The first “modern” type of vehicular bridge was built in Hong Kong China in the 1920s. The need for an efficient transportation system to cope with population growth and enable economic development has demanded the construction of more and more bridges since the middle of the 20th century. By 2007, Hong Kong had a total of about 1300 vehicular bridges. Four of these bridges, including the Tsing Ma Bridge, Kap Shui Mun Bridge, Ting Kau Bridge, and the cable-stayed bridge on the Hong Kong- Shenzhen Western Corridor, are considered to be major bridges supported by cables. Currently, the Stonecutters Bridge on Route No. 8 is under construction and is expected to be completed in late 2009. At the same time, the Hong Kong-Zhuhai-Macao Bridge will be in its detailed design stage soon. While efforts have been made by bridge builders to construct these giant structures, the upkeeping of these valuable assets at a high standard and ensuring their continuous functioning and performance during their intended lifespans will be another important task for bridge engineers. Wind and structural health monitoring system (WASHMS) will play a key role in this respect.
Looking back in history, Hong Kong, China began in the middle of the 19th century as a fishing village where one could hardly find any bridges on what Lord Palmerston (the then British Foreign Secretary) had disparagingly described as “a barren island, with hardly a house upon it”. Subsequently, Hong Kong started to develop into an industrial and manufacturing center after the Second World War. Transportation networks were also developed to cope with ever-increasing economic and social activities. More and more bridges have since been built. To transform Hong Kong from an industrial city to an international financial center in the late 1990s, Hong Kong was in need of a world-class international airport and an efficient transportation network to connect the airport with the various parts of the city. A number of major cable supported bridges were built on the airport access routes as a result.
Upon completion of the airport project, the focus of road transport projects was transferred to cross boundary crossings and the improvement of the local strategic road network. The Hong Kong-Shenzhen Western Corridor (HK-SWC) was completed 10 years after the territory’s return to the Mainland. Meanwhile, the Stonecutters Bridge on Route No. 8 is currently under construction and is expected to be completed in late 2009.
Looking to the near future, the mega Hong Kong-Zhuhai-Macao Bridge will soon enter its detailed design phase for target commencement in 2010. Experience gained in constructing the earlier major bridges will definitely contribute to building this mega bridge.
Hong Kong has a steep and hilly terrain. In the old days, most of its inhabitants could only cluster along the northern shore of Hong Kong Island and on the Kowloon Peninsula. Throughout the 19th century, sea transport was the only means of transport in Hong Kong. All movement of goods within or outside the territory had to be done by sea as all warehouses lay along a 5 km stretch of the Island’s northern shore. The first motor car was delivered to Hong Kong in 1915, but it was not until the 1920s that steps were taken to cater for motor vehicles specifically. This was the time when the first “modern” vehicular bridge (Kwong Fuk Bridge) in Tai Po was built (Fig. 1) [ 1]. Even in the immediate post-Second-World-War years, Hong Kong had very few local industrial and economic activities. Apart from railway, the most common types of land transport at that time were rickshaws and sedan chairs. Bridges were usually built using primitive materials like timber, stone, or cast iron.
The situation changed in the 1960s when Hong Kong experienced a sudden surge in population growth due to the influx of immigrants from the Mainland. As industry developed and population began to multiply, Hong Kong was faced with the urgent task of providing homes and infrastructures for its people. An ambitious program was implemented to provide new homes in the new towns of the New Territories for about 1.8 million people in mid 1980s. One of the foremost items in this program was to provide better transport connections among these new towns. As a result, more and more bridges and viaducts were constructed as part of the program. With such a boom in the bridge construction industry and advances in construction technology and material science, bridges were usually designed as reinforced/prestressed concrete structures. Moreover, the precast segmental construction method was introduced to Hong Kong in the 1980s for constructing bridge viaducts over busy roads or above the sea near the shoreline.
 | Fig.1 Kwong Fuk Bridge in Tai Po |
China started to implement its open-door policy in the late 1970s. As a result, industrial activities in Hong Kong were gradually shifted northwards to areas in the Pearl River Delta such as Dongguan. Hong Kong underwent a rapid transition from an industrial and manufacturing center to a service-based economy in the 1980s and then matured to become a financial center in the 1990s.
