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(2).
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.
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. click to see figure 40
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 (figure ). 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 t he structural potential of reinforced concrete arches (click to see figure).
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 Ba umgart 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 (figure ). In building the 62 meter span Lahn Bridge at Balduinstein in Germany in 1950, Ulrich Finsterwalder significantly advanced th is 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 s tructure 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, i ncluding 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 (figure ). 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 o f 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 unad orned 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 (figure ), 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 segment ally-cast bridge innovators and Maillart's pioneering reinforced concrete arches.
Although it has often been suggested otherwise , Calatrava's bridge designs are not the direct progeny of those that proceed them. The masterpieces of 20th century engineering are antecedents to rather than progenitors of Calatrava 's work. Technical elegance is of course central to Calatrava's work, but unlike Schlaich, Calatrava does not create his forms by beginning with a technical idea which is then formally refined. And unlike Maillart and Baumgart, he does not search for ne w structural paradigms. In fact, unlike most classic bridges, Calatrava's forms cannot be described in terms of structural typology. Using his training as both architect and engineer, and his skills as a sculptor, Calatrava brings a broad set of concerns to the problem of bridge design and produces works that transcend issues of engineering without disregarding them. His bridges are both mega-sculptures and public places, formally defined by a complex intertwinement of plastic expression and structural revelation.
Calatrava's Bach de Roda Bridge in Barcelona (click to see figure 7) demonstrates this. In elevation, the choice of twin arches supporting the roadway by a set of suspender cables presented Calatrav a with the structural achilles heel of this form: its susceptibility to buckling (click to see figure 8). In plan he was given the problem of resolving two arches that cross the train tracks below at askewed (approximately 60 degree) angle. The buckling problem is often solved by a horizontally oriented truss between the main arches (figure ). This solution applied to the skewed plan would certainly work structurally, but formally it would introduce a competing system, unrelated to the arches. Calatrava's solution to the structural problem was to place secondary arches of equal height next to the main arches on either side of the bridge (click to see figure 6). The secondary arches lean inward from their base, beyond the bridge's edge, and are connected to the main arches by fins near their apex, thus bracing both arches against buckling. This resolution at once gives the bridge a unique three dimensional form, obviates the need to place a bracing truss between the two main arches, and allows the problem of skew to be solved by simply shifting the arches in plan.
The secondary arches also have a purely architectural purpose. Angled suspenders, lying in the plane of these arches, help support a pedestrian walkway at the roadway level (click to see figure 10). The walkway's edge is bowed outward in plan, reflecting the arch's elevation while creating a pedestrian plaza at the center of the bridge. The sloped suspender ropes at the walkway's edge and the main roadway suspenders themselves define the limits of the plaza in three dimensions. The concrete abutments of the secondary arches are flanked by concrete stairways, descending from the pedestrian platform to a park below (click to see figure 13). In this work, Calatrava is not just using his structural prowess and sculptural abilities to generate a provocative form. With the introduction of the pedestrian plazas and circulation system, he transforms, as Kenneth Frampton notes "the mere commission for a bridge into an occasion for creating a place." 
While the bridge's secondary arches solve a central structural problem and help to transform it into an occupiable civic icon, their proportions and detailing are less resolved than the bridge's overall concept. The cross-section of these arches, for example, seems at odds with loads they carry. Via their canted suspender ropes, they are loaded with only half the weight of the pedestrian plaza. The main arches, made of almost the same cross-section, support half of the much heavier roadway, its weightier traffic, and half of the pedestrian walk. The weight carried by the main arches is explicitly portrayed by the massive, exposed roadway girders that they support (via the main suspender rods). The lighter load carried by the canted arches is clearly a lluded to in the knife-edge thinness of the pedestrian plaza's perimeter. While the large cross-section of the secondary arches leaves uncompleted the formal relationship between loads carried and structural bulk begun at the roadway-plaza level, their massiveness gives the upper level of the bridge a slightly heavy, static appearance (click to see figure 8). The sloped arches also terminate abruptly at an arbitrary point a few feet above the plaza level, where they land on attenuated concrete abutments (click to see figure 14). These awkwardly detailed elements are at odds with the general dynamic expression of the bridge, which emphasizes movement in the leap of the main arches across the train tracks, and in the streamlined railing details shooting across the bridge's deck (click to see figure 18).
If the Bach de Roda bridge underscores Calatrava's synthetic use of structure to help solve both formal and spatial problems, his La Devesa pedestrian bridge in Ripoll, completed in 1991, epitomizes his structural inventiveness and his playful exploration s of statics (click to see figure 63). This tilted-arch span connects the town's train station to a new park and housing development on the far side of the Ter river. Glimpsed from a distance, the Ripoll Bridge's structural system seems incomplete, and its stability precarious. Standing on its wood-planked walkway, the bridge's solidly-proportioned superstructure is reassuring, although the source of the bridge's stability remains mysterious (click to see figure 65). In fact, the structure's equilibrium is achieved by a complex amalgam of statical systems (click to see figure 92). Lying in the plane of the canted arch, steel tension arms pick up the walkway loads and transfer them to it. The steel arch in turn delivers these forces to the existing concrete retaining wall and new concrete pylon at either side of the river. Because they do not lie in a vertical plane, the tension arms must resist both horizontal and vertical force components to remain in pure tension (click to see figure 68). (Keeping the tension arms loaded in pure tension ensures that the arch-shape is loaded in its plane, which in turn ensures that it works as an arch-structure instead of as a curved beam). The vertical force component in the tension arms comes from the weight of the walkway; the horizontal component is developed by a truss set in a plane just below the walkway (click to see figure 64). This truss keeps the walkway from translating sideways toward the arch. In doing so, it develops a horizontal force component, which is balanced by the corresponding component in the tension arms.
