Concept and requirements of sustainable development in bridge engineering

Abstract

The concept of sustainability is described in this paper using a single sustainable principle, two goals of sustainable development, three dimensions of sustainable engineering, four sustainable requirements and five phases of sustainable construction. Four sustainable requirements and their practice in China are discussed in particular. The safe reliability of bridges is first compared with the events of bridge failure in China and in the rest of the world and followed by structural durability, including the cracking of concrete cable-stayed bridges, deflection of concrete girder bridges and fatigue cracks of orthotropic steel decks. With respect to functional adaptability, lateral wind action on vehicles and its improvement are introduced regarding a sea-crossing bridge located in a typhoon-prone area. The Chinese practice of using two double main span suspension bridges and a twin parallel deck cable-stayed bridge is presented in discussing the final sustainable requirement: capacity extensibility.

Keywords

sustainable engineering / safe reliability / structural durability / functional adaptability / capacity extensibility

Introduction

Ever since John Smeaton of England called himself the first civil engineer in the world in the 18th century, civil engineers have been facing new challenges each century. In the 19th century, civil engineers created methods for the efficient use of structural materials such as steel and concrete to build bridges and structures. During the 20th century, engineers learned that lifetime maintenance costs often exceed original construction costs, prompting engineers to develop new design methods to consider the effective maintenance and repair of bridges as part of the initial design problem. Entering the 21st century, we will be challenged with not only the whole life-cycle design of major bridges but also the minimization of construction impact at the end of the lifetime of a bridge. This challenge is sustainable engineering: this century’s challenge throughout the world, particularly in China with its unprecedented development in transportation systems.

Concept of sustainability

“Sustainability” comes from the Latin word “sustinere”, which means “to bear” or “to endure” and can be translated simply as long-term compatibility. Today, the concept of sustainability might be more meaningfully described by the following conceptual framework, which includes a single sustainable principle, two goals of sustainable development, three dimensions of sustainable engineering, four sustainable requirements and five phases of sustainable construction, shown in Fig. 1.

Single sustainable principle

The sustainable principle originally comes from a general law in ancient forestry that absolutely less timber be taken from a forest than the amount it can reproduce. The precautionary principle of sustainability has been valid in forestry for centuries. The first historical document to address this principle was the forestry regulation of a monastery in Alsace dating back to the year 1144; another example is the well-known treatise “Silvicultura Oeconomica” written in 1713 (http://www.projektwerkstatt.de, 2003). These documents have generally defined the basic sustainable principle as not gaining more products from a field than it can regenerate. This general principle should be followed not only in forestry but also in agriculture, stockbreeding, industry and other fields.

Two goals of sustainable development

Since the end of World War II, however, short-term gains have been the main focus of economic action, which has led to the unlimited consumption of materials and fossil energy with all of the adverse consequences such as great amounts of waste and emissions. Increasing resource consumption and the resulting emissions led to the famous 1972 report to the Club of Rome “Limits of Growth” [1]. In 1987, the “Brundtland Commission” established by the United Nations published their report, “Our Common Future” and defined “Sustainable Development” for the first time as a “Development that meats the needs for the present without compromising the ability of future generations to meet their own needs” [2]. Five years later, the World Business Council on Sustainable Development defined sustainable development at the Earth Summit of the UN in Rio De Janeiro in 1992 as an “economic process that can be maintained long-term in line with the earth’s carrying capacity” [3]. Sustainable development, therefore, is specifically defined as long-term economic growth without compromising the earth’s future capacity, which implies two goals of sustainable development: long-term growth and providing for future generations.

Three dimensions of sustainable engineering

Since then, numerous attempts have been made all over the world to implement sustainability in various activities of daily life. As one of the most important world-wide associations in civil engineering, the International Association for Bridge and Structural Engineering (IABSE) issued two very important governing documents, “IABSE Declaration for Sustainability Development” in 1996 and “Ethical Principles for the Practice of Structural Engineering” in 2002, and published the series “Sustainable Engineering-Putting It into Practice” in Structural Engineering International in 2004 [4].

As these documents indicate, sustainable engineering covers three dimensions: the ecological, economic and social dimensions. Ecological sustainability is not a measurand but a paradigm. To measure the level of ecological sustainability, some indicators are used to reflect the impact of construction activities on the environment, but this measure can be used only partly quantitatively and partly qualitatively. According to the International Organization for Standardization (ISO), the types of impact on ecology can be identified by resource consumption, whose indicators include renewable materials (biotic) and non-renewable materials (abiotic), such as water and land, and emissions produced during production, utilization and removal, which are hazardous to the ecosystem and human health (main topic of building biology) [5]. The purpose of economic sustainability is the minimization of costs, including not only those associated with construction at any given moment but also the entire life span of a building in terms of quality and requirements. Usually, the investor, who finances the project, and later the user have different interests with respect to costs. While the user is interested in low operating costs, the investor expects a high return of investment and therefore low production costs. Economic sustainability could motivate both of them to focus on the optimization of life-cycle costs. Social sustainability indicates the consideration of the present and future states of society and prioritizes spirit over matter, person over object and ethics over technology.

