GB2443192A - Bridge structure - Google Patents

Abstract

The bridge structure, particularly for extending over railways etc., comprises a number of portal frames supported at least one footing 8 at ground level, each portal frame comprising a central span 2 and two backspans 4 joined to the central span, with the or each footing being supported by a minipile support 14. More than one footing is preferably provided, each of which may comprise a low level ground beam that acts as a retaining wall and may be shared by more than one portal frame. Each footing is preferably cast from reinforced concrete around the top of one or more minipiles, which are preferably orientated at an angle to the vertical. The portal frames are preferably prefabricated from steel sections and delivered to site part assembled. The central and backspan portions are preferably connected using joints 6 formed from integral splice plates. Also claimed is a method of erecting a bridge structure.

Description

A BRIDGE STRUCTURE

The invention relates to a bridge structure, and more specifically but not exclusively to a bridge structure for use as a railway overbridge.

Level crossings are a common sight where roads and railways cross. Their popularity stems from their comparatively simple and inexpensive construction, however they are not without their problems.

Firstly, the maximum speed of a train is limited by a level crossing. This problem has become more apparent and significant with the advent of faster trains and tracks. A longer standing and more serious concern is one of safety. Level crossings are a major source of risk on a railway system. In the UK for example, annual fatality figures regularly reach double figures and several hundred further near misses are reported every year. Safety can be improved by the provision of manned crossings and by regular safety checks and maintenance of equipment, but not much can be done about the single biggest cause of accidents, human error.

Furthermore, by their very nature, level crossings cause congestion. For motorists forced to wait at crossings, this can lead to frustration and errors of judgement. It is these errors of judgement that constitute the largest single cause of accidents at level crossings.

One possible proposal is to replace at least some of the most dangerous level crossings with bridges.

Bridges are beneficial in a number of ways. They are safer than level crossings and have no direct effect on the railway line. Accordingly, they do not limit the operating speed and capacity of the railway (or the roads) in the same way that level crossings do. It has also been argued that, as a result of lower running costs, a bridge can be more cost effective than a level crossing in the long term.

Nonetheless, the high initial costs of erecting a bridge, and the associated short term disruption to the road and rail networks, means that typically bridges are only considered for a limited number of applications.

Accordingly, it is an object of the present invention to provide a bridge structure which may be constructed at a lower initial cost and with minimal disruption to the transport network.

According to a first aspect of the present invention there is provided a bridge structure comprising a plurality of portal frames, supported on a footing at ground level, each portal frame comprising a central span and two backspans joined to the central span, and the footing being supported by a minipile support.

Typically bridges comprise massive fill retaining abutments and wingwalls with heavy simply supported decks. The use of a portalised construction and the elimination of large abutment wall structures improve structural efficiency and eliminates much of the dead weight of the structure. As a result, the use of minipile supports becomes feasible, where usually it is precluded. This is beneficial both in reducing construction costs and in minimising the disruption caused during construction. Minipiles may be bored into the ground with a lighter plant, during normal traffic hours, thus avoiding the cost and disruption caused by typical high capacity piling rigs. The use of ground level supports also eliminates the need for dedicated bearings at the road surface, further reducing complexity and cost.

For the sake of simplicity the footings may be shared by more than one portai frame, being formed, for example, as low level ground beams. These ground beams would advantageously also act as retaining walls, prefereably low retaining walls, for a fill embankment, and may be cast from reinforced concrete, preferably around the tops of the minipile supports.

For stability, it is advisable to orient the minipiles at an angle to the vertical. This angle is preferably around 10 .

The portal frames may be of steel fabrication, and to ease construction they would preferably be fabricated in 3No elements namely 2 backspans to either side, and a central spanning section. The central span and/or backspans may be pre-fabricated prior to delivery to site. The erection of the bridge structure is thereby simplified. Alternatively, if it is required for ease of transportation, the backspans may be only part assembled prior to delivery.

