Firth of Forth Railway Bridge

Firth of Forth Railway Bridge

Scotland

Nine miles west of Edinburgh, Scotland, the mouth of the River Forth is spanned by Europe’s first all-steel, long-span bridge. Completed in 1890 it was then the longest bridge in the world. Until 1917 it was also the largest metal cantilever, and at the beginning of the twenty-first century it remains the second largest ever built. It was a major accomplishment of Victorian engineering.

The extension of the railroad along Scotland’s east coast, to complete the direct route between Edinburgh and Aberdeen, was hampered for most of the nineteenth century by two broad inlets of the North Sea: the Firth (mouth) of Tay and the Firth of Forth. The River Forth rises near Aberfoyle and widens into its firth about 50 miles (80 kilometers) from the ocean.

Vessels up to about 300 tons (270 tonnes) could navigate as far as Alloa, about 16 miles (26 kilometers) inland; those up to about 100 tons (91 tonnes) could reach Stirling, a little further on.

After earlier aborted proposals—a tunnel in 1806 and a bridge in 1818—for crossing the firth, little more was attempted for fifty years. In 1865 an act of Parliament sanctioned a bridge across the Queens-ferry Narrows, where the river passes between steep banks at the neck of the firth. Four railroad companies—North British, North Eastern, Midland, and Great Northern—formed a consortium in 1873 and commissioned Thomas Bouch, engineer for North British, to design the bridge. He proposed a suspension structure with twin spans of 1,600 feet (480 meters). The project was delayed for five years because of lack of funds; by spring 1879 only one pier had been started.

When the much-vaunted Tay Railway Bridge, also designed by Bouch and less than two years old, collapsed in a gale on 28 December 1879 with the loss of seventy-five lives, work on the Forth bridge was immediately suspended by another act of Parliament. In January 1881 a British Board of Trade inquiry found that the Tay disaster was caused by inadequate design and poor supervision. Bouch’s Firth of Forth scheme was abandoned. Within months the engineer died, a broken man. The engineers of the Forth consortium’s member railways, Thomas Harrison, William Barlow, John Fowler, and Benjamin Baker, had to develop a new design. In May 1881 Fowler and Baker submitted a plan for a continuous girder, or balanced cantilever, structure. In July 1882 yet another act authorized construction. The Tay bridge affair had so undermined public confidence in railroads that the legislation insisted that the Forth bridge should “enjoy a reputation of being not only the biggest and strongest, but also the stiffest bridge in the world.” There was to be no vibration, even as trains passed over it. Consequently, it was greatly over-engineered.

Before 1877 steel bridges had been banned by the Board of Trade because the Bessemer conversion process produced steel of unpredictable strength. The Siemens-Martin open-hearth process, developed by 1875, bad changed that, yielding material of consistent quality. That kind of steel was used in the Forth bridge, heralding the transition from cast and wrought iron in such structures. A smaller steel cantilever bridge had been built in Germany, but the Scottish project was on a larger scale than had been seen before. There is little doubt that its designers owed much to a U.S. model of several years earlier. Between 1869 and 1874 James B. Eads had designed and built the world’s first steel bridge, over the Mississippi at St. Louis, Missouri. Its three-arch superstructure, with a center span of 520 feet (156 meters) and side spans of 502 feet (150 meters), supported by four massive limestone piers, carried a railroad and a road for other traffic on two levels. Other pioneering features of Eads’s bridge were adopted by the British: the use of pneumatic caissons (large diving bells fed with compressed air) to excavate the foundation, tubular steel structural members, and a balanced cantilever design that allowed construction to proceed without temporary supports that would have obstructed the waterway.

In December 1882 the contract for the Forth bridge was awarded to a consortium led by Tancred Arrol, an experienced and respected company headed by William Arrol, which already had contracts for the Caledonian Railway Bridge over the Clyde and the replacement Tay bridge. At the height of building activity, there would be 4,600 Britons, Italians, Germans, and Austrians working shifts around the clock. The construction of the foundations and piers took until the end of 1885. Each of the bridge’s three cantilever towers stands on four 70-foot-diameter (21-meter) granite piers, founded on the bedrock. Eight of the piers are in water, and their foundations were excavated by men working in wrought-iron pneumatic caissons, sunk up to 90 feet (27 meters) below the river surface. The massive cylinders were prefabricated in Glasgow, then dismantled and taken to Queensferry, where they were reassembled. Once excavation was complete, the air shafts and the working spaces were filled with concrete, and the granite piers rose above them.

