1. INTRODUCTION

This paper reports on bridges from past to the future with their artistic and structural implications created by the architect, the engineer or both. New bridge forms that will appear with the utilization of new materials are also discussed.

1.1. Bridge Building

"… It was Michelangelo who is recorded to have said: "A bridge ought to be built as though it were intended to be a cathedral, with the same care and the same materials…" 14 (Watson, Stewart C., p.49)

"Bridge is a structure that is built over a river, road, or railway so that people or vehicles can cross one side to the other" (COLLINS Dictionary, p.89)

"Bridges are architecture, but architecture of a very special kind, unique in its single-mindedness. Ordinarily the art of architectural or landscape design consists in the creation of space, and structure is finally the means to that end. But since the function of a bridge is simply the continuation of a roadway over a void, its structure is both means and end, and its reality lies not in space enclosed, but in structure itself. Since a bridge does not define space, but cuts through it, it is free of all the intricate psychological considerations that must be taken into account when space is molded or enclosed. Thus, paradoxically, a bridge is at once the most tangible and most abstract of architectural problems. As such, it is capable of extraordinary purity, though it may perhaps never achieve the richness and depth of expression that are possible in buildings of more complex human motivation." 8 (Mock, Elizabeth B., p.7)

1.2. History of Structural Art

1.2.1. Iron Age

"… A full understanding of the new material was not immediate. When iron first appeared on the European scene as a likely structural medium for bridges, in eighteenth century England, the impulse was to treat it like stone. When a material is so new that its own individual nature is not yet understood, the usual tendency is to handle it in the same manner as more familiar materials. Some iron chain bridges were built, as early as 1741, but the other early iron spans were all arches, and generally assembled like stone vaults of small panels of cast iron…" 8 (Mock, Elizabeth B., p.41)

"…iron seems to have been welcomed from the beginning as an honorable material, capable of a new and startling beauty of its own, and the transition from stone to metal, from mass to line, was accomplished with a minimum of esthetic fumbling…" 8 (Mock, Elizabeth B., p.41)
1.2.2. Reinforced Concrete Age

"The second period of structural art begins in the 1880s when steel prices dropped and reinforced concrete was developed. Engineers soon began to explore new forms with these materials…" 2 (Billington, David P., p.7)

"Concrete is a building material consisting of gravel or crushed stone, sand, cement and water. Because it is a fluid mass when mixed, it can be cast into forms of any shape desired before hardening. Reinforced concrete is made by casting the concrete over a cage of steel bars. An unreinforced concrete beam will bend downward under its own weight, especially at the center, causing the top to compress together but stretching the bottom apart in tension. Concrete carries compression easily, but under very low tension it cracks. However, if reinforcing steel bars are embedded along its bottom inside, the bars can carry tension while the concrete in the top part of the beam can carry the compression."4 (Billington, David P., p.9)

"Joseph Monier, a French gardener, was one of he first to think of reinforced concrete. Around 1867 he strengthened concrete tanks and pipes by casting the concrete over a skeleton of iron, … It was a civil engineer, G.A. Wayss of Berlin, who recognized the potential of reinforced concrete as a large-scale building material when he saw some of Monier's work at the Antwerp Exhibition of 1885." 4 (Billington, David P., p.9)

"The use of reinforced concrete become widespread only after 1894, however, and the leading European figure, along with Wayss, was François Hennebique (1842-1921)… In 1902 Paul Christophe, a former member of Hennebique's staff, published Reinforced Concrete and Its Applications, in which he gave a detailed picture of the uses of reinforced concrete ten years after Hennebique's method was patented… In the United States, progress in reinforced concrete building from 1894 to 1904 was summarized at an international engineering congress held in St. Louis in 1904. Sponsored by the American Society of Civil engineers (ASCE), the congress was intended to be a "review of progress during the past decade" in thirty-seven subjects, one of which was "concrete and concrete-steel," i.e., reinforced concrete." 4 (Billington, David P., p.9-10)

"… François Hennebique of France and Robert Maillart of Switzerland who were successful in creating structural shapes eloquent of the unique powers and properties of the wonderful new material." 8 (Mock, Elizabeth B., p.84)

