WO1999032727A1

WO1999032727A1 - Improvements in and relating to suspension bridges

Info

Publication number: WO1999032727A1
Application number: PCT/GB1998/003699
Authority: WO
Inventors: Gordon Michael Rose
Original assignee: Steel Construction Institute
Current assignee: Steel Construction Institute
Priority date: 1997-12-18
Filing date: 1998-12-10
Publication date: 1999-07-01
Anticipated expiration: 2000-06-18
Also published as: AU1496999A; GB9726717D0

Abstract

A suspension bridge comprises one or more decks (1) suspended by a system of suspender cables (2) which in turn are supported from lengthwise-extending main (3) and secondary catenary cables (4) which pass over and are supported by saddles mounted on spaced apart cable supporting structures (6). Side support cables (12) are provided which extend lengthwise and one to each side of the deck(s) (1). Laterally extending linking cables (5, 15) extending between the side support cables and the or each deck. The lengthwise extending cables are laced together by a plurality of laterally extending cables (14).

Description

IMPROVEMENTS IN AND RELATING TO SUSPENSION BRIDGES

This invention relates to suspension bridges and methods of constructing the same. The term "suspension bridge" embraces cable- stayed and combinations of suspension and cable-stayed bridges.

Suspension bridges are, of course, well known and generally comprise two or more spaced tower structures each typically comprising two vertical side-by-side steel or concrete columns embedded in concrete foundations and connected together by diagonal bracings and horizontal cross girders positioned at the top of the structure and below a deck suspended from vertical catenary main suspension cables stretching between ground anchorages and over saddles carried by the several tower structures. Each side of the deck is connected to the respective vertical catenary main suspension cable by a multiplicity of vertical suspender cables. Conventionally, the outermost sides of the tower structures are vertical or near vertical with each catenary main suspension cable and the respective suspender cables lying in a generally vertical plane which also contains the outermost face of the respective tower structure.

The term "deck" embraces any form of deck and includes decks for roads railways, tramways, pipelines, power lines, pedestrian walkways and the like.

The span lengths and load bearing capacity of the deck supported between any given pair of tower structures is limited inter alia by criteria such as its vertical, lateral and torsional resistance to static and dynamic forces from wind, earthquakes and other forces of nature.

Previous proposals for increasing the length of suspension bridges have neglected consideration of the fundamental bending and torsion frequencies of the finished bridge. A prerequisite design condition to be satisfied is that a bridge must not be subject to the possibility of a failure similar to the documented "galloping" failure of the Tacoma Narrows bridge.

Previous proposals for increasing the span have addressed this requirement by increasing the cross-sectional dimensions of the deck. To do so, however, increases the cost and self weight of the deck. Thus, as the span increases, much of the cable capacity is used to carry the deck and not the payload, until an optimum span is reached where the cables will carry only the deck and no payload.

Furthermore, previous proposals have tended to concentrate on the stability of the finished bridge, but that, at least initially, is not the priority. Any improvement in bridge design must ensure that the whole structure at every moment during its construction will resist predetermined wind and seismic forces and vibrations. A satisfactory final design is of no value if the structure risks failure during construction.

The present invention sets out to provide a suspension bridge having increased load carrying capacity and vertical, lateral and torsional resistance with consequent potential increases in the length of the deck between neighbouring tower structures. Potentially, a span length between tower structures in excess of 4000 metres can be achieved.

This has the significant advantage that in most cases the supporting towers can be located on dry land. This eliminates maritime working for the towers and their foundations and enables all work to be carried out on dry land. A further advantage is that the required tower height will be the height above deck and not height above water surface. This will lead to less loaded tower structures which are cheaper and easier to construct and less susceptible to wind and seismic damage during construction. The invention therefore also provides a way of increasing the load carrying capacity, lateral and torsional resistance of existing suspension bridges and other bridge forms.

The invention also provides a way of increasing the load carrying capacity, lateral and torsional resistance of existing suspension bridges and other bridge forms.

The invention also sets out to provide a suspension bridge able to resist relatively high wind and earthquake forces and vibrations during construction.

In one aspect, the invention provides a suspension bridge which comprises one or more decks supported by a system of suspender cables depending from lengthwise-extending main catenary cables which pass over saddles mounted on spaced apart cable supporting structures, lengthwise extending secondary catenary cables which pass over and are supported by the cable supporting structures and are positioned one to each side of the main catenary cables, side support cables extending lengthwise and to each side of the bridge deck, and laterally extending linking cables which are secured to and interconnect the lengthwise-extending catenary and the side support cables, the spacing between the lengthwise-extending support cables being greater than the width of the deck(s) .