Four sustainable requirements

The realization of the sustainable principle and goals of construction tends to depend strongly on three dimensions of sustainable engineering: the ecological, economic and social dimensions. There is a growing need for the coordination and widespread acceptance of the concepts and fundamentals of these three dimensions. Therefore, it is now necessary to transform this current process of forming opinions and drafting a methodological framework into the preparation of appropriate, precise and manageable requirements. The first sustainable requirement is the safe reliability of a structural system, which should not only apply to the “design for the moment” but the life-cycle design as well. The structural system is normally the most difficult part of a bridge to repair or replace, so it should be designed and constructed to last the entire lifetime of the bridge, which constitutes the second requirement: structural durability. As the third requirement of functional adaptability, structural systems should be designed and constructed to be adaptable to allow for the functional changes or upgrades that almost inevitably occur during their lifetimes. The last but not least sustainable requirement is capacity extensibility, which is related to spanning longer bodies of water and providing wider traffic passageways for bridges due to the ever-growing increase in traffic demand. These four sustainable requirements and their implementation in China are discussed in detail in the following sections.

Five phases of sustainable construction

According to the concept of sustainability, the entire life cycle of a bridge or structure can be divided into five phases, “from cradle to grave”: planning, design, construction, operation/maintenance and removal. An overview of all five phases considering the sustainable principle, the goals of sustainable development, the ecological, economic and socio-cultural effects of construction activities, and sustainable requirements including safe reliability, structural durability, functional adaptability and capacity extensibility is required. This new approach is called “sustainable engineering”; when further reduced to the construction sector, it is called “sustainable construction”. Obviously, sustainable construction is nothing new. Within the context explored in this paper, however, emphasis is placed on how sustainable construction focuses attention not only on the construction phase but also on the entire life spans of structures and to the understanding of engineers’ responsibilities.

Safe reliability

As a sustainable requirement, the safe reliability of bridges can be simply evaluated by monitoring the events of bridge failure under construction and during service. Approximately 60 failures of bridge structures in China and the rest of the world between 2001 and 2008 were studied. The age of the failed bridges ranged from 0 years (during construction) to 68 years in China and 140 years in the rest of the world. The most frequent causes of bridge failures were attributed to construction and maintenance, which constitute 38% and 36% of total bridge failures in China and in the rest of the world, respectively. Other frequent principal causes are overloading and ship collision.

Information collection of bridge failures

The data regarding bridge failures are very important not only to forensic investigation but also to structural design itself. Detailed information about failed bridges or bridges subjected to severe conditions is often not available or complete, mostly due to legal reasons and the fear of ruining the reputation of persons or companies related to the construction of such bridges, which is particularly the case in China. The transportation authorities of the United States, such as the New York Department of Transportation (NYDOT), may have the most effective database in this regard. Despite a dearth of information concerning this important issue, several specific studies have been conducted on bridge failures in the United States, for example, that of Harik et al. for the period of 1951-1988 [6], that of Wardhana1 and Hadipriono for the next 12 years spanning the years 1989-2000 [7], and so on.

This paper follows the above-mentioned studies to investigate and analyze the failures of bridges that have occurred in this century, i.e., the period between 2001 and 2008. The information contained in this paper is collected mainly from websites, such as Wikipedia, Sina News and the home page of Ministry of Communications; from newspapers; from engineering journals and magazines; from personal experience; and through e-mail contacts. Although the information base about bridge failures is far from being complete, the authors believe that the information assembled here is sufficient to draw some useful conclusions, especially in the comparison between China and the rest of the world. Though there is no clear definition of bridge failure in China, in this paper, the term failure refers to two conditions, total collapse or substantial partial collapse of a bridge, in which full or partial replacement may be needed.

Failure occurrences

Collective studies on bridge failures reveal that 26 and 33 bridges of various types failed in the past eight years (2001-2008) in China and the rest of the world, respectively. The figures for China do not include the bridge failures due to the Wenchuan Earthquake on May 12nd, 2008, which resulted in more than 6100 bridge collapses. Out of the total recorded failures in China, 20 cases of bridge collapse were found through the internet, and all bridge failures in the rest of the world were simply recorded from the Wikipedia website.

