WO2005050054A1

WO2005050054A1 - Stay cable damper

Info

Publication number: WO2005050054A1
Application number: PCT/CA2004/001942
Authority: WO
Inventors: Trevor C. Haskett
Original assignee: Motioneering Inc.
Priority date: 2003-11-19
Filing date: 2004-11-10
Publication date: 2005-06-02
Also published as: CA2450089A1

Abstract

An improvement in damping vibrations in the cables (16) of cable stayed bridges involves the provision of one or more viscous damping elements (26) within the space (20) between the cable (16) and a surrounding stay tube (18). The direction of action of the damping element(s) is generally parallel to the longitudinal axis of the cable. In one embodiment a toggle mechanism (36) is provided at each end of a viscous damping element (26) to connect the element to both the stay tube and the cable. Each toggle mechanism (36) is pivotally connected to the viscous damping element (26) as well as to the stay tube (18) and cable (16) so that transverse vibrations occurring in the cable are transmitted via the toggle mechanism to the viscous damping element so as to effectively dampen such vibrations. In a typical scenario the viscous damping element (26) will be a piston and cylinder arrangement with a viscous fluid retained within the cylinder, similar to an automotive shock absorber

Description

STAY CABLE DAMPER

The present invention relates to long span bridges in general and to cable- stayed bridges in particular. The invention is primarily concerned with damping vibrations in the stay cables themselves.

BACKGROUND OF THE INVENTION Long span bridges continue to be constructed around the world with larger and larger spans. Historically, long span bridges were generally of the suspension type, but cable-stayed bridges are becoming more common with advances in materials and aerodynamics. A cable-stayed bridge supports the deck or roadway from a plurality of cables that are connected at one end either to the top of a support tower or along the vertical extent of such a tower. The cables are connected at the other end thereof to the deck or roadway, along the edges, or perhaps along the centerline of the deck. Such bridges require less cable than typical suspension bridges; they can be constructed from identical pre-cast concrete sections; and they take less time to build than suspension bridges. They also have an aesthetic appeal that can influence the awarding of construction contracts. Notwithstanding that cable-stayed bridges have become popular, there is still a problem with the behavior of the long sloping cables used with such structures. In particular, under certain wind and rainfall conditions, the aerodynamics of the cables becomes unstable and large amplitude vibrations of the cables can occur.

To date, no known significant damage has occurred to a bridge, but this problem continues to be the focus of the design community and the owners of such bridges. Several devices for increasing the damping of the cables have been developed and patented. US Patent No. 6,334,608 shows a passive viscous device in which, at the end of a "stay tube", which is commonly used in the construction of a cable-stayed bridge, a device is attached between the cable and the stay tube to dissipate energy. Another similar approach is found in US Patent No. 5,857,712 wherein a viscoelastic material is positioned between the cable and the stay tube to dissipate energy. US Patents Nos. 6,435,323 and 6,292,967 show an alternative approach which introduces added damping along the length of the cable itself and not through the connection(s) nearer to the deck or the tower.