In another aspect, the invention provides a suspension bridge which comprises one or more decks supported by suspender cables which, in turn, are supported by lengthwise-extending main catenary cables which pass over saddles mounted on spaced apart cable supporting structures, secondary catenary cables positioned one to each side of the main suspender cables, side support cables extending lengthwise of the bridge along each side thereof and laterally extending linking cables connected to each of the lengthwise-extending catenary and side support cables, the spacing between the lengthwise-extending support cables and the secondary catenary cables being greater than the width of the deck(s).

All lengthwise extending cables are preferably laced together by a triangulation of cables which extend across the width of the deck and are joined to the lengthwise-extending cables at their points of connection to the linking cables. This triangulation contributes significantly to the stiffness of the overall structure.

Each cable supporting structure may comprise a first pair of upstanding columns which carry the saddles over which the catenary main cables pass, and a second pair of upstanding columns positioned outwardly from the first such pair over which the secondary catenary cables pass. Each column may be vertical or inclined to the vertical. The second pair of columns may be separate from, secured to or form part of the supporting structure which comprises the first pair of columns.

The height of the second pair of columns is typically less than that of the first such pair.

In another embodiment, each cable supporting structure comprises a pair of columns spaced outwardly from the sides of the deck and supporting an arch from which the various lengthwise extending cables are supported.

The invention also provides a method of constructing a suspension bridge as described in the foregoing paragraphs.

In a further aspect, the invention provides a foundation for a cable supporting structure of a bridge, the foundation comprising an open-topped flotation chamber whose base is designed to support the cable supporting structure and whose sides, in use, are at least partially submerged within a mud bed, anchorages extending downwardly from the flotation chamber to anchor points below the mud bed.

In a still further aspect, the invention provides a suspension bridge which comprises one or more decks suspended by a system of suspender cables supported from lengthwise-extending main and secondary catenary cables which pass over and are supported by saddles mounted on spaced apart cable supporting structures, side support cables extending lengthwise and one to each side of the deck(s), a plurality of laterally extending linking cables connected to each of the catenary and side support cables and a plurality of lacing cables spaced along the length of the bridge which are joined to the lengthwise extending cables at their points of connection to the linking cables.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which:-

Figure 1 is a side view of a section of a suspension bridge in accordance with the invention;

Figure 2 is a plan view from above of the suspension bridge section shown in Figure 1 ;

Figure 3 is a view looking along the length of the bridge illustrated in Figures 1 and 2 from a tower towards the centre of the main bridge span;

Figure 4 is a perspective view of a bridge in accordance with the invention;

Figures 5 and 6 schematically illustrate an alternative bridge in accordance with the invention, Figure 5 showing supporting cables along one side only of a bridge;

Figure 7 schematically illustrates an alternative cable supporting structure in accordance with the invention; and

Figure 8 schematically illustrates a foundation for a supporting structure of a suspension bridge in accordance with the invention.

The suspension bridge illustrated in Figures 1 to 3 of the drawings includes a deck 1 supported by generally vertical suspender cables 2 from main catenary cables 3 which stretch lengthwise of the bridge and are secured at their ends to ground anchorages (not shown). Secondary catenary cables 4 are provided which also extend lengthwise of the bridge and are secured at their ends to ground anchorages (not shown) . Inclined suspender cables 5 extend downwardly from the secondary catenary cables 4 and are secured to the deck 1 to provide additional support therefor. The cables 3, 4 pass over saddles carried by two or more tower structures 6. As mentioned, these cables define catenaries between the tower structures 6, the catenaries defined by secondary cables 4 being positioned outwardly of the catenaries defined by the main cables 3. Each tower structure 6 essentially comprises a pair of spaced steel, timber or concrete columns 7 connected by cross girders 8 to define an arch. Additional columns 9 spaced outwardly from the columns 7 are provided, these being connected to the columns 7 by horizontal and inclined girders 10, 1 1 respectively. The columns 9 are preferably vertical; they may however be inclined towards or away from one another.

In a conventional suspension bridge, each catenary main suspension cable, the suspender cables suspended therefrom and the side of the deck supported thereby lie only in or adjacent to a vertical plane which also contains the respective side faces of the tower structure. As will be seen from Figures 2 and 3 this is not the case with a suspension bridge in accordance with this invention. As shown in Figures 2 and 3, the distance between the secondary catenary cables 4 as they pass over the saddles of the tower structure 4 is substantially greater than the width of the deck 1 ; this is achieved by increasing the width of the tower structures. This has the effect of increasing the lateral stability and torsion resistance of the bridge.