Table 1 shows the incidence of failure of over 7 bridge types, which range from arch to suspension. Approximately 12% of these bridges in China could not be identified; hence, they are classified as miscellaneous. The components of these bridges are primarily made of masonry, concrete and steel. Most of these bridges found in the rest of the world featured unknown bridge types and materials. It can be seen from Table 1 that the dominant types of failed bridges in China are concrete beam/girder and concrete arch bridges, with 11 (42.3%) and 6 (23.1%) incidences, respectively, which constitute over 65% of the total number of bridge failures.

The types of failures and the phase during which these failures took place are listed in Table 2. The number of failures that occurred during service life (19 occurrences in China and 20 occurrences in the rest of the world) is greater than that during construction (7 and 10 occurrences). This phenomenon is expected for most bridges because the number of bridges currently in service is much greater than that being constructed. Also, the duration of the service life is much longer than that of the construction of bridges. Furthermore, loads applied to bridges increase with time, while efforts to upgrade and maintain bridges remain relatively the same throughout the years. The types of failures classified as partial collapse and total collapse are also presented in Table 2. Between these two failure types, the number of partially collapsed bridges (13 out of 26 occurrences in China and 22 out of 33 occurrences in the rest of the world) is greater than that of totally collapsed bridges.

Causes of failures

The causes of bridge failures are classified into two principal causes, which include both enabling and triggering causes. Enabling causes are related to specific man-made mistakes in design, detailing, construction, maintenance and materials; triggering causes involve external events, for example, overload, collision, flood, strong wind, explosion, fire, landslide, and terrorist acts, as shown in Table 3.

Table 3 reveals that 13 (59%) out of 22 collapses in China and 14 (54%) out of 26 collapses in the rest of the world were attributed to enabling causes, while less than half of collapses (9 cases or 41% in China and 12 cases or 46% in the rest of the world) were due to triggering causes. Both statistics are very different from the results of the dozen-year studies in the United States [7], which show that the majority of bridge collapses (415 cases or 85%) were due to triggering causes.

Detailed information regarding enabling causes shows that the dominant specific causes are construction- and maintenance-related deficiencies, causing 10 out of 13 enabling collapses in China and 12 out of 14 enabling collapses in the rest of the world, respectively. Among the specific causes due to external events, the leading causes of bridge failures are overload, collision, flood and strong wind. The most dominant figures are those related to overload (4 out of 9 cases in China and 3 out of 12 cases in the rest of the world), followed by collision failures (3 out of 9 in China and 2 out of 12 in the rest of the world). The dozen-year studies [7], however, concluded that flood and scour predominantly caused the bridge failures (243 out of 415 triggering causes), and the next highest cause was the overloading (44 occurrences or 9%) of various types of bridges.

Structural durability

The durability of bridge structures is defined as their ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration. A durable concrete or steel structure will retain its original form, quality, and serviceability when exposed to its respective environment. According to current Chinese standards, the durability of bridge structures should be 100 years. Actually, the appearance of serious defects on load-bearing members necessitates bridge reconstruction well under the supposed lifetime. Some bridges are totally rebuilt previous to the half-time of their presumed life span. The most popular problems in durability, cracking of concrete cable-stayed bridges, deflection of concrete girder bridges and fatigue cracking of orthotropic steel decks, are discussed in the following sections.

Cracking of concrete cable-stayed bridges

The construction of cable-stayed bridges began in China in 1975. Since then, more than 250 cable-stayed bridges have been completed, and over 90% of them have been built with prestressed or reinforced concrete girders. According to statistics obtained via the internet, there are approximately 40 long-span concrete cable-stayed bridges with main spans over 200 m, and the longest span length is the 500-m span of the Jinsha Yangtze River Bridge in Hubei Province completed in 2002.

Among long-span concrete cable-stayed bridges, more than 20% of them have shown cracking in their concrete girders, which is one of the most severe problems of structural durability in concrete bridges. Through various investigations, most cracks can be identified in top plates, bottom plates, web plates and diaphragms; these four types of cracks are believed to be typical cracks for concrete cable-stayed bridges. Table 4 shows eight long-span concrete cable-stayed bridges suffering from numerous cracks in their girder plates and diaphragms [8]. The age of the cracked cable-stayed bridges ranges from 11 to 27 years, with an average of 19 years, which is far from the presumed lifetime of 100 years. Taking the Jinan Yellow River Bridge as an example, Table 5 presents details regarding crack distribution in box girder elements and longitudinal bridge spans [9].