"The leadership of France in reinforced concrete construction has been due preminently to Hennebique to Auguste Perret, the architect, and to Eugene Freyssinet, celebrated for his vaulted hangars of 1924 at Orly, his great arched bridges, and now for his work with prestressed concrete." 8 (Mock, Elizabeth B., p.101)

"The prestressing of concrete consists in artificially creating stresses approximately equal and opposite to those that are produced by dead weight and live load in the completed, functioning structure. In the Freyssinet system this is effected through the pressure exerted by a series of parallel steel wires of high tensile strength that are stretched to the limit of their elasticity and embedded in the concrete, thus creating permanent compression. Such construction uses at least 70% less steel than ordinary reinforced concrete, an economy of financial importance; and it uses 30 to 40% less concrete, an economy of esthetic importance, for it implies unprecedented slenderness." 8 (Mock, Elizabeth B., p.101)

"The prestressing of concrete produces virtually a new material for construction which is able to resist tension forces that concrete could not otherwise sustain, and it has a useful gain in stiffness." 13 (Shirley-Smith, H., p.206)

2. ARCHITECTURE OF BRIDGE DESIGN

2.1. The Nature of Architectural Engineering

"Architectural engineering can be seen as a collection of disciplines related to the technical aspects of building design and construction. The National Society of Architectural Engineers (NSAE) … defines architectural engineering as "the profession in which a knowledge of mathematics and natural sciences, gained by study, experience and practice, is applied with judgment of the development of ways to use, economically and safely, the materials and forces of nature in the engineering design and construction of buildings and heir environmental systems." 15 (Belcher, M.C.)

"Architecture and Engineering are two dissimilar disciplines which must work together due to the vast array of aesthetic and technical needs of a complex modern building. Architectural engineering can be thought of as bridging the gap between the two… The type of person who chooses architecture as a profession tends to be highly creative, curious (in both senses of the word), and inclined to use a thinking style known as "synthesis". The synthetic thinker approaches a problem by immediately proposing a global solution which may be later refined or abandoned in light of emerging information. This character sketch is in sharp contrast with the type of person who is typically attracted to the traditional fields of engineering. The engineer tends to be pragmatic, unimaginative, and rational, usually adopting the thinking style that has been labeled "analytical"." 15 (Belcher, M.C.)

"The engineer approaches a problem by compiling vast quantities of data and proceeds through it in a systematic, linear manner to arrive, eventually, at the single, final, irrefutably "best" solution. The architect's penchant for false starts, backtracking, and arriving at a design through iteration is a source of constant irritation to the traditional engineer… In school, architects are given assignments like "create something" whereas engineers are given assignments like "learn this equation". Architectural engineers, however receive training in both analysis and synthesis."15 (Belcher, M.C.)

2.2. Art of the Structural Engineer

"Everything that is beautiful and noble is the result of reason and calculation. -BAUDELAIRE" 13 (Shirley-Smith, H., p.209)

"The modern bridge engineer has to be an artist and a poet as well as mathematician, scientist, financier and contractor. - D.B.STEINMAN" 13 (Shirley-Smith, H., p.206)

"The disciplines of structural art are efficiency and economy, and its freedom lies in the potential it offers the individual designer for the expression of a personal style motivated by the conscious aesthetic search for engineering elegance" 2 (Billington, David P., p.5)

"The modern world tends to classify towers, stadiums, and even bridges as architecture… Here even the word is a problem because "architect" does come from the Greek word meaning chief technician. But beginning with the Industrial Revolution, structure has become an art form separate from architecture… Sometimes the engineers have worked with architects just as with mechanical or electrical engineers, but the forms have come from structural engineering ideas." 2 (Billington, David P., p.14)

"There are three types of designers who work with forms in space: the engineer, the architect, and the sculptor. In making a form, each designer must consider the three dimensions or criteria (scientific, social, symbolic)… The first, or scientific criterion, essentially comes down to making structures with a minimum of materials and yet with enough resistance to loads and environment so that they will last… The second, or social criterion, comprises mainly analyses of costs as compared to the usefulness of the forms by society… Finally, the third criterion, the symbolic, consists of studies in appearance, along with a consideration of how elegance can be achieved within the constraints set by the scientific and social criteria… For the structural designer the scientific criterion is primary (as is the social criterion for the architect and the symbolic criterion for the sculptor). Yet the structural designer must balance the primary criterion with the other two…" 2 (Billington, David P., p.17)

