18-12-2012, 05:28 PM
Aerodynamic Challenges of Major Chinese Bridges
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Introduction
Since the implementation of China’s reform and open-door policy in 1978, the country’s economy has
soared for the past three decades with 9% average annual growth rate of Gross Domestic Product. This
has created a great demand for development of transportation infrastructure, in particular the country’s
highway system. During the golden period of highway construction that began in 1988, unprecedented
development of highway bridge construction has been experienced in the country. By the end of 2008,
the total number of highway bridges will increase to 550 000, for a total length of 21 000 km. Both
figures represent an almost fourfold increase since 1988. Among these highway bridges are numerous
long-span bridges, whose construction began with the great success of Shanghai’s Nanpu Bridge (Fig.
1a) completed in 1991, whose cable-stayed span of 423 m was then the third longest in the world. By
the end of 2008, there were 51 long-span bridges with a main span over 400 m in China, including 16
suspension bridges, 28 cable-stayed bridges, and 7 arch bridges. These are listed in Table 1 (Xiang,
Chen & Ge, 2003). Shanghai’s Lupu Bridge (Fig. 1b) with a 550 m main span holds the world record
span length for arch bridges. The main span lengths of cable-supported bridges include the 1088 m
span of the Jiangsu Sutong Bridge (Fig. 1c), the longest cable-stayed bridge span in the world, and the
1650 m span of the Zhejiang Xihoumen Bridge (Fig. 1d), the longest box-girder suspension bridge
span in the world (Ge & Xiang, 2006a).
Aerodynamic Stabilization of Suspension Bridges
The construction of long-span suspension bridges around the world has experienced considerable
development over the past century. Beginning with the 483 m span of the Brooklyn Bridge, built in
1883, the main span length of suspension bridges increased to 1280 m with the construction of the
Golden Gate Bridge in 1937, i.e., by a factor 2.7 over 54 years. For the next 44 years, the incrase
slowed to a factor of 1.1, following completion of the Humber Bridge’s main span of 1 410 m in 1981.
Completion in 1998 of the Akashi Kaikyo Bridge, with a main span of 1 991 m, represents an increase
by a factor of 1.4 over 17 years.
Single Box Girder with Central Stabilizer
The Jiangsu Runyang South Bridge, completed in 2005, is the second longest suspension bridge in
China and the fourth longest in the world. The bridge connects Zhenjiang City and Yangzhou City over
the Yangtze River in the eastern Chinese province of Jiangsu. The main section of the bridge was
designed as a typical three-span suspension bridge with a span arrangement of 510 m + 1490 m + 510
m as shown in Fig. 2. The deck cross section is a traditional closed steel box, 36.3 m wide and 3 m
deep, and carries three 3.75 m wide traffic lanes in each direction with 3.5 m wide shoulders on both
sides for emergencies (Fig. 3). The box girder is equipped with classical barriers and sharp fairings
intended to improve aerodynamic streamlining as well as aesthetic quality (Chen et al., 2002).
Twin Box Girder with Central Slot
The Zhejiang Xihoumen Bridge will become the longest suspension bridge in China and the second
longest in the world behind the Akashi Kaikyo Bridge. This bridge is part of the Zhoushan
Island-Mainland Connection Project linking the two islands of Jintang and Cezi in Zhejiang Province.
It crosses the Xihoumen Strait, one of the most important national deep waterways. The bridge route
was selected at the narrowest point of the Xihoumen Strait, where the distance between Jintang and
Cezi is about 2200 m and where a small island near Cezi, Tiger Island, can be used to support a pylon
for a cable-supported bridge. The other pylon is located on the slope forming the shore of Jintang
Island. The precise location of the pylon foundation on Jintang was the subject of a detailed study, with
consideration of its effect on main span length. A foundation above the water level with main span of
1650 m, a foundation 20 m under water with a main span of 1520 m, and a foundation 35 m under
water with a main span of 1310 m were among the combinations studied. To avoid constructing
deep-water foundations, the Xihoumen Bridge was finally designed as a suspension bridge two
suspended spans and a main span of 1650 m, as shown in Fig. 5 (China Highway Planning and Design
Institute, 2003).