The durability problems of concrete cable-stayed bridges can also be grouped into two main causes: enabling causes, which involve design, detailing, material and construction, and triggering causes, which involve overload, thermal load, concrete creep and shrinkage, among others. Despite great investigative efforts, however, it is still very difficult to conclude the general cause of concrete cracking in cable-stayed bridges, and only case studies can reveal specific causes for particular bridges. Based on the case study of the Jinan Yellow River Bridge, longitudinal cracks in the top plate and vertical cracks in the diaphragms mainly resulted from the absence of lateral prestressing and uneven thermal loading between top and bottom plates, while the cracks in the bottom and web plates were attributed to frequent overloading and concrete shrinkage [8].

With the experience gained in studying the above-mentioned bridges, the Chinese bridge community is paying increasing attention to the structural durability of concrete cable-stayed bridges. As an example of a new bridge project, the Ningbo Yongjiang Bridge was originally designed as a concrete cable-stayed bridge with a main span of 468 m and the twin-rib cross section shown in Fig. 2(a). At the evaluation meeting of the preliminary design, the expert evaluation committee did not approve the concrete twin-rib section of bridge deck, which may have some durability problems. Following the instructions of the committee, the design team changed the initial concrete deck to a new deck section, a composite structure with two steel boxes and a prestressed concrete plate, as shown in Fig. 2(b). Of course, cracks on the bottom of the girder and in transverse diaphragms can be completely avoided with steel structures, and deck plates become replaceable members if necessary, through which structural durability is greatly improved [10].

Deflection of concrete girder bridges

Two main types of concrete girder bridges are widely used as long-span bridges in China: prestressed concrete continuous girder bridges and prestressed concrete continuous frame bridges. Both PC girder bridges can span lengths between 150 m and 300 m, and PC continuous frame bridges may have longer bridging capacity. There are approximately 50 PC girder bridges with a main span of over 200 m in China, the longest of which is the secondary navigation channel section of Humen Bridge with a main span of 270 m that was completed in 1997; though a steel and concrete hybrid, Shibanpo Yangtze River Bridge in Chongqing, has a main span of 330 m.

The main problem related to durability in long-span PC girder bridges is excessive deflection at the middle of the main span, which is attributed to the cracking of the PC box girder. Structural deflection results in more cracks in the bottom plate of the PC box girder, and an increase in the number of cracks reduces structural rigidity accordingly to further exacerbate deflection. Table 6 lists nine long-span PC girder bridges, three in China and six in the rest of the world, suffering from excessive deflection in the middle span. The duration of deflection of these PC girder bridges is between 3 and 28 years, with an average of 11 years, which is excessively short compared with the target life cycle of 100 years.

Fatigue cracks of orthotropic steel decks

Orthotropic steel decks are light-weight systems, as compared to concrete deck slabs, and have priority in being used in long-span bridges. Although the technology of orthotropic steel box girders was developed in the 1970s, it was first employed in China in 1997 in a long-span bridge, the Humen Bridge in Guangdong Province, which is a suspension bridge with a main span of 888 m. There are approximately 30 cable-supported bridges with orthotropic steel box girders with a main span over 400 m in China. The longest of these is the Zhoushan Xihoumen Suspension Bridge with a main span of 1650 m, which was opened to traffic in October 2009.

Because the decks are flexible orthotropic steel plates and bear traffic loads directly, actual stresses due to traffic loads are very severe, which result in the main durability problem: various fatigue cracks. The orthotropic steel box of the Humen Bridge is shown in Fig. 3. The total width of the cross section including fairings is 35.6 m, and the depth is 3.0 m in the middle. The top plate of the box girder is 12 mm thick and is stiffened by U-ribs 8 mm thick spaced at intervals of 620 mm, as shown in Fig. 4. After 12 years of service life, the bridge shows so many fatigue cracks in the orthotropic steel box that the whole bridge cannot be properly opened to traffic.

The typical fatigue cracks observed in the orthotropic steel deck can be classified into five patterns, Pattern A to E, described in Fig. 5<FootNote>

(Wu C. Personal communication, 2009)

</FootNote>.

Pattern A is found in the top plate along the welding line connecting the top plate and longitudinal U-rib. The cracks propagate from the bottom to the top of the top plate inside or outside the U-rib. There are three types of initial fatigue cracks. The first one originates from the welding intersection of the diaphragm and longitudinal U-rib shown in Fig. 6(a) and constitutes 75% of Pattern A cracks. The second is from the middle of two adjacent diaphragms in Fig. 6(b) and is the longest crack among the three initial cracks. The third is from a filed connection of the deck plate in Fig. 6(c) and is very difficult to find without removing the wearing surface.

