Arup Fellow and Director, Hong Kong
ABSTRACT: The paper describes the author’s personal experience and involvement in the design of large sea-crossing bridges. The approach to design has been based on construction methods that allow fast-track quality construction with use of large pre-fabricated bridge elements in both steel and concrete.
1 INTRODUCTION
1.1 Need for large sea-crossing bridges
Infrastructure plays a key and vital role in the economic development and well-being of a region or country. In many parts of the world waterways whether wide rivers, bays and estuaries have meant large detours and/or use of ferries thus severely hindering the movement of people and goods and limiting economic development. This has led to the bridging of these waterways with road and rail bridges and since the 1990’s several large sea-crossing bridges have been built mainly in the Far-East. The methods of construction have varied for these crossings but the driver has invariably been fast construction with pre-fabricated elements.
Another feature of the crossings has been bridging of navigation channels associated with long waterway crossings which has led to the development of long span cable supported bridges. 1.2 Development and features of crossings
The features and development of the crossings which also depends upon its location is described with reference to Oresund Crossing Denmark-Sweden, Shenzhen Crossing Hong Kong, Incheon Bridge Korea, Hong Kong Zhuhai Macao Bridge, Queensferry Bridge Scotland and Brunei Temburong Bridge, all of which are sea-crossing bridges. The primary feature of all these bridges is that foundation and substructure construction can proceed in parallel with precast or prefabricated superstructure elements off-site in factory conditions, thus appreciably shortening the construction period. Design for ship impact is also an important feature of the navigation channel bridges, that are invariably associated with long sea crossings, and various protection methods have been devised to safeguard the bridge towers adjacent to the navigation channels.
2 ORESUND CROSSING DENMARK-SWEDEN 2.1 Location and general features
Oresund Crossing across the Danish Straits connects Copenhagen in Denmark with Malmo in Sweden and was opened in July 2000. The Danish Straits are of special importance because they provide the only natural connection between the Baltic and the open seas. The straits also function as hydraulic links and are profoundly important for the maintenance of water quality and survival of marine life within the Baltic. Any scheme for the crossing had to ensure that obstruction of water flow was as little as possible. The total length of the link is 16 km and comprises of an 4 km immersed tunnel, 4 km of artificial island and 8 km of bridge. The final alignment is shown in (Fig. 1).
Figure 1. Figure 2.
Figure 3. Figure 4.
2.2 Alignment and standard spans
The Treaty specified an alignment which was simply a straight line from the artificial island south of Saltholm to the landfall in Sweden. We proposed a S-curve alignment for the bridge to give users of the Link more interesting views (Fig. 2). The bridge reference design in the treaty comprised of single level bridge structures carrying two tracks of high speed rail and a dual 2-lane with hardshoulder road. In the ARUP competetion winning design the road and rail was separated with the road above and rail below. With this arrangement the most economical structural solution was to use steel trusses with diagonals connecting the upper and lower decks. These trusses are uniform throughout the bridge, but modified at the cable-stayed main spans so that every other diagonal has the same direction as the cables. The 20 m bay length of the truss is constant along the bridge and imposes a modular discipline on all the spans. The deep composite girders lead naturally to longer spans, which have environmental as well as visual and construction advantages. Longer spans meant less obstruction to water flow to meet the limit to blocking set by the environmental authorities in both countries at 0.5%.
We were aware of the special floating heavy lift ‘the Swanen’ which can lift a 7200 t payload out of which 6000 t can be a structural element. The water depth along the alignment is shallow and foundation bedrock is also at shallow depth. Pad foundations could be used and hence precast cellular foundations were designed which could be lifted into place by the Swanen. The Swanen was also able to lift precast hollow columns and whole 140 m long bridge spans into place (Figs 3, 4). The chosen construction method meant that both the substructure and superstructure could comprise of large concrete and steel bridge elements that could be precast or prefabricated in controlled factory type environment with good quality control thus ensuring a long life durable structure in an aggressive marine environment.
2.3 Navigation span
ARUP proposed a single navigation span of 490 m over Flintrannan instead of the 330 m and 290 m spans over Flintrannan and Trindelrannan specified in the Treaty. A truss sufficiently deep
to accommodate the railway is naturally stiff enough to act as a deck for a cable-stayed span considerably longer than that required by the brief, so the opportunity was taken to provide only one navigation span at Flintrannan.