To enhance the lateral stability of the bridge further, side support cables 1 2 are provided which extend lengthwise of the bridge one to each side of and spaced from the deck 1 . The side support cables 1 2 are supported from the secondary catenary cables 4 by lateral cables 14 and are connected to the deck by generally horizontal or inclined linking cables 1 5. The side support cables 1 2 are supported by the columns 9 whose foundations are capable of carrying horizontal thrusts. These foundations may be separate from or form part of the foundations of the main tower structures. The columns 9 may, as shown, be secured to the columns 7 of the tower structures 6 or may be separate therefrom.

Additional lengthwise extending catenary cables 1 6 are provided which essentially bisect the spacing between the secondary catenary cables 4 and the side support cables 1 2. The cables 1 6 are supported by the columns 9 and are connected to the deck 1 by inwardly inclined suspender cables 1 7. The columns 9 are capable of resisting vertical and horizontal loads imposed by the cables 1 6.

Thus the deck is suspended and/or supported on each side by four sets of main suspension cables, these being the main and secondary catenary cables 3, 4, the side cables 1 2 and the additional catenary cables 1 6. Additional main catenary and suspender cables may be provided, each being connected to the deck by suitably aligned suspender cables. To maintain the catenary cables in their correct positions all cable sets are tied together with lateral cable loops 14. These cable loops are secured to each of the lengthwise extending cables. To increase the lateral stability and torsional resistance of the bridge even further, all cable sets are laced together with lacing cables 1 8 to produce a web-like structure. The lacing cables 1 8 extend widthwise of the bridge and are secured to each of the lengthwise extending cables at their points of connection to the linking cables. The resulting triangulation between the catenary cables and the lateral cables makes a substantial contribution to the torsional stiffness of the overall structure.

One or more vertical or near vertical cables anchored in suitable foundations may be attached to the underside of the deck to complete the web-like structure.

The bridge section illustrated in Figure 4 is similar to that illustrated in Figures 1 to 3. Some integers have been omitted for the sake of clarity but otherwise like integers to those illustrated in Figures 1 to 2 have been used in Figure 4. In the Figure 4 embodiment, however, the tower structures comprise two central outwardly inclined columns 20, two outer outwardly inclined columns 21 and two sideways extending columns 1 9. All six columns are supported by a central foundation and may stand vertically or may be inclined towards one or other bridge end. Two decks 1 are shown in Figure 4.

The following procedure may typically be employed for the erection of the bridge illustrated in Figures 1 to 3.

After erection of the tower structures 6 and the catenary main suspension cables 3, a circular, or other profile, collar is supported from the midspan position of the cables 3. The side support cables 1 2 are then spun over the tower saddles and pass through saddles on the midspan collar. The generally horizontal linking cables 1 5 are then constructed between the cables 1 2 and supported by the vertical suspender cables 2.

The additional catenary cables 16 are then spun over the tower saddles and pass through saddles on the midspan collar, and the inclined suspender cables 1 7 attached to the intersection of the cables 2 and the cables 1 5 at deck level.

The secondary catenary cables 4 are then spun over the tower saddles and passed through the saddles on the midspan collar, and the suspender cables 5 attached to the intersection of the cables 2 and the cables 1 5 at deck level.

At appropriate stages, throughout, the cables are constructed to complete the "web" network.

The web system when in place is resistant to vertical and horizontal loads, both static and dynamic.

At this stage the whole web system, main cables, suspenders, etc. may be used to upgrade the static and dynamic strength of an existing bridge otherwise incapable of meeting modern requirements.

For new bridges the deck 1 is then constructed to hang from the intersections of the cables 1 5, 1 7, 2 and 5, the static and dynamic stability of the deck not being reliant upon the form of deck construction. The deck 1 may be erected in sections of any sequence either from below of from each end, thus eliminating the need for maritime working in cases where the bridge spans across water.

In an unillustrated embodiment, following construction of the cables 1 5, the suspender cables 2 are attached to the cables 3 and the cables 1 5 such that the line of the connections between the cables 1 5 and 2 in plan is not a straight line between towers. In this embodiment, the cables 2 are not vertical, but lie on a curved line between towers. The cables 1 5 are not of equal length on both sides of the deck, and the line of the deck when constructed consequently is a curve between towers.

Figures 5 and 6 illustrate an alternative cable supporting structure in accordance with the invention. Main catenary suspension cables 24 support suspender cables 25 to which a deck (not shown) is attached. Lengthwise extending secondary catenary cables 26 extend between and are secured to neighbouring tower structures 8, 9 and are connected to the deck sides by suspender cables 27. Lengthwise extending side support cables 28 extend between and are secured to neighbouring tower structures and are connected to each deck side by lateral cables 29. The several lengthwise extending cables are laced together by a triangulation of laterally extending cables 30 to define a web-like structure. The positions of saddles over which the catenary cables pass may lie, or be caused to lie, on the profile of an arch defined by each tower structure. Thus, the saddle support columns referred to above may be added to or eliminated by an arch of steel concrete, timber or like material, the feet of the arch continuing down to suitable foundations.