"The Engineer, inspired by the law of economy and governed by mathematical calculation, puts us in accord with universal law. He achieves harmony. The Architect, by his arrangement of forms, realizes an order which is pure creation of his spirit. [Le Corbusier]" 3 (Billington, David P., p.121)
2.3. Creation and Designing of Bridges

"Since the reality of a bridge lies in its structure, the art of bridge building lies in the recognition and development of the beauty latent in those structural forms that most effectively exploit the strength and special properties of a given material." 8 (Mock, Elizabeth B., p.7)

"We must not assume that the simple application of these rules will in itself lead to beautiful bridges. The designer must still possess imagination, intuition, and a sense for both form and beauty… The rules, however, provide us with a better point of departure and help us with the critical appraisal of our designs, particularly at the model stage, thus making us aware of aesthetic design errors. [Menn]" 5 (Burke, Martin P., p.1255)

3. BRIDGE AESTHETICS AND STRUCTURAL EXPRESSION

3.1. Principles of Bridge Aesthetics

Martin P. Burke identifies and synthesizes the major abstractions appearing in the 1991 book Bridge Aesthetics Around the World by 24 of the world's best-known authorities on bridge-design aesthetics, and he says: "… a number of authors suggested some basic principles that should be observed in the conception and design of every bridge. Chief among these principles are "bridge/place integration," and the employment of a "structurally expressive form"." 5 (Burke, Martin P., p.1252)

3.1.1. Bridge/Place Integration

"We must respect the natural balance and form of our structures in a manner that leads to the least possible disturbance of the landscape. This is especially important if bridges are built near outstanding scenery. [von Olnhausen]" 5 (Burke, Martin P., p.1252)

"Italy, crossed by major and minor roads, railways and canals, is extremely varied and very beautiful. The problem of inserting bridges into this natural environment is of great significance. As a result, aesthetics has always played a role of considerable importance in the design of Italian bridges…[de Miranda]" 5 (Burke, Martin P., p.1252)

"… we recognize the need to integrate a structure into its environment, landscape or cityscape, particularly where the dimensional relationships and scale are concerned. Many mistakes have been made during the past decades by placing massive concrete blocks in the heart of older areas of a city… Sometimes, long-span bridges with deep, heavy beams spoil lovely valley landscapes or towns… [Leonhardt]" 5 (Burke, Martin P., p.1252)

"… The present stage of bridge design may be characterized as a comprehensive scheme-in short, the "the contextual approach." This design philosophy incorporates the spatial, social, and historical meanings of a place, as well as the design theme of a town. [Nakamura and Kubota]" 5 (Burke, Martin P., p.1253)

3.1.2. Structurally Expressive Form

"The most important principle in bridge architecture is to achieve a clean and well-defined anatomical construction, devoid of deception and unnecessary detail, and with a directness of line both pleasing to the eye and responsive to the senses… [Ghaswala]" 5 (Burke, Martin P., p.1253)

"The final principle is that of veracity-the form should follow function. When this principle is obeyed, the style of a structure should be conditioned by the force system and properties of the applied materials. [Glomb]" 5 (Burke, Martin P., p.1253)

"The structure also needs to reveal itself as a pure, clear form and thus impart a feeling of confidence and stability. Here, we must seek simplicity. The form of the basic structure must also correspond to the materials used. Brick masonry and timber each dictate different forms from those for steel or reinforced concrete. [Leonhardt]" 5 (Burke, Martin P., p.1253)

"… bridge forms should not be shaped for their appearance without considering the efficiency with which they will support their loads. Rather, the two primary objectives, function and appearance, are inextricably bound together, and the choice of a particular form for a specific bridge should evolve from a design process that alternately and continuously considers the effect of each decision on both the function of the form and the appearance and the appearance of the form. Ultimately, a form will emerge that-at least in the opinion of the designer- optimizes both aspects of the design. Both function and appearance determine form. A bridge designer who neglects one or the other will be constructing the wrong bridge in the wrong place at the wrong time." 5 (Burke, Martin P., p.1253)