The inherent stiffness of the truss deck was also a factor in choosing a harp configuration for the cables. The live load moments in a slender deck are sensitive to the vertical stiffness of the cable system, which strongly suggests a fan arrangement. This does not apply to the truss deck. Its repeating geometry has also a natural affinity with the harp, which can be emphasised by adjusting the angles of the diagonals to match those of the cables.
The harp system has a visual formality, particularly apparent when cable planes are vertical, and the towers were designed to express this. The effect is further enhanced because each cable plane is supported by independent towers unconnected above deck level and was a major visual feature of the bridge.
2.4 Ship impact protection
As the water was relatively shallow, approximately 12 m at the navigation span and for aesthetic purposes the ship protection to the towers is provided by submerged artificial earth mounds.
3 SHENZHEN CROSSING AND DEEP BAY LINK HONG KONG 3.1 Location and general features
The Shenzhen Crossing and Deep Bay Link (DBL) connects Yuen Long Highway and the future Route 10 in Hong Kong with Shenzhen in China (Fig. 5).
DBL is approximately 5.4 km long and Shenzhen Crossing across Deep Bay is approximately 5.0 km long. The design and construction of the bridge whilst being of fast-track nature also had to take into account the minimisation of ecological damage. Following the concept ARUP had developed for the Oresund Crossing between Denmark and Sweden, ARUP proposed an S-shaped horizontal alignment for the bridge as shown onFigure 8.
3.2 Marine viaduct
The marine viaduct is formed in modules of bridges. Each bridge consists of 8 continuous span concrete boxes with 75 m internal spans and 70 m end spans (Fig. 6). The constant depth viaducts were built by the precast segmental balanced cantilever technique using a combination of external and internal prestressing cables. The precast segments were constructed in China and delivered to the bridge site by barge. Precasting allowed quality construction of the segments.
3.3 Navigation spans
There are two navigation channels in the bay, and each of them is bridged with a single inclined tower cable stay bridge. One cable stay bridge is in Chinese waters and the other in Hong Kong waters. The inclination of the towers are deliberately inclined towards each other to indicate amity between the Shenzhen and Hong Kong people (Fig. 7).
The deck of the navigation channel bridges are orthotropic steel box girders. Almost the full length of the main was fabricated off-site, brought to site by barge and lifted by strand jacks into place.
3.4 Ship impact protection
The foundations of the towers are large diameter bored piles. Piled dolphins are used for protecting the towers (Fig. 7).
Figure 5. Alignment.
Figure 6. Figure 7.
4 INCHEON BRIDGE KOREA 4.1 Location and general features
Incheon Bridge is a 12.3 km long sea crossing in South Korea. It connects the new Incheon Inter- national Airport on Yeongjong island to Songdo (New City). The majority of the length of the bridge is constructed as low level viaduct structures with pretensioned precast 50 m long concrete box girder spans. Where the alignment rises to cross the navigation channel, precast segmental balanced cantilever approach bridges with 145 m spans link the viaducts to the cable stayed bridge which provides the 800 m long navigation span (Fig. 8).
4.2 Low level marine viaducts
The low level viaducts consist of 50 m spans and 250 m long five span bridge units. The soffit of the bridge is typically 4.5 m above H.H.W.L. and the substructure generally consists of pile bents with pile caps only adopted in deeper water. The 50 m spans are pre-tensioned and precast in a single pour in the contractor’s specially constructed casting yard. The spans are then erected using the Full Span Launching Method (FSLM). Since much of the viaduct is in shallow water and tidal flats which are inaccessible by floating cranes a self launching overhead gantry system was used to erect the deck (Fig. 9). However, the end of the viaduct is in deeper water and so each 1350 t precast span is lifted by floating crane onto multi-wheel transporter units which then deliver the span to the erection front.
4.3 Navigation span bridge
The cable stayed bridge is a 1480 m long structure with an 800 m main span. Two planes of stay cables support a 33.4 m wide orthotropic steel box girder. The pylon is a reinforced concrete hollow section in a diamond configuration which provides torsional stability to the main span and minimises the size of foundation which must be protected from ship impacts.
