GB2546779A

GB2546779A - Suspension bridges SB1

Info

Publication number: GB2546779A
Application number: GB1601614.9A
Authority: GB
Inventors: Michael Corney John
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-01-28
Filing date: 2016-01-28
Publication date: 2017-08-02
Anticipated expiration: 2036-01-28
Also published as: WO2017129937A1; GB2546779B; GB201601614D0

Abstract

The suspension bridge comprising a first and second towers, with a region spanning between the two towers, a box girder deck, a suspension cable extending between the tops of said towers, at least the end-most portions retaining a coherent form and a multiplicity of hangers, at least some of which are transversely inclined with respect to the longitudinal axis of the deck and extend from the suspension cable to the bridge deck. The hangers may be in an inverted V-shape and support the deck along its respective sides. The suspension cable may extend over the complete span of the bridge and include vertical hangers supporting the deck along its centre line over the central region of the span. The towers at the end of the span may be in the form of an A and the hangers may take the shape of an inverted Y.

Description

SUSPENSION BRIDGES

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.

BACKGROUND

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.

While these parallel deck arrangements are aerodynamically stable, they do not help the issue of deck tilt due to asymmetric loads which may present problems with extreme span bridges due to the relative flexibility of the suspension cables.

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, and also offers the possibility of ‘contra-flow’ operation to cater for road works or traffic problems. As far as cost is concerned, this will helped by the lower manufacturing cost of the simpler deck structure, although the overall cost would be expected to be dominated by the reduced costs of the cable needed to support the lighter deck.

It is these benefits of the single deck that has led to the concept of structural triangulation of the deck and its suspension elements in order to augment the torsional stiffness of the deck so as to mitigate the effects both of flutter and deck tilt. This is the basis of the invention described below.

SUMMARY OF THIS INVENTION

According to the invention, a suspension bridge comprises (a) first and second towers, (b) a box girder deck, (c) a suspension cable means extending between the said towers from the tops thereof, and in which at least end-most portions of said cable means are of, and remain of, a coherent form, and (d) a multiplicity of hangers, some of which are transversely inclined with respect to the deck longitudinal axis, extending from the said suspension cable means to the said bridge deck.

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

Several embodiments of the invention as hereinbefore stated are set out in the accompanying drawings of which:

Figure 1 shows part of a conventional bridge deck supported by vertical hangers attached to two cables

Figure 2 shows part of a bridge deck supported by inclined hangers attached to a single cable

Figure 3 shows a cross-section of a conventional bridge deck with two cables and vertical hangers

Figure 4 shows a cross section of a deck with a single cable and inverted ‘V’ shaped hangers

Figure 5 shows a cross section of a conventional deck reacting to an applied torque

Figure 6 shows the cross section of a deck with a single cable and inverted ‘V’ shaped hangers reacting to an applied torque

Figure 7 shows a cross section of a deck with a single cable and vertical hangers supporting the deck along its centre-line

Figure 8 shows a cross section of a deck with a single cable and inverted Ύ’ shaped hangers

Figure 9 shows part of a deck with a single cable and an interspersed mixture of hanger types supporting the deck

Figure 10 shows one half of the span of a bridge with a single cable and inclined hangers supporting the deck along its sides

Figure 11 shows one half of the span of a bridge with a single cable attached to the middle of the deck at mid-span with a combination of inclined and vertical hangers

Figure 12 shows the central part of a bridge deck with the cable attached to the centreline of the deck and with vertical hangers supporting this portion of the deck

Figure 13 shows the central part of a bridge deck with the cable located above the centre-line of the deck and with vertical hangers supporting this portion of the deck

Figure 14 shows the side view of a deck supporting road traffic, and indicating the locations of the cable both attached to the centre-line of the deck and located above the centre-line of deck

Figure 15 shows an end view of a deck supporting road traffic, and indicating the locations of the cable attached to the deck and located above the deck

Figure 16 shows one half of the span of a bridge with a bifurcated cable attached to the sides of the deck at mid-span

Figure 17 shows the central part of a bridge deck with the bifurcated cables attached to the sides of the deck

Figure 18 shows the central part of a bridge deck with the bifurcated cables located above the deck

Figure 19 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 above the deck

Figure 20 shows an end 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 above the deck

Figure 21 shows the end view of a conventional tower with two cables and vertical hangers supporting a bridge deck along its sides

Figure 22 shows the end view of an ‘A’ shaped tower with a single cable supporting a bridge deck along its sides

EMBODIMENTS OF THE INVENTION

Suspension bridges in accordance with the invention may be regarded each as a radical departure from the conventional suspension arrangement with its two suspension cables linking the tops of two Ή’ shaped towers to the respective sides of the deck via vertical hangers. Instead, a single cable is suspended between the tops of a pair of ‘A’ shaped towers, with hangers supporting the bridge deck from the said single cable.