3.2. Robert Maillart's Bridges

"Maillart was a great inventor and technician, but by the very fact that he exploited the technical possibilities to the utmost limit, his bridges above all reach beyond the sphere of merely technical achievement. They are conceived in such a daring and uncompromising spirit that they rise above the purely technical to genuine artistic vision. Maillart had the creative power of an artist, who always conjures up something new through the means of expression of his time and by making use of all the possibilities at his disposal." 1 (Bill, Max, p.31)

"Maillart's fundamental idea was that structure should be liberated from mathematical analysis; but, at the same time it should be disciplined by the results of physical testing and visual observation…" 4 (Billington, David P., p.107)

Salginatobel Bridge

"It may seem that Maillart was largely reacting to the criterion of monumentality and not presenting any criteria of his own… However, Maillart was expressing a view of right relationships, even though not in the traditional language of architecture. We cannot expect every artist to set out his aesthetic theory while working to create a new style. Maillart left as complete a record as we have any right to expect." 4 (Billington, David P., p.121)

"Structural art, coming from the imagination of the engineer, has three basic ideals: efficiency, economy and elegance. For Maillart, efficiency meant the use of as little material concrete and steel) as possible consistent with a large margin of safety. Economy, for Maillart, implied competitive construction costs as well as relative freedom from maintenance… In structural art, elegance is not achieved by giving up the disciplines of minimum materials and competitive costs in favor of some separate search for beauty… Maillart never did such things. Elegance comes from within the disciplines; Maillart played with his forms to refine them and to enliven them without adding to cost or materials…" 3 (Billington, David P., p.116-117)

4. FUTURE TRENDS

4.1. New Materials and Technology

"President D. Roosevelt stated on October 18, 1931, "There can be little doubt that in many ways the story of bridge building is the story of civilization. By it we can readily measure an important part of a people's progress." 11 (Podolny, W., Jr., p.10)

"As we prepare to enter into the 21st Century it is appropriate to contemplate how to bridge technology of the future will evolve. Research is already underway on improving various materials that may have dramatic impact on the bridge industry. Applications of new systems to existing bridge types of bridge structures are being attempted…" 11 (Podolny, W., Jr., p.1)

"Our transportation infrastructure is literally crumbling away at an ever increasing faster pace… Construction materials and structural forms that were established in this century, have for the most part, remained largely unchained. Existing materials have been found inadequate. They are less durable and more difficult to maintain than expected. We are in need of more reliable, lower maintenance solutions. Researchers and engineers around the world are seeking new materials, methods and techniques for life-cycle cost reduction." 12 (Podolny, W., Jr., p.14-1)

4.1.1. High Performance Aluminum Alloys (HPA)

"High performance aluminum alloys can bring to the bridge market significant advantages such as: light weight, high strength, superior corrosion resistance, ready fabrication, recyclability, workability and process capability, and toughness and strength at sub-zero temperatures. The use of high performance aluminum extrusions and forgings can provide virtually maintenance-free components for bridges. Due to its relatively light weight, aluminum bridges provides proportionally less inertial momentum to the destructive energy of earthquakes and, therefore, has the potential to significantly reduce earthquake damage repair costs…. The use of aluminum in bridges is not new. As for as it is known, its first use was for the aluminum deck on the Smithfield Bridge in Pittsburgh in 1933. It has been used in nine bridges in the United States as a major structural component or for the total superstructure…" 12 (Podolny, W., Jr., p.14-5)

"Aluminum bridge deck replacement projects have these goals: speedy completion, minimum life-cycle cost, and long term durability." 12 (Podolny, W., Jr., p.14-5)

4.1.2. High Performance Concretes (HPC)

"High performance concretes HPC) are all too often simply associated with high strength. Although high strength is a desired attribute it is not the only one. Other qualities are improved constructability, improved durability, and improved mechanical properties. Typical concrete strengths currently utilized range from 4 to 6 ksi (27.5 to 41 MPA). Increasing these strength levels by a factor of two or three (103 MPA for example) reduces the volume of concrete required and opens up significant design possibilities…" 12 (Podolny, W., Jr., p.14-5)