Like Pattern A, Pattern B is in the U-rib plate along the welding line connecting the top plate and longitudinal U-rib, as shown in Fig. 7. Most of the Pattern B cracks are from the filed connection of the U-rib. Pattern C is in the welding connection between the U-rib segments in Fig. 8. Pattern D is in the U-rib at the welding toe connected to the diaphragm, as shown in Fig. 9. Pattern E is in the diaphragm at the welding toe connected to the U-ribs. The cracks propagate from cope holes along the diaphragm horizontally or diagonally or along the welding connection, as shown in Fig. 10.

After a series of on-site and laboratory investigations, the main reason for fatigue cracks was attributed to frequent overloads, including the volume and the weight of vehicles (Wu C. Personal communication, 2009). The daily traffic volume through the deck depicted in Fig. 11 shows that the number of daily vehicles increased from 14928 in 1997 to 62439 in 2008, representing an increase factor of 4.2. The six-lane bridge deck has experienced 155 million vehicles over the past 11 years, and each lane has serviced 25.8 million vehicles on average, which is approximately 13 times the deck’s fatigue life cycle. The distribution of vehicle weight was also measured and is shown in Fig. 12. There are three peaks of vehicle weights in Fig. 12: 50, 150 and 400 kN. The design vehicle load for the bridge is 200 kN, and almost half the volume of vehicles exceeds the designed weight (Fig. 12).

Functional adaptability

Bridges are structures constructed mainly of steel, concrete or a composite of the two that provide a way to overpass obstacles, such as rivers, canals, railways, and highways. The main function of bridge structures is to build a passageway for vehicles, pedestrians or pipelines. With the increase in bridge span and deck elevation to water level, bridge structures become more flexible and sensitive to vibration, and bridge decks become much more susceptible to wind action during operation, which may require the consideration of functional adaptability for vehicles and pedestrians on bridge decks. Lateral wind action on vehicles and accounting for its effects in bridge design are discussed using a typical example, a sea-crossing bridge located in a typhoon-prone area.

Definition of design wind speed

As a major sea-crossing bridge in the Zhoushan Mainland and Islands Linking Project, the 21-km-long Jintang Bridge, the third longest sea-crossing project in China, is located in a typhoon-prone area with a design wind speed of 40.44 m/s, which is defined as the maximum speed at the standard level of 10 m above water level for a weighted average time of 10 min for the return period of 100 years.

To evaluate lateral wind action on vehicles passing through the Jintang Bridge, the basic wind speed for the safe operation of vehicles on the bridge deck should be determined based on several related specifications or references. According to AASHTO and Chinese Specifications for highway bridges, the basic wind speed for vehicle operation on a bridge deck is 25 m/s. The authority of Zhejiang Highway Administration has also established a wind speed limit of 25 m/s for highway operation. However, because the maximum wind speed for current ferry operation in bridge sites is up to Grade 10, or approximately 26.5 m/s, the owner of the Jintang Bridge has ultimately prescribed the basic wind speed and allowable wind speed for vehicle operation on the deck of the Jintang Bridge to be U_b = 27 m/s and [U_a] = 25 m/s, respectively [11].

In the atmospheric boundary layer, wind speeds above sea level increase with elevation, which can usually be described by a power-law relation as follows:

(1)U_{\mathrm{H}}=U_{\mathrm{b}}{\left({\displaystyle \frac{h+\mathrm{1.5}}{\mathrm{10}}}\right)}^{\alpha },

where α is the exponent of the power law related to field roughness, and α = 0.16 for the Jintang Bridge; h is the deck level with respect to the water surface, and H = h + 1.5 approximately represents the level of the vehicle center on the bridge deck. Based on Eq. (1), Table 7 lists the wind speeds at the key levels when the basic wind speed is U_b = 27 m/s.

Lateral wind speeds acting on vehicles on a bridge deck vary with the level of the deck and are particularly influenced by the deck cross section. To calculate the lateral wind speed, the equivalent lateral wind speed is defined under the same total wind pressure as follows:

(2)U_{\mathrm{e}\mathrm{q}}=\sqrt{{\displaystyle \frac{\mathrm{1}}{z_{\mathrm{r}}}}{\displaystyle {\int }_{\mathrm{0}}^{z_{\mathrm{r}}}{U}^{\mathrm{2}}(z)dz}},

where z_r is the calculated height; z_r = 4.5 m for trucks, and z_r = 2.0 m small cars; U is the lateral wind speed at the level z. With an equivalent lateral wind speed U_eq on the bridge deck, the lateral wind speed U_H at the vehicle level and the allowable wind speed [U_a], reduced wind speed coefficient β and its allowable value [β_a] can be defined as follows:

(3)\beta ={\displaystyle \frac{U_{\mathrm{e}\mathrm{q}}}{U_{\mathrm{H}}}}\mathrm{a}\mathrm{n}\mathrm{d}[{\beta }_{\mathrm{a}}]={\displaystyle \frac{[U_{\mathrm{a}}]}{U_{\mathrm{H}}}}.

