WO2001023670A1

WO2001023670A1 - Reinforcement system for a bridge

Info

Publication number: WO2001023670A1
Application number: PCT/GB2000/003622
Authority: WO
Inventors: Richard Charles Connor
Original assignee: Qinetiq Limited
Priority date: 1999-09-29
Filing date: 2000-09-21
Publication date: 2001-04-05
Also published as: AU7303000A; GB9922985D0

Abstract

The invention relates to the field of bridges and more particularly to those that are relatively quickly deployable, as required in military applications. Prior art bridge reinforcement systems utilise a series of rigid reinforcement links which are suspended from the underside of the bridge to be reinforced. Such systems comprise a large number of individual tension elements all of which must be pre-installed and tensioned. This type of arrangement which is complex results in deployment of the reinforcement system being equipment and man intensive. Furthermore, this type of reinforcement system projects by as much as two metres from the underside of the bridge which means that it cannot be used where there are obstructions at this level. The invention proposes the use of a number of aramid fibre tendons which are attached to the underside of the bridge and held under tension.

Description

REINFORCEMENT SYSTEM FOR A BRIDGE

This invention relates to the field of bridges and more particularly to those that are relatively quickly deployable, as required in military applications.

In military and, recently, many peace-keeping operations there is often a need to bridge terrain swiftly in order to assist rapid transport of vehicles and personnel. In a hostile environment when permanent vehicular bridges are either destroyed or were never built, it is often the case that a strong, temporary bridging structure is required.

Various examples of deployable bridges which are constructed from modules exist in the prior art. In the undeployed state the modules are kept separate or folded to facilitate stowage and ease of transport. Various techniques and apparatus exist to enable the modules to be linked together over a gap. The span of the bridge or, alternatively, its load-carrying capacity can be increased by some system of reinforcement that increases the section stiffness in order to decrease loads and deflections in the bridge girders. This is particularly important in military applications if the bridge is required to support heavy armoured vehicles.

Prior art military bridges use an underslung method of reinforcement (see for example the patent GB 1587204). In the UK, both the Medium Girder and Christchurch Bridges are reinforced using this principle. A series of inverted U-shaped modules are linked to form bridge trackways. Rigid reinforcement links form chains which are suspended underneath each trackway and cross the span from one end to the other. Either a single V-shaped King post or twin Queen posts are deployed between the reinforcement links and trackways to separate the two by a significant distance along the total trackway length. When tensioned, the trackway / post / chain system is rather like a strung bow and arrow. The track is forced upwards by the tension of the chain acting on the King (or Queen) post which itself is in compression. If a load is then driven onto the bridge, its weight causes the chain to tension further with the result that the reactive force upwards, through the post(s) is increased, and bridge reinforcement is thereby increased.

There are however numerous disadvantages to this form of reinforcement. Such bridges rely on a large number of individual tension elements connected together, held at some distance below the bridge lower chords (the larger the distance the greater the reinforcement effect) and tensioned mechanically or hydraulically before any live load is applied. All these tension elements must be pre-installed before the bridge is built across the gap and must be raised or lowered either by manual or mechanical means. Reconfiguration of the trackway modules from an unreinforced to a reinforced form can take many men several hours. Deploying the reinforcing links once the bridge is across the gap is both equipment and manpower intensive. Such reinforcement must also be significantly tensioned mechanically or hydraulically to be efficient, and this increases the complexity of the bridge structure. Moreover, the underslung reinforcement can extend as much as 2 metres below the bridge lower chord, possibly impinging significantly on the geometry of the gap profile. Such bridges cannot be used when the gap contains obstructions at this level. Moreover, bridges reinforced in this way cannot be used over wet gaps because of the potential for the reinforcement to enter flowing water. Adverse hydrodynamic loads may catastrophically disturb the bridge's balance, particularly when the reinforcement picks up debris and weeds from flowing water. There is a perceived need to provide an alternative form of reinforcement for a deployable bridge.

It is an object of this invention to provide an alternative reinforcement system which will enable a deployable bridge to support an equivalent load across a similar span as prior art bridge reinforcement systems, whilst overcoming at least some of the aforementioned disadvantages of the prior art.

Permanent bridge structures such as road bridges can be susceptible to damage, for example from general wear and tear or in the case of road bridges from colliding vehicles. Repair to such bridge structures is often a costly and time consuming process during which the bridge may be out of operation. There is therefore a perceived need to provide a reinforcement system for fixed bridge structures.

It is therefore a further object of this invention to provide a reinforcement system capable of providing reinforcement to a fixed bridge structure.

Accordingly this invention provides a reinforcement system for a bridge characterised in that the reinforcement system comprises one or more continuous aramid fibre tendons and tensioning means therefor arranged such that, when deployed, each tendon is attached at two end or near-end portions of the bridge and extends parallel to its span underneath the bridge and is held under tension by the tensioning means

In an alternative aspect, this invention provides a deployable bridge comprising linkable bridge modules and a reinforcement system which is connectable to terminal or near-terminal bridge modules characterised in that the reinforcement system comprises one or more continuous tendons of aramid fibre and tensioning means therefor arranged such that, when deployed, each tendon extends underneath the bridge and is held under tension by the tensioning means.