4.1.3. Advanced Fiber Reinforced Polymer Composites (FRP)

"Advanced fiber reinforced polymer composites (FRP's) offer the potential of eliminating the problem of excessive dead load of long span bridges as well as the adverse environmental effects resulting in corrosion problems of metals and concrete reinforcement. For suspension bridges , the most promising application of FRP's is for cable stays and the main cables of catenary suspension bridges, and as a replacement of concrete bridge decks if not the total superstructure girder." 12 (Podolny, W., Jr., p.14-6)

"Fiber materials used for FRP's include glass, carbon and Kevlar… The composite material is fabricated from these thin fibers and polymer resins as binders. Ultimate tensile strength range from 160 to 800 ksi (1100 - 5500 MPA). Primary advantages of a FRP composite deck is its lighter weight, corrosion resistance and potential for fabrication in modular units that would allow rapid erection… Several current cross-section configurations are being investigated which would reduce the weight of a conventional concrete deck by approximately 70 to 80 percent." 12 (Podolny, W., Jr., p.14-6)

"Cables are one of the main components to inhibit the extension of suspension bridge spans. As spans become progressively longer and dead load increases, the steel cables become longer and heavier. The relationship between center span length and dead load is shown in the figure, for a three-span catenary suspension bridge with a stiffening truss girder. What this indicates is that as the center span length increases the cable weight increases at a faster rate than the dead weight of the suspended structure. Stated another way, as the span increases there is a decreasing percentage capability of the cable to carry live load. Higher strength and lighter cables will be required for future spans exceeding today's technology." 12 (Podolny, W., Jr., p.14-6)

"A promising solution to this problem is the use of carbon fiber composite cables (CFCC). A seven-wire carbon fiber composite cable has been develop in Japan that is similar to conventional prestressing strand. Material strength is 300 ksi (2070 MPA). However the unit weight of this material is approximately one-fifth that of steel. For comparative purposes the cable material required for suspension bridges, of both the catenary and stayed type, of steel and carbon fiber composite cables is shown in the figure… It is apparent that a cable material with a 80 percent reduction in unit weight would have enormous impact in the design and construction of suspension and cable-stay bridges." 12 (Podolny, W., Jr., p.14-7)

4.1.4. High Performance Steels (HPS)

"… New High performance Steels (HPS) will provide enhancements in weldability, toughness, corrosion resistance, ductility, fatigue resistance, fire resistance, formability and strength. Higher strength steels with good weldability are likely to dramatically reduce structure cost. Improved welding electrodes and processes will be, or are being developed. These two developments, along with improved confidence levels, may allow the use of one sided and field welding, which will lead to lower erection time and decrease costs. Improved ductility and toughness of HPS will lead to greater utilization in bridges where seismic design is of concern. Improved corrosion resistance will reduce life-cycle costs and lead to greater utilization of HPS in bridges. However, to maximize the potential and usage of HPS new innovative structural forms will have to be developed… Incorporated with other materials and techniques such as polymers, concrete, prestressing tendons; these new steels will provide bridges with improved aesthetic qualities and reduced cost of construction." 12 (Podolny, W., Jr., p.14-10)

4.2. Bridge Form In The Future

A cable stayed bridge in which the towers are multi-storey buildings with leisure facilities, offices and shops accessed from the main bridge. 7 (Head, P.R., p.29)

"Approximately 70% of the natural light would penetrate the plastic tube and supplementary lightning would be provided… The plastic tube bridge would protect the environment from traffic and traffic from the environment. Roadway surface conditions would be protected from the climate, the roadway would be dry, fog and ice-free at all times, increasing safety. Maintenance would be reduced as snow plowing and deicing chemicals would not be required." 6 (Gordon, S. , Podolny, W., Jr., p.216)

