WO2017129937A1

WO2017129937A1 - Long span suspension bridges - cable geometry

Info

Publication number: WO2017129937A1
Application number: PCT/GB2017/000009
Authority: WO
Inventors: John Michael Corney
Original assignee: John Michael Corney
Priority date: 2016-01-28
Filing date: 2017-01-25
Publication date: 2017-08-03
Also published as: GB2546779B; GB2546779A; GB201601614D0

Abstract

A bridge comprises a deck (1) supported by a single cable (3) over the outer regions of the span which bifurcates into a pair of cables (6, 7) supporting the deck over the central region (A) of the span.

Description

LONG SPAN SUSPENSION BRIDGES - CABLE GEOMETRY

This invention relates to suspension bridges, and is particularly applicable to those with very long spans in excess of 2 kilometres, and with aerodynamically slim box girder decks.

INTRODUCTION

Two of the problems associated with very long span suspension bridges are 'flutter' (coupled torsional and flexural oscillations) at high wind speed, and deck tilt due to asymmetric traffic and other loads.

For a given bridge deck in zero wind conditions, the natural torsional frequency is typically much higher than the natural flexural frequency, and these two oscillatory modes are largely independent of each other. As the wind-speed across the deck increases, the aerodynamic forces and moments acting on the deck result in a progressive coupling between these two modes until their frequencies converge, beyond which destructive flutter occurs.

For a long span bridge it must be ensured that flutter will not occur within the

'design wind speed', for which the deck needs to have adequately high torsional stiffness, and this becomes progressively more difficult to achieve as the span increases. The point is reached when the required torsional stiffness cannot be achieved within a practical deck mass. It is then usual to provide some form of aerodynamic stabilisation, by replacing the conventional single deck with an assembly consisting of two or more laterally disposed individual deck sections.

For example, the Xihoumen Bridge (1650m span, 2009, China) and the Yi Sun Sin Bridge (1545m span, 2012, South Korea) each has two parallel decks where the airflow through the intervening void helps to stabilise the twin deck assembly at high wind speed.

Future extreme span suspension bridges also propose such parallel deck arrangement. These include the Messina Straits Bridge (3,300m span, Italy) with its triple decks, and the Sognefjord Bridge (3,700m span, Norway) with its dual decks.

Compared with such dual or triple decks, it is considered that the traditional single deck offers benefits in respect of manufacturing simplicity, lateral and longitudinal wind forces, structural stiffness, fatigue between the decks and their lateral support structure, along with the possibility of 'contra-flow' operation to cater for road works or traffic problems.

When one contemplates extremely long span bridges, of perhaps 4km in span or more, the dynamics of the deck and suspension system as a whole become dominated by the cables which are now likely to be significantly more massive than the deck.

The suspension system according to this invention includes a cable arrangement which provides improved torsional stiffness compared with the conventional suspension system with its two cables throughout the span. Here, the suspension system consists of a single cable at one end of the span which bifurcates into two cables over the central region of the span, and which are then joined together to form a single cable again at the other end of the span, in an overall symmetric arrangement. In a practical implementation, dual cables would be installed throughout the length of the span, but would be attached to each other over the outer regions of the span, giving what amounts to a single cable in these end regions. Compared with an otherwise identical conventional bridge, the flexural stiffness of the suspension system remains unchanged, while the torsional stiffness is now increased by a factor depending on the ratio of the span of the bridge to the length of the dual cables section between their points of bifurcation.

Similarly the natural torsional frequency of the cables will be higher than the natural flexural frequency by the same ratio, which is consistent with the acknowledged criterion for good flutter alleviation, that the torsional natural frequency of the deck and suspension as a whole should be significantly greater than its flexural natural frequency.

It is worth noting that for a given bridge, any improvement in the torsional stiffness of the suspension system will not only help with flutter alleviation, but will also reduce the amount of deck tilt due to asymmetric traffic and other loads.

SUMMARY OF THIS INVENTION

A suspension bridge, including a continuous cable means whose ends are attached to ground anchors and which passes over the tops of two towers located at the respective ends of the span of the bridge which supports the deck by means of hangers distributed at intervals along its span, and which is in the form of a single cable in the outer regions of the span and is bifurcated to form two cables in the central region of the span. The foregoing and other specific features of suspension bridges in accordance with this invention, being features hereinafter described with reference to the accompanying drawings, are the subject of claims of the claims schedule hereof.

DESCRIPTION OF THE DRAWINGS Embodiments of the invention as hereinbefore stated are set out in the accompanying drawings of which:

Figure 1 shows one half of the span of a bridge with a bifurcated cable attached to the sides of the deck at mid-span

Figure 2 shows the central part of a bridge deck with the bifurcated cables attached to the sides of the deck

Figure 3 shows the central part of a bridge deck with the bifurcated cables located above the deck

Figure 4 shows the side view of a deck supporting road traffic, and indicating the locations of the bifurcated cables both attached to the sides of the deck and located at two alternative positions above the deck

Figure 5 shows an end view of a deck supporting road traffic, and indicating the locations of the bifurcated cables attached to the sides of the deck and located at two alternative positions above the deck

Figure 6 shows one of the two outer regions of the bridge where the deck is supported by a single cable, and where there is an interspersed mixture of hanger types supporting the deck

Figure 7 shows the end view of a conventional Ή' shaped tower with two cables and vertical hangers supporting a bridge deck along its sides

Figure 8 shows the end view of an TV shaped tower with a single cable supporting a bridge deck along its sides

EMBODIMENTS OF THE INVENTION

A suspension bridge in accordance with the invention may be regarded as a radical departure from the conventional suspension bridge with its arrangement of two suspension cables linking the tops of two Ή' shaped towers to the respective sides of the deck via vertical hangers. As a point of clarification, a 'cable' may include an arrangement of two or more immediately adjacent and mutually attached sub-cables which together perform the functions of a single cable.

