Before the development of expansion joints, traffic simply rolled over open gaps whose edges were reinforced by steel profiles in order to extend their lifetime. As bridges grew in size and hence the gaps grew wider, steel profiles with interlocking finger-plates were fitted between them in order to enlarge the movement capacity.

But dirt and water became a severe problem for such sensitive structures; to overcome this, cover-plates were installed above the expansion joints. At first these plates were fixed to one edge and slid over the other, but as loads and movements increased, the rollershutter joint with multiple, hinged cover-plates was developed.

Today’s huge suspension and cable-stayed bridges require expansion joint systems capable of bridging gaps in the range of metres. To cope with this, a type of expansion joint which divided the total movement capacity into several smaller gaps by the use of lamellas was designed - the modern modular expansion joint.

Each joint type has advantages and disadvantages for the bridge designer, but certain types convince because they are able to fulfill most requirements (see table). The rollershutter, for example, cannot compete with the other joint types in the lower movement range, but its main strength lies in its high movement capacity. On the other hand, the finger joint is valued because traffic can cross it quietly and comfortably, although it has limited capacity to absorb movements and rotations in any of the three axes. The mat-type joint is a simple, low noise joint with strong limitations in terms of adaptability and bridge movement. Modular expansion joints are strong on many counts such as water-tightness, cleaning requirements, geometric adaptability, and most importantly, movement and rotation capacity. However, these joints need complex design and advanced workmanship.

In the future, requirements will become even more demanding as larger and more complex bridges are built. This evolution will increase the demands on expansion joints which will be required to provide extreme movement and rotation capacity, to be low cost, quiet, maintenance free, to fulfill safety requirements and to have seismic features. Modular expansion joints are best placed to meet these demands.

The latest development is a modular expansion joint with sinusoidal plates, designed by specialist supplier Mageba to reduce noise emissions from bridges in urban areas. Traffic generates more noise on bridges than it does on roads, as sound is produced not only on the top of the deck, but also below it. The noise created by traffic crossing expansion joints is particularly disturbing. This new expansion joint combines the advantages of a normal modular expansion joint with those of a finger joint and it is achieved by welding or bolting sinusoidal plates on top of the lamella beams.

The results are convincing; the over-rolling tyre no longer hits the transverse edge, but instead it travels along a curved, diagonal line. As a result, the vehicle tyre does not create as much noise - test results show that the noise can be reduced by 80%.

In the future, there will be many reasons why the modern modular expansion joint will continue to be the bridge builder’s favourite choice, whether for highly populated areas with low noise requirements or for extremely long span bridges which demand extreme movement capacities.

EVALUATION OF MAIN EXPANSION JOINT TYPES:

Finger joints

Dating from 1911, the finger joint represents one of the oldest ways of bridging a gap. Its main elements are cantilevering or sliding steel plates, each connected to one side of the gap. Designed with a finger-like shape, these steel plates can easily move into each other. The main movement of this joint type is along the axis of the fingers themselves.

Advantages:

* Low noise emission thanks to finger elements in traffic direction

* Smooth surface offering good over-rolling comfort for fast and heavy traffic

* Suits refurbishment work well

Disadvantages:

* Openings in the joints can be dangerous for cyclists and motorcyclists

* Limited geometric adaptability to follow kerb units and central reserve construction

* Finger may project into the carriageway if there is settlement or tilting of the bridge

* High maintenance costs due to dirt accumulation below the finger

Rollershutter

This mechanical joint was first introduced in the mid-1930s: the heart of the rollershutter is the sliding plinth. On its circular surface hinged tongue-plates move in and out, to compensate for the closing and opening of the bridge gap. A pendulum plate connects the moving tongue-plate to the other side of the gap.

Advantages:

* Large allowable movement range

* Good over-rolling comfort, even for fast and heavy traffic

* Long lifetime if maintenance is carried out properly

Disadvantages:

* Watertightness not possible at road surface

* Limited adaptability to the bridge structure

* Risk of corrosion of the steel elements

* High initial investment and high maintenance cost

Mat-type joints

The gap is bridged by reinforced elastomeric elements which expand and contract with the movement of the bridge. These elements are connected to a supporting steel structure which itself is anchored into the concrete structure of the bridge. The elastomeric elements consist of prefabricated mats.

Advantages:

* Easy construction with few movable parts

* Watertight system protecting the structure from dirt and water

* Low noise emission

Disadvantages:

* Large horizontal forces are transferred into the bridge structure due to the high resistance of the elastomeric elements

* Difficult transition of the elastomeric element to the asphalt surface or concrete

* The joint surface can be damage by heavy duty vehicles such as snow ploughs

Modular joints

Developed around 1970, the modular expansion joint divides the total movement capacity across several smaller gaps. Lamella beams which are oriented transversely to the direction of traffic form the load-carrying elements and are supported by joist beams. The two are connected by elastomeric elements that transfer the traffic loads into the bridge structure. Elastic or mechanical gap control systems ensure that all the gaps are of equal widths and the joint is made watertight through elastomeric strip seals being connected between the lamellas.

Advantages:

* Watertight system, protecting the structure from dirt and polluted water

* Movement and rotations in all three axes are possible through elastomeric and spherical elements

* High adaptability to the bridge structure

* Seismic protection of the joint is possible by using a ‘fuse box’ which allows serious damage to the bridge to be avoided

Disadvantages:

* Numerous movable parts

* Mechanically operated gap control system can lead to poor functioning of the joint, especially when individual gaps are blocked by stones

* Noise generation through transversely located lamellas, although new developments address this problem

* Attractive overall cost; reasonable initial outlay with low maintenance costs

Gianni Moor is deputy general manager of Mageba

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