Santiago Calatrava's stunning Alamillo Bridge, its 167 meter canted tower almost palpably straining against its cable-stays, opened in 1992 as part of Seville's extensive infrastructure expansion for the 1992 Worlds Fair. For Valencian engineer-architect Calatrava, the full-scale, steel and concrete manifestation of the bridge marked the culmination of an arduous and often frustrating process begun in 1987. The design of the structure was controversial since its conception that year, when Calatrava was invited to design new vehicular and pedestrian passages over the Guadalquivir river, across the island housing the Worlds Fair, and over the Meandro San Jeremino into Seville. Calatrava's comprehensive proposal for two mirror-image, canted-tower, cable-stayed bridges - connected by a viaduct over the desert-like terrain between them - was rejected by Andalucian authorities as too daring. Thinking it unwise to commission two of the unprecedented cable-stayed bridges at once, the officials commissioned a more conservative work for the Guadalquivir crossing, and allowed Calatrava to proceed with the design of the viaduct and the bridge over the San Jeremino. Prior to its completion, the Alamillo Bridge had to survive reports by three consulting engineers who claimed its construction at least imprudent if not hopelessly impractical, the analysis of its performance in several Ph.D. theses, last minute changes to the tower's materials and construction methods, and alleged cost overruns .
Since the industrial revolution, the art of bridge design has undergone radical change. From short-span, masonry arch structures to aerodynamically profiled steel suspension bridges spanning more than 2,000 meters, the rapid evolution of bridges during the past two centuries reflects the drastic improvements in both material technology and analytical tools used in their design and construction. Abraham Darby's pioneering use of cast-iron in his 30 meter Coalbrookdale Bridge of 1799 prophesied changes in the course of bridge design made possible by emerging, high-strength materials. During the 19th century, the unprecedented iron and steel arches of Gustave Eiffel and others demonstrated the ability of these materials to span distances unheard of less than 100 years earlier. Eiffel's Garabit viaduct of 1884, features a 178 meter long iron arch that is still considered one of the most dramatic examples of this type of construction. At the turn of the 20th century, the works of innovators like Eugene Freyssinet in France and Robert Maillart in Switzerland, demonstrated how another new, high-strength material - reinforced concrete - would again transform the art of bridge-design. Maillart's hollow box arch forms, for example, first used for his 38 meter long Zuoz bridge in 1901, introduced the capabilities of this material to generations of civil engineers.
Although the increased strength of reinforced concrete, iron, and steel allowed bridges to span ever longer distances, they also encouraged the creation and refinement of new bridge forms. Steel's ability to resist both tension and compression, for example, allowed long-span bridges to be made for the first time of trusses, and propelled the evolution of the suspension bridge. Thomas Telford's 177 meter long Menai Straights Suspension Bridge of 1825, helped catalyze rapid advances in suspension forms. The 2,220 meter long Humber Bridge in Britain, completed in 1981, demonstrates how far these advances have come. 
In addition to illustrating the continuing advances in steel strength that made its unprecedented length possible, the structure's shape is tribute to the technical ingenuity of its designers. By deciding to resist wind forces with a knife-edge profile rather than massive truss-work, the bridge's designers made it aerodynamically efficient, reduced its weight, and therefore lowered its cost.
In searching to make the Humber bridge as efficient (and economical) as possible, its designers produced a structurally inventive and elegant solution that both advanced bridge design technology and produced a new, streamlined bridge form. This process is similar to that employed by Eiffel, Maillart, Freyssinet, and in fact almost every important recent bridge-design innovator. These similarities of process underscore the unchanging value held by virtually all preeminent post-industrial-revolution bridge designers: that the primary goal of bridge engineering is to solve the problem of span, as economically as possible, in both the technical and fiscal senses of the word. 'Elegant' design has come to be characterized in these terms, and is often understood today to follow from structural brevity and economy.  These sentiments are reflected, for example, in the writing of structural engineer and critic David Billington, who extols "the engineer's ideals: efficiency in materials, economy in construction, and elegance in form." 