The potential benefits of this invention are considerable. Aramid fibre ropes have a high strength to weight ratio which optimises fibre efficiency and also a high value of Young's Modulus. They do not corrode, have very good tension / tension fatigue resistance and high ultra-violet resistance. Moreover, such ropes are sheathed with a repairable, abrasion resistant material which ensures minimum build-up and ease of release of mud, ice, etc. Furthermore, such ropes are lighter than conventional arrangements.

Conveniently the fibre ropes may be constructed of Kevlar ^®. A suitable type of fibre rope is the Parafil^® rope (manufactured by Linear Composites Limited) which comprises kevlar fibre. However, it should be recognised that the rope is not intended to be limited to ropes from any particular source. Any variation of kevlar/aramid fibre and grade could usefully be used with this invention. The primary requirement, which is satisfied by Parafil rope, is that the ropes have the required combination of axial stiffness and flexibility. For example, a steel wire rope is flexible but disadvantageously heavy (certainly for portability considerations). Carbon fibre reinforced plastic links are light, but too rigid to be as advantageous as a kevlar/aramid rope.

Conveniently, the tensioning means can be provided by means of mechanical or hydraulic tensioning means. However, the attachment points which connect the tendons to the bridge can be arranged to hold the tendons under tension without the need for a separate mechanical or hydraulic tensioning system. In this case the tensioning means comprises the attachment points. Once deployed the fibre tendons are held under tension on the underside of the bridge. Under the weight of the bridge itself or as a result of the weight due to a load crossing the bridge the bridge will sag and press down either directly or indirectly on the tendons. The tendons which are under tension are resistive to this force and an upwards reaction force is generated which reduces the sag of the bridge. The force is transmitted to the bridge at the points at which the tendons are attached to the bridge and at any point where there is a load path into the underside of the bridge, e.g. where the tendons are in contact with the sagging bridge.

The tendons, once deployed, are located beneath the bridge span but are generally parallel to it. This is in contrast with prior art systems which project down from the underside of the bridge. The reinforcement system of the invention therefore provides similar functionality to the reinforcement systems of the prior art but without the associated disadvantages.

A further advantage of the reinforcement system of the invention is that it can be used with both deployable bridges and permanent bridge structures.

Conveniently, the position of each tendon relative to the underside of the bridge is maintained by means of guides on the bridge. These guides and/or clips are attached to the underside of the bridge and the tendons are threaded through them thereby preventing lateral slippage of the tendons which could potentially reduce the load reinforcement.

As well as preventing any excessive movement of the tendons relative to the bridge span the guides may also be arranged such that in use they provide a load path into the bottom of the bridge. In this variant of the invention the tendons are held beneath the bridge span but substantially parallel to it by the guides (The separation between the tendons and the underside of the bridge is small compared to the length of the bridge span). As the bridge sags under loading the guides will press down on the tendons and the upwards reaction force generated by the tensioned tendons will be transmitted back into the bridge via the guide structures. In the case of deployable bridges and in comparison with the prior art, the inherent lightness and flexibility of the rope reinforcement system as compared with discrete rigid links, means that the ropes may be deployed simultaneously with the bridge as it is launched. Single length strops may be coiled and deployed from a spool. Alternatively, the ropes could be located immediately below the bridge modules in guides, or constrained to provide a specific profile along the bridge. This latter embodiment is particularly useful if the bridge launching process uses a system of rollers.

In its undeployed configuration, there are a number of potential benefits to the deployable bridge of the invention. It obviates the need, experienced in prior art bridges, to reconfigure unreinforced bridge modules to fit reinforcing links, winches and straps, etc. to the inside of every panel. This saves the cost and weight of numerous components used to restrain and deploy the reinforcement, in addition to the weight of all the prior art links themselves. This invention can be implemented with only a single attachment to terminal bridge modules, although there may be a requirement for more if more reinforcing tendons are required for a trackway.

The King post (or Queen posts) and associated assembly occupy significant space and weight in the prior art undeployed King Post bridge module. Additionally, a single rope, running the length of the bridge span, removes the need for joints in the reinforcement links at module junctions, at which reinforcing links are pinned together. The joints, King post and assembly all add to the weight of the prior art system, and are therefore disadvantageous to mobility.

The prior art reinforcement system presents a bridge which is more vulnerable to damages and is more likely to be detected if covert operation is required.

Finally, a preferred feature of the present invention is that the reinforcement is deployable directly into its optimum position: immediately under the chords as the bridge is deployed; and tensioningis not required until after deployment. This significantly reduces the time, manpower and / or equipment required to deploy and dismantle the bridge since it obviates the need first to deploy and then to recover reinforcing links. ln order that the invention may be more fully understood embodiments thereof will now be described with reference to the accompanying drawings in which

Figure 1 illustrates schematically a bridge employing the reinforcement scheme of the invention.

