This invention is concerned with the stabilisation of bridges comprising a deck supported

by tensile supports and provides both a stabilised bridge structure and a method of stabilising an existing bridge

BACKGROUND ART

Various types of bridge have a deck supported by tensile supports from towers, or similar structures, erected at, or intermediate, the ends of the bridge In the case of a suspension bridge the tensile supports are typically vertical cables, rods or chains interconnecting each

longitudinal side of the deck to a corresponding catenary suspended between the towers

A cable-stayed bridge also comprises a deck supported by tensile supports, usually in the form of rods or cables, extending from the longitudinal sides of the deck directly to the towers

It is well known from the Tacoma bridge disaster in 1940 that a suspension bridge can

suffer dramatic structural failure due to fluttering instability in a sustained modest wind

loading which caused a resonant oscillation of the deck which built up progressively until

destruction occurred The problems associated with wind loading of suspension bridges,

and indeed all bridges comprising a deck supported by tensile supports, become much

more severe as the span of the deck increases With a very long span, for instance that

proposed for the Straights of the Messina, the wind loading along the span can vary

substantially and can promote substantial asymmetric pitching and heaving of the deck

Since the Tacoma bridge disaster, various proposals have been made to address this

problem. For instance, in European Patent 0233528, it has been proposed that a

suspension bridge, comprising a suspension structure formed of cantenary wires and

vertical stays and a substantially rigid planar deck structure hung onto the suspension

structure, could be stabilised by aerodynamic elements which are shaped like aerofoils and

are rigidly fixed to the bridge structure to control the action of the wind on the structure,

the aerodynamic elements consisting of wing control surfaces which have a symmetrical profile and an aerodynamic positive or negative lifting reaction together with a flutter

speed considerably higher than the flutter speed proper to the bridge structure, the wing

surfaces being fixed just under the lateral edges of the deck structure of the bridge, with

their plane of symmetry inclined in respect of the horizontal plane, the bridge structure and the wing control surfaces interacting dynamically in order to shift the flutter speed of the

whole at least above the top speed of the wind expected in the bridge area.

Instead of using aerofoils rigidly fixed to the bridge structure, International Patent Application PCT/GB93/01862 (Publication Number WO 94/05862) teaches that a bridge

deck can be made less stiff than the decks of existing bridges by using flaps, or ailerons,

provided at the lateral edges of the bridge deck, the flaps or ailerons being pivoted from

the bridge deck for articulation between extended and retracted positions, and being

computer controlled to regulate the forces on the deck in response to wind loading.

Intemational Patent Application PCT/DK-93/00058 (Publication Number WO 93/16232)

teaches a system for counteracting wind induced oscillations in the bridge girder on long

cable supported bridges, wherein a plurality of control faces are arranged substantially symmetrically about the longitudinal axis of the bridge and are adapted to utilise the

energy of the wind in response to the movement of the bridge girder for reducing said

movement, the control faces being divided into sections in the longitudinal direction of the

bridge, and a plurality of detectors are provided for measuring the movements of the

bridge girder, and a local control unit is associated with each control face section and is

adapted to control the control face section in question in response to information from one

or more of the detectors These detectors are arranged to measure the movements or

accelerations of the bπdge at the point concerned and to transmit a signal to a control unit,

such as a computer, which uses an algorithm to apply a signal to a servo pump controlling a hydraulic cylinder to rotate the associated control face section In this manner, each

control face section can be adjusted continuously in response to the movements of the

bπdge girder at the point in question as measured by the detectors which are in the form of accelerometers This invention essentially requires the provision of a complex electronic system incorporating a significant number of accelerometers connected by extensive wiring along the bπdge girder to the computers, and an associated hydraulic

system for driving the control faces

Therefore, from these pπor art documents it is known for a bπdge to comprise a deck

supported by tensile supports, and aerofoil stabilisers pivoted about respective axes

generally longitudinal of the deck for articulation to a position to improve stability of the deck