Figure 8. Incheon bridge. Figure 9. Viaduct and launching gantry.
4.4 Ship impact protection
Ship impact protection is provided in the form of circular sheet piled dolphins filled with crushed rock and tied together with a reinforced concrete cap. The dolphins were designed to provide both deterministic and probabilistic protection, the former being to stop a 100,000 DWT design vessel travelling at 4.5 m/s directly towards the cable stayed bridge pylon and the latter being to reduce the annual collapse frequency to less than 1 in 10,000 when considering a distribution of design vessels heading towards any point on the bridge axis in any direction.
The dolphins work by dissipating energy through various mechanisms; crushing of the ships bow, local deformation of the dolphin, passive resistance of the soil and friction between ship and dolphin. A reliable way to estimate impact dissipation in soil structures is through testing of a physical model in a centrifuge which allows earth pressures to be correctly modelled at a reduced scale. However, due to the time and expense required for centrifugal model testing it is preferred to use the results to calibrate a non-linear finite element analysis which will then allow analysis of different configurations. This method, which had previously been adopted for Stonecutters Bridge (Lee & Peiris 2004), was followed for the design of the Incheon Bridge ship impact protection. 5 HONG KONG ZHUHAI MACAO BRIDGE
5.1 Location and general features
The alignment of the bridge is shown in (Fig. 10). Currently the delta is bridged approximately 50 m upstream of the mouth of the delta and the journey time between Hong Kong and Zhuhai/Macao is about 4 hours by road and 1 hour by fast ferries. The new link will reduce the journey time to approximately 30 minutes and more importantly provide a safe and fast link to Hong Kong International Airport and Hong Kong Shipping Container Terminals. The link with a total length of 42 km will have one of the longest sea-crossing bridges in the world.
Generally the orientation of the alignment has been kept normal to the water flow in order to minimize obstruction to flow of water and horizontal curves have been introduced to provide interesting views of the main bridge as seen by the driver and occupants of the vehicles. The alignment starts from the Boundary Crossing Facilities opposite Zhuhai & Macao, runs in open waters and ends at the Boundary Crossing Facilities at the north-east tip of Hong Kong Airport.
The general arrangement of the link in mainland waters comprises of: 75 m short span viaducts approximately 7 km in length; 110 m long span viaducts approximately 14 km in length; Jiuzhou Navigation Bridge approximately 500 m in length; Jianghai Navigation Bridge approximately 700 m in length; Qing Zhou Navigation Bridge approximately 900 m in length; Approximately 5 km of immersed tunnel with two artificial islands.
The Hong Kong section is approximately 12 km long and generally comprises of viaducts with spans ranging from 70 m–180 m.
5.2 Marine viaducts
A number of long sea and river crossings have recently been constructed in China such as Sutong Bridge, Donghai Bridge, Hangzhou Bridge. In all of these bridges two separate prestressed concrete
Figure 10. HKZMB components.
Figure 11. Figure 12. Figure 13.
box girder decks have been used, each supported by single column piers under each deck. For this bridge, from environmental considerations, it was decided to use a single column piers to support either two separate decks or a single wide deck. The reason for this is to provide the least obstruction to water flow specially as the water flow is not always normal to the alignment of the bridge. To further minimize obstruction to water flow, the piles are going to be buried in the sea-bed. 5.2.1 Short span viaducts
The construction and whole life cost of the structure is dependent upon the cost of site establishment and preliminaries such as fabrication and assembly yards, transportation, availability and cost of large floating cranes, launching gantries, maintenance costs etc. The exercise showed that whilst the precast concrete boxes were possibly the cheapest, the single wide composite box with a span of 75 m is the optimum solution (Fig. 11).
5.2.2 Long span viaducts
Cost analyses of concrete, composite and orthotropic steel boxes showed that the single wide orthotropic steel box girder with a span of 110m is the optimum solution taking into consideration quality construction, construction equipment, and construction period. (Fig. 12).
5.2.3 Foundations
The substructure comprises of piles, pile-caps and pier columns with large diameter bored piles, buried pile-caps using precast housing as permanent formwork for the pile-caps and precast hollow pier columns. This construction method limits insitu concrete construction to piles and pile-caps, thus helping in minimising environmental impact and shortening the construction period.