SUMMARY OF THE INVENTION The present invention attempts to overcome the technical limitation of the devices described above. The present invention utilizes a traditional viscous damper, such as a shock absorber (piston and cylinder arrangement). One might expect to align such a damper normal to the cable axis to provide the desired damping, but it becomes clear that there is very little space between the outer surface of the cable and the inner surface of a stay tube. Thus, with the present invention the viscous damper is aligned parallel to the cable within the annular region between the cable and stay tube. A toggle-like mechanism, or a series of such mechanisms, is utilized to translate the transverse motion of the cable into motion in the direction parallel to the cable longitudinal axis, which is also the axis of the viscous damper. This allows the viscous damper to be fully enclosed within the stay tube as opposed to another possible approach where it might be connected between the cable and the deck itself, as shown in US Patent No. 4,955,583. If necessary, additional amplification of the damper motion can be provided by adding amplification into the toggle mechanism. An "off-the -shelf" damper can be sourced to provide the necessary energy dissipation in the arrangement of the invention. An arrangement utilizing three viscous dampers within the stay tube space, at 120° intervals is preferred but the actual number required will depend on the expected vibrations to be dissipated. The arrangement of the present invention is simple, using conventional, proven, viscous damper technology, and provides ease of installation, inspection and maintenance. It is possible to scale the cable motion if necessary, either amplifying or reducing the associated damper stroke. Broadly speaking the present invention may be considered as providing an arrangement for damping vibrational forces between a first movable component of a structure having an axis and a second fixed component of the structure spaced from the first component. The invention provides a viscous damping element positioned within the space between the components with the direction of damping action of the element being generally parallel to the axis of the first component. A toggle mechanism is positioned at each end of the element, each toggle mechanism being pivotally connected to both the element and to each of the first and second components, whereby vibrations induced in the first component relative to the second component are transmitted to the viscous damping element by way of the toggle mechanisms such that the viscous damping element will dampen such vibrations. In a particular aspect thereof the invention relates to an arrangement for damping transverse vibrations in a cable of a cable-stayed bridge, the cable extending between an associated tower and a deck of the bridge, the cable having a stay tube surrounding the cable at at least one end thereof, defining an annular space between the outer surface of the cable and the inner surface of the stay tube. At least one viscous damping element is positioned within the annular space such that its direction of damping action is generally parallel to the longitudinal axis of the cable. A toggle mechanism is provided at each end of the viscous damping element to connect the element at each such end thereof to the cable and to the stay tube. Transverse vibrations occurring in the cable are transmitted to the viscous damping element by way of the toggle mechanism, causing the viscous damping mechanism to compress or lengthen and to thereby dampen the cable vibrations. In a preferred embodiment there are three such viscous damping elements located within the space between the cable and the stay tube, located approximately 120° apart. Each such viscous damping element is connected to the cable and the stay tube by its own set of toggle mechanisms. The invention will be described in greater detail with reference to the drawings provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates schematically a prior art representation of a cable-stayed bridge. Figure 2 illustrates a cable extending through a stay tube at one end of a cable. Figure 3 illustrates an end view of a cable and stay tube configuration with a damping arrangement according to this invention mounted thereto. Figure 4 illustrates a cross-sectional view of the damping arrangement of this invention on the line A-A of Figure 3. Figure 5 illustrates a cross-sectional view of an alternative damping arrangement for this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 illustrates in a schematic sense a cable-stayed bridge 10 of the prior art. In this case the bridge 10 includes a deck or roadway 12 extending from shore to shore across a river R. A pair of towers 14 extend vertically above the deck 12, either in the centre of the deck width or adjacent the edges of the deck. The towers are anchored to the ground and support the full live weight of the bridge and its load. A plurality of cables 16 extend from each tower to be anchored at the deck itself. Different arrangements for the cables, as for example the parallel attachment pattern as shown and a radial attachment pattern (not shown) can be employed. As indicated above wind forces can induce vibrations in the cables, which could lead to premature wear and perhaps even failure of a cable itself or of its attachment point or points. Figure 2 shows a perspective view of a cable 16 at one end thereof wherein it is seen that the cable passes through a so-called "stay tube" 18 which protects the cable adjacent its end anchor points. The stay tubes are fixed to the deck and/or to the tower, such that the cables will move transversely relative thereto at one end of the tube when vibrating under the influence of high winds or other forces. With particular reference to Figure 3 it will be seen that an annular space 20 is created between the outer surface 22 of the cable and the inner surface 24 of the stay tube 18. With the present invention at least one linear or non-linear acting viscous damping element 26 is positioned within the space 20 with its direction of action being generally parallel to the longitudinal axis 28 of the cable 16. The direction of action is shown by the double headed arrow 30. The viscous damping element can be an "off-the-shelf" product, similar to an automotive shock absorber. Typically it will have a piston and cylinder structure, with a rod 32 connected to a piston within a cylinder 34. The cylinder contains a viscous damping fluid which inhibits the reciprocal movement of the piston and rod structure. The fluid will have limited compressibility and the piston will be provided with aperture means which allows a small volume of fluid to pass therethrough during reciprocal movement of the piston and rod. At each end thereof the viscous damping element will be connected to the cable and stay tube by a toggle mechanism 36. In its simplest form as shown each toggle mechanism will involve a pair of links 38, 40 with the links 38 pivotally connected to the viscous damping element and to the inner surface 24 of the stay tube 18 and the links 40 pivotally connected to the viscous damping element and to the outer surface 22 of the cable 16. Although a single damping arrangement in accordance with this invention will operate to reduce or dampen vibrations within the space 20 it is preferred that a minimum of three such arrangements be located within the space 20 as is clearly seen in Figure 3, the arrangements being equiangularly spaced apart by about 120°. In operation it is clear that if the cable vibrates transversely relative to the stay tube the vibrations will be transmitted by the links 38, 40 to the viscous damping element or elements within the space 20. The links will act much like a scissor jack such that if the space above the cable in Figure 4 diminishes the space below the cable will expand and the effect will be for the upper links to push the ends of the viscous damping element connected thereto towards each other and for the lower links to pull the ends of the viscous damping element connected thereto further apart. The damping effect of the viscous damping fluid within the relevant damping elements will counteract the vibration forces, back through the links of the toggle mechanisms, so as to reduce or dampen the vibrations occurring in the cable. With the present invention it is possible to achieve significant damping effects within the protected space 20 between the cable and the stay tube. This reduces the deleterious effects of weather and wear on the elements involved. The invention utilizes relatively inexpensive components and the components are available for inspection, maintenance, and repair as required. An alternative arrangement for mounting a viscous damping element within the space 20 is shown schematically in Figure 5. In this case the viscous damping element 26 is pivotally connected to a lug 42 mounted to either the inner surface of the stay tube 18 or to the outer surface of the cable 16, as by a link 44. The rod 32 of the element 26 is pivotally connected to another link 46 which is pivotally connected to a second lug 48 mounted to the inner surface of the stay tube. The opposite end of the link 46 is slidably received in a slot 50 of a flange 52 mounted to the cable 16. The operation of this arrangement is similar to the operation of the first embodiment in that transverse vibrations of the cable 16 relative to the stay tube 18 are transmitted to the viscous damping element 26 which in turn will counteract those vibrations. The principles of the present invention, although developed for a cable-stayed bridge, are not restricted to such an application. For example, the invention could be applied as a tuned mass damper for a chimney or other similar structure, wherever an annular ring of steel or other material is suspended near the top of the chimney or stack. A plurality of viscous damping arrangements in accordance with the invention could be utilized to reduce or eliminate vibrations created by wind or other forces. In reality, for any situation in which, due to space constraints, one would be precluded from using a conventional viscous damper, the present invention will allow the motions to be transformed into another plane where space is often available for the use of such a viscous damper, including but not restricted to car seats and washing machines.

