are used to regulate the width of the gaps between the joint's lamella beams. But these systems are not effective when the movement capacity is very large, due to friction and other forces which arise as the joint opens and closes. To overcome this problem, and ensure that the movement of the joint will be evenly distributed across its individual gaps, an alternative, asymmetrical control system has been developed by Mageba. This incorporates a staggered layout of the control springs, with the number of springs being increased at one end of the joint to counteract the build-up of friction forces.
Similarly, these control springs are subject to additional loading in an expansion joint experiencing extreme movements, and must be adapted to suit. For instance, Mageba has optimised the rubber mixture in its control springs, in order to improve the overall performance and durability by a factor of 2.5. This improvement was verified by independent testing.
Another problem with large scale joints is the increase in the level of noise generated by traffic crossing the joint, caused by the increased duration of contact between the vehicle's wheels and the joint. In many cases, for example on elevated highways in urban areas, it may be necessary to apply suitable surfacing to reduce the noise.
One solution involves fixing profiled steel plates - so-called 'sinus plates' due to their shape - to the top surface of the joint. These plates eliminate any straight edges perpendicular to the direction of travel, and ensure that vehicles travelling over the joint continuously grip the surface, greatly reducing the noise generated by traffic on the joint.
Noise measurements carried out by an independent body have shown that modular expansion joints with sinus plates can cut the amount of noise generated by traffic by up to 70% compared to other types of expansion joints.
Large expansion joints require some form of surface treatment to improve tyre grip, particularly in wet weather. One such proven anti-skid surface is Robo-Grip, a five-layer laminate coating that is applied cold in liquid resin form. It was originally developed for British Royal Navy aircraft carriers, and offers a friction coefficient of up to 0.9. It also guarantees that this coefficient will be at least 0.5 over its full service life, even under the most adverse traffic and weather conditions. It is also resistant to pollution and ultra-violet radiation.
In addition to using modern and high quality expansion joints it is also important to reinforce the adjoining asphalt surfaces, for example using a system such as Robo Dur. Vertical slots are cut at a 45o angle to the joint's edge profiles and filled with the Robo-Dur high strength epoxy mortar, forming support ribs which strengthen the road surface and protect it against deformation. These support ribs absorb the vertical forces of the traffic and the shear forces caused by braking vehicles, preventing deformation of the road surface. They improve driving comfort, and increase the service life of the joint, guaranteeing proper functioning of the joint for many years and reducing renovation and maintenance expenditure.
Depending on their location, large bridges may also be affected by earthquakes, which can destroy the bridge's expansion joints and may even cause severe damage to the structure itself. Mageba's 'Fusebox' system allows the connection between the expansion joint and the main structure to break in a controlled manner in the event of an earthquake. This permits the expansion joint to close during an earthquake without being destroyed, and to settle afterwards in such a way as to allow emergency vehicles to cross the joint.
Large expansion joints are proportionately more complex and have more moving parts than smaller joints, and are likely to be installed on major bridges where the condition of the joint and the bridge itself must be guaranteed at all times. Therefore a bridge owner may wish to incorporate an automated monitoring system into the expansion joint to provide real-time information on the condition of the joint or the bridge, or other data.
Remote monitoring can provide continuous records of almost any variable relating to a bridge's condition, such as the position or length of any part, or the force acting on that part. Modern automated systems can also be configured to analyse the data gathered, present it in tabular or graphic format, and make it available to an authorised user anywhere in the world via the internet. Automatic notification by email or SMS if a defined alarm level of any measured variable is reached, can also be provided.
The initial cost of expansion joints for large-scale bridges is usually a very small percentage of the total cost of the bridge. But data from bridges around the world shows that maintenance and repair costs during the lifetime of an expans