Work on the superstructure began in 1886 using 64,800 tons (54,860 tonnes) of steel from two steelworks in Scotland and another in Wales, fixed with rivets from a Glasgow foundry. All the structural

members were fabricated in on-site workshops, pre-drilled, test-assembled—exact dimensions were needed in a riveted structure—and then dismantled to be painted and carried to the site for erection. Each of the 331-foot-high (99.3-meter) cantilevers consists of two inward-sloping trusses fabricated from huge, internally stiffened tubular members up to 12 feet (3.6 meters) in diameter. They support 680-foot-long (204-meter) cantilever arms that are linked midspan by suspended girders of about half that length, making the distances between the towers about 1,700 feet (540 meters). The length of the bridge between the end piers is about 5,300 feet (1,600 meters). Together with the approach viaducts and arches at each end, the bridge carries the double-track railroad for 2,765 yards (2,490 meters), 150 feet (45 meters) above the surface of the Firth of Forth. The central gap was closed on 14 November 1889, and the Prince of Wales ceremonially opened the bridge on 4 March 1890.

Engineering Building | Leicester University, England

Engineering Building

Leicester University, England

The Scots architect James Frazer Stirling (1926–1992) formed a partnership with James Gowan (b. 1923) in 1955 after winning a commission for a low-rise housing development in Ham Common, Middlesex (1955–1958). The design started a trend in England for broadly finished brick and exposed concrete. There followed a couple of domestic scale projects, and in July 1959 their more influential work: the Engineering Building at Leicester University (completed 1963), which has been called the “pinnacle of their mutual achievement.” The seminal building, which juxtaposes a glazed office tower with red-tile facings on the massive cantilevered lecture theaters and a single-story workshop, was unlike any postwar architecture elsewhere and broke the hold of Le Corbusier upon British architects. The critic Reyner Banham coined the name “New Brutalism” to describe the new style, which exposed concrete, steel, and brick and rejected the polished and elegant finishes and geometric regularity of the International Modern Movement. The character of the Engineering Building was quickly and widely emulated in Britain; its influence persisted even longer in Japan.

Leicester University was founded as a university college in 1921 and granted its Royal Charter in 1957. The administration appointed the Cambridge engineer Edward Parkes to set up a new engineering faculty, to commence with 200 students. The university also commissioned Leslie Martin to produce a master plan for developing the 9-acre (3.6-hectare) campus; Stirling and Gowan’s building was its first major postwar facility. By the end of 1959 they had produced two alternative preliminary designs. The final scheme was approved in March 1960, although the two architects disagreed over the glazing of the tower block. In fact, their partnership was dissolved as soon as the building was completed.

The building has two main elements: a complex, multistory main building that houses two lecture theaters, laboratories, and offices, and a lower level housing workshops. Two cantilevered reinforced concrete lecture theaters (attributable to the structural engineers), their sloping seating expressed on the outside of the building, are set at right angles to each other and are joined by a diagonal ramp. Four stories of laboratories rise beside the smaller theater on tall concrete columns; surfaces are faced with deep red Accrington brick and red Dutch tiles. Above the larger theater—also brick and tile clad—is a six-story, fully glazed office tower, its narrow rectangular form modified by cut-off corners, crowned by a water tank. The spiral staircase that serves it penetrates the cantilevered block. The adjacent ground-level heavy-machinery workshops, covering over two-thirds of the site and designed mainly by Gowan, are clad in part with translucent glass and roofed with long, diagonal, north-facing glass trapezoidal prisms. One historian has commented that “a mannerist taste for distortion and paradox” permeates the building, and that the “diversity of forms 1/4 is a pretext for the liveliest interplay of masses.” Such a cynical view undervalues the work of one of Britain’s—the world’s—greatest twentieth-century architects; indeed, a winner of the prestigious Pritzker Architecture Prize (1981) and “a leader of the great transition from the Modern Movement to the architecture of the New.”

Empire State Building | New York City

Empire State Building

New York City

For forty-one years from 1931, the Empire State Building was the tallest tower in the world. That distinction has since been wrested and rewrested by a series of successors. The 102-story building, covering its 2-acre (0.8-hectare) Park Avenue site and soaring to 1,252 feet (417 meters), was completed in the incredibly short time of 1 year and 45 days; in fact, the time from the decision to build to the letting of office space was only 27 months. Because of the precise planning and exacting project management that achieved such efficiency, this most familiar of all skyscrapers is one of the great architectural feats of the twentieth century.

The Empire State Company was formed in 1929 by John Jacob Raskob (General Motors’ chief executive), the industrialist Pierre S. du Pont, the politician Coleman du Pont, Louis G. Kaufman, and Ellis P. Earl. Raskob invited Alfred A. Smith, the New York State governor until 1929, with whom he had political ties, to become president of the corporation. The two men became the prime movers of the project. The 35-year-old Waldorf-Astoria Hotel, on the corner of Fifth Avenue and Thirty-fourth Street, was bought for about $16 million from the Bethlehem Engineering Corporation and demolished to make way for the new building. The architects Richmond H. Shreve, Arthur Loomis Harmon, and William Lamb (who did much of the design work) were initially commissioned to create a 50-story, 650-foot-high (195-meter) office block. But the scheme would go through more than 15 revisions before emerging as an 86-story, 1,252-foot (375-meter) tower. Last-minute revisions would further increase it to 102 floors and a height, including its mast, of 1,472 feet (450 meters). The structural engineers were H. G. Balcom and Associates.

