GB2344370A - A tubular multiple cable bridge for long spans - Google Patents

Abstract

The bridge comprises three sets of cables, all permanently in tension. The cables are tensioned between piers 1, which are hoop-shaped. The longitudinal cables 2 are embraced by two sets of cables 4 and 5 following helical paths around the first set. When the helical cables are drawn in at the piers, a taut waisted tubular membrane is formed by the three sets of cables, on which may rest the modular deck 6. The tube so formed may be strong and stiff enough to be supported by piers supported on pontoons floating on water.

Description

A TUBULAR MULTIPLE CABLE BRIDGE FOR LONG SPANS This invention enables cables to be configured into tubes capable of spanning very long distances, and particularly suitable for traversing long stretches of water with multiple spans supported by floating piers.

The currently favoured form of bridge for long spans is the suspension bridge. The longest span of any suspension bridge is 1,990 metres, that of the Akashi Kaikyo bridge in Japan. While longer spans are possible, stretches of water many kilometers wide will require several spans and the provision of piers in potentially deep water. The expense of building islands or deep water structures upon which to place the piers can be reduced if the piers are placed on floating pontoons. However, while a pontoon will provide the buoyancy necessary to support the weight from the bridge, it is unable to provide as rigid a fondation as is conventionally expected from a bridge pier. Without such rigid supports, the structure between them needs to be stiff and strong enough to resist the pitching, rolling and yawing of its piers. The suspension bridge is ill equipped by its nature to do this because its towers, cables and hangers are completely unbraced and as a result are seriously lacking in stiffness. Without a comparatively deep deck construction the suspension bridge's dynamic performance, even with rigid piers, is inadequate, as was demonstrated by the failure of the Tacoma Narrows suspension bridge which collapsed in 1940 from wind-induced oscillations. This invention promises to overcome these weaknesses and offer sufficient stability from the cable system itself without recourse to a deep deck.

This invention provides three sets of cables, a first set of cables running in the longitudinal direction of the bridge, embraced by two sets of cables each following helical paths wrapped tightly around the first set. This configuration constitutes a waisted tubular membrane, taut, stiff and strong in all directions, along the bottom of which rests the bridge's deck which supports traffic.

FIGURE 1 shows a preferred embodiment of the tubular multiple cable bridge. The bridge is likely to have several spans, each identical to each other. Each pier 1 is a hoop to which is anchored a multiplicity of longitudinal cables 2. Each longitudinal cable runs between two piers, anchored at the same position around the hoop at each pier, such that the straight line joining the ends of each cable is parallel to the bridge's axis. The spacing of these cables is least at the tops of the hoops, increasing towards the bottom, with a cluster of cables 3 under where the deck is positioned. The longitudinal cables define a tube around which two sets of smaller helical cables 4 and 5 are wrapped. Each set is the mirror image of the other, one following a left-handed helical pattern 4 and the other a right-handed one 5. Each set of helical cables contains a multiplicity of separate cables that run approximately parallel to each other as they wind around the longitudinal cables 2. Each helical cable is anchored at each end to a pier. The pitch of the helical cables varies smoothly and symmetrically about the mid-span section. The pitch is at its smallest at mid-span. All three sets of cables are taut in tension, and the tension of the helical cables causes the tube to be waisted, as shown in Figure 1, have its smallest diameter at mid-span. The deck 6 rests on the inside of the bottom of the tubular net formed by the three sets of cables. It is modular, each module being supported by a number of helical cables. At each end of the bridge the longitudinal cables are anchored to the ground.

The benefits of such a configuration over that of a suspension bridge are: * The friction between the helical cables 4 and 5 and the longitudinal cables 2 allow the cables to perform like a stiff membrane, possessing stiffness in all directions tangential to its surface.

* The tension in the cables provides the membrane with stiffness normal to its surface. Hence the tube wall is stiff in all directions.

'The bridge deck is not required to perform a primary structural role, so may be comparatively light.

There are no joints in the primary structure since all interactions of the primary structure rely on contact pressure and friction. This is beneficial, as joints are expensive to fabricate and maintain.