Various embodiments of this invention represent different means of dealing with the central region of the span which will depend on the specific application. Considerations that will affect this central area will depend on whether the deck is a single or dual carriageway, the desired clearance between the cables, hangers and the deck.

Figure 1 depicts a section of a conventional suspension bridge with its deck [1] supported by hangers [2] and [3] attached to the sides of the deck at their lower ends and attached to the two main cables [4] and [5] at their upper ends, while Figure 2 shows a section of a similar bridge deck [1] suspended in accordance with this invention, with an arrangement of inclined hangers [6] and [7] whose lower ends are attached to the sides of the deck and whose upper ends are attached to the single cable [8]. This suspension arrangement of is common to all the embodiments of the invention and is implemented from the towers and over at least the outer portions of the span.

Cross sectional views of these two versions are shown in Figure 3 and Figure 4 respectively. In Figure 3 it will be noted that the bridge deck [1] is supported by vertical hangers [2] and [3] whose upper ends are attached to the respective suspension cables [4] and [5]. This is the conventional arrangement for suspension bridges, while in Figure 4, the outer regions of the deck [1] are now supported by inwardly inclined inverted ‘V’ shaped hangers [6] and [7] which are respectively attached to the single cable [8].

The effects of a torsional load when applied to the bridge deck in both the conventional suspension arrangement and in the suspension arrangement according to this invention can be compared qualitatively. In the conventional deck (Figure 5) a clockwise (say) torque applied to the deck will be reacted by a reduction in the tension within the hanger [2] and the cable [4], along with an increase in the tension within the hanger [3] and the cable [5]. In other words the torsional deflection Φι will depend on the stiffness of the suspension elements (hangers plus cable) on the respective sides of the bridge deck.

In the deck shown in Figure 6, it is clear that the flexibility reflected by the single cable [8] no longer affects the torsional stiffness of the deck, including its suspension elements. The flexibility of the cable only influences the flexural stiffness. One would therefore expect the angular deflection Φ2 to be less than the angular deflection Φτ of the conventional deck.

Alternatively the said inverted ‘V’ hangers could be partially replaced by vertical centre-line hangers. Figure 7 shows the deck [1] supported along its centre-line [10] by vertical hangers [9] attached to the single cable [8]. This arrangement offers a reduced overall number of hangers, and also provides an open view from the deck to the external scenery. The issue of the torsional stiffness of the deck structure would be expected to limit such centre-line hangers to limited portions of the span.

An arrangement of inverted Ύ shaped hangers is also possible as an alternative to the said inverted ‘V’ shaped hangers. Figure 8 shows the deck [1] supported by inclined hangers [13] and [14] which are attached to the centre-line vertical hangers [11] at point [12] and supported by the single cable [8]. This hanger arrangement may offer benefit compared with the inclined ‘V’ arrangement in terms of structural triangulation of the deck along with the inclined sections of the hangers, although there may be issues relating to dynamic operation under turbulent conditions in a given application.

In an optimised arrangement it may be possible to install a mixture of inverted ‘V’ hangers and inverted Ύ’ hangers. Figure 9 shows such an arrangement of interspersed hangers by way of illustration. The deck [1] is supported by a mixture of inverted ‘V’ shaped hangers [6] and [7], inverted Ύ shaped hangers whose inclined portions [13] and [14] are attached to the vertical hangers [11] at point [12] and vertical hangers [9] attached to the centre-line of the deck at point [10] and the upper ends of all the said hangers are attached to the single cable [8].

Figure 10 shows one embodiment of the invention and is a perspective view of part of a suspension bridge 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 its length between the tops of the towers, only one of which [2] is shown in this half span view. This arrangement of deck and its suspension elements provides the required structural triangulation along the complete span. Flowever it will be noted that there is an irreducible minimum clearance Ή’ between the deck and its overhead cable around mid-span, and this clearance is determined by the impact on the headroom of the deck traffic resulting from the inclined hangers.

However, as a further embodiment of the invention, this clearance Ή’ can be reduced to zero by attaching the cables to the centre-line of the deck around mid-span. Figure 11 shows a perspective view of part of a suspension bridge between the midspan and the tower [2] 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’. Between the said point ‘B’ and the centre of the span at ‘A’, the deck is supported by vertical centre-line hangers [5].

Figure 12 shows this embodiment in more detail in the mid-span portion of the bridge. It shows the central region of the deck [1] with the single suspension cable [3] attached to the centre-line of the deck at its mid-span position [2] by means of vertical hangers [5]. Beyond this mid-span region, the single cable supports the deck by inclined hangers [4].