Shreve, Harmon, and Lamb produced a steel-framed, art deco tower whose marble-clad, five-story base covered the whole site. From a 60-foot (18-meter) setback at the fifth floor, it rose uninterrupted to the 86th floor. The upper levels were faced with silver buff Indiana limestone and granite, and the verticality of the facade was emphasized by continuous mullions of chrome-nickel steel. The office floors were served by seventy-three elevators.

The esthetics of the design were hardly remarkable, and the building was either ignored or criticized by the aficionados of the sterile European Modernism—so-called international architecture—then being touted in North America. For the present purpose, the Empire State’s artistic qualities are inconsequential, because its significance lies in the fact that the architects made a design that, in the contractor’s words, was “magnificently adapted to speed in construction.” And speed was of the essence: the clients announced an 80-story building in August 1929 and forecast the completion date: 1 May 1931.

The firm of Starrett Brothers and Eken won the contract, estimated at $50 million. The Waldorf-Astoria Hotel was demolished within a month, and site excavation began on 22 January 1930, digging 55 feet (16.7 meters) below ground to the gray Manhattan bedrock. Construction started just under two months later, and through the meticulous construction scheduling of the chief engineer, Andrew Eken, it proceeded at record pace. Materials suppliers were asked to deliver goods as they were needed, so there was no need for on-site storage in the downtown area. When materials arrived on-site—at the busiest time, that meant almost 500 deliveries daily—they were immediately hoisted to the appropriate floor and transported by railways to their final location for

fixing. The steel frame rose an average of four and a half floors a week, on a forest of 210 steel columns. One fourteen-story section was completed in a week! Altogether, 69,600 tons (58,930 tonnes) of structural steel were placed in only six months. By the middle of November 1930 the building’s masonry skin was fixed. This unprecedented logistical feat was achieved by an average workforce of 2,500, which at times reached 4,000. Together, they worked 7 million carefully monitored man-hours, including Sundays and public holidays, to meet the deadline. In fact, the building was completed a few days ahead of its rigorous schedule. On 1 May 1931 President Herbert Hoover pressed the switch in Washington, D.C., that turned on the skyscraper’s lights.

The Empire State was one of the last gasps of New York’s real-estate boom. From late in the 1800s more than 180 tall buildings, none under twenty stories, had been erected in Manhattan. As that phase was drawing to a close about thirty years later, New York City saw what might be described as a three-sided “skyscraper war.” The antagonists were the Empire State, the Bank of Manhattan, and the Chrysler Building. The “cold and nondescript” Bank of Manhattan, designed by H. Craig Severance and completed in April 1929, was, at 927 feet (278 meters), the world’s tallest building—at least momentarily. The Chrysler Building, then being built for the automobile tycoon Walter P. Chrysler, was originally planned to be crowned with a dome, bringing it to within 2 feet (0.6 meter) of the height of the bank. Its architect William van Alen obtained permission to add the spire that is now recognized as the building’s most distinctive feature. Its components were prefabricated inside the upper floors, and it was placed in just one and a half hours in November 1930, bringing the height of the Chrysler Building to 1,048 feet (314 meters). With the advantage of playing a little behind the game, Raskob and Smith had their architects add six stories to the 1000-foot (300-meter) Empire State Building, originally intended to terminate in a flat observation deck. Above it all soared a 200-foot (60-meter) tower, bringing its total height to 1,250 feet (375 meters).

It was mooted that this tower would serve as a mooring mast for airships. The 86th floor would house passenger lounges, airline offices, and baggage rooms, and the vessels would be moored at the 106th level. One attempt to moor a dirigible succeeded for just three minutes, and a near disaster with a U.S. Navy blimp in September 1931 finally led to the abandonment of the scheme—a decision tragically validated by the fiery destruction of the Hindenberg at Lakehurst, New Jersey, in 1937. The two observation decks remained just that, and the mast later formed the base of a television tower.

The Empire State Building cost $24.7 million. Optimistically conceived during a real-estate boom, the success of the venture was dashed by the Wall Street crash of 1929. When the building was opened its owners were hard-pressed to find tenants for the 2.1 million square feet (199,000 square meters) of office space, and some witty New Yorker coined the nickname “Empty State Building.” Apart from the impact of the Great Depression, the 350 Fifth Avenue address was too far from the central business district. Eighteen months after opening, only a quarter of the space had been rented; six months later, there were still fifty-six vacant floors and the problems continued throughout the 1930s. After World War II the commercial center of gravity of New York was the Rockefeller Center, the last of whose nine towers was completed in 1940. Although the Empire State achieved 85 percent occupancy by 1944, even now it has a vast number of tenants renting small areas. Over 15,000 people work in it, and up to 20,000 clients, shoppers, and tourists visit daily. Every year, over 3.8 million sightseers and tourists visit the observation levels.