The bridge's tension arms restrain the arch from moving much out of its plane, which helps prevent it from buckling. They also help the arch resist buckling by dictating the way it deforms under load. As gravity loads tend to deflect the walkway and tension arms as shown in figure 93, the arms rotate the arch into a more upright position, which slightly stiffens it and further protects it against buckling. This principle - increased stiffness under increasing load - is a well known feature of suspension structures, whose cables derive their stability partially from the weight of the bridge they support. Making the same principle work for an arch - whose normal tendency is to become less stabile with increasing load - exemplifies Calatrava's unique structural ingenuity.
Because the weight of the walkway and tension arms are not centered under the arch, they tend to rotate below it as gravity loads pull them downward. This rotation is resisted by a large pipe (click to see figure 68), which acts as a torque-tube, collecting torsion at each strut and delivering it to the concrete pylon and retaining wall at the arch's ends (click to see figure 63). With this move Calatrava exploits an often-neglected property of closed sections (pipes and tubes) loaded in torsion - that they, like columns, are very stiff and do not deform much under load. Using the pipe section as a torque tube, Calatrava builds on experiments he began with the Lucerne Post Office Depot and the Stadelhoffen Railway Station canopies (figure ). The Ripoll Bridge is by far the boldest of the three explorations, because torsional resistance is here a necessary feature of a major spanning element, and because the span itself is so long.
Although Calatrava's use of the pipe is an elegant statement of its torsional capabilities, it was of course not employed as the most technically elegant solution to an unavoidable structural problem. The pipe celebrates a little used property of closed sections, without regard to the bridge's technical economy. Torque-tube action is also just one interesting feature of the bridge's complicated, nebulous structural system. The system eschews a clear explanation of structural behavior in favor of elision, surprise, and invention. What a contrast to the laconic expressions of structural behavior offered by the works of Maillart, Baumgart and Finsterwalder!
The structural system of the La Devesa Bridge is so intricate as to be incomprehensible to laymen, leaving it to be appreciated purely in formal, architectonic terms. In this regard it is also quite a striking work (click to see figure 62). Discussing this bridge, Calatrava says that for him, "making a simple thing is often very difficult." Accepting that Ripoll is, at least formally, a fairly straightforward structure, it shows that Calatrava is indeed able to create a simple bridge that is both beautiful and provocative. The delicate proportioning of the structure illustrates Calatrava's strong sense of form. The carefully balanced relationships among its major components - the smooth steel superstructure, plain wood decking, and plank-formed reinforced concrete pylons - show his sophisticated articulation of systems through material choices and shapes.
But to characterize this bridge as simple, even expressionistically, is to ignore the complex web of relationships among components, and to neglect their simultaneous functions as compositional tools, spatial delineators, and structural subsystems. The seemingly straightforward reinforced concrete pylons, for example, serve three interrelated functions (click to see figures 70) (click to see figure 64). Rising from the park, one of these canted stalagmites serves as an abutment, creating a landing point for the steel arch at the same elevation as on the far bank. The second pylon anchors a flanking concrete stair (similar to the abutment employed less decisively on the Bach de Roda bridge), used to descend from the bridge's walkway to the park below (click to see figure 63). Taken together, the pair of pylons also formally resolve the difficult problem of terminating a symmetrical structure at asymmetrical end conditions.
The most unfortunate feature of this bridge is the awkward concrete appendage that receives the arch and walkway on the station side of the bridge (click to see figure 69). This crude corbel was invented by Calatrava's Spanish structural consultant to accommodate a field change in the location of the twin pylons on the opposite bank. Local construction authorities insisted on moving the pylons a couple of feet back from the edge of the river, making the bridge's span a couple of feet longer. Rather than re-proportioning the arch or pylons to accommodate the new length, Calatrava allowed the problem to be solved by the hastily designed, box shaped cantilever. In accepting this ad-hoc solution, Calatrava exposes an impatience with the vagaries of construction. This is particularly unfortunate in a bridge relying so heavily on harmony among all its pieces.
This lack of follow-through can sometimes have technically deleterious consequences. In an effort to cut costs, Calatrava's pile-supported foundation design for the Bach de Roda Bridge was changed by local authorities to a spread footing scheme underneath the side-span supports (click to see figure 7). To Antonio Carreras, Calatrava's associated engineer for the bridge, this change was responsible for the minor settlement problems evident on porti ons of the bridge today. Though the settlement is slight, it could have been avoided altogether by a greater insistence on the use of piles.