Table 7 also gives the allowable wind speed [U_a] and the allowable reduced wind speed coefficient [β_a]. After determining U_eq, the reduced wind speed coefficient β can be derived. If β>[β_a], some countermeasures to reduce the lateral wind speed should be adopted for the bridge deck. Otherwise, it is safe enough for vehicles to support lateral wind action.

Determination of lateral wind speeds on vehicles

A two-dimensional investigation of lateral wind speeds around the bridge deck was conducted using computational fluid dynamics software, LBFlow based on the Lattice Boltzmman method, and the key numerical results were verified by sectional model wind tunnel tests. The Jintang Bridge is composed of a steel box girder bridge, a cable-stayed bridge with a main span of 620 m, and several concrete box girder bridges with similar sections but different depths, from 3.45 m to 13.45 m. Therefore numerical calculations and wind tunnel tests were carried out for four types of cross sections, including one steel box and three concrete box sections with depths of 3.55, 7.65 and 11.95 m, as shown in Fig. 13 [11].

Table 8 lists the reduced lateral wind speed coefficients for four cross sections with and without railings and/or anti-collision walls on the bridge deck. With respect to the steel box girder, it is necessary to adopt some countermeasures to reduce lateral wind speeds for trucks with z_r = 4.5 m because β>[β_a] = 0.71; however, this is not the case for small cars with z_r = 2.0 m because β<[β_a] = 0.70. For the concrete box girder, some countermeasures should be used along most of the 3.55-m-deep girder and a small part of the 7.65-m-deep girder with z_r = 4.5 m; the rest of the bridge does not require any countermeasures.

Conceptual design of wind barriers

Because the reduced lateral wind speed coefficients for the steel box girder and the concrete box girder with a depth of 3.55 m are greater than the allowable values, wind barriers used as countermeasures should be studied and designed for these two types of cross sections. After comparing more than 10 types of wind barriers for the steel box girders and 20 types for the concrete box girders, three types of steel box girders and six types of concrete box girders were selected for further consideration, whose reduced lateral wind speed coefficients are listed in Table 9.

Among these three types of cross sections of barriers, the ellipse was chosen as the final scheme, as shown in Fig. 14, although the most effective one is the rectangle. Turbulent wind speeds around bridge decks and the mean wind profiles above bridge decks using these wind barriers are shown in Figs. 15 and 16.

Capacity extensibility

The structural engineering challenge of bridging larger obstacles has entered a new era characterized by crossing wide rivers and sea straits. One of the most interesting challenges has been identified as bridging capacity, particularly that of cable-supported bridges such as cable-stayed bridges and suspension bridges, which are two bridge types with potentially the longest spans. Traditionally, bridging capacity has referred to the longitudinal bridge span records set by the Akashi Kaikyo suspension bridge and the Jiangsu Sutong cable-stayed bridge and the transversal deck width currently used of approximately 40 m. Bridging capacity extensibility involves not only invention in making spans longer or decks wider but also the furthering or developing of traditional concepts to implement improvements in the proper way. The concept of a continuous multiple main-span scheme for crossing longer bodies of water is innovative, as is the idea of using several parallel bridge deck configurations to provide wider passageways.

Double main-span suspension bridges

Bridging wider rivers or sea bodies requires longitudinal bridging capacity extensibility to create not only longer single spans but also multiple main spans due to ever-growing navigational requirements and very deep water environments. As is the case for traditional three-span suspension bridges with a single main span of up to 2000 m, it is necessary to develop double or even multiple main-span schemes for suspension bridges to increase longitudinal bridging capacity in this new century. The technical details of the Jiangsu Taizhou Bridge and Anhui Maanshan Bridge across the Yangtze River, both with double main spans of 1080 m, are now discussed in terms of static and dynamic performance.

From a conceptual design point of view, a double main-span suspension bridge is an extensible single main-span structure that features an additional supporting pylon at mid-span to improve static and dynamic structural performance. To make a conceptual comparison between a double main-span suspension bridge and a corresponding single main-span structure, the Anhui Maanshan Bridge has been taken as a typical model of a double main-span suspension bridge, with a span arrangement of (360+ 1080+ 1080+ 360) m and a sag-to-span ratio of the main cable of 1/9, as shown in Fig. 17(a) [12]. The corresponding model used for comparison is a traditional three-span suspension bridge with a span arrangement of (720+ 2160+ 720) m, which has the same sag-to-span ratio of 1/9, as shown in Fig. 17(b). With the same steel box deck, these two bridge models have been investigated using different schemes of the pylon stiffness as follows:

Scheme A-1: single main-span structure with infinite stiffness (IS) pylons;

Scheme A-2: single main-span structure with proper stiffness (PS) pylons;

Scheme B-1: double main-span structure with IS pylons;

Scheme B-2: double main-span structure with PS side pylons and an IS center pylon;

Scheme B-3: double main-span structure with PS pylons.