By limiting the mass of each pre-fabricated section to five tonnes, and preferably to three tonnes or less, light duty cranes may be used to erect the structure. This avoids the need for larger capacity cranes which would need to sit on prepared footings.

It is preferable that the joins between the backspans and the central span are spliced. Ideally the underside splice plates to the backspan bottom flanges would be integrally formed with the backspans for ease of assembly of the central span.

A road deck may also be provided. This would preferably be cast insitu on substantial permanent formwork, and would be secured to the portal frames by means of welded shear studs. These would ensure composite portai action between portal frames and road deck.

The bridge structure is particularly suitable for use as a railway overbridge.

According to a second aspect of the present invention there is provided a method of erecting a bridge structure, comprising steps of boring minipiles, casting ground level supports, constructing approach embankments, erecting and securing pre-fabricated backspans, erecting and securing pre-fabricated central spans and forming a road deck.

This method of manufacture, made possible by the characteristics of the bridge design, minimises disruption to the transport network.

It is preferable if the road deck is of compsite construction and is formed above the central spans. It is time and cost effective to form the supports as ground beams. It is preferable that these ground beams are designed and located so as to additionally function as small retaining walls for the approach embankments. This is simplified if the approach embankments are raked back from the supports. The need for abutments is also eliminated as a result. Where the embankments are raked back in this way, it is beneficial that the backspans of the bridge are triangulated to support the roadway above the sloping fill.

To further minimise the impact of the construction process, the use of light duty cranes is preferred. This is made possible by the mass of each pre-fabricated section being limited, preferably to five tonnes, and ideally to three tonnes or less.

There follows a detailed description of a preferred embodiment of the present invention. This description is given for illustrative purposes only and is not intended to limit the scope of protection sought. Throughout the description reference is made to the accompanying drawings, in which: Figure 1 shows a longitudinal cross-sectional view of one end of a bridge structure according to the present invention; Figure 2 shows a detailed view of a join between a backspan and the central span in an unassembled state; Figure 3 shows the join of Figure 2 in an assembled state with a road deck applied; Figure 4 shows a lateral cross-sectional view of a bridge structure according to the present invention; and Figure 5 shows a longitudinal cross-sectional view of a bridge structure according to the present invention spanning a two track railway line.

Referring to Figure 1, one end section of a bridge is shown. The drawing shows a central span 2 and one backspan 4, which are joined together by a splice joint 6 shown in detail in Figures 2 and 3. The backspan 4 is supported on footings 8 at ground level.

In this preferred embodiment, the backspan is formed with one large main frame member 20 and two smaller leg frame members 21. Each leg frame member 21 is fixed to one end of the main frame member 20 and angled towards the other leg frame member 21 so as to form a triangulated backspan. The apex formed by the two leg frame members 21 is supported on the footings 8. The triangulated shape of the backspan allows the embankment fill 12 from the approaching roadway to be raked back at an angle, eliminating the need for large abutments. Indeed, in this example the footings 8 are constructed as a ground beam provided with a dwarf retaining wall 10 to retain the embankment fill 12. The footings 8 are supported on minipile supports 14 which are bored into the ground. The minipiles 14 are preferably of around 300mm in diameter and are raked alternately, preferably at around 10 to the vertical.

Where the backspans 4 meet the approach road at the top of the fill embankment 12 an expansion joint 16 is provided. A cill beam 18 is also shown at the top of the fill embankment 12. The main frame member 20 and the leg frame members 21 of the backspan 4 are of steel fabrication and the assembled backspan 4 is preferably limited to five toimes in mass to allow lifting by a light duty, preferably rough terrain crane. Additional splices 22 may be provided in the leg frame members 21 of the backspan 4 to allow for transportation to the site prior to final assembly as a fully triangulated backspan 4.

The central span is also formed from a large frame member. This central frame member 23 is of similar cross sectional size and construction to the main frame member 20 of the backspan 4. As with backspan 4 it is preferable that the total mass of the central span does not exceed five tonnes.