Calatrava followed the design and construction of the Alamillo Bridge more closely (click to see figure 35). This is of course partly because stakes were higher here - the bridge was the subject of an enormous amount of technical scrutiny and features the longest clear span Calatrava has constructed to-date. The result of Calatrava's persistence is a bridge bearing a remarkable resemblance to the model he produced in 1987 (click to see figure 32), featuring technical details that have silenced his critics.
More than most of Calatrava's bridges, the form of the Alamillo bridge is comparable to modern structures designed by conventional methods. Since the roadway is supported periodically along its length by a series of suspender cables, which are in turn ti ed back to a tower, the bridge can be classified as a cable-stayed structure. Louis Wintergast's bridge over the Rhine River near Speyer in Germany, for instance, epitomizes contemporary, asymmetrical cable-stayed forms (figure )< a href="#fn20">. But unlike Wintergast's bridge, how the Alamillo Bridge works is not easily perceived. Lacking the backstays that stabilize the tower of Wintergast's bridge, the Alamillo bridge suggests incipient movement more than static suppo rt. Like Calatrava's bridge in Ripoll, Alamillo seems to be held in static equilibrium by sleight of hand (click to see figure 40). On one hand, without back-stays, why isn't Alamillo's tower bent toward the water by the pull of its roadway stays? On the other hand, considering the somewhat massive profile of its tower, why doesn't the whole structure tip over backward? Answers are provided by the tower's mass itself, which plays the role of the backstays used on most cable-stayed bridges (click to see figure 94). The tower's own weight pulls it downward, and by virtue of its backward cant, counteracts the tendency of the roadway-stays to b end the tower toward the water. The downward and backward pull of the tower is delicately controlled by its tonnage, which is calculated so that it will not pull on the cables with enough force to tip the bridge over backward.
Using the weight of the tower to counter the pull of the cable-stays is an interesting inversion of a principle of Gothic construction, where the weight of vertically projecting pinnacles is used to counter the outward thrust of flying buttresses. Anothe r interesting feature of this seemingly meta-stable form is that the horizontal force that the stay cables put into the roadway deck is equilibrated by the analogous force they put into the tower. These two forces equilibrate each other where they meet a t the base of the tower. The net result is that (unlike most asymmetric cable-stayed bridges) the foundation of the Alamillo bridge must resist only vertical loads, and not also horizontal ones. Although Calatrava's structural system draws on both cable -stayed and gothic structural paradigms, it is not simply an amalgam of the two and the resulting bridge does not reflect any clear structural type.
The structural principle of the Alamillo's tower also shows how Calatrava uses structural experiments with small sculptures as inspirations for his bridges. Calatrava often explores "toys and games that can give plastic expression to the principles of s tatics," and goes on to say "exercises such as these are the generators of many of my ideas about formal language which later, as full sized structures, inhabit a real landscape." His 1979 toros sculpture (figure ) shows how this process helped generate the form of the Alamillo Bridge.
The sleek appearance of the Alamillo Bridge contrasts sharply with Wintergast's structure, which emphasizes clearly articulated components and the structural actions of thrust, counter-thrust, compression and support. Calatrava's structural components a re melded together by smooth, doubly curved forms to emphasize the bridge's flowing silhouette rather than clearly delineating their structural purpose. The intersection of the bridge deck, tower and abutment, for example, is made with a smoothly sculpte d, integrally cast moment connection, without changing materials or using discrete pieces (click to see figure 31). The result is a continuous transition zone, which emphasizes the continuity and pl asticity of the bridge without visually explaining the transfer of forces among the components it connects.
Overall, the appearance of Calatrava's bridge is that of a precisely machined form, more reminiscent of Jean Prouve's bent-steel furniture designs (figure ) than most civic engineering structures. The bridge's plastic, finely scul pted image (which is echoed in the guardrail and traffic bollards shown in the presentation drawings and model of the Bach de Roda Bridge) also bears a close resemblance to the manufactured metal fins of Detroit's automobiles or the streamlined designs of Norman Bel Geddes (figure ). But unlike these machined precedents, that evoke the image of motion while concealing the guts of the machines they house, the Alamillo Bridge's form not only houses the structure, but is the structur e itself. In this way, Calatrava avoids the treatment of structure as a hidden armature evident in much postmodern architecture, while he simultaneously provides an alternate to the so called "high-tech" expression of Michael Hopkins, Norman Foster or Re nzo Piano (click to see figure).