From the 1980s to the late 1990s, Hong Kong was at its golden age of bridge building during which a number of world class bridges were built. In the late 1980s/early 1990s, Hong Kong decided to move its airport from the city center of Kowloon to Chek Lap Kok on the Lantau Island. The new airport necessitated the construction of a major new strategic highway of approximately 34 km in length between the western part of Hong Kong Island and the airport. This was the first physical connection between the Kowloon Peninsula and Lantau. The key part of the connection is the spectacular 4 km long Lantau Link, comprising the 1377 m span Tsing Ma suspension bridge and the 430 m span Kap Shui Mun cable-stayed bridge. These two bridges carry both road and rail traffic.
Shortly after the commissioning of the Airport Access Links, the Route No. 3 (Country Park Section) was commissioned in 1998 to connect traffic from the north-west New Territories to the Lantau Link. The 1177 m long Ting Kau cable-stayed bridge forms part of this route. It is a unique and visually impressive structure, which continues to attract world-wide and local interest.
The Airport Core Programme was an extremely complex and fast tracked program. To ensure successful implementation of this ambitious program, Hong Kong looked for imported technology including different levels of expatriate personnel from experts down to construction supervisors and skilled laborers. As a result of the technology transfer, Hong Kong has nurtured a number of experts in bridge construction and project management. The quality and standard of the local construction industry have also been improved significantly. The following major bridges were constructed during the golden age of bridge building history in Hong Kong.
 | Fig.2 Elevation of the Tsing Ma Bridge |
Tsing Ma Bridge has a main span of 1377 m and an overall length of 2160 m (Fig. 2). Obviously, a bridge of this span length would be extremely flexible and wind sensitive. The following discussions will focus on this aspect. As the Tsing Ma Bridge will be the only road/rail link to the airport, it is essential that this access be maintained in all but the most severe weather conditions. This “all-weather” capability is achieved by locating the two railway tracks and two protected roadways in the lower deck, where they will be protected by stainless steel cladding. The final design uses a double-deck steel box construction with truss stiffening and non-structural edge fairings (Fig. 3).
 | Fig.3 Hybrid arrangement of stiffening truss and box of the Tsing Ma Bridge |
Learning from the experience of the Tacoma Narrows Bridge collapse in 1940, the deck of the Tsing Ma Bridge was streamlined. Extensive wind tunnel tests confirmed that the adoption of a streamlined deck section with faired edges and central air vent would ensure aerodynamic stability in an extreme wind speed of 95 m/s (one-minute mean). In addition, investigations were undertaken to ensure that the air flow within the lower deck would be at low speeds, as required for highway and railway operation. It has been established by wind tunnel measurements that the wind speed in the sheltered deck will be approximately 40% of the external wind speed. Graduated wind shields are provided on the upper deck adjacent to the bridge towers.
The Kap Shui Mun Bridge has a main span of 430 m and an overall length of 750 m (Fig. 4). Like the Tsing Ma Bridge, it carries a six-lane highway on the upper deck and twin railway tracks plus two sheltered road lanes on the lower. It is supported by two planes of stay cables. The bridge is a record-breaker in its own right, but is often overshadowed by its giant neighbor on the same link.
 | Fig.4 Elevation of the Kap Shui Mun Bridge |
There are a number of unique features on this bridge. It is one of the stiffest structures ever to be incrementally launched, and has perhaps the most heavily-loaded cable stay towers ever built. The middle 387 m of the 430 m central span is a steel/concrete double-composite box section (Fig. 5). The upper and lower concrete decks are cast onto the prefabricated steel webs at the Lantau side of the site, before being floated out and hoisted into position. The side span and the transition elements of the central span are of post-tensioned in situ concrete. The transition elements are jacked into position, through the middle of the towers, by the incremental launching method. This method was used for the first 45 m of the central span coming out from each of the towers, i.e. up to a defining point where water in the Kap Shui Mun Channel would be deep enough to allow the composite units to be floated out.