Figures 2 and 3 provide a detailed view of the splice joint 6 between the central span 2 and a backspan 4. The main frame member 20, leg frame members 21 and central frame member 23 are all shown as I' section beams. Also shown are the ends of a number of L' section cross-members, forming crossframes 24 which are provided between adjacent portal frames to stabilise the bridge framework. These crossframes 24 are shown in more detail in Figure 4.

Figure 2 shows the splice joint 6 in an unassembled state. Shear studs 26 are provided on both the main frame members 20 of the backspan 4, and on the central frame member 23 of the central span 2, to bond the deck of the road to the framework. The bottom flange splice plate 30 is shown as integrated with the main frame member 20 of the backspan 4 for ease of assembly.

Figure 3 shows the assembled splice joint 6 with the road deck 32 applied. The central frame member 23 of the central span 2 is positioned onto the bottom flange spline plate 30 of the main frame member 20 of the backspan 4. The top flange splice plate 31 of the central frame member 23 of the central span 2 accordingly rests on the top of the main frame member 20 of the backspan 4. Once the splice joint 6 is thus assembled and suitably fixed the road deck 32 is applied.

Figure 4 shows a lateral cross sectional view of a single lane carriageway bridge according to the invention. The crossframes 24 are clearly shown between the portal frames 40 of the bridge structure. It should be understood that the cross section of the bridge structure, as shown in Figure 4, is substantially unchanged along the length of the assembled structure. The portal frames 40 are formed from two backspans 4 joined to one central span 2. Accordingly, depending on the point at which the lateral cross section is taken, each I' shaped member of each portal frame 40 may correspond to either the main frame member 20 of a backspan 4, or to the central frame member 23 of a central span 2. In the shown embodiments only two portal frames 40 are shown but in practice the number used will depend on the specific application. Tubular edge girders 42 are provided on either side of the portal frames 40 and run along the entire edge of the roadway of the completed bridge structure. The edge girders 42 are connected to the mainframes by the crossframes 24, and are spliced in similar positions and in a similar way to the mainframes 40.

Parapets 44 are provided along both edges of the roadway of the completed bridge structure. The parapets may consist of railings fixed to vertical extensions of the concrete road deck 32 above the edge girders 42. The height of the railings a, and of the vertical road deck extensions b will vary according to the application, as will their construction. In this particular embodiment these dimensions are 1000mm and 320mm respectively. The dimensions d between the centres of the edge girders 42 and the portal frames 40, and e, between centres of adjacent portal frames 40, will likewise vary according to the width requirements of the specific bridge, although the distance e will typically be in the region of 2400mm, and the distance d will typically be in the region of 1000mm. Roadway details 46 showing the finished road surface with kerbs and pavements running along either side are provided mainly for illustrative purposes.

Figure 5 shows a side view of a complete bridge in a preferred application as a railway overbridge. Two backspans 4 and a central span 2 as previously described are shown spanning railway lines 48. Part of the approach gradient 50 leading up to the bridge from the right hand end is also shown. It can be seen from Figure 5 that the central span 2, and the top of each backspan 4, is curved to follow the road profile. Each triangulated backspan 4 is shown resting on footings 8, which incorporate a dwarf retaining wall 10 as earlier described. The embankment fill 12 on each side is shown raked back from the railway lines 48 and is suitably retained by said dwarf retaining walls 10. Below the footings 8, minipiles 14 are shown extending down into the ground at an angle x to the vertical. The minipiles 14 are shown schematically, with the angle x in this case being 100. The height of the central span 2 above the railway lines 48 is designated f, while the distance from the outside of a railway line to the closest leg frame member 21 of a backspan 4 is designated g. The dimensions f and g will vary depending on the precise application, but for a crossing over a non-electrified two-track UK railway as shown, suitable dimensions are 4800mm and 4500mm respectively.