The execution of the Alamillo Bridge demonstrates Calatrava's approach to construction. Borrowing the technique pioneered by Baumgart and Finsterwalder, Calatrava proposed building the Alamillo Bridge using a slip-formed, successive cantilever method. This would have allowed the bridge to be erected without scaffolding, by building it simultaneously upward and outward from the base of the pylon. Each new segment of the tower would balance a new segment of roadway; stay cables would connect the two se ctions and balance the loads between them. This scheme was abandoned by the bridge's contractors, who were relatively unfamiliar with slip-formed techniques and had their own ideas about how to build the bridge. Without objection by Calatrava, the contractors used a huge crane to lift sections of the tower into place, and they built the roadway on top of false-work. By allowing this change, Calatrava shows himself more interested in getting his work built than how it is built.< p>
This pragmatic attitude was also applied to the construction of the Bach de Roda Bridge. The bridge's contractor erected both the main and secondary arches using abutment-to-abutment false-work (click to s ee figure 16). The arches were installed in segments, using a crane to place them onto the scaffolding. While this brute-force method of construction worked, it is not technologically refined. The structures of Eugene Freyssinet, a French cont emporary of Maillart's, demonstrate developments in construction methods made by structural engineers more concerned with assembly than Calatrava. Over the course of his career, Freyssinet developed many elegant construction schemes that are still admire d today for their simplicity and economy. To solve the problem of constructing an arch form under similar circumstances as at the Bach de Roda Bridge (over land), Freyssinet developed a scaffold-less method to build his parabolic blimp hangars at Orly (f igure ). Construction of the hangers proceeded by construction machines that crawled up the unfinished edges of the vaults, extending the structure as they went. This system allowed the unfinished vaults to be braced with single struts, rather than with continuous false-work.
Although Calatrava's interest in construction is demonstrated by the erection methods he proposes for many of his projects, he is not obsessed with elegant assembly. Unlike Freyssinet, he is not trying to innovate in the field of construction technology. And unlike Finsterwalder, he does not employ construction techniques as a primary inspiration for formal expression. Instead of trying to maximize technical elegance in assembly, Calatrava is content to use any reasonable construction techniques to pro duce his work; instead of refining a form derived from a particular construction process, Calatrava employs various erection methods (that are often not reflected in the final form of this work) to help realize a particular artistic vision. In this way, Calatrava uses construction technology as a means rather than an end, reflecting his stronger interests in utility, program and expression.
In a proposed bridge over the Thames in London, Calatrava shows one way he would pursue his bridge-design ideals at an even grander scale than at Alamillo (click to see figure 75). His 380 meter lo ng East London River Crossing would open a new major thoroughfare between Thamesmead and Docklands, while standing as a monumental gateway to the entire city (click to see figure 74).
As in almost all of Calatrava's bridges, a sense of movement is immediately conveyed by the bridge's elevation. But unlike Ripoll and Alamillo, the motion suggested here is derived from a few strong, sweeping gestures, instead of the brinksmanlike testin g of statical limits (click to see figure 73). The high elevation of the roadway and low profile of the arch are responses to meeting the clearance requirements of boats below and planes above. The side-span pylons are easily identifiable as a variation of the V-shaped twin pylon form first employed at Ripoll (click to see figure 63). But here the pylon system works a bit differently. On th is bridge, each set of pylons acts with the roadway girder as a huge cantilevered truss, supporting the traffic above them and the gravity reactions of the center-span arch (click to see figure 95). Each truss is anchored in the river at the base of its twin, canted pylons, and on each riverbank under the large, vertical pylons. The weight of the center-span arch is delivered to the trusses as concentrated loads at their tips. Working like a divin g board with a swimmer at its end, each truss delivers this load to the base of the twin pylon, while the vertical pylon beyond pulls downward, acting as a tension strut to resist the tendency of the truss to tip over. The ends of the center-span arch ar e tied together by the roadway girder below it, which prevents the arch's horizontal thrust from being transferred to the pylons. Although the center strut connecting the roadway and the arch's apex is first noticed as a formal marker for the center of t he span, and for the rhythm it establishes with the vertical side-span pylons, it also serves a central structural role. Connected rigidly to the roadway girder, it acts as a beam to resist horizontal movement of the arch and thereby stiffens it against buckling. This element highlights Calatrava's continued search for novel solutions to the problem of arch buckling that he began with the Bach de Roda and Ripoll bridges.
Despite its intriguing structural system, the East London River Crossing's form does not focus attention on how it works. As with Alamillo, one reads the bridge's plastic shapes as parts of a continuous, single object (click to see figure 75). Visually, the bridge appears as a sculptural statement rather than a structural diagram. Calatrava's emphasis on the bridge's overall form rather than its structural details stems from his gestural design me thods. Upon obtaining a commission (or upon entering a competition), Calatrava first visits the site. After this he produces a series of sketches, often rendered in watercolor, representing trial bridge designs. The margins of these sketches are someti mes filled with structural calculations, reflecting Calatrava's dual search for a particular visual gesture and structural viability (figure ). Settling on a form, Calatrava passes copies of the final sketches to his staff archite cts and engineers. The architects develop the profile into presentation drawings while the engineers analyze the design's structural performance. In using this method for the London Crossing design, Calatrava established a strong visual reading of the b ridge before the structural details were fleshed-out. The inevitable result is a bridge emphasizing its form over its structural detail. Calatrava's design method works opposite from that used to produce most major bridges, in which the choice of struc tural system is decided first, and is immediately followed the development of the structure's technical details. This method produces a bridge whose silhouette emerges primarily from the details of its structural design.
Calatrava's presentation models also underscore his emphasis on overall form over structural elaboration. The models are crafted with exceptional precision, yet their monochrome palette and mirrored ground planes make them seem unreal or otherworldly. T hese miniature forms do not reveal structural particulars or materials as much as they evoke an abstracted, aesthetic vision.