 | Fig.5 Steel/concrete double-composite box section of the Kap Shui Mun Bridge |
The Ting Kau Bridge has an overall length of 1177 m with two cable-stayed spans measuring 475 and 448 m (Fig. 6). One of the outstanding features of this “landmark bridge” is the three towers, with heights of 172, 200, and 163 m above the Hong Kong Principal Datum, located on the Ting Kau Headland, on the reclaimed land, and on the north-west Tsing Yi shoreline, respectively. In addition to transverse stabilization cables, the 200 m tall central tower is stabilized by a pair of longitudinal stay cables connecting the tower head to the deck sections adjacent to the side towers. According to the bridge designer, the special design of the bridge towers was somewhat analogous to “working and appearing like masts of sailboats”. They may make the road users feel like “entering a ship crossing the Rambler Channel and then leaving it again.”
 | Fig.6 Elevation of the Ting Kau Bridge |
The two separated bridge decks supported by four planes of cables on both sides of the three towers contribute to the slender appearance of the bridge, and are considered to be aerodynamically favorable. Owing to a short design and construction period, a composite structure was adopted for the deck design. The steel/concrete deck comprises a steel grid of two main outer girders with steel cross girders spanning 18.77 m at 4.5 m spacing and a concrete slab on top formed by 4.4 m × 4.6 m precast panels of 230 mm thick and cast in situ joints (Fig. 7).
 | Fig.7 Erection of the deck of Ting Kau Bridge |
In 2001, with increasing economic and social activities between Hong Kong and the Mainland, the three existing Hong Kong-Shenzhen vehicular border crossings became nearly saturated. The governments on both sides recognized the need to construct the fourth crossing to alleviate the traffic congestion at the existing border crossings and to facilitate the flow of people and cargo between the two places. It was decided to place the fourth crossing to link up the North-west New Territories with Shekou of Shenzhen across the Deep Bay, i.e. the Shenzhen Western Corridor (HK-SWC).
 | Fig.8 Elevation of the Shenzhen Western Corridor Bridge |
Deep Bay has a high ecological value because of its extensive low-lying inter-tidal mudflats and mangrove forests. The alignment of the corridor was chosen carefully through the cooperation of governments on both sides of the border. The Hong Kong section of the HK-SWC includes the construction of a cable-stayed bridge with a 159 m tall single concrete inclined tower and a 210 m long steel main span supported by a single plane of cables (Fig. 8). The deck is a single steel box girder 38 m wide and 4 m deep (Fig. 9).
 | Fig.9 Erection of the 38 m wide and 4 m deep steel box girder |
The design and construction of the 1018 m span Stonecutters Bridge represent the present situation of major bridge development in Hong Kong. The bridge is a key element on Route No. 8 leading to the airport in Hong Kong. The route is being built to relieve traffic congestion on the existing airport access Route No. 3 and is expected to be completed in late 2009.
Hong Kong presents a unique maritime setting that makes it one of the world’s remarkable places. The Stonecutters Bridge will be a major landmark in the western area of the harbor and will be visible from many parts of Hong Kong. Hence, the Highways Department was determined to deliver this mega bridge project at the highest aesthetic standards and raise it to an iconic status with symbolic value. The following is a brief account of what have been done with a view to achieving this goal.
For the first time in Hong Kong, the conceptual design of the bridge was obtained through the conduct of an open international design competition. The main objective of the competition was to secure a reference scheme that would make the Stonecutters Bridge stand out among the world’s long span bridges and become a fitting landmark of the harbor and a gateway for the container terminal, thereby underlining and promoting the image of Hong Kong as a vibrant and important center of international trade.
The competition was conducted in two stages, with 27 entries in Stage 1 which were whittled down to 5 in Stage 2. The entries were assessed by two committees, i.e. the Technical Evaluation Committee and Aesthetic Evaluation Committee, each of which comprised international and local experts as judges. The winning design was a cable-stayed bridge with two mono-column pylons each 298 m high and an aerodynamic twin deck. The total length of the bridge is 1596 m with a main span of 1018 m. It has four back spans having lengths of 79.75, 70, 70 and 69.25 m at each side of the main span, respectively. The towers are in concrete up to level+ 175 m and in steel-concrete composite from level+ 175 m to level+ 293 m with the outer skin being stainless steel. The top 5 m are a glass covered steel structure, which acts as an architectural lighting feature and provides storage space for maintenance equipment. The bridge deck at the central span and in the vicinity of towers will be of steel while the side spans will be of concrete. The twin longitudinal deck girders are 14.3 m apart and are connected by cross girders at 18 m and 10 m intervals in the central span and side spans, respectively. The two planes of stay cables take a modified fan arrangement, anchored at the outer edges of the deck also at 18 m spacing in the central span and 10 m spacing in the back spans. Please refer Figs. 10 and 11 for the general arrangement of the bridge.