The embodiments described above are for illustrative purposes only. By virtue of its design and standardised construction the bridge according to the invention is readily customisable for any number of situations. The height, length, size and number of portal frames may be varied depending on specific requirements fo bridge height, width, span, skew and approach gradient, inter alia. The approach gradient may be modified depending on the type and volume of traffic, and crash barriers may be included or omitted as appropriate. Furthermore, the choice of steel section used for fabrication of the various bridge components should not be considered to be limited to that described and shown in the drawings but should encompass all reasonable alternatives known to one skilled in the art.

Claims (36)

1. A bridge structure comprising a plurality of portal frames, supported on a footing at ground level, each portal frame comprising a central span and two backspans joined to the central span, and the footing being supported by a minipile support.

2. A bridge structure according to claim 1, wherein more than one footing is provided.

3. A bridge structure according to claim 1, wherein the or each footing is shared by more than one portal frame.

4. A bridge structure according to claim 2 or 3, wherein the or each footing is a low level ground beam.

5. A bridge structure according to claim 4, wherein the or each ground beam acts as retaining wall to a fill embankment.

6. A bridge structure according to any of the preceding claims wherein more than one minipile is provided.

7. A bridge structure according to any of the preceding claims, wherein the or each footing is formed from reinforced concrete.

8. A bridge structure according to claim 7, wherein the or each footing is cast around the tops of one or more minipile supports.

9. A bridge structure according to any of the preceding claims, wherein the or each minipile support is oriented at an angle to the vertical.

10. A bridge structure according to claim 9, wherein the angle is 100.

11. A bridge structure according to any of the preceding claims, wherein the portal frames are fabricated from steel sections.

12. A bridge structure according to any of the preceding claims, wherein the backspans are pre-fabricated and delivered to site ready for erection.

13. A bridge structure according to any of claims 1 to 11, wherein the backspans are part assembled and delivered to the site for final assembly.

14. A bridge structure according to any of the preceding claims, wherein the central span is pre-fabricated and delivered to the site ready for erection.

15. A bridge structure according to any of claims 12 to 14, wherein the mass of each pre-fabricated section is no more than five tonnes.

16. A bridge structure according to claim 15, wherein the mass of each pre-fabricated section is no more than three tonnes.

17. A bridge structure according to any of the preceding claims, wherein the joins between the backspans and the central span are spliced.

18. A bridge structure according to claim 17, wherein flange splice plates are integrated with the backspan fabrication.

19. A bridge structure according to claim 17 or 18, wherein flange splice plates are integrated with the central span fabrication.

20. A bridge structure according to any of the preceding claims, further comprising an in situ deck.

II

21. A bridge structure according to claim 18, wherein the in situ deck is debonded from the frames in the region local to the joins between the backspans and the central span.

22. A bridge structure according to claim 20 or 21, wherein an expansion joint is provided in the deck.

23. A bridge structure according to any of the preceding claims, wherein the bridge is a railway over-bridge.

24. A method of erecting a bridge structure, comprising steps of boring at least one minipile, casting a ground level footing, erecting and securing pre-fabricated backspans, erecting and securing pre-fabricated central spans and forming a road deck.

25. A method according to claim 24, wherein more than one footing is cast.

26. A method according to claim 24 or 25, wherein the or each footing is cast as a ground beam.

27. A method according to any of claims 24 to 26, further comprising constructing approach embankments.

28. A method according to claim 27, wherein the or each footing is cast at the base of an approach embankment and acts as a small retaining wall to the approach embankment.

29. A method according to claim 27 or 28, wherein the approach embankments are raked back from the or each footing.

30. A method according to claim 29, wherein the backspans are triangulated and the embankments are raked back at an angle substantially similar to that formed by the backspans.

31. A method according to any of claims 22 to 30, wherein the mass of each pre-fabricated section is no more than five tonnes.

32. A method according to claim 31, wherein the mass of each pre-fabricated section is no more than three tonnes.

33. A method according to claim 31 or 32, wherein light duty cranes are used to erect the pre-fabricated sections.

34. A according to any of claims 24 to 33, wherein the road deck is of composite construction.

35. A bridge structure substantially as herein described with reference to and as shown in the accompanying drawings.

36. A method of erecting a bridge structure substantially as herein described.