Calatrava's highly conceptual models leave much open to imagination, and heighten one's anticipation about the tectonic details of his full-size structures (click to see figure 5). Viewing a model b efore visiting the completed bridge, one wonders about the actual structure's materials and construction as much as one anticipates the experience of observing and moving through it at full-scale (click to s ee figure 6).
Although not discernable from his models, Calatrava develops a clear idea of the materials for his commissions long before construction begins. Calatrava feels that one cannot design a structure without at least postulating what it is made of and why. Even for his most schematic competition entries, he decides what his design will be made of. In his small commissions, material choices often make a strong contribution to the work. Their particular contribution varies widely amo ng projects, depending on the specific materials employed. Calatrava's precast concrete piers supporting the roof of his Kuwait Pavilion at Seville's Expo 92 (figure ), for example, contribute to the expression of the pavilion as much by their impeccably smooth finish as by their specific form. These virtually identical piers echo the smooth, repetitive forms of the wood finger trusses they support. By contrast, the rough texture and idiosyncratic individuality of the plank-for med, cast in place concrete of the Bach de Roda Bridge serves as a foil for the smooth, machined steel superstructure above it.
Calatrava's design for the East London River Crossing shows that specific material choices are less important to him in his larger commissions (click to see figure 76). This is particularly true whe n a change in materials will improve the constructability of the work. Calatrava envisioned the pylons of the East London Crossing in steel, but he would allow them to be built of reinforced concrete if a contractor claimed this would significantly reduc e its cost. Similarly, the Alamillo bridge's tower was designed in cast-in-place concrete (click to see figure 37), but at the contractor's behest, Calatrava allowed it to be built of a concrete-enc ased steel shell instead (click to see figure 36). Except for some minor changes in geometry at the base of the tower, this change did not affect the general form of the bridge. However, its close- up, tactile relationship to pedestrians passing over the bridge was rather drastically altered by this move. The slightly rough, honeycombed surface characteristic of slip-formed concrete has been replaced by smooth, slightly wavy sheets of steel. The boundary between sheets is clearly marked by groove welds that have not been ground flush. Although the result is not objectionable, it is very unlike the original design.
Calatrava's attitude toward the materials used in his large commissions contrasts strongly with some of his contemporaries, most notably Peter Rice. In describing Renzo Piano's De Menil Museum in Houston, Rice, who designed the building's structure and n atural light control systems, reports
We wanted to use ferro cement as the principle element in the design of the light louvers which were a part of the roof (figure ). This decision in turn gradually determined and dictated the design of the building... After we mad e the decision to use ferro-cement, we went to Houston to find out how we would make it...the typical reaction from manufacturers was "Well, we can do it out of pre-cast and it will only be 1/4 of an inch thicker. Isn't that going to be sufficient?" We couldn't find anyone who understood the tactile quality we were seeking from these pieces.
In the end, Rice and Piano found the material they wanted at a small manufacturing plant in England. In this design Rice and Piano used a particular material technology to at least frame, if not determine, the form of the work. By contrast, Calatrava's Alamillo and East London Crossing bridges show that he is willing to sacrifice tactile detail to ensure the realization of his primary gestures - at least in his larger commissions.
As a body of work, one of the most remarkable things about Calatrava's bridges is the diversity of their forms, structural ideas, and materials. This is particularly true in comparison to Freyssinet and Maillart, whose bridges are visually similar and ea sily attributable. Calatrava's bridges do have a lot in common, however. Their similarities are based in their shared concepts, rather than in their literal forms and materials. Calatrava's bridges suggest four common ideas: they all represent a break with the idea of a bridge as a technical-utilitarian artifact; they use structural principles, materials, and construction methods as springboards for formal expression, rather than for creating the monumental expression of a structural paradigm; they are designed as architectonic mega-sculptures, not as simple pathways; and they celebrate the role of bridges as civic icons in the urban landscape. By exploring these ideas in overlapping, complex ways, Calatrava creates structures that defy conventional c lassification.
Calatrava's design ideals both distinguish him from his contemporaries and provide him with a new and iconoclastic conceptual model of what a bridge should be. His ideals also suggest a new direction for the art of bridge design. Calatrava's success in pursuing this new direction is attributable to many sources, but being in the right place at the right time has possibly helped him most of all. His opportunities have depended on the altruism of his Western-European clients and their view that the infra structure should be used to make civic gestures. Eleven of Calatrava's almost thirty bridge projects were first designed as competition entries. Similar competitions are routinely held by communities throughout Western Europe to help them meet their ut ilitarian needs and improve their public spaces. Cost is certainly an issue in these venues, but often the most technically-efficient entry is passed over for a technically-plausible entry that also addresses other urban issues more completely.