A finite-element idealization of these five schemes was attempted using finite beam elements for the stiffening girder, pylons, and cable elements by considering the geometric stiffness of the main cables and hangers. Having performed a dynamic finite-element analysis, the first and second natural frequencies of the structures were extracted and compared for these five schemes in Table 10, and the following conclusions were made [13].

1) The first and second frequencies of the double main-span scheme (B-1) are tremendously enhanced compared with the single main-span scheme (A-1) under the condition of infinitely stiff pylons. This is the most important reason why a double main-span suspension bridge exhibits better structural dynamic performance than a single main-span structure.

2) The stiffness of the side pylons has little influence on the first and second frequencies of lateral bending and torsional vibration for the single main-span bridge model and the lateral and vertical bending vibration for the double main-span bridge model, but it does influence the vertical bending and torsional frequencies of the single and double main-span structures to some extent.

3) It should be noted that the central pylon stiffness of the double main-span structure greatly influences the fundamental frequencies, particularly those of the vertical bending modes and torsional modes, and should be carefully determined by considering some other structural characteristics such as static performance.

As concluded in the foregoing comparison, the most important structural characteristic of double main-span bridges is likely the longitudinal bending stiffness of the central pylon, R_p, defined as

(4)R_{\mathrm{p}}={\displaystyle \frac{\left|T_{\mathrm{1}}\cos {\alpha }_{\mathrm{1}}-T_{\mathrm{2}}\cos {\alpha }_{\mathrm{2}}\right|}{{\delta }_{\mathrm{p}}}},

where T₁ and T₂ are the cable forces at the central pylon top, α₁ and α₂ are the cable angles at the central pylon top, and δ_p is the longitudinal displacement of the central pylon top. Under the most unfavorable load conditions, those in which only one main span is loaded, the longitudinal bending stiffness of the central pylon dominantly controls bridge structure performance, including the displacements and stresses of the central pylon and deck as well as the sliding resistance between the main cable and saddle pad. Accordingly, the selection of the longitudinal stiffness of the central pylon should be carefully verified using the following four decisive factors [14].

1) δ_d = Vertical displacement of the mid-span deck.

2) δ_p = Longitudinal displacement of the central pylon top.

3) σ_m = Maximum or minimum working stresses in the central pylon.

4) K_s = Safety factor of sliding resistance between the main cable and saddle pad, which can be defined as

(5)K_{\mathrm{s}}={\displaystyle \frac{\mu \theta }{\ln (T_{\mathrm{1}}/T_{\mathrm{2}})}},

where μ is the friction factor between the main cable and saddle pad, μ = 0.2 based on various experiments, and θ is the angle of saddle arc. According to the current design code for highway suspension bridges in China, the value of K_s should be greater than 2.

Among the four factors considered in longitudinal stiffness selection, two of them, δ_d and δ_p, will be at an advantage with a large value for the longitudinal bending stiffness of the central pylon, whereas the other two factors, σ_m and K_s, will be at a disadvantage. Therefore, these four decisive factors should be carefully compared under various conditions.

With regard to the longitudinal bending stiffness, the longitudinal shapes of central pylons can be broadly divided into two types: A-shaped central pylons with the greatest stiffness, described in Fig. 18(a), and I-shaped central pylons with the smallest stiffness, described in Fig. 18(b). The advantages and disadvantages of both A-shaped steel pylons and I-shaped concrete pylons were combined into the optimal shape for a central pylon, which is the inverse Y-shaped steel pylon shown in Fig. 18(c), in the Jiangsu Taizhou Bridge shown in Fig. 19 [15].

Considering the knowledge gained from studying the Jiangsu Taizhou Bridge, if we can make a double main-span suspension bridge with an A-shaped central pylon more flexible or that with an I-shaped central pylon more rigid, the same objective can also be achieved without adopting an inverse Y-shaped central pylon. Because the I-shaped central pylon has the smallest longitudinal stiffness among the three pylon types, a very large cross section was chosen for the Anhui Maanshan Bridge, and a fixed connection between the central pylon and the steel box deck was made, though the connections between decks and pylons are typically hinged. To compare the stiffness of two structures with fixed and hinged connections between the deck and central pylon, a dynamic finite-element analysis was carried out, and the first and second natural frequencies of the structures were identified and compared for these two connections with respect to the Jiangsu Taizhou Bridge; the results are shown in Table 11, from which following conclusions are drawn [10].