Figure 1 shows a perspective view of part of a suspension bridge, according to one embodiment of the invention, between the mid-span and the tower at one end of the span. The deck [1] is supported by a single cable [3] via inclined hangers [4] along part of the length of the cable from the top of the tower at 'B' and a point part-way along the span at 'P'. In this mid-span region between 'P' and 'A' the deck is supported by hangers [5] attached at their lower ends to the edges of the deck and at their upper ends to the respective bifurcated cables [6] and [7], while at the mid-span position 'A' the two bifurcated cables are attached to the respective sides of the deck.

Figure 2 shows this embodiment in more detail in the mid-span portion of the bridge. It shows the bifurcated cables [6] and [7] attached to the sides of the deck [1] at its mid-span position, and supporting the deck along its sides by means of hangers [5] whose upper ends are attached to the respective bifurcated cables. At the points of bifurcation, the deck is supported by inverted 'V profile hangers [4].

There may be an issue of traffic obstruction caused by the large diameter cables around the mid-span position where the cable is attached to the sides of the deck. This is due to their inwardly inclined locus between the attachment points at the sides of the deck and the point of bifurcation above the centre-line of the deck. As a variation of this embodiment, the bifurcated cables are raised with respect to the deck by a relatively small amount as shown in Figure 3. Again, the deck [1] is supported over its mid-span region by means of hangers [5] attached to the sides of the deck, and whose upper ends are attached to the said bifurcated cables [6] and [7]. Again, at the points of bifurcation, the deck is supported by inverted V profile hangers [4].

Figure 4 shows the side view of the bridge deck [1] according to these embodiments around its mid-span. It shows the position of the cables [7] when attached to the sides of the deck, and it also shows the cable position [5] when raised above the deck so that the minimum elevation of these bifurcated cables is above the clearance line [2]. This clearance line is determined by the height of the highest vehicles [3] likely to be encountered on the bridge. It also shows the bifurcated cables installed in an intermediate position [6], the rationale for which is best described by reference to the end view shown in Figure 5. Figure 5 shows the end-view of the bridge deck [1] around its mid-span according to these embodiments. It shows the position of the cables [7] and [10] when attached to the sides of the deck, while the dotted lines [15] and [17] show the loci of these cables as the location of the cross section moves away from the mid-span region of the deck. This indicates that there may be a clearance issue for the high-sided vehicle [3] and [4] over a significant length of the mid-span part of the bridge. Also shown is an embodiment where the cable positions [5] and [8] are raised above the deck so that their minimum elevation is above the clearance line [2], allowing the high- sided vehicles [3] and [4] to pass along the bridge below the cables. The intermediate cable positions [6] and [9] show the positions of the bifurcated cables when subject to outward lateral forces by means of cables [13] and [14]. The benefit of the outwardly displaced cable locations is indicated by the loci [16] and [18] which remain well clear of the tops of the high sided vehicles. The lateral cables [13] and [14] assume that there is the possibility of anchoring the bridge deck to give it a lateral constraint around its centre-span region; such a feature is not part of this invention.

Figure 6 illustrates the various possible hanger arrangements in the outer regions of the deck which are supported by a single cable. The deck [1] is supported by a mixture of inverted 'V shaped hangers [3] and [4], inverted Ύ' shaped hangers [5] and [6], and vertical hangers [7], [8], [9], [10] and [11] attached to the centre-line of the deck. Figure 7 is an end view of the planned Messina Straits Bridge, where the Ή' shaped tower [1] supports the cables at the tops of the two limbs of the tower at [5] and [6], while the end view of the cables [3] and [4] support the planned triple deck [2] along its sides.

Figure 8 shows an 'A' shaped tower [7] supporting a single suspension cable at its top [12] and the outer part of the cable [9] connected to the bifurcated cables [10] and [11] supporting a single deck [8]. An 'A' shaped tower would be better able to resist lateral loads resulting from cross-winds affecting the cables and the deck, than would be the case with the conventional Ή' shaped deck. This is due to the triangulated main structural elements. It would also have a lower centre of gravity making the structure more stable, and a lower centre of pressure from wind forces meaning lower overturning moment due to such forces. There may also be some aesthetic attraction in the simple 'A' profile towers with their single cables at the ends of the span.

Claims

A suspension bridge, including a continuous cable means whose ends are attached to ground anchors and which passes over the tops of two towers located at the respective ends of the span of the bridge which supports the deck by means of hangers distributed at intervals along its span, and which is in the form of a single cable in the outer regions of the span and is bifurcated to form two cables in the central region of the span.

A suspension bridge according to Claim 1 , in which one or more of the hangers attached to the single cable at the outer regions of the span has an inverted V profile with the plane of this profile normal to the direction of the span, and which support the deck along its sides.

A suspension bridge according to Claim 1 , in which one or more of the hangers attached to the single cable at the outer regions of the span has an inverted Ύ' profile with the plane of this profile normal to the direction of the span, and which support the deck along its sides.

A suspension bridge according to Claim 1 , in which one or more of the hangers attached to the single cable in the outer regions of the span has a vertical T shaped profile and supports the deck at or along its centreline.

A suspension bridge according to Claim 1, in which the bifurcated cables are attached to the respective sides of the deck in the region of its mid-span position.

A suspension bridge according to Claim 1, in which the hangers attached to the two respective bifurcated cables in the central region of the span wholly support the deck along its respective sides.

A suspension bridge according to Claim 1 in which the towers at the ends of the span are in the form of an Ά', with the plane of this Ά profile being normal to the direction of the span.