Figure 14. Options to stabilize the internal towers
n a multi-span cable-stayed bridge. Figure 15.
5.3 Navigation channel bridges
There are three navigation channel bridges. Jizhou Bridge near Zhuhai with a cable stayed main span of 260 m is visually the most interesting design with a distinctive central sail tower and composite deck,Figure 13.
6 FORTH REPLACEMENT CROSSING (QUEENSFERRY BRIDGE) 6.1 Location and general features
The Forth Replacement Crossing is currently being built across the Firth of Forth to maintain and improve reliability of a vital transport link in Scotland. The total length of the new bridge, including approach viaducts, is approximately 2.7 km. The cable stayed section will include two 650 m spans to cross the two major navigation channels – the Forth Deep Water channel and the Rosyth Navigation channel. Beamer Rock divides the two channels, and forms the location for the central tower. The cable-stay bridge is a unique 3-tower cable stayed bridge with a pair of 650 m main spans across and overlapping stay cables in the middle of the main spans to stabilize the central tower.
With two main spans required over the navigation channels, a major challenge the design had to address was the stability of the internal tower. As the internal tower is not connected to a stiff back span structure, out of balance live loading on only one of the main spans causes a significant sway of the tower resulting in large deflections of the tower and deck and large bending moments in the tower.
This issue is well known for multi-span cable stayed bridges, and there are a number of config- urations which can be adopted to stabilise the internal tower. The simplest of these is to adopt a very stiff deck, or very stiff towers. Other configurations are shown inFigure 14in the following order: Provide anchor piers; Tie the top of the towers with stabilising horizontal cables; Use sloping stabilizing cables from the top of each internal tower to the junction of the deck with the adjacent towers Use overlapping stay cables.
The option to extend the length of the stay cable fans beyond mid-span, so that they overlap in the central region of each span was investigated in detail to ascertain if this configuration provided a good solution. Parametric studies were carried out to investigate the ability of the overlapping cables to provide the stiffness required. As the length of the overlapping zone is increased, the system becomes stiffer, and the bending moments in the tower and deck reduce. The arrangement adopted in the final scheme is to overlap the stay cables over approximately 25% of the main span. (Fig. 15).
6.2 Ship impact protection
The southern main span crosses the Forth Deepwater Channel, the main access to the upstream ports (Fig. 16). The northern main span crosses the approach into Rosyth port. A quantitative
Figure 16.
Figure 17. Ship impact – workflow.
marine collision risk assessment (Carter et al. 2010), based primarily on Eurocode, was carried out to determine the appropriate design impact forces for the foundations and substructures.
This risk assessment, based primarily on BS EN 1991-1-7 (2006), forms the backbone for determining the actual ship impact forces on the structure (Fig. 17). The navigational conditions in the vicinity of the bridge are complex, with bends in the navigation channels and significant obstructions, not least of which is the existing Forth Rail Bridge as shown in Figure 16. The holistic model of AASHTO (2009) was not adequate to address this and a semi-holistic model was developed following the principles of Eurocode and taking account of specific features of the site. The semi-holistic model considers: vessel aberrancy at any point on the transit paths in the vicinity of the bridge leading to a large number of aberrancy scenarios (defined solely by the point of aberrancy); post-aberrancy behaviour of the vessel in a holistic manner without attempting to explicitly track the path and velocity of the vessel taking into account specific human, mechanical and metocean factors.
7 TEMBURONG LINK BRUNEI 7.1 Location and general features
The new 30 km Cadangan Projek Jambatan Temburong (Temburong Bridge Project) in Brunei will connect the relatively isolated district of Temburong with the more developed Brunei-Muara district (Fig. 18). link will comprise 14,6 km long marine viaducts, two cable stayed bridges across
Figure 18.
navigation channels in Brunei Bay, 12 km of low height short span viaduct across the Temburong peat swamp forest and a small area of mangroves, and approximately 3,6 km road in Brunei-Muara district where 3 lengths of tunnels are required as well as at-grade roads and viaduct ramps to link with the existing road network. The key challenges include very soft ground conditions, shallow