In contrast to those who proceed him and many of his contemporaries, Calatrava does not consider efficiency and economy as design ideals, but instead necessary aspects of a design. Also, Calatrava often uses the technological state-of-the-art in his unusual bridge designs, but he is not primarily concerned with trying to advance it or to derive new forms from it. Although Calatrava's use of material and construction technologies are often efficient and economical, the elegance of his bridges derives from a broader set of concerns, which are rooted in his unusual design methods and values. The Alamillo bridge both demonstrates Calatrava's design ideals and contrasts them from those of many other bridge designers.
As the canted tower demonstrates, Calatrava emphasizes structural inventiveness over structural brevity; the elevated positioning of the pedestrian walkway and its careful enframent within the cable-stays reflects a concern with creating architectural space that is rare among bridge designers; and his original bridge - viaduct - bridge proposal underscores his belief that bridges can modulate the urban fabric while standing as civic monuments. By treating his bridge commissions as public places, civic icons, and opportunities for structural invention, Calatrava challenges the tenets of contemporary bridge designers while he encourages all of us to reexamine the potential of our infrastructure.
More than any other civic engineering works, bridges derive their formal expression from the idioms of structural typology. Their unadorned, rational forms stand as full-scale structural paradigms developed in response to solving the technical problem of span. Their silhouettes and materials reflect the engineer's understanding of mechanics and construction techniques at the time they were built. John and Washington Roebling's extraordinary Brooklyn Bridge in New York is an elegant example. figure
Except for the towers' Gothic portals, the bridge conveys a direct, pure expression of the principles of structural mechanics and construction methods that make it possible: the bridge's four main steel cables present the classic profile of a catenary suspension system; the towers and anchorages illustrate both the compressive capabilities of stone and the block by block assembly process by which they were erected; the construction of the stiffening trusses, which spread non-uniform loads among several adjacent suspender ropes, is clearly revealed through its connections; the inclined cable-stays reflect the need to restrain the bridge against laterally unbalanced loads.
The Brooklyn Bridge is equally important as an example of the perfunctory treatment of pedestrian access and urban siting that often characterizes contemporary bridge design. Though Roebling's original design provided barely adequate movement on and off the bridge  , subsequent rehabilitation of the bridge's pedestrian paths has made entering Manhattan even more difficult. Currently, there are only two ways to descend from the structure into Manhattan by foot: via a decrepit, subterr anean subway tunnel, or along a concrete traffic island that ends abruptly at Centre Street's busy stream of traffic. This bizarre circulation system transforms the procession from one of New York's famous civic icons to the heart of its downtown into a confusing - if not frightening - experience.
If the Brooklyn Bridge's renovated access-ways symbolize the contemporary bridge designers' indifference to placemaking, the work of the best 20th century engineers demonstrates their relentless search for improved technical prowess. As new structural ma terials, construction methods, and empirical data on bridge performance have emerged, existing bridge systems have been improved, and occasionally, new structural types have been created. Robert Maillart's work demonstrates how a development in material technology led to the creation of a new type of bridge. As one of the first bridge designers to study the properties of reinforced concrete, Maillart developed new "hollow box" arch forms that reflected his desire to exploit the structural properties of this new material and to correct problems he had observed with existing reinforced concrete bridges modelled after their masonry precursors.  
Maillart's bridges were considered radical at the time they were designed, and were sometimes criticized harshly by pre-eminent engineers of the day. Despite the criticism of his peers, his bridges are for the most part performing adequately to this day, and are now considered particularly didactic examples of the structural potential of reinforced concrete arches.
Twentieth century developments in construction technology have also led to the creation of new bridge types. To reduce increasing scaffolding costs, and to allow river traffic to continue unimpeded during bridge construction, Brazilian engineer Emilio Baumgart developed a system for forming reinforced concrete bridge-decks that required no false-work.  By extending the structures's roadway-formwork outward from previously completed bridge segments, Baumgart erected the first such " cantilever-construction" bridge, with a main span of 69 meters, over the Rio de Peixe in 1930. 