Claims

1. An arrangement for damping vibrational forces between a first movable component of a structure having an axis and a second fixed component of the structure spaced from the first component, comprising: a viscous damping element positioned within the space between said components with the direction of damping action of said element being generally parallel to said axis of said first component; and a toggle mechanism at each end of said element, each toggle mechanism being pivotally connected to both the element and to each of said first and second components, whereby vibrations induced in said first component relative to said second component are transmitted to said viscous damping element by way of said toggle mechanisms such that the viscous damping element will dampen such vibrations.

2. An arrangement for damping vibrational forces between an elongated cable of a cable-stayed bridge, said cable having an axis, and a stay tube fixed to said bridge and surrounding said cable at at least one end thereof so as to define an annular space between the cable and the stay tube, comprising : a viscous damping element positioned within the space between the cable and the stay tube, with the direction of damping action of said element being generally parallel to said axis of said cable; and a toggle mechanism at each end of said component, each toggle mechanism being pivotally connected to both the element and to each of said cable and said stay tube, whereby vibrations induced in said cable relative to said stay tube are transmitted to said viscous damping element by way of said toggle mechanisms such that the viscous damping element will dampen such vibrations.

3. The arrangement of claim 2 comprising at least three of said damping elements equiangularly spaced about said cable within said annular space, each said damping element being connected to said cable and said stay tube by respective said toggle mechanisms.

4. The arrangement of claim 2 or claim 3 wherein each said toggle mechanism comprises a first link pivotally connected to said damping element and to said stay tube and a second link pivotally connected to said damping element and to said cable.

5. An arrangement for damping vibrational forces between an elongated cable of a cable-stayed bridge, said cable having an axis, and a stay tube fixed to said bridge and surrounding said cable at at least one end thereof so as to define an annular space between the cable and the stay tube, comprising : a viscous damping element positioned within the space between the cable and the stay tube, with the direction of damping action of said element being generally parallel to said axis of said cable; first linkage means pivotally connecting one end of said element to one of said stay tube and said cable; and second linkage means pivotally connecting the opposite end of said element to both said stay tube and said cable, whereby vibrations induced in said cable relative to said stay tube are transmitted to said viscous damping element by way of said second linkage means such that the viscous damping element will dampen such vibrations.

6. The arrangement of claim 5 comprising at least three of said damping elements equiangularly spaced about said cable within said annular space, each said damping element being connected to said cable and said stay tube by respectivefirst and second linkage means.

7. The arrangement of claim 5 or claim 6 wherein each said second linkage means comprises a link pivotally connected at one end to said stay tube, the other end of said link being slidably received in an elongated slot of a flange fixed to said cable, and means pivotally connecting said link intermediate the ends thereof to said damping element.