1) The first and second vertical bending frequencies of a bridge with a fixed connection between the deck and central pylon are greater than those with a hinged connection between the deck and pylon; they are also greater than those of the Jiangsu Taizhou Bridge.

2) The first lateral bending frequency of a bridge with a fixed connection is greater than that with a hinged connection and that of the Jiangsu Taizhou Bridge; the second frequency retains the same value in the three structures.

3) The first and second torsional vibration frequencies exhibit no difference between bridges with fixed and hinged connections and are only slightly lower compared to those of the Jiangsu Taizhou Bridge.

Twin parallel deck cable-stayed bridges

The engineering practice of constructing long-span cable-supported bridges has given rise to bridge deck widths that are up to 40 m long to support six-lane traffic. With ever-growing traffic demands, it is at times necessary to build a new bridge adjacent to an existing one or to build two parallel bridges from the onset to provide a wider bridge deck for more traffic lanes, i.e., eight or ten lanes, which results in another extension of bridging capacity: twin or multiple, parallel deck bridges. The most challenging type of twin parallel deck bridge to construct is a cable-supported bridge, specifically a cable-stayed bridge. The Ningbo Yongjiang Bridge in China, which features a twin parallel deck spanning 468 m, is introduced as an example of such a bridge with regard to the conceptual design of its pylon type and deck section.

The Ningbo Yongjiang Bridge was designed as a cable-stayed bridge with the span arrangement (63+ 132+ 468+ 132+ 63) m, as shown in Fig. 20, due to navigational requirements and the twin parallel decks that are approximately 2 × 24 m wide that were prescribed by a feasibility study [16]. Aiming at low construction costs in China and high structural damping for dynamic performance, both the pylons and the bridge deck were proposed to be made of reinforced and prestressed concrete. During the preliminary design stage, a further comparison of pylon types and deck sections should be made based on structural performance, particularly with respect to dynamic and aerodynamic characteristics. Two types of pylons, including the twin diamond shape shown in Fig. 21(a) and the twin H shape shown in Fig. 21(b), and three types of deck sections, including the closed box shown in Fig. 22(a), the twin separated boxes shown in Fig. 22(b) and the twin side ribs shown in Fig. 2(a), have been numerically compared with respect to their structural dynamic characteristics. The fundamental frequency values of six combinations are listed in Table 12 [17].

A comparison of the six alternatives shows that the fundamental frequencies of the vertical vibrations are almost the same for the different types of deck sections and pylon shapes. Although the fundamental lateral frequencies of the twin-diamond-shaped pylons are approximately 13% lower than those of the twin-H-shaped pylons, a comparison of the two pylon types shows that the fundamental torsional frequencies of the twin-diamond-shaped pylons are approximately 2% to 8% higher than those of the twin-H-shaped pylons. The twin-diamond-shaped pylons were selected for further design by considering the simpler force resistance in the pylons, especially for the central column. Among the three types of bridge deck sections, the fundamental lateral frequency of the twin side rib cross section is approximately 13% and 23% higher than that of the twin separated box and the closed box cross section, respectively, but the fundamental torsional frequency of the twin side rib section is approximately 26% and 36% lower than that of the twin box section and the single box section, respectively.

Conclusions

The concept of sustainability is exemplified by a conceptual framework that features the single sustainable principle (do not take any more products from the field than it can regenerate), two goals of sustainable development (long-term growth and providing for future generations), three dimensions of sustainable engineering (ecological, economic and social sustainability), four sustainable requirements (safe reliability, structural durability, functional adaptability and capacity extensibility) and five phases of sustainable construction (planning, design, construction, operation/maintenance and removal).

Four sustainable requirements and their implementation in China are discussed. The safe reliability of bridges is first introduced by comparing incidences of bridge failure in China and in the rest of the world. The most frequent causes of bridge failures were attributed to construction and maintenance, which constitute 38% of total bridge failures in China and 36% in the rest of the world. Other frequent principal causes are overloading and ship collision. Problems related to the structural durability of large bridges are quite severe in China, particularly the cracking of concrete cable-stayed bridges, deflection of concrete girder bridges and fatigue cracking of orthotropic steel decks. Functional adaptability may be related to the serviceability of bridges, including considerations made for structural vibrations, lateral windy decks for vehicle operation, among others. The main results of the investigation and mitigation of lateral wind action on vehicles are revealed using the Jintang Bridge, a sea-crossing bridge located in a typhoon-prone area, as an example. For the last sustainable requirement, capacity extensibility, lessons and experiences gained from two double main-span suspension bridges and a twin parallel deck cable-stayed bridge in China are presented.

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