It is also known from these documents to provide a method of stabilising a bridge having

a deck supported by tensile supports including mounting aerofoil stabilisers about respective axes generally longitudinal of the deck

DISCLOSURE OF INVENTION

It is an object of the present invention to enable a bndge to be stabilised without the use

of an extensive electronic sensing and control system

According to one aspect of the invention each stabiliser is connected to be dπven by a mechanism operable by angular movement between the deck and an adjacent tensile

support about a longitudinal axis of the bridge, and each mechanism being arranged such

that, when there is angular movement between a portion of the deck and the adjacent tensile support, the associated stabiliser will be articulated to a position which will generate a force on its deck portion, in the presence of a cross wind In this manner it is possible to stabilise a bπdge by minimising the coupling between rotational and vertical

movements of the deck, thereby damping any tendency of the structure to flutter

Preferably each mechanism includes a lever which is secured to the associated tensile

support and is pivoted to the deck about an axis generally parallel to the pivot axis of the

associated stabiliser Each mechanism may be arranged to amplify the articulation of its associated stabiliser with respect to the angular movement

At least some of the stabilisers may be pivoted about their respective axes directly to the

deck and be arranged to be articulated by respective links pivoted to their respective

levers

At least some of the stabilisers may be pivoted about their respective axes directly to the

deck and be positioned to modify the aerodynamic properties of the deck Alternatively

at least some of the stabilisers may be pivoted above their respective axes either from the

tensile supports or from their respective levers In this case each stabiliser is preferably

arranged to be articulated by a link pivoted to the deck

At least one of the stabilisers may be provided with an independently adjustable control

surface In this manner the control surface can be adjusted relative to the stabiliser

thereby alteπng the force that will be generated by the stabiliser and applied to the deck

Preferably the stabilisers are arranged in pairs which are mounted on opposite sides of the deck and are counter-balanced by an interconnecting link In this case the interconnecting

link is preferably arranged operatively between the mechanisms of the pair of stabilisers

According to another aspect of the invention a method of stabilising a bridge having a

deck supported by tensile supports and having aerofoil stabilisers mounted about

respective axes generally longitudinal of the deck which includes using angular movement between the deck and the tensile supports about a longitudinal axis of the bπdge to

articulate the stabilisers to positions which will generate a force, in the presence of a cross

wind, to reduce the overall aerodynamic lift on the deck

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, with reference to the

accompanying drawings, in which -

Figure 1 is a diagrammatic transverse section through the deck of a bridge stabilised in

accordance with the present invention,

Figure 2 is a view similar to Figure 1 but illustrating the movement of a pair of stabilisers

during angular movement in one direction between the deck and an adjacent tensile

support about a longitudinal axis of the bridge.

Figure 3 is a view similar to Figure 2 but illustrating the movement of the stabilisers during

angular movement in the opposite direction between the deck and an adjacent tensile

support,

Figure 4 is an enlargement of the left-hand portion of Figure 2 illustrating one form of mechanism operable by angular movement between the deck and the adjacent tensile

support,

Figure 5 is a view similar to Figure 4 but showing a modification to the aerofoil stabilisers,

Figure 6 is a view similar to Figure 1 but illustrating the counterbalancing of a pair of

stabilisers, and

Figure 7 is a view similar to Figure 1 but illustrating an alternative mounting for the

stabilisers on a different bridge deck.

DESCRIPTION

It is well known that long span suspension bridges have a tendency to suffer from flutter¬

like instability during conditions of very high winds. One approach to this problem has

been to increase the torsional stiffness of the bridge deck, thereby increasing the wind

speed at which instability occurs. This is achieved by conventional structural techniques

which inevitably increase the weight of the bridge deck and consequently also increase the weight of the suspension cables and their supporting structure. An alternative approach

has been to augment stability of the bridge deck by means of actively controlled aerofoils.

Such active stabilisation closely follows practice already adopted in aircraft control systems, where aerofoils, or other control services, are appropriately deflected by means of hydraulic, pneumatic or electrical actuators in response to the sensed motion of the vehicle, which in this case is the local part of the flexible bridge deck structure being

stabilised.