An alternative tower structure is illustrated in Figure 7. In this embodiment, the two-dimensional arch described above is expanded to a three-dimensional arch-type structure which has a significant dimension in the lengthwise direction of the bridge.

The illustrated arch 31 comprises a triangulated space-frame lattice of members in three dimensions, continuing down to four suitable foundations. The geometry of the space-frame may be adjusted to produce three, four or more foundation points. In all of the embodiments described, the deck 1 may or may not touch or be attached to the support structure, the deck being supported by the suspender cables and catenary cables which are in turn fixed to and supported by the support structure.

A foundation having increased seismic resistance for the tower structures is illustrated in Figure 8. The foundation comprises an open- topped flotation chamber 32 anchored through cables 33 to a bedrock 34 below a layer 35 of mud and gravel. The layer 35 may be covered by water 36. The base 37 of the chamber provides a support for the respective tower structure. Once assembled, the upper surface of the chamber may be sealed against the ingress of water. In use, the chamber 32 is maintained at a predetermined height above the bedrock 34 by the mud/gravel layer 35, lateral stability of the chamber being provided by the anchored cables 33.

It will be appreciated that the foregoing is merely exemplary of suspension bridges in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention. Thus, each tower structure may be of conventional form but of increased width to enable the catenary main suspension cables and the suspender cables to be inclined to the vertical. To increase the lateral stability, torsional resistance and load carrying capacity of existing suspension bridges the width of each tower structure can be increased such that the main suspension cables and suspender cables are positioned as discussed above.

Claims

1 . A suspension bridge which comprises one or more decks supported by a system of suspender cables depending from lengthwise- extending main catenary cables which pass over saddles mounted on spaced apart cable supporting structures, lengthwise extending secondary catenary cables which pass over and are supported by the cable supporting structures and are positioned one to each side of the main catenary cables, side support cables extending lengthwise and to each side of the bridge deck(s), and laterally extending linking cables which are secured to and interconnect the lengthwise- extending catenary cables and the side support cables, the spacing between the lengthwise-extending support cables being greater than the width of the deck(s) .

2. A suspension bridge which comprises one or more decks supported by suspender cables which, in turn, are supported by lengthwise- extending main catenary cables which pass over saddles mounted on spaced apart cable supporting structures, secondary catenary cables positioned one to each side of the main suspender cables, side support cables extending lengthwise of the bridge along each side thereof and laterally extending linking cables connected to each of the lengthwise-extending catenary and side support cables, spacing between the lengthwise-extending support cables and the secondary catenary cables being greater than the width of the deck(s).

3. A bridge as claimed in Claim 1 or Claim 2 wherein the lengthwise extending cables are laced together by a triangulation of cables which extend across the width of the deck or decks and are joined to the lengthwise extending cables at their points of connection to the linking cables.

4. A bridge as claimed in any one of Claims 1 to 3 wherein each cable supporting structure comprises a first pair of upstanding columns which carry the saddles over which the main catenary cables pass, and a second pair of upstanding columns positioned outwardly from the first such pair.

5. A bridge as claimed in Claim 4 wherein each column is inclined to the vertical.

6. A bridge as claimed in Claim 4 or Claim 5 wherein the second pair of columns forms part of the supporting structure which comprises the first pair of columns.

7. A bridge as claimed in any one of Claims 4 to 6 wherein the height of the second pair of columns is less than that of the first such pair.

8. A bridge as claimed in any one of Claims 4 to 7 wherein the catenary main suspension cables pass between saddles on the first pair of columns.

9. A bridge as claimed in any one of Claims 1 to 3 wherein each cable supporting structure comprises a pair of columns spaced outwardly from the sides of the deck and supporting an arch from which the various lengthwise extending cables are supported.

10. A foundation for a cable supporting structure of a bridge, the foundation comprising an open-topped flotation chamber whose base is designed to support the cable supporting structure and whose sides, in use, are at least partially submerged within a mud bed, anchorages extending downwardly from the flotation chamber to anchor points below the mud bed. A suspension bridge which comprises one or more decks suspended by a system of suspender cables supported from lengthwise- extending main and secondary catenary cables which pass over and are supported by saddles mounted on spaced apart cable supporting structures, side support cables extending lengthwise and one to each side of the deck(s), laterally extending linking cables extending between the side support cables and the or each deck, and a plurality of laterally extending cables which lace together the lengthwise extending cables in a triangulation.