The invention relates to a bridge deck system. It is known to provide bridge deck systems in which a bridge deck is supported by cables, such systems being used in cable stayed and suspension bridges. As well as being subject to traffic loading, such bridges are also subject to wind loading. The danger of wind loading on the bridge leading to fluttering instability and structural failure was dramatically illustrated by the collapse of the Tacoma Narrows suspension bridge in the State of Washington, USA on 7 November 1940. Thus in order to keep the movements of the bridge deck due to wind to acceptable levels, it is the normal practice to design the deck with a certain degree of stiffness, for example by the use of stiffening girders.

Viewed from one aspect the invention provides a bridge deck system comprising a bridge deck supported by cables, the bridge deck having a cross-section substantially in the form of an inverted aerofoil section.

Such a bridge deck will tend to produce a downward force on the deck in response to wind loading, keeping the cables taut and preventing excessive movement of the deck. Thus the deck can be less stiff than the decks of existing bridges, producing savings in the quantities of material in the deck (in particular, the stiffening girders thereof) and in the cables. The system is suitable for use in cable stayed and suspension bridges, for example.

Preferably flaps are provided at the lateral edges of the bridge deck. These can be used to control the forces on the deck in response to wind loading. The positioning of the flaps may be computer controlled, by techniques which are well established in the aircraft industry. In a preferred arrangement, there is provided

means for detecting movements of the bridge deck, means for inputting signals corresponding to the detected movements to a computer, and means for controlling the positioning of the flaps in response to signals received from the computer. The computer will be programmed to control the flaps to keep bridge deck movements to a minimum.

Although one flap may be provided on each lateral edge of the bridge deck, preferably a plurality of flaps are provided along each edge. This will enable more precise control of the wind induced forces on the deck. Preferably the flaps are retractable so that when wind flows across the bridge deck a flap at the leading edge thereof can be retracted whilst a flap at the trailing edge thereof can be in use. This will enable the bridge deck to perform aerodynamically whatever the wind direction. The flaps will normally be pivotally mounted in order to adjust their positioning, and preferably they are retractable by pivoting. For example, a flap may pivot through approximately 180° between the position of use and the retracted position. It may advantageously be positioned between adjacent transverse girders of the bridge deck when retracted. Much smaller pivoting movements will normally be required when the flap is in use.

Ailerons may be provided at the lateral edges of the bridge deck. These are movable control surfaces and can be used to improve the precision with which the force on the deck in response to wind loading is controlled. The ailerons may be pivotally mounted. They are preferably capable of pivoting upwardly or downwardly from a neutral position. In a preferred embodiment, a plurality of ailerons are provided along each edge of the bridge deck. Under wind loading,- the ailerons at the trailing edge may be moved up or down to counter undesired movements of the bridge deck. The operation of the ailerons is preferably computer

controlled. Thus a preferred embodiment comprises means for detecting movements of the bridge deck, means for inputting signals corresponding to the detected movements to a computer, and means for controlling the positioning of the ailerons in response to signals received from the computer.

In a simple form of the invention, ailerons are provided, but not flaps. The downward pull produced by the cross-sectional shape of the bridge deck under wind loading can then be modified as necessary by movements of the ailerons. Alternatively, the ailerons may be mounted on the flaps, for example pivotally. In general, the ailerons will be smaller than the flaps and capable of faster movements; they will therefore be particularly suitable for responding quickly to gusts of wind.

A further problem in bridge design arises from the fact that the loads applied by traffic or wind are often not evenly distributed, so that the bridge deck and the cables must be designed accordingly. According to a preferred feature of the present invention, therefore, the bridge deck system comprises means for adjusting the strain of the cables. With such an arrangement, a better distribution of load can be maintained under operating conditions and further savings can be made in the quantities of material in the stiffening girders and the cables. For example, if the stress in a given cable exceeds a certain value, the strain adjuster for that cable may act to reduce the strain and thereby reduce the stress. Alternatively or additionally, the strain in other cables may be increased to relieve the stress in the given cable. Preferably, therefore, strain detecting means is provided for the cables. Again, the strain adjusting means is advantageously computer controlled, so that optimum load distribution can be achieved.

It will be appreciated from the above that the provision of cable strain adjusters is advantageous independently of the bridge deck design. Thus viewed from another aspect the invention provides a bridge deck system comprising a bridge deck supported by cables, and means for adjusting the strain of the cables.

The strain adjusting means may for example be a device provided at one end of a cable or at an intermediate point along its length. Strain adjustment is preferably effected by adjusting the length of the device.

A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a bridge having a bridge deck system according to the invention; and

Figure 2 is a transverse cross-section through the bridge deck system.

Figure 1 shows a cable stayed bridge having a bridge deck system 1 consisting of a bridge deck 2 supported by cables 3. The cables 3 are secured to a bridge pier 4 and are each provided at an intermediate point along their length with a cable strain adjuster 5.

The bridge deck 2 has a cross-section substantially in the form of an inverted aerofoil section and has along both lateral edges a row of flaps 6. The flaps are arranged to pivot about an axis 7 so as to be retractable as shown on the right hand side of Figure 2. Each flap is provided on its upper surface (when in the non-retracted position) with an aileron 8 (i.e. a movable control surface) . The bridge deck 2 includes internal structural members 9 surrounded by an external skin 10 which provides the deck with an aerodynamic shape. The structural framework design may be discrete or integral with the skin plates or it may be in plate girder form or combinations of these configurations. Adjacent to each lateral edge of the deck 2 there are

provided a pair of longitudinally extending girders 11 which extend upwardly beyond the external skin 10. A central traffic barrier 12 and two peripheral traffic barriers 12 are provided to define six traffic lanes, three in each direction. The width between the peripheral traffic barriers is about 24m, whilst the width of the deck 2 with the flaps on both sides retracted is about 36m.

Figure 2 shows the arrangement which might be adopted with the wind flowing in direction A. The flaps 6 at the leading edge of the bridge deck are retracted between transverse girders, whilst the flaps 6 at the trailing edge are in their advanced positions. Thus the overall cross-section is similar to that of an upside down aircraft wing to produce a downward force on the bridge deck which advantageously keeps the cables 3 in a taut condition. The ailerons 8 can be relatively quickly adjusted in order to deal with gusts of wind or sudden changes in wind direction. Thus any distortions in the bridge shape can be kept to a minimum with a reduced overall level of stiffness and reduced quantities of the materials required to provide such stiffness. The cable strain adjusters 5 respond to the loading on the bridge deck so as to tend to even out the load distribution. The strain adjusters 5 operate by varying the effective length of each cable between its upper end where it is secured to the bridge pier 4 and its lower end where it is secured to the bridge deck 2.