Europe's emphasis on the symbolic importance of its infrastructure stands in stark contrast to the U. S., where there is little association between cultural vision and civic engineering. Competitions for bridges, except to help decide the lowest cost str ucture, are virtually unheard-of in America. The commissioning of infrastructure works in the U. S. is driven by a kind of economic rationalism, in which the least expensive solution is considered not only the most meritorious, but often the only accepta ble one. From this puritanical point of view, "design" embraces only the most narrowly defined issues of technology and utility, and the resulting structures are reduced to mere commodities. Predictably stodgy and often ugly, these products fall short o f their potential to convey technical elegance or stimulate visual interest (click to see figure). The American system has effectively kept Calatrava from attempting to build a bridge in the St ates, although he has been asked to submit preliminary designs on at least two occasions. Calatrava's approach has been more welcome in American architectural circles, as demonstrated by his recently receiving the competition-determined commission for th e completion of a portion of the Cathedral of St. John the Divine in New York City.
Besides being helped by the design climate in Europe, Calatrava also benefits by the revolutionary post-war advances in computer technology and related fields. Numerical methods, computational techniques and automated graphics systems quickly provide con temporary engineers with an unprecedented wealth of analytical information about a proposed structure. The structural analyses of turn of the century designers were restricted to closed-form, analytical solutions provided by classical physics and differen tial calculus. Determinant structural types (forms whose forces can be found by applying the laws of static equilibrium alone) were employed in the design of most major civil engineering works, as seen in such disparate structures as Maillart's Salginato bel Bridge and Contamin's Gallerie des Machines (figures  and ). Determinant types were used because the solution of an indeterminant structure's internal forces and reactions was so complicated. In the so metimes critical calculations of deflection, the mathematical difficulties posed by indeterminant structures were often insurmountable. In an epoch when the most advanced numerical aide was a slide rule, using a three-hinge system for the Gallerie des Ma chines and the Salginatobel Bridge arguably made possible the computations required to ensure their safety.
Similarly, analytically described geometries were often the easiest and sometimes the only shapes which could be analyzed easily and constructed full-scale from design drawings. The form of the Salginatobel Bridge, for example, was both inspired and limi ted by the methods of structural analysis available to Maillart. As David Billington points out, in addition to working with determinant systems, Maillart chose forms for which he could justify critical assumptions about the behav ior of his bridges, which made their analysis and design relatively simple. In the pre-computer era, problems of determinance and geometry effectively limited the possible forms of civic engineering commissions to either representations of existing struc tural types, or - as in Maillart's case - the creation of new ones.
Today, the rise of the Finite Element Method of structural analysis, and the Stress Field Theory for simplifying reinforced concrete calculations have made the structural analysis of any form essentially only as difficult as the problem of representing a structure's geometry and loading. Parallel developments in computational and presentation graphics have made non-analytic geometries easy to represent. While many engineers have taken advantage of these developments simply to represent and analyze a str ucture more expediently, Calatrava uses these technologies to develop his complicated geometries and ensure their structural plausibility.
Calatrava has also avoided the loss of structural design skills sometimes associated with the extensive use of computers. As has been often noted by older engineers, the use of computers to the exclusion of "by-hand" analytic methods has resulted in the loss of a qualitative feel for structural behavior. This is an especially ominous development because some of the best engineer's attribute their success to their qualitative grasp of structures. It has been reported that the master engineer Felix Cande la, for example, advocates "the intuitive feeling in the old cathedral master . . . for handling material and forces." Calatrava's clear qualitative grasp of structural behavior is illustrated by his ingenious solutions to the arc h-buckling problems of the Bach de Roda, Ripoll, and East London River Crossing bridges. His feel for structural proportion is the stuff of legend among his engineering staff, who more often than not do not need to modify the size of the components depic ted in his early sketches.
Calatrava's bridge-design practice also flourishes in part because he applies both his broad architectural and engineering training to his commissions. Calatrava's facility with structure, form and program makes him an ideal individual to design civic en gineering works, and he has few similarly trained competitors. Most of today's structural and civil engineers have a narrowly defined, almost exclusively technical education, which limits their ability to tackle many larger design issues. The highly spe cialized engineering education and professional practice are largely responsible for this. A typical engineer's undergraduate training in the U. S., for example, includes five one-semester courses in liberal arts and usually includes no introduction to t he problems of site, program and context. A typical graduate program for a master's degree has no room for courses outside structural theory, technical design and construction techniques. Graduate training in Europe is similar. Engineering careers have also become increasingly specialized, to the point where a single individual may devote his entire career to the application of one material in either civil engineering or building commissions. This system encourages engineers to develop only very limit ed design skills, and has resulted in today's ironic situation in which the professionals entrusted with the creation of a significant portion of our built environment are unprepared to consider its compositional and programmatic aspects.
Calatrava's contributions to the field of bridge engineering suggest consideration of his methods as a new paradigm for civil engineering practice. In a time when design professionals are grappling with the increased complexity of their disciplines by sp ecializing, Calatrava's approach suggests that by focusing on a particular field, a designer with a broad background can paradoxically produce a richer, more compelling body of work than those trained exclusively in that field. A general extension of Ca latrava's methods, however, should of course be approached carefully. Calatrava is a quick learner who has unusual facility in a number of disciplines; most designers do not have the time, or arguably the talent, to become so well-rounded. Few engineers or architects can muster the range of skills needed to work in Calatrava's ouvre. Frenchman Marc Mimram is one of a handful of Calatrava's contemporaries who are trained as both engineer and architect, and can compete with Calatrava without the help of consultants. In 1991, Mimram received 2nd prize in the competition Calatrava won for a bridge over the river Spree in Berlin (figure ).