* The longitudinal cables are smaller in diameter then the main cables of a suspension bridge, and therefore easier to install.

There is significant structural redundancy in the primary structure. The structure can accommodate the loss of one cable, and possibly several cables, in each span.

Cables can be replaced and deck modules maintained with minimal disruption.

FIGURE 2 shows a typical sequence of construction, which is as follows: 1. Construct and tether the piers.

2. Construct and position the longitudinal ropes.

3. Install a temporary saddle-shaped moving gantry around the outside of a pier that will run along the longitudinal cables.

4. Moving from one pier to another, lay one set of helical cables.

5. Moving back again, lay the other set of helical cables.

6. Draw the helical cables in at each pier, to lift the lower longitudinal cables, at the same time adjusting the tension in these lower cables, thus forming the waisted tube.

7. From the gantry make adjustments to cable positions as necessary.

8. Place deck modules, not consecutively, but in an order to distribute the load evenly across the span. At the same time release some of the tension in the lowest cables to compensate for the added deck loading.

9. From the gantry, make adjustments to cable positions as necessary, and check the geometry and the forces in the primary structure.

10. Remove the gantry, or park it over a pier.

11. Untether the piers.

To keep the whole bridge balanced, each of these stages must be performed in unison on every span of the whole bridge.

On completion the group of longitudinal cables beneath the deck 3 may be unstressed.

The deck itself may be stressed in compression.

Typically, the deck follows a gentle vertical curve with a maximum slope of about 1 in 20 adjacent to the piers, not too steep for road traffic.

The successful structural performance of this form of construction depends on its detail design. Provided the helical cables are finally following curves on the tube which are close enough to geodetic curves, then any slip of the cables due to more extreme loading is going to be in the direction of the helical cables. Slip in this direction will only alter their alignment minimally, and therefore hardly alter the geometrical configuration of the cable net.

While friction alone may be enough to maintain the integrity of the bridge when transient loading is within limits, in harsh environments and with particularly heavy vehicles some additional restraint may be needed to prevent unreasonably large movements of the cables relative to each other. In which case, crossing cables may be lashed together where they contact each other. It may be appropriate to lash a proportion of the cable contacts, possibly those below deck level, which are the most accessible. Below deck the helical cables may also be lashed to the tubular structure forming the deck modules.

The same principles can be applied to a wholly convex pressurised tube. Such a tube might find application outside the earth's atmosphere in a weightless environment, when the role of the piers as supports might be secondary. A single"span"might provide the appropriate volume of containment, with the longitudinal cables bellying out at mid- span, not waisted. Construction might be made easier if the longitudinal members were slender rods rather than cables. The sequence of construction would be similar to that already described, with the two sets of helical cables being drawn in to bend the longitudinal members into a convex profile. The section of the tube would be circular, and the"pier"rings would be in tension, so they, too, could be cables.

Claims (6)

1. CLAIMS 1. A structure, kept permanently taut in tension, comprising three sets of slender members, such as cables, the first set oriented in the longitudinal direction disposed so as to create a tubular formation between supporting hoops at each end, and the other two sets of cables, one following left and the other right-handed helical paths all wrapped tightly around the first set, constituting a tubular membrane, taut, stiff and strong in all directions, along the bottom of which may rest a deck for supporting traffic.
2. 2. The process of forming a structure as claimed in Claim 1 by drawing in the two sets of helical members at the supports, sliding them over and tightening them against the longitudinal members.
3. 3. A multiple span bridge, being a series of structures as claimed in Claim 1, separated by piers, where the tubular membrane is waisted at mid-span, and anchored to the ground at the extreme ends of the bridge.
4. 4. A multiple span bridge as claimed in Claim 3 where the piers are supported by pontoons that may or may not be tethered.
5. 5. A structure as claimed in Claim 1, where the tubular membrane is subjected to internal pressure and bellies out at its centre.
6. 6. A structure substantially as herein described and illustrated in the accompanying drawings.