2. Description of Scheme Options 2.1 Functional Cross Section 2.2 General Arrangements 2.3 Deck Type 2.4 Tower Forms 2.5 Approach Bridge Type 2.6 Foundations
2. Description of Scheme Options
2.1 Functional Cross Section
The functional cross section of the bridge is required to incorporate the following items:
- Dual two lane motorway with 3.3m hard shoulders (70 mph design speed)
- Footway / cycleway with provision for maintenance vehicle access
- Multi-modal (public transport) corridor (70 mph design speed)
- Vehicle restraint systems and pedestrian / cyclist parapets
- Windshields (designed to be difficult to climb)
- Highway lighting
At the concept stage, a number of alternative configurations for the functional cross section were considered and assessed against a range of criteria including:
- Road connectivity
- Multi-modal connectivity
- Operational considerations
- Tower aesthetics
- Foundation costs
Two configurations were selected for further development
- Three Corridor Option
- Double Level Option
Functional Cross Section Options
The two options are illustrated in Drawings FRC/C/076/S/021 and FRC/C/076/D/031 contained in Appendix B. For the Three Corridor option the bridge deck is a single level with the multi-modal corridor located in the centre of the bridge. Stay cables and tower legs are located in a structural zone between the multi-modal corridor and the main motorway carriageways. For the double deck option the multi-modal corridor is located at a lower level with stay cables anchored close to the edge of the deck. In both arrangements the footways / cycleways are located at the edges of the deck.
2.2 General Arrangements
The general arrangement of each option has been developed to be largely consistent with the FRCS Reference Design with the bridge having two cable stayed main spans, each of approximately 650 m and three towers with the central tower founded on Beamer Rock. The general arrangements are illustrated in Drawings FRC/C/076/S/001 and FRC/C/076/D/011 contained in Appendix B.
A fundamental consideration for the general arrangement of the bridge is the provision of two cable stayed main spans. This arrangement requires special consideration of how to stabilise the central tower. Appendix D describes the studies carried out during the conceptual design development to select two different options for central tower stability corresponding to the Functional Cross Section options:
Functional Cross Section |
Central Tower Stability |
|---|---|
Three Corridor |
Achieved by crossing cables in the centre of the main spans |
Double Level Deck |
Use the combined deck and tower stiffness to achieve sufficient static and dynamic global stiffness. |
2.3 Deck Type
A key driver for the Three Corridor option is the requirement for the deck to be torsionally stiff due to the significant span of the bridge combined with the stay cables being anchored relatively close to the centre of the deck. The only feasible deck in this case is a box girder. Two alternatives are considered which are illustrated in Drawing FRC/C/076/S/101 in Appendix B:
- Steel orthotropic box girder
- Steel-concrete composite box girder
Considering past practice it is known that composite decks can be more economical for short to medium span cable stay bridges but become uneconomical at long spans. The current longest composite span is the recently completed Qingzhou cable-stayed bridge over the Ming River, Fuzhou, China with a span of 605 m. The 650 m span proposed would therefore be a world record and is at the upper end of the boundary between medium span and long span by this definition. It is anticipated that the relative economy of the deck types will be marginal and will be determined by market conditions, material costs at the time of construction and the preferred working arrangements of the tendering contractors.
Since the two deck options will be aesthetically the same the aim of the design is that both types be developed. Provided that during the design development neither deck type is proven to be unfeasible it could be considered to tender the project on both deck types, allowing the tendering contractors to select the one for which they are able to provide the most competitive price for. If this strategy is followed then the tower design should be developed to have the same external shape for both deck options although the wall thickness and other internal details may vary.
Orthotropic Box Girder Section
Composite Box Girder Section
2.3.2 Double Level option
The double level options utilise a deep stiffening truss that assists with stabilising the central tower. The key driver for the overall behaviour of the suspended decks is the relative stiffness of the tower and the deck. The deck must be stiff enough to accommodate the deformations at mid-span under asymmetric live loading and reduce bending effects in the tower such that the tower can be kept relatively slender and elegant. Three alternatives are considered which are illustrated in Drawings FRC/C/076/D/111 to FRC/C/076/D/113 in Appendix B:
- 2 Plane Warren Truss
- 4 Plane Warren Truss
- 2 Plane Vierendeel Truss
The logical truss arrangement is a Warren truss which is more elegant than a Pratt truss. The shear forces are also reversible in most parts of the deck which means that structurally the Pratt truss is not particularly relevant since its defining feature is that the bracing arrangements relate to the direction of the shear force. Providing two planes of trusses, one beneath each plane of cables, creates a torsionally stiff and robust structure. However an alternative with four truss planes is also considered in order to triangulate the transverse span to reduce the cross beam depth and also reduce the section size of the bracing members.