"Another tubular highway concept that has been advanced in recent years is that of the submerged floating tunnel (SFT), or "underwater bridge." The concept is simply one of a tubular superstructure that is sufficiently submerged under water to provide adequate clearance for marine vessels that would pass over the structure. Support for the superstructure would be provided by cables anchored to the bottom of the lake or sea, as this proposal for crossing a Norwegian fjord near Hogsfjord, Norway…" 11 (Podolny, W., Jr., p.7)

"…As compared to a intrusive above water bridge structure it would result in uninterrupted scenic vistas with virtually no impact on the environment… The concept of the submerged floating tunnel-sometimes called Archimedes bridge because of the way it uses buoyancy to carry the load…" 11 (Podolny, W., Jr., p.10)

"To bridge the strait of Gibraltar is an engineering challenge which will require unprecedented span lengths because of the extreme water depths… The objective has been to develop schemes with realistic explorations of known state of the art technology as well as adopting new combinations of known technology." 10 (Ostenfeld, K.H., Peterssen, A., Forsburg, T., p.241)

"In the U.K. … a new structural form called SPACES, has been developed and is being promoted by a team effort of materials suppliers, manufacturers, fabricators, bridge designers and contractors. The system consists of a tubular three dimensional space frame acting compositely with a lightweight deck slab. When necessary, the structure can be post-tensioned. The space frame is enclosed by a participating aerodynamically profiled shell in advanced composite material." 12 (Podolny, W., Jr., T., p.14-13)

Richard Rogers, Hungerford Bridge proposal

Team Luscher, Lake Geneva proposal ... Terry Farrel, Blackfriars Bidge proposal

Antoine Grumbach, Thames Bridge proposal

5. CONCLUSION

"… Overcrowding and environmental problems are driving engineers to look for more durable materials derived from sustainable resources to build our bridges. The structures have to be more flexible to accommodate change of use and must be able to be maintained without disrupting their use. New bridge forms are emerging and the beginning of the 21st Century will be fascinating period of change in bridge technology." 7 (Head, P.R., p.31)

"The utilization of high strength and increased durability will allow us to take new and emerging structural form concepts from the drawing boards into reality." 11 (Podolny, W., Jr., p.31)

"The past and present have been exciting, the future promises to be even more exciting. The Winds of Change are changing technology at an ever-faster pace in the effort to overcome and provide solutions to current problems and obstacles. To change is to advance. Change is inevitable. However, fear of change should not be used as an excuse to inhibit advancement and innovation." 12 (Podolny, W., Jr., p.14-4)

6. BIBLIOGRAPHY

1. Bill, Max; "Robert Maillart: Bridges and Constructions"; Frederick A.Praeger Publishers, NY, 1969

2. Billington, David P.; "The Tower and The Bridge: The New Art of Structural Engineering", Princeton University Press, Princeton, New Jersey, 1985

3. Billington, David P.;" Robert Maillart and The Art of Reinforced Concrete", the Architectural History Foundation, NY, the MIT Press, 1989

4. Billington, David P.;" Robert Maillart's Bridges - The Art of Engineering", Princeton University Press, 1979

5. Burke, Martin P.; "Bridge Aesthetics: World View", Journal of Structural Engineering; (121/8), August 1995, pp.1252-1257

6. Gordon, S., Podolny, W., Jr.; "Future Trends in Suspension Bridges", Proceedings, "Bridges Into the 21st century", Hong Kong, 1995, The Hong Kong Institution of Engineers.

7. Head, P.R.; "Bridge Materials, Construction Technology, Public Use and Financing - The Dynamics for Change in the 21st Century", Proceedings, "Bridges Into the 21st century", Hong Kong, 1995, The Hong Kong Institution of Engineers.

8. Mock, Elizabeth B.; "The Architecture of Bridges", The Museum of Modern Art, NY, 1949

9. Murray, Peter; Stevens, Mary Anne; "Living Bridges: the Inhabited Bridge, Past, Present and Future", Prestel, Munich*NY, 1996

10. Ostenfeld, K.H., Peterssen, A., Forsburg, T.; "Suspension Bridge Development Trends Based on Little Belt, Great Belt, Höga Kusten Bridge and the Gibraltar Crossing", Proceedings, "Bridges Into the 21st century", Hong Kong, 1995, The Hong Kong Institution of Engineers.