 | Fig.10 Elevation of the Stonecutters Bridge |
 | Fig.11 Steel deck section of the Stonecutters Bridge |
For a twin deck structure like the Stonecutters Bridge, vortex shedding actions may be amplified as the shed vortices drift across the central air gap and impinge on the downwind girder. One effective way to mitigate vortex induced oscillation is to provide guide vanes at the flow separation point (at the knuckle line) of the soffit in order to guide the air flowing underneath the deck to prevent or diminish rhythmic vortices formed at the upwind knuckle line of the windward deck [ 3]. This method has successfully been adopted in mitigating the vortex induced oscillation of the Storebaelt Bridge. Figure 12 illustrates the concept. Like many other projects, the study of vortex shedding vibration of the Stonecutters Bridge deck employed sectional models of a scale of 1∶80. The test results indicated that the guide vanes had not been effective in mitigating vortex shedding vibration. It was thought that with the low Reynolds no. ( Re) employed in the 1∶80 scale wind tunnel test, the boundary layer growing along the soffit plate becomes rather thick, thus limiting a high flow rate through the vanes, making the vanes ineffective [ 4]. Hence, it was decided to pioneer some high Re tests in order to further investigate the effectiveness of the guide vanes. Sectional model tests using a bigger scale of 1∶20 were then carried out in a bigger wind tunnel with higher wind speeds with a view to raising Re by one order of magnitude. The guide vanes design which had failed in the 1∶80 scale tests proved to be very efficient in the 1∶20 scale tests. The vortex shedding vibration response was completely eliminated. A number of interesting findings were also revealed in determining the steady-state aerodynamic force coefficients under different Re [ 5].
 | Fig.12 Anticipated flow pattern without and with guide vanes |
While great efforts have been made to obtain a high standard and quality design through the conduct of an international design competition, it is equally important to ensure that the bridge will be durable and that the special features built into the design can be long lasting with minimum maintenance effort. The conduct of a proper durability assessment of the design will provide the necessary framework for achieving that goal. Some special measures have been adopted in the Stonecutters Bridge project from a durability point of view, such as:
1) The use of micro-silica to make the concrete of lower tower less permeable, coupled with the use of Grade 304 stainless steel reinforcement for the outermost layer and the ties.
2) The use of duplex grade stainless steel for the steel/concrete composite upper tower structure.
3) The use of a dehumidification system for the inside of steel deck to avoid the use of a sophisticated painting system, which would be costly to apply and maintain.
It is a basic principle in the design that all important parts of the structure should be accessible. Access facilities therefore are designed to meet this principle. Access to all important parts of the structure must be achieved without any disruption to traffic, which given the strategic nature of the Stonecutters Bridge as a major route to the Hong Kong airport and container terminals, is a major consideration. Apart from the routine maintenance access provided in the earlier major bridges such as the rack and pinion lift for maintenance inside the bridge towers and the underslung gantries for maintenance outside the steel deck and stay anchorages, the Stonecutters Bridge will also be equipped with a shuttle train running along the entire length of the deck for personnel and equipment (Fig. 13). The exterior of the towers above deck level is accessible from a cradle suspended by a permanent derrick on the tower top.