In building the 62 meter span Lahn Bridge at Balduinstein in Germany in 1950, Ulrich Finsterwalder significantly advanced this technique by post-tensioning newly cast bridge segments. This method allowed Finsterwalder to cantilever large sections of the bridge at once, and, by allowing him to use higher strength concrete, it reduced the amount of material needed to make the structure safe. The cost of the bridge was greatly reduced by the consequent savings in both construction time and materials. The technical and economic advantages of post-tensioning inspired Finsterwalder to execute a number of longer-span structures, including the 114 meter span of his Rhine Bridge at Worms, in Germany of 1952, and his Rhine Bridge at Bendorf, in Germany of 1965, featuring a 208 meter long main span. 
By the 1970's, the methods pioneered by Baumgart and Finsterwalder, commonly called slip-forming, had spread over much of Europe and were being refined by many bridge designers. The Swiss engineer, Christian Menn, for example, erected his Aare River Bridge at Felseau in Switzerland using lip-formed methods for both the structure's piers and roadway.  By segmentally cantilevering the bridge first upward then outward, Menn completed the entire bridge with virtually no ground supported scaffolds.
These segmentally-cast bridges make as technologically radical a break from their conventionally constructed antecedents as Maillart's bridges made in breaking away from masonry arches. Also, the similar forms of Baumgart's, Finsterwalder's and Menn's bridges all illustrate their construction methods, and the structural principles that guided their erection. With their common silhouettes symbolizing their technological advances, these bridges mark the emergence of a new typological bridge form, based on an elegant method of construction.
Although treating their bridge commissions as opportunities for placemaking was not a primary concern for Baumgart, Finsterwalder or Menn, the formal appearance of their structures has certainly played an important role in their design. The silhouettes of many of their bridges, as well as Maillart's works, are striking and frequently admired by both architects and engineers. But the gracefulness of these structures is determined largely by their seemingly simple structural systems and their largely unadorned and unclad concrete detailing. Their bridges are the product of their common search for technically elegant solutions to particular technical problems.
This is not to say that engineers never design without a specific aesthetic agenda. Joerg Schlaich, the talented and innovative German engineer, has pointed out that "if an engineer has a strong aesthetic preference, he can utilize his professional background to find a technical justification for it,  and has employed this credo to refine the finish and texture of some of his work.
In his remarkable cable stiffened, membrane cooling towers at Schmehhaussen, Schlaich wanted the cable net outside the membrane to "give scale to the surface and demonstrate the cable net structure," although this would expose the net to more severe weather than it would face if located on the inside.  Schlaich justified his choice based on wind tunnel tests that showed that the rough surface produced by the cables reduced the concentrated wind forces on the membrane, allowing for smaller diameter, less expensive cables. As the clearly articulated hyperbolic shape of the towers attests, Schlaich used his formal ideas to elaborate and refine the expression of a structural paradigm, instead of using them as tools in the form making process. In this sense Schlaich's towers are kindred spirits to the forms of the segmentally-cast bridge innovators and Maillart's pioneering reinforced concrete arches.
the scintillating essay continues...
Anthony C. Webster is the Director of Building Technologies and an Assistant Professor Columbia University Graduate School of Architecture, Planning and Preservation
."Engineers Rise to the Occasion", Engineering News Record, October 14, 1991, p 35.
.David Billington. The Bridges of Christian Menn. Catalog to the exhibition held at Princeton University; September, 1978. Page 9.
.David McCullough, The Great Bridge. Simon and Schuster, 1972. pp 543, 544.
.David Billington, Robert Maillart's Bridges: The Art of Engineering. Princeton University Press, 1979. Pages 34-36 and appendix B.
Rhine Bridge at Bendorf, Germany(?). Ulrich Finsterwalder and Dyckerhoff & Widmann, 1965.
.Deborah Gans, et al, eds, Bridging the Gap. Van Nostrand Reinhold, 1991. Page 116.