Stabilization for Super Long Spans
As a long-time dream and an engineering challenge, the technology of bridging larger obstacles has
entered into a new era of crossing wider sea straits. Examples of these bridge sites include, for
example, the Messina Strait in Italy, the Qiongzhou Strait in China, the Tsugaru Strait in Japan, and the
Gibraltar Strait linking the European and African Continents. One of the most interesting challenges in
this regard has been to determine the limit of suspension bridge span length. The dominant concerns in
the design of super long span bridges are technological feasibility and aerodynamic considerations.
From the perspective of aerodynamic stabilization for longer span lengths, a typical three-span
suspension bridge with a 5 000 m central span and two 1 600 m side spans is considered to be the
effective limit of span length (Fig. 7).
Aerodynamic Concerns of Cable-Stayed Bridges
Cable-stayed bridges can be traced back to the 18th century. Many early suspension bridges were of
hybrid suspension and cable-stayed construction, such as the Brooklyn Bridge. One of the first modern
cable-stayed bridges was a concrete-decked structure built in 1952 over the Donzere-Mondragon Canal
in France, but it had little influence on later development. The steel-decked Stromsund Bridge,
designed by Franz Dischinger and built in Sweden in 1955 with a main span of 183 m, is therefore
more often cited as the first modern cable-stayed bridge. It took 31 years for the span length of
cable-stayed bridges to increase to 465 m with the construction of the Annacis Bridge in Canada in
1986. In the last decade of the twentieth century, record span length increased rapidly. Notable
examples of record spans are the 1991 Skarnsund Bridge (520 m), the 1993 Yangpu Bridge (602 m),
the 1995 Normandy Bridge (856 m) and the 1999 Tatara Bridge (890 m). With the completion of the
1088 m main span of the Jiangsu Sutong Bridge in 2008, the record increased again by almost 200 m.
Record-Breaking Cable-Stayed Bridges
The cable-stayed bridge has become the most popular type of long-span bridge in China for the past
two decades. In 1993, Shanghai’s Yangpu Bridge with a main span of 602 m became the longest span
cable-stayed bridge in the world for a brief time. Although this record was quickly surpassed by the
Normandy Bridge in 1995 and the Tatara Bridge in 1999, China already has about 30 cable-stayed
bridges with main span longer than 400 m, and is currently constructing three record-breaking span
length cable-stayed bridges, including the recently completed 1088 m Jiangsu Sutong Bridge as well as
the 1018 m Stonecutters Bridge in Hong Kong and the 926 m Hubei Edong Bridge to be completed
respectively in 2009 and 2010 (Ge & Xiang, 2007).
Critical Flutter Speed
There were two great moments in the history of cable-stayed bridges when the record span increased
dramatically. The first was in 1995, with the 254 m leap from the 602 m span of the Yangpu Bridge to
the 856 m span of the Normandy Bridge. The second was in 2008 with the 198 m increase of the
record from the 890 m span of the Tatara Bridge to the 1088 m span of the Sutong Bridge. Is it
possible to make further significant increases of the span length of cable-stayed bridges? Apart from
structural materials and construction technology, among the most important concerns related to this
question are dynamic and aerodynamic characteristics.
In order to study the dynamic characteristics of a cable-stayed bridge, the finite-element method is
generally used to calculate the natural frequencies of an idealized structure. The finite-element
idealization of a cable-stayed bridge is basically created with beam elements for longitudinal girders,
transverse beams and pylon elements, and cable elements that account for the geometric stiffness of
stay cables.
Rain and Wind Induced Vibration of Stay Cables
The most common problem suffered by the long-span cable-stayed bridges listed in Table 9 relates to
the aerodynamics of long stay cables under windy and/or rainy weather conditions. Wind tunnel testing
of prototype cable sections was carried out in dry-wind and rain-wind situations, as for example for the
Sutong Bridge with outer diameters of 139 mm (the most common cables) and 158 mm (the longest
cables). These studies showed that cable vibration is much more severe under the rain-wind condition
than under the dry-wind condition for both cable sections shown in Fig. 13, and the maximum
amplitudes of these two cables exceed the allowable value of length/1700 (Chen, Lin & Sun, 2004).