 | Fig.13 Shuttle train inside the southern girder of the Stonecutters Bridge |
Bridge health monitoring serves an important role in predicting the structural behavior of long span bridges and helping bridge owners to maintain their valuable assets. The Highways Department has ten years of experience in operating a sophisticated wind and structural health monitoring system for long span cable-supported bridges. WASHMS was first installed on the Tsing Ma Bridge, the Kap Shui Mun Bridge and the Ting Kau Bridge in 1997. With the experience gained in operating this system for a few years, the second generation was evolved and put into practice on the cable-stayed bridge in HK-SWC. Again, with the experience gained in implementing the HK-SWC WASHMS, the Highways Department has designed the third generation WASHMS, which is being installed on the Stonecutters Bridge (Fig. 14). The third generation system addresses the problem encountered in the earlier systems where data retrieval from storage for processing and analysis was inefficient. The system also enhances the correlation and regression analyses to be carried out in different views varying from two-dimensional to multi-dimensional views thus making the structural health monitoring system more user-friendly [ 7].
 | Fig.14 Wind and structural health monitoring system of the Stonecutters Bridge |
The planning, design and construction of the Stonecutters Bridge posed many challenges to bridge engineers. The high quality expectations in a landmark structure justifies the conduct of an international design competition to obtain an elegant design. After that is achieved, the next important mission is to ensure that such a design will be buildable, durable, easy to maintain, and safe to operate. Much effort has been spent in various stages of the project to achieve these goals. With its construction commenced in April 2004, the Stonecutters Bridge is now at the most critical stage of its life where all the planned and designed measures will be implemented to ensure the health of the bridge when it is born in 2009.
The implementation of the new airport and its access route 10 years ago is a milestone in Hong Kong’s major bridge construction history. At the same time, the transportation network of Hong Kong started to take shape. The completion of HK-SWC in 2007 and the full commissioning of Route No. 8 including the Stonecutters Bridge by end of this year will refine the network. To further enhance Hong Kong as a transportation hub, there are a number of mega scale road and railway routes being planned for completion by the middle of the next decade. The majority of these projects, however, are tunnels. As far as bridges are concerned, the focus will be placed on the Hong Kong-Zhuhai-Macao (HKZM) Bridge and its link road to the existing transportation network. The HKZM Bridge comprises a proposed series of bridges and tunnels that would connect the west side of Hong Kong with Macao and the neighboring city of Zhuhai which is situated on the west side of the Pearl River Delta (Fig. 15). The proposed 29 km bridge is comparable with the world’s longest bridge, the Second Lake Pontchartrain Causeway in the United States, which is 38.4 km long. Construction is expected to commence in late 2009 but no later than 2010. The bridge will be completed around 2016.
 | Fig.15 The Hong Kong-Zhuhai-Macao Bridge |
Looking to the farther future, we have devised preliminary conceptual plans to build more strategic routes to link up Lantau Island with the West New Territories. These preliminary plans may opt for long span bridge solutions. However, it might be too early at this stage to even make a prediction on what will happen.
To ensure that Hong Kong will continue to enjoy a sustainable development in the years to come, one important aspect is to upkeep our valuable assets to a high standard. WASHMS will play a key role in this exercise. WASHMS in Hong Kong was born ten years ago. The systems as well as their master bridges therefore are still at their young ages. Up to this point, major structural defects still have not been observed. We will take this opportunity to plan and schedule the work of WASHMS such that we could prepare ourselves for the near future before our valuable stock of elegant bridges gets older and suffers from health problems. While they are young and strong, we can carry out investigation work on the structural performance of the bridges and set up computer bridge models to predict/evaluate the bridge responses during in-service (mainly for fatigue assessment) and extreme events (mainly for stability and damage assessments). Such investigation and development work will provide valuable information and solutions to the bridge operators as the bridges get “older and weaker”. As time goes by, the number of damaged or deteriorated components in old bridges due to degradation will increase. The understanding of the deterioration process will not only improve the efficiency of the corrective/preventive maintenance, but also provide solutions for predictive/condition-based maintenance. There are therefore still a lot of potential for further research and development in WASHMS.
Bridge construction is critical to the development of Hong Kong and its economy. Bridges narrow the gaps between one region and another throughout the territory. Bridges speed up cross-region and cross-border movement of people and goods and improve the overall efficiency of the transport system. During the past years, bridges were built because of better economic conditions and more bridges also bring about better economy to Hong Kong.
The bridge construction techniques in Hong Kong became mature with the completion of several large-scale bridges over the years. Like many other developed countries, the focus of bridge engineering will unavoidably be shifted to the maintenance and operation of the completed bridges. WASHMS plays a key role in this respect.
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