The ability to treat bridge designs as opportunities for structural invention, while enriching them both formally and programmatically has been successfully pursued more often by architect-engineer teams. James Carpenter, who studied architecture at the Rhode Island School of Design before taking a degree in sculpture, recently accepted a commission to design a bridge over the Mississippi river in St. Paul, Minnesota (figure ). Carpenter is working out the structure's conceptual details with engineer Joerg Schlaich, and will entrust the preparation of its construction documents to Sverdrup Engineers. Working with another engineer (Peter Rice), Richard Rogers took second place in Paris' Gentile Bridge competition over the Seine i n 1987. Rogers' team approach to bridge design follows his proven success in working collaboratively on programmatically more complex commissions. Rogers, who feels that "use of new technologies and sculptural, rather than decorative composition [are] s ome of the most enduring legacies of the Modern Movement," sees technology and spatial design best pursued collaboratively. He says "our office calls together all the relevant specialists, including consulting engineers, the momen t we get a project." This collaborative model is clearly more accessible to most architects and engineers than embodying both sets of skills in one person, and is probably necessary in solving the more complicated problems of larg e architectural commissions.
Cost also provides a basis for questioning the extension of Calatrava's methods to mainstream practice. Some of Calatrava's bridges are relatively inexpensive. The Ripoll Bridge, for example, cost about $700,000, and his design for the East London cros sing is estimated to cost at most 10% more than a standard box-girder bridge. But as often as not, it seems "if you want a bridge by Calatrava, you have to pay for it," as one of his staffers says. Escalated costs are sometimes a n inevitable and understandable byproduct of new and unique structures. On the other hand, many of the twentieth century's best civic engineering achievements have been obtained through competitions whose judges selected winners proposing elegant and ine xpensive structures. Pier Luigi Nervi's exquisite Italian State Monopolies Warehouse project in Tortona Italy (figure ), for example, was selected for construction through a competitive bidding process, as were many of this other projects. Many of Maillart's and Freyssinet's commissions were obtained by similar means. Maillart's Salginatobel Bridge is perhaps his most famous example. While Europe's cultural climate has nurtured Calatrava's career, its he ritage demonstrates that commissions obtained by cost-based competitions need not be banal.
Calatrava has also of course experienced his share of problems in executing his bridges. For example, Dragados y Construcciones SA, the Alamillo bridge's contractor, claimed that the change in the bridge's tower from slip-formed concrete to a concrete fi lled steel shell was not just a construction expedient. Dragados maintained that Calatrava's proposed, slip-formed construction method was unbuildable, and that the cost of the bridge was much more than predicted. The modificatio n in the tower's design was partially responsible for Dragados' claims of escalated costs.
The difficulties Calatrava has experienced in realizing some of his larger projects seem inevitable in the face of the vast amount of knowledge required to practice - let alone innovate in - the art of civil engineering. Joerg Schlaich reflects a commonl y held view when he says "if you want to be a structural engineer who can contribute innovative ideas and doesn't want to be repetitive, then you have a life's work keeping up with the developments in the field." Presumably one's work improves with experience over time. By this test, Calatrava is "too young to be a good engineer. But he can catch up." Schlaich certainly has a point. The complexity of the structural engineering (and architectural) fields is in-part responsible for the collaborative design process being pursued by Rogers and others. Today, because there is so much for an engineer to learn about structures, it is commonly accepted that his technical education is just beginning when he completes his university st udies. As Schlaich points out "At the end of his studies, a structural engineer is happy if he can [design] a beam on three supports in prestressed concrete. If you introduce a cable or a complicated support, he will be lost." I n the face of this complexity, structural innovations are often the result of years of thought and refinement by the brightest professionals, who frequently work almost exclusively with one structural material. Maillart's extraordinary advances in reinfo rced concrete construction bear testimony to this; his new structural forms were developed over the course of years, and were applied primarily to reinforced concrete bridges. Calatrava's most ambitious works also underscore this truth. His most innovat ive construction scheme (for the Alamillo Bridge) was borrowed from techniques developed by Baumgart and Finsterwalder more than 30 years earlier. When carefully scrutinized by those who specialize in building the designs of others, it was rejected as im practical. Also, some of the most advanced structural analyses of Calatrava's bridges (for example the arch buckling studies produced for the Bach de Roda and Ripoll bridges) were performed with computer programs whose details are understood by only a ha ndful of theoreticians.
Viewed more broadly, Schlaich's analysis of Calatrava misses the point. It is based on the assumption that engineering innovation is the goal. But unlike those who proceed him, Calatrava has not tried to specialize in the pursuit of structural innovatio ns in a strict technical sense. He has trained himself more generally, combining his architectural background and artist's sense of form with his engineering skills. Bringing all his training to bear, Calatrava approaches his work from a wide perspectiv e. This perspective gives him the opportunity to advance the field of bridge design in a broader sense than that embodied in the technical progress of his predecessors.