The present invention provides an alternative approach to active stabilisation by

controlling aerofoils mechanically by means of linkages connected to the bridge deck

suspension members. In this manner stabilisation can be achieved without the use of a

plurality of accelerometers and the associated wiring, computer control and service

systems which have been proposed for articulating aerofoils by means of hydraulic,

pneumatic or electrical actuators.

With reference to Figures 1, 2 and 3, a suspension bridge comprises a deck 10 supported

from a pair of unshown catenaries by two series of tensile supports 1 1 and 12 which are

conveniently formed as rods or cables. The bridge deck can be of any convenient

construction known in the art and typically comprises a box girder 13 defining

camageways 14, 15 separated by raised curbs 16, 17 and 18 Irrespective of its specific

cross sectional profile, the deck 10 has aerodynamic properties when exposed to a cross

wind and its stability is controlled by two seπes of aerofoil stabilisers 19 and 20 positioned

along each longitudinal edge of the deck 10 Each stabiliser is connected to the deck 10

by a pivot 21 for articulation about an axis which is generally longitudinal of the deck,

thereby allowing articulation of the stabiliser 19, 20 to a position which will generate a

force, in the presence of cross wind, to reduce the overall aerodynamic lift on the associated portion of the deck 10

The lower ends of the tensile supports 11, 12 are very firmly attached to the ends of levers

22 which are also secured to the deck 10 by respective pivots 23, thereby permitting

angular movement between each tensile support 11 or 12 and the deck 10 about the axes of the pivots 23 which are generally parallel to the axis 21 of the associated stabiliser

As will best be seen from Figure 4, a link 24 is connected by a pivot 25 to the stabiliser

19 at a point spaced from the pivot 21, and also by a pivot 26 to the lever 22 at a point

spaced from the pivot 23, the pivots 21, 23, 25 and 26 being parallel In this manner, any

angular movement between the deck 10 and the tensile support 1 1 will cause relative

angular movement of the lever 22 about its pivot 23, thereby causing the link 24 to

transmit this motion to the stabiliser 19 which will rotate in the same direction about it

pivot 21 It will be noted that the effective lever arm between the pivots 23 and 26 is greater than that between the pivots 21 and 25 whereby the relative angular movement of the lever 22 causes an amplified movement of the stabiliser 19 It will also be noted that

the lever 22 and the link 24, together with their associated pivots 21, 23, 25 and 26 form

a mechanism operable by angular movement between the deck 10 and the adjacent tensile

support 11.

In this manner any torsional movement of the bridge deck 10 relative to any of the tensile supports 1 1 or 12 will cause articulation of the adjacent stabiliser 19 or 20, thereby

modifying the aerodynamic properties of the deck 10 Thus, in Figure 2, counter¬ clockwise rotation of a portion of the deck 10 simultaneously causes the left hand

stabiliser 19 to be lifted whilst the right hand stabiliser 20 is lowered. In this manner the stabilisers 19 and 20 will exert a restoring couple to the deck 10 irrespective of whether the cross wind is from the left or from the right.

In Figure 3 the deck 10 has been rotated clockwise and it will be noted that the movement of the stabilisers 19 and 20 are similarly reversed so that they will again exert a restoring couple on the deck 10.

It should be particularly noted that the deflection of the stabilisers 19 and 20 will always

augment the stability of the deck 10, regardless of whether the wind is blowing from the

left or the right.

The ratio of the distances between the pivots 23 and 26 and the pivots 21 and 25 will

depend on the dynamics of the deck 10 and its suspension 1 1, 12 and can be determined

by wind tunnel tests and/or theorical calculations. The ratio will, for some bridge

constructions, depend upon the span- wise position of the particular stabiliser 19 or 20

In Figure 5, most of the components are equivalent to those in Figure 4 and have been

identified with the same reference numerals as they have the same function. The only

modification is that the outer end of the stabiliser 19 is provided with an independently

adjustable control surface 126 which is connected to the stabiliser 19 by a pivot 27 which

is parallel to the axis of pivot 21. The control surface 126 can be articulated, about its

pivot 27, relative to the stabiliser 19, by a power actuator 28 which is housed within the

stabiliser 19 as shown and drives the control surface 126 through a linkage 29. The power actuator can be operated mechanically in order to set the control surface 126 in a position to give the stabiliser 19 a desired characteristic for the portion of the deck to which it is

attached, or can be operated electrically, pneumatically or hydraulically whereby the

characteristics of the stabiliser 19 may be continuously adjusted.

