Bridge structure comprising tower, bridge beam, main/suspension cable, suspending bars, and diagonal cable-stays
The present invention relates to bridges for spans exceeding about 500 m, said bridge constituting an alternative to conventional suspension bridges and cable- stayed bridges.
For spans in the range of 500 - 1000 m both suspension bridges and cable-stay bridges are technically possible. The present invention will most likely involve cost savings for spans in this range.
Until today only suspension bridges have been used for spans over 1000 m, and worldwide it has been built a number of bridges with spans ranging from 1000 - 2000 m. A typical suspension bridge comprises a central bridge beam that is suspended from two parallel suspension cables with vertical suspension hangers between the bridge beam and the suspension cable. The suspension cables are sup- ported by a tower on each end of the bridge span, and continue over these towers and are finally anchored in rock or a concrete foundation. The towers are normally designed as an A or an H, with one leg on each side of the centrally running bridge beam, and with crossbeams between the legs both under and above the bridge beam.
For spans exceeding 1000 m, wind constitutes the main loading on the structure, and it is crucial to choose a constructional concept that makes the bridge beam able to withstand the wind forces, at the same time as the structure must be made aerodynamically stable in all acting wind conditions. These aspects eventually determine the extent of material use, weight and cost.
For long spans, the dead weight of the bridge structure becomes the larger predominant static load as compared to traffic loads at the same time as the wind climate affect the dynamic behaviour and aerodynamic design of the bridge. With increasing spans, the traditional suspension bridge structure reaches a dead weight and a mass that economically preclude a realization of the projects. To
illustrate this, the load ratio between bridge beam, suspension cables and traffic loads for a 1000 m bridge with two lanes is approximately 60/20/20%, but for a similar 2000 m bridge, the corresponding figures are 55/35/10%. The total specific weight to be carried has at the same time increased from approximately 12 tons per meter of the bridge beam, to 16 tons per meter of the bridge beam.
There are many potential crossings with spans in excess of 1000 m that have not been realized because of insurmountable costs. It would be of great benefit to reduce the cost for such structures. This must be obtained by reducing the self weight of the bridge, at the same time as the aerodynamic stability is maintained. It is also a requirement that the structure can be built in a predictable and rational manner that comply with strict requirements to health, environment and safety.
A number of bridges built in the 19th and 20th century used a combination of diagonal cable-stays and suspending bars. The most well-known is the Brooklyn Bridge, finished in 1883 and with a span of 486 m. Diagonal stays and suspension hangers overlap over the entire span, making it a highly redundant static system. The structure combines the stiffer cable-stayed bridge system with the flexible suspension bridge system in a manner that is not optimal.
NO 107352 teaches a combination of diagonal stays and suspension cables, but not a centrally carrying element in combination with split lanes.
So-called hybrid bridges, where diagonal stays close to the towers have been combined with a suspension bridge in the middle part of the span, have also been built. Examples are the footbridges Sidi Me Cid (164 m) in Algeria and Nagisia Brigde (110 m) in Japan.
Diagonal stays alone combined with split lanes and a centric mono tower is used on the Stonecutter Bridge in Hong Kong, which has a span of 1018 m, but the diagonal stays are arranged on the outside of the bridge deck. With longer spans this structure type will become increasingly expensive and yield stability problems during construction.
The present invention solves the problems involved with wind stability by utilizing split bridge decks with a wide air gap in between, in addition to a cantilever concrete bridge section close to the towers, followed by a cable-stayed section and then the central suspension span, contributing to a more rigid bridge deck. The distance between the bridge decks determines the transversal stiffness of the bridge. This distance can be varied and contributes to reducing the materials and mass of the bridge. A variation of this kind is in principle not possible for a traditional suspension bridge structure. Furthermore, the split bridge decks in combina- tion with centric towers and diagonal stays and centric suspension cable with inclined suspension bars will form a triangular structure that in itself increases torsio- nal stability of the bridge deck. The increased torsional stability improves the aerodynamic stability of the bridge. In regard to requirements for safety during construction, the cantilever bridge section from one side (cantilever concrete bridge and diagonal cable-stayed bridge) will experience increasing stability problems during construction that is proportional with the square of the unsupported length, while the suspension bridge yields a much more stable structure because the suspension cables are arranged first by aerial spinning on site. The suspension cable span from one tower to the other and contribute to the overall stability while other elements, such as hangers, stays and bridge girder segments are quickly assembled element by element simultaneously with the cable spinning.
A cantilever concrete structure is the cheapest alternative for the part of the bridge closest to the towers, but this method alone gives very limited spans (maximally 400 m span determined by cost). The diagonal cable-stay method is somewhat more expensive for shorter spans, but can be used for considerably longer spans (maximally approximately 1000 - 1200 m determined by cost). The suspension bridge method requires a relatively high price per meter for shorter spans, but can be used for very long spans. The longest span ever built is 1991 m (Akashi -
Kaikyo) from 1998. The cost rate of suspension bridges increases greatly with increasing spans, mainly because of increase in main cable tonnage and increasing wind stability problems.
The present invention reduces the constructional costs for long span bridges considerably by utilizing a serial combination of cantilever, diagonal cable-stay and suspension bridges. This combination results in a considerable reduction of materials as compared to a pure suspension or diagonal cable-stayed bridge.
The goal of the present invention is reached by means of those features described in the characterizing clause of claim 1. Further advantageous features and properties are described in the dependent claims.