There may be an issue of traffic obstruction caused by the large diameter cable around the mid-span position where the cable is attached to the centre-line of the deck, between the two carriageways. As a variation of this embodiment, the suspension cable is raised with respect to the deck by a relatively small amount as shown in Figure 13. Again the deck [1] is supported over its mid-span region by means of vertical hangers [5] attached to the cable and supporting the deck along its centre-line. Beyond this centre-span region the deck is supported along its sides via inclined hangers [4].

Figure 14 shows the side view of the bridge deck [1] around its mid-span. It shows the position of the cable [4] when attached to the deck and it also shows the cable position [5] when raised above the deck so that its minimum elevation 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.

Figure 15 shows the end-view of the bridge deck [1] around its mid-span. It shows the position of the cable [4] when attached to the deck within its central reservation, and it also shows the cable position [5] when raised above the deck so that its minimum elevation is above the clearance line [2], allowing the high-sided vehicles [3] to pass along the bridge below the cable. The dotted line [6] shows the locus of the attached cable [4] as the location of the cross section moves away from the mid-span region of the deck, and indicates that there may be a clearance issue for the high-sided vehicle over a significant length of the mid-span part of the bridge when the cable is installed within the central reservation of the bridge deck.

Figure 16 shows a further embodiment of the invention and again is a perspective view of part of a suspension bridge 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 partway along the span at ‘P’. Between the said point ‘B’ and the centre of the span at ‘A’ the said clearance Ή’ (referred to in Figure 10) is here reduced to zero by bifurcating the cable at point ‘P’ and attaching these bifurcated elements to the sides of the deck at the mid-span position at ‘A’. In this mid-span region between ‘P’ and ‘A’ the deck is supported by inclined hangers [5] attached at their lower ends to the edges of the deck and at their upper ends to the respective bifurcated cable elements [6] and [7].

Figure 17 shows this embodiment in more detail in the mid-span portion of the bridge. It shows the bifurcated cable elements [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 inclined hangers [5] whose upper ends are attached to the respective bifurcated cable elements. Beyond this mid-span region, the deck is supported by inclined 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 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 cable elements are raised with respect to the deck by a relatively small amount as shown in Figure 18. Again, the deck [1] is supported over its mid-span region by means of inclined hangers [5] attached to the sides of the deck, and whose upper ends are attached to the said bifurcated cable elements [6] and [7].

Figure 19 shows the side view of the bridge deck [1] according to this embodiment 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 cable elements 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 cable elements installed in an intermediate position [6], the rationale for which is best described by reference to the end view shown in Figure 20.

Figure 20 shows the end-view of the bridge deck [1] around its mid-span according to this embodiment. 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 a variation of this 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 cable elements when subject to outward lateral forces by means of the cables [13] and [14]. The benefit of the outwardly displaced cable locations is indicated by the loci [16] and [18] which remain 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 21 is an end view of the planned Messina Straits Bridge, where the Ί-Γ 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 22 shows the ‘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 elements [10] and [11] supporting a single deck [8]. An ‘A’ shaped tower is likely to be better able to resist lateral loads on the towers resulting from cross-winds affecting the cables and the deck than would be the case with the conventional Ή’ shaped deck due to its triangulated nature of the 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.

Claims

CLAIMS 1 A suspension bridge which comprises: (a) first and second towers, (b) a box girder deck, (c) a suspension cable means extending between the said towers from the tops thereof, and in which at least end-most portions of said cable means are of, and retain, a coherent form, and (d) a multiplicity of hangers, at least some of which are transversely inclined with respect to the deck longitudinal axis, extending from the said suspension cable means to the said bridge deck.
2. A suspension bridge according to Claim 1, in which the said cable means is of coherent form throughout its length and extends over the complete span of the bridge and includes hangers arranged in an inverted ‘V’ configuration, supporting the deck along its respective sides.
3. A suspension bridge according to Claim 1, in which the said cable means is of coherent form throughout its length and extends over the complete span of the bridge and include vertical hangers supporting the deck along its centre-line over the central region of the span.
4. A suspension bridge according to Claim 3 in which the suspension cable is attached to the centre-line of the deck at or around its mid-span position.
5. A suspension bridge according to Claim 1 in which the central region of the cable means is laterally bifurcated and in which the hangers attached to the two respective bifurcated elements of the cable means support the deck along its respective sides.
6. A suspension bridge according to Claim 5 in which the bifurcated elements of the cable means are attached to the respective sides of the bridge deck at or around its mid-span position.
7. A suspension bridge according to the preceding claims in which the towers at the ends of the span are in the form of an ‘A’.
8. A suspension bridge according to Claims, 1, 2, 3, 4 and 5 in which the hangers include those of an inverted Ύ’ shape supporting the bridge deck along its sides.
9. A suspension bridge according to Claims, 1, 2, 5 and 6 in which the suspension system includes vertical hangers supporting the deck along its centre-line.
10. A suspension bridge substantially as hereinbefore with reference to Figures 2, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and 22 of the accompanying drawings.