The breadth of Calatrava's interests, of course, prevents him from pushing the structural state of the art, or grappling with programmatically complex commissions on his own. In one sense, this makes Calatrava a specialist while it prevents him from inno vating. But in a larger sense, he becomes both a generalist and an innovator. By addressing the issues of technology, program and formal expression, he becomes a generalist in his domain. By successfully synthesizing these elements in his work, he trans forms utilitarian civil engineering structures into both sculptural statements and important public spaces. In executing this transformation Calatrava becomes an innovator in a design field that is neither architectural nor structural but is richer than either alone. Equally important, he does not forfeit the use of the technical state-of-the-art or forgo the ability to work on architecturally complex commissions with collaborators. But most important is the fact that his bridges exist (click to see figure 69). Their presence challenges us to re-evaluate our infrastructure's civic potential as they provide a basis for questioning the brief of the professionals who design it.
New York: John Pachuta
Spain: Roxanna Matticoli
. Director of Building Technologies / Assistant Professor
Columbia University Graduate School of Architecture, Planning and Preservation
. "Engineers Rise to the Occasion", Engineering News Record, October 14, 1991, p 35.
Severn Bridge. Britain, 1968. (click to see figure).
. For example:
David Billington, "Maillart and the Salginatobel Bridge." Structural Engineering International, April 1991, p 46.
Sigfried Giedion, Space, Time, and Architecture. Harvard University Press, 1949. pp 383-385.
. David Billington. The Bridges of Christian Menn. Catalog to the exhibition held at Princeton University; September, 1978. Page 9.
Brooklyn Bridge. General View. (click to see figure).
. David McCullough, The Great Bridge. Simon and Schuster, 1972. pp 543, 544.
Tavanasa Bridge, Robert Maillart. 1905. (click to see figure).
. David Billington, Robert Maillart's Bridges: The Art of Engineering. Princeton University Press, 1979. Pages 34-36 and appendix B.
. Hans Wittfoht, Building Bridges, Beton Verlag, Dusseldorf, 1984. p 196.
Bridge over the Rio de Peixe. Emilio Baumgart, Brazil, 1930. (click to see figure).
Rhine Bridge at Bendorf, Germany(?). Ulrich Finsterwalder and Dyckerhoff & Widmann, 1965. (click to see figure).
. David Billington. The Bridges of Christian Menn. Catalog to the exhibition held at Princeton University; September, 1978. Pages 7 to 9.
. Deborah Gans, et al, eds, Bridging the Gap. Van Nostrand Reinhold, 1991. Page 116.
Membrane cooling Tower. Schmehhaussen, Germany. Joerg Schlaich. (click to see figure).
. For Example:
Dennis Sharp, "Felix Candella and Santiago Calatrava," World Architecture. Issue 13, 1991, p 35.
"Expressive Engineering", Architectural Review. September 1987, p 51.
Arched Roadway Bridge Showing lateral bracing system between arches. (click to see figure).
. Werner Blaser, ed, Santiago Calatrava: Engineering Architecture. Page 17.
Stadelhoffen Station. Steel and glass canopy over train platform. (click to see figure).
Bridge Over the Rhine River near Speyer, Germany. Louis Wintergast, 1968. (click to see figure).
. Fritz Leonhardt, Bridges - Aesthetics and Design. Deutsche Verlags-Anstalt, Stuttgart, 1984. pp 257 - 278.
. Bridging the Gap, Page 123.
Tower Toros Sculpture. (click to see figure).
Bent Steel Chair Design. Jean Prouve. (click to see figure).
Streamlined Designs. Norman Bel Geddes. (click to see figure).
. Interview by the author with Dr. Calatrava. Zurich, June, 1992.
Blimp Hangar at Orly, Eugene Freyssinet. Construction photo. (click to see figure).
Bridge, Santiago Calatrava. Early Sketch. (Image currently unavailable).
. Interview by the author with Dr. Calatrava. Zurich, June 1992.
Kuwait Pavilion. Precast concrete piers and wood finger trusses.
(click to see figure). (click to see figure).
. Bridging the Gap. Pages 92 - 94.
The De Menil Collection, Renzo Piano and Peter Rice. Ferro cement light louvers. (click to see figure).
Salginatobel Bridge. Robert Maillart. (click to see figure).
Gallery des Machines. Contamin and Dutart. (click to see figure).
. Billington, Robert Maillart's Bridges. p 92.
. Werner Blaser, ed, Santiago Calatrava: Engineering Architecture. Page 11.
Bridge over the River Spree, Berlin. Marc Mimram. Competition entry, 1991. (click to see figure).
Wabasha Street Bridge, James Carpenter with Joerg Schlaich. St Paul MN, 1991.
(click to see figure).
(click to see figure).
. Richard Rogers, Architecture, a Modern View. Thames and Hudson, 1990, p 16.
. Bridging the Gap. p 140.
. Charles Knevitt, "Arched Bridge Could Be Gateway to the Capitol." The London Times, 4 July, 1990.
Turin Exhibition Hall, 1940 - Pier Luigi Nervi. (click to see figure).
. Luigi Nervi, Structures. F. W. Dodge, 1956. pp 67, 80.
. "Engineers Rise to the Occasion", p 35.
. Bridging the Gaps. Pages 162, 163.
. Bridging the Gaps. Page 162.