The benefit of a mechanically linked stabiliser arrangement, such as that described with

reference to Figures 1 to 4, is the absence of any large power actuators which would

obviously need a continuous available source of energy, even in the midst of hurricane force winds, and the absence of computers and accelerometers. However, an active

control approach, in common with comparable aircraft systems, is extremely flexible as

changes to the control system can be accommodated with relative ease, and functional

complexity can be provided as necessary.

The attraction of the combined implementation taught by Figure 5 is that the best features

of both appr :hes can be included. In this manner, the benefit of large mechanically-

driven stabilisers 19, 20 can be achieved and their function can be augmented by small

actively controlled surfaces 126 in a similar manner to a trim tab on an aircraft elevator.

In this manner the bulk of the stabilisation will be performed by the large mechanically

operated stabilisers 19 and 20, whilst the small actively controlled surfaces 126 would

finely tune performance whilst being undemanding in terms of size, cost, power

requirement and integrity, when compared with a stand-alone active control system.

Figure 6 shows a construction which is generally the same as that already described with reference to Figures 1 to 4, and accordingly the same reference numerals have been used

to denote the equivalent components. The difference is that the masses of the stabilisers 19 and 20 are balanced by interconnecting links 30 which have their outer ends connected

to extensions 31 of the stabiliser mounting by respective pivots 32 of which the axes are parallel with the pivots 21 and 23. The inner ends of the links 30 are joined by a common pivot 33 to a link 34 which is allowed to rotate about a pivot 35 carried by the bridge deck

10. In this manner, the masses of a transversely aligned pair of stabilisers 19 and 20 are

counter-balanced irrespective of their articulation.

In Figure 7 the bridge deck 10 is of somewhat different construction insofar as the levers

22 are mounted on pivots 23 positioned inboard of the outer longitudinal edges of the

deck 10, thereby defining walkways 36 and 37. The aerofoil stabilisers 19 and 20 have

also been moved so that they are now connected for articulation about pivots 38 which

extend longitudinally of the deck 10 and are carried by the respective levers 22. The

stabilisers 19 and 20 are articulated by respective links 39 which are pivoted as shown

between the deck 10 and the stabilisers 19 and 20. It will be noted that the links 39 cross

the levers 22 to ensure that the angular movement between the deck 10 and the adjacent

tensile supports 1 1 and 12 will cause the stabilisers 19 and 20 to be articulated in the

appropriate direction. With this arrangement it will be appreciated that, rather than

modifying the aerodynamic properties of the deck 10, the stabilisers 19 and 20 exert

compensating forces to the deck 10 via their respective levers 22. If desired, the

stabilisers 19 and 20 may alternatively be mounted directly on the tensile supports 1 1 and 12.

In the case where the tensile supports are formed by suspension rods, the rods themselves

would be connected to an appropriate trunnion which would receive the pivots 23, whereby the tensile support bar 11 or 12 would replace the upper arm of the lever 22, the trunion being designed to provide the mounting for the pivot 26.

The mechanisms taught by Figures 4 and 7 may be replaced by any other convenient mechanism or gearing which will drive the stabilisers 19 and 20 as required.

If desired, a bridge deck 10 can be fitted with the stabilisers 19 and 20 of both Figures 4 and 7.

In addition to providing a bridge structure having a novel form of stabilisation, it will be

noted that the arrangements taught herein can be used to modify existing bridges having

a deck supported by tensile supports and that this can be achieved without the need for

completely dismantling the bridge.