The Vierendeel truss alternative is proposed in order to create a visually less complex structure since the bracing members of the Warren truss are inclined in two different directions and result in possible visual interference effects when viewed from certain angles. However, it is well established that Vierendeel trusses are less efficient than triangulated trusses and the feasibility of this proposal has been carefully studied.
4 Plane Warren Truss
2 Plane Vierendeel Truss
2.4 Tower Forms
Three alternative tower forms have been developed, in each case the tower is a reinforced concrete hollow structure with a fabricated steel anchor box to house the upper stay anchors. Provision is made within the towers for access during construction and for inspection and maintenance.
Tower Forms
Each tower form is to be considered with a particular functional cross section as tabulated below:
Tower Form |
Functional Cross Section |
|---|---|
Needle |
Three Corridor |
Inverted Y |
|
H Shape |
Double Level |
2.4.1 Needle
The concept of the Needle tower is to have a single vertical element centrally located but with a hole punched through it to allow the multi-modal corridor to pass. The aesthetic development of this option has therefore focussed on developing the tower to be a single object rather than a collection of legs, base and anchor box.
2.4.2 Inverted Y
Above deck level, the Inverted Y tower is somewhat similar to the Needle tower with split legs located towards the middle of the deck with the multi-modal corridor passing between the legs and the main motorway carriageways outside. However, the aesthetic concept and practical considerations on foundation size lead to slightly different proportions above deck compared to the Needle tower.
2.4.3 H Shape
In concept, the H Shape has two legs which are inclined towards each other and connected in the region of the stay anchorages without any cross beam below deck level so that the deck floats through the tower. The aesthetic development of the concept has studied the inclination of the legs to achieve reasonable aesthetic proportions and foundation size as well as the number and composition of the cross beams. Due to the relative complexity of the truss deck form associated with this option, the tower itself is developed to be simple in form.
2.5 Approach Bridge Type
The Reference Design shows the southern approach viaduct to be 635 m long, consisting of 9 spans with a maximum span of 80 m. The northern approach viaduct is 115 m long and is a two span structure.
The key issues for the further development of the approach viaducts are visual continuity with the cable stayed bridge and long spans to reduce the numbers of piers to be constructed in the environmentally sensitive channel and inter-tidal zone.
Three types of approach bridge are being considered:
Approach Bridge Type |
Functional Cross Section |
|---|---|
Composite Box Girder |
Three Corridor |
Concrete Box Girder |
|
Truss |
Double Level |
For the Three Corridor Option visual continuity is best provided by a composite approach bridge with a single wide box girder of the same basic cross section as the cable stayed bridge deck. However, this may be a relatively costly solution and since the approach bridge is a significant structure in its own right a concrete alternative has been developed with three box girders.
For the Double Level option the only solution that can provide visual continuity is to continue a truss of similar form to the cable stayed bridge deck into the approach viaducts. This has the advantage of offering long spans and thus reducing the number of piers.
2.6 Foundations
The ground conditions have yet to be determined at all foundation locations and, together with water depths, will vary along the crossing alignment. These, as well as constructability, will be key drivers for the selection of foundations schemes. The side span and approach span pier locations will depend on the structural form adopted for the deck, whilst the foundation geometry at the main towers will depend on the tower form selected. Consequently there are a range of conditions and foundation solutions that may be appropriate which will require further investigation and development in the next stage.
The most challenging foundations will be those for the main towers. The south tower foundation will be in the deepest water and, with 650 m main spans, will be located where the river bed level is about -22 mOD. The ground conditions are currently uncertain but it is anticipated that there will be a substantial thickness of soft alluvium overlying variable glacial deposits before reaching bedrock. This foundation will also have the most onerous ship impact loading. The FRCS Reference Design shows caisson foundations taken to rockhead at the main towers. However, based on the information currently available these would be difficult to construct at the south tower.
Preliminary studies have shown the alternative of a piled foundation to be feasible. This would comprise a group of large diameter piles socketed into the rock. It is envisaged that a precast pile cap could be used and that techniques developed in the offshore industry could be adopted for forming connections between the piles and pile cap underwater.
A similar piled foundation with precast cap may also be suitable for the north tower but would require some initial dredging to install the pile cap. Alternatively in situ pile cap construction within a temporary cofferdam could be considered.
The central tower foundation will be located on Beamer Rock and it is expected that a spread foundation will be suitable for this location. The overall sizing of the foundation will depend on the form of tower selected.
The side and approach spans will require foundations in varying depth of water or on land with rockhead at varying level below bed level. Either piled or pad foundations may be appropriate depending on the conditions at each location.