Below follows a non-limiting description and examples of the present invention under reference to the enclosed drawings, in which:
Fig. 1 shows a side view of a bridge according to the present invention,
Fig. 2 shows a cross-section of the suspension bridge section according to the present invention,
Fig. 3 shows a cross-section of the diagonal cable-stayed section according to the present invention,
Fig. 4 shows a cross-section of the cantilever concrete section according to the present invention, and
Figs. 5 and 6 show perspective views of the bridge according to the present invention, with different cable anchorings.
Fig. 1 shows a bridge according to the present invention comprising two mono towers 1 , one central main suspension cable 2, cantilever concrete bridge sections 3, diagonal cable-stayed sections 4 and a suspension bridge section 5. The canti-
lever concrete bridge sections 3 can protrude from for example 50 m to maximally 200 m, the diagonal cable-stayed section 4 continue after the cantilever section 3 and can protrude until about 500 m, while the suspension bridge section 5 spans over the remaining gap up until about 2000 m in this example. For a bridge with a total span of approximately 1300 m, the respective lengths may measure approximately 100 m, 250 m and 600 m (i.e. 2 x 100 m + 2 x 250 m + 600 m = 1300 m). Corresponding numbers for a bridge with a span of 2700 m may be for example 100 m, 450 m and 1600 m = 2700 m). The cantilever bridge sections 3 form stiff elements that continue into the somewhat more flexible diagonal cable-stayed sec- tions 4, while the suspension bridge section 5 forms the most flexible part between the diagonal cable-stayed sections 4. This combination of a cantilever bridge, diagonal cable-stayed bridge and a suspension bridge, wherein the suspension bridge part is reduced, results in a considerably lighter and thinner main suspension cable, greatly reducing the total cost of the bridge.
Fig. 2 shows a cross-section of a bridge according to one example of the present invention, wherein the cross-section is taken through the suspension bridge part of the bridge. Two bridge decks 6 run in varying transverse distance and are connected with each other by means of cross-beams 7 on the inside of the bridge decks 6. Hangers 9, fixed to the main suspension cable 2, extend down to each end of the cross-beams 7 on the inside of the bridge decks 6. Since the hangers 9 (and diagonal cable-stays 8) are fixed on the inside of the bridge decks 6, an inspection vehicle with a long, flexible arm will easily be able to access underneath the bridge decks 7 and easily be transported along the direction of the bridge deck 6, without meeting obstacles such as hangers 9 (or diagonal cable-stays 8) that require the arm to constantly being drawn in and pushed out into position under the bridge deck 6.
Fig. 3 shows a cross-section of a bridge according to one example of the present invention, wherein the cross-section is taken through the diagonal cable-stay part 4 of the bridge. The bridge decks 6 are connected by means of cross-beams 7. The distance between the bridge decks are increased as compared to fig. 2. The diagonal cable-stays run from the cross-beams 7 up to the mono-towers 1.
Fig. 4 shows a cross-section of a bridge according to one example of the present invention, wherein the cross-section is taken through the cantilever part 3 of the bridge. The bridge decks 6 are further apart because they must run on each side of the mono-towers 1 (the distance between the bridge decks 6 corresponding to the outer dimensions of the mono-tower 1 at that height). The bridge decks 6 are fixed to the mono-tower by means of a strong cross-beam 7. According to the present invention, the bridge decks 6 curve inwards on one side or both sides of the mono-towers 1 , thereby keeping the length and weight of the cross-beams 7 down. In addition, a design of this kind tends to give an aesthetically pleasing and elegant appearance.
In stead of mono-towers, it is possible to use alternative tower designs according to another example of the present invention. Possible designs are A-towers, H- towers, X-towers or V-towers. In addition, other tower designs are also possible.
The construction can be carried out in the following manner: - Towers formed as hollow box with varying width and wall thickness are built with sliding or climbing form. Traditional time and cost consuming cross- beams can be avoided. All material transport and worker access can be located inside the tower sheltered from wind and weather conditions. An outside tower crane is not necessary (this is an important aspect because the tower will top about 170 m for a bridge with a span of 1000 m and about 300 m for a bridge with a span of 2000 m). - The construction of the concrete cantilever parts of the bridge beam can begin as soon as the right height is reached, and largely be carried out simultaneously with the further raising of the tower. - The construction of the diagonal cable-stay part can be carried out at the same time as the main suspension cable is made by aerial spinning. - It is much more efficient to arrange only one centric main suspension cable than the traditional two cables placed on each side of an H - tower. - The bridge decks in the suspension bridge part are pre-fabricated and assembled in larger units.
The present invention results in shorter construction time and a less weather sensitive construction process than building a conventional suspension bridge, this resulting in considerable cost savings. The connection of diagonal cable-stays and hangers on the inside of the bridge decks eases the access of a "bridge lift" for inspection and maintenance of the bridge boxes. The centric main suspension cable provides a large and safe platform for inspection and maintenance of cable and hangers.
According to the present invention, a serial combination of a cantilever bridge, diagonal cable-stay bridge and suspension bridge provides a bridge structure with a combination of flexible and rigid properties that gives advantageous transitions between the various elements (from rigid close to the towers till flexible far from the tower), and results in an optimal ability to carry large vertical and horizontal loads. At the same time, each bridge part is used in its most optimal, cost saving position. The present invention provides a synergy between various constructional principles, obtaining an optimal structure, considerably shorter construction time, considerably lower cost, as well as easier maintenance.