Figure 2 is a cross section of the deployed bridge of the invention taken along line 2-2 of Figure 1.

Figure 1 illustrates a bridge 10 in accordance with the present invention in its deployed position, indicated generally by 20. The bridge 10 extends across a gap in terrain 14 and is constructed from bridge modules (not shown), such as known in the prior art, which extend from a home (defined by the bankside from which deployment is made) to a far bank. At bridge ends are home- 16 and far- 18 bank terminal modules of ramped construction to assist vehicles in driving on and off the bridge 10. Pairs of anchoring connectors 21 , 22 are attached to terminal modules 16, 18, one connector of each pair being at each bank end. Two sheathed Parafil ropes 24 are held, spaced apart, between each pair of connectors 21 , 22 and so extend along almost the entire span of the bridge 10. Parafil^® is a kevlar fibre rope with particular terminations known as barrel and spike. The connectors 21 , 22 therefore use these terminations which are particularly suitable for holding fibre ropes under tension. Each rope 24 acts in tension by means of the anchoring connectors 21 , 22 and in its deployed position runs via guides (not shown) immediately beneath a lower rim of the bridge.

Figure 2 illustrates a cross-section taken along the line 2-2 of Figure 1. The bridge 10 comprises two trackways 30, 32, each trackway being inverted U-shaped with two girder chords 34, 36, 38, 40. A tensioned Parafil rope 24a, 24b, 24c, 24d, as described with reference to Figure 1, is guided, but not otherwise attached, immediately beneath each of the four girder chords 34, 36, 38, 40. One anchorage connector 21 , 22 is used for each pair of ropes supporting the one trackway. The two trackways 30, 32 provide a roadway for vehicles and personnel, although they may support a platform decking 50 if another roadway surface is desired. With reference to Figures 1 and 2, the bridge 10 is deployed as followed. One anchorage connector 22 is attached to each far-bank terminal module 18 as the bridge 10 is launched. The second anchorage connector 21 of each pair is attached to the respective home-bank terminal module 16 towards the end of the launch process, before the bridge is lowered and Parafil ropes tensioned. The degree of tension in these flexible ropes is adjustable, depending on the span and load requirements of the bridge. In an alternative embodiment a hydraulic system (not shown) is used to provide additional tensioning of the ropes after attachment, if required.

Once the ropes 24a, b, c, d, are attached at each end, the whole bridge construction is reinforced along its entire length. Under their own, or any additional, weight the trackways 30, 32 will press down on the ropes 24. This will place a distortive force on the ropes which, as a result of their high tension are resistive to this force. The ropes therefore generate reaction forces upwards along the entire underside of the bridge, reducing its sag. The greater the downwards force, for example at the midpoint of the span, or as a result of vehicles crossing, the greater the reaction force and therefore reinforcement. Since the Parafil ropes 24 are tensioned all along their length, the reaction force is capable of acting at any point along the bridge span. Based on a tensile load of approximately 200 tonnes per trackway, four Parafil ropes are required to support the equivalent weight / span capability of the prior art Christchurch Bridge.

Each Parafil rope has a barrel and spike termination for connection to the bridge itself. Such terminations are simple and reliable and have been proven in the prior art. In addition their design has been optimised for use in the No 10 Close Support Bridge in which Parafil ropes are used for opening and closing the two halves of the bridge.

The optimum position for the Parafil ropes 24 is immediately below the girder chords 34, 36, 38, 40. It is necessary to keep the rope in place, in contact with the girder, to prevent any lateral slippage hindering its load reinforcement. Suitably positioned guides and / or clips are therefore attached to the bridge modules and the ropes threaded through. The ropes are therefore guided relative to the girder chords, but not attached at any but the anchorage points.

Claims

1. A reinforcement system for a bridge characterised in that the reinforcement system comprises one or more continuous aramid fibre tendons and tensioning means therefor arranged such that, when deployed, each tendon is attached at two end or near-end portions of the bridge and extends parallel to its span underneath the bridge and is held under tension by the tensioning means. .

2. A reinforcement system according to Claim 1 characterised in that each tendon is a Parafil rope attached to the bridge by means of a barrel and spike termination.

3. A reinforcement system according to Claim 1 or 2 characterised in that the position of each tendon relative to the lower portion of the bridge is maintained by means of guides on the underside of the bridge.

4. A reinforcement system according to Claim 3 characterised in that the guides on the bridge provide a load path into the bridge.

5. A reinforcement system according to any preceding claim characterised in that the tensioning means comprises hydraulic or mechanical tensioning means for further tensioning the tendons.

6. A deployable bridge comprising linkable bridge modules and a reinforcement system which is connectable to terminal or near-terminal bridge modules characterised in that the reinforcement system comprises one or more continuous aramid fibre tendons and tensioning means therefor arranged such that, when deployed, each tendon extends underneath the bridge and is held under tension by the tensioning means.