In 1955, the American Society of Civil Engineers named the Empire State Building one of the “Seven Modern Wonders of the Western Hemisphere,” and on the occasion of its Golden Jubilee in 1981 it was, not without reason, designated an official New York City landmark.

Eiffel Tower | Paris, France

Eiffel Tower

Paris, France

The Eiffel Tower was built between 1887 and 1889 as the entrance arch to the International Paris Exhibition, held to celebrate the centenary of the French Revolution. Conceived in 1882 by Gustave Eiffel’s chief research engineers Maurice Koechlin and Emile Nouguier, and constructed in collaboration with architect Stephan Suavestre, the tower is a graceful and imaginative puddled iron lattice pylon. It soars to 1,020 feet (312 meters), the first building in almost 5,000 years to surpass the height of the Great Pyramid.

Preliminary sketches were made in June 1884 and in September Eiffel, suddenly interested in the project, registered a patent “for a new configuration allowing the construction of metal supports and pylons capable of exceeding a height of 300 meters.”

A careful and innovative assembly of over 18,000 small lightweight parts, the Eiffel Tower demonstrated to fullest advantage the structural possibilities of wrought iron. The world’s tallest structure until the Chrysler Building was constructed in 1929 in New York City, it became (and still is) a landmark synonymous with Paris. Intended as a temporary exhibit and scheduled for demolition in 1909, it was saved by its tourist potential and its usefulness as a communication antenna. A radio tower added in 1959 increased its height by 56 feet (20 meters).

Eiffel had specialized in metal construction during his studies at the École Centrale des Arts et Manufactures in Paris. Prior to the acceptance of his design for the tower, he had built in iron and steel, notably the Maria-Pia railway bridge over the Douro River in Oporto, Portugal; the Truyere Bridge near Carabit, France; locks on the Panama Canal; and the internal frame for the Statue of Liberty. Whilst the Parisian tower drew on the outcomes of these projects, it was nonetheless a unique scientific and engineering challenge: its great height meant that wind loads had to be calculated in the design as well as the effects of gravity, Eiffel chose open lattice and splayed legs so that the wind would pass through the structure. In gale-force winds the movement of the tower is estimated to be a mere 4.5 inches (11 centimeters). Speedy and safe transportation of workers and materials (and later of visitors) was another challenge. Eiffel installed elevators that ran on inclined tracks within the tower’s legs; the guide rails were used as tracks for climbing cranes during construction.

The Eiffel Tower weighs over 13,200 tons (11,180 tonnes), more than 70 percent of which is metal. Its 412-foot-square (126-meter) base is defined by the four huge masonry foundation piers set in bedrock; each supports a leg, and the legs converge to form the shaft. Eiffel employed a team of 50 engineers to prepare 5,300 drawings to his specifications, 100 workers to fabricate the components in the Eiffel factory at Levallois-Perret on the outskirts of Paris, and between 150 and 300 site laborers. His calculations were so precise that no revisions were required during construction. Work began on 1 July 1887 and the project was finished in a little over twenty-six months. Eiffel was awarded the French Legion of Honor.

On the tower’s completion, opposition to its erection was silenced. An earlier protest published in Le Temps had been signed by such illustrious Frenchmen as the writers Guy de Maupassant and Alexandre Dumas Jr. and the architect Charles Garnier. Others had described the proposal as a “truly tragic street lamp” and a “carcass waiting to be fleshed out with freestone or brick, a funnel-shaped grill, a hole-riddled suppository.” But it was an instantaneous popular success. In the last five months of 1889, over 1.9 million people visited it. Each paid an entrance fee to help defray the cost—a little under Fr 8 million (about U.S.$1.5 million).

Three viewing platforms—at 186, 376, and 900 feet (57, 115, and 276 meters)—were provided for visitors. At the first, where there were restaurants and a theater, arches linked the four legs; applied after the construction of the legs and platform, they were purely ornamental. Visitors were taken to the first and second platforms in double-deck, glass-enclosed hydraulic elevators. Stairs led to the third platform, and an elevator gave access to the top of the tower, where Eiffel originally had his studio and office (now restored). Each level offered a panoramic view of Paris and beyond for about 50 miles (80 kilometers). From the Eiffel Tower, people were afforded, for the first time, the unique opportunity of seeing the earth from far above.

When the Société de la Tour Eiffel’s original operating concession expired in 1980, the city of Paris assumed direct control of the tower through a company called Société Nouvelle d'Exploitation de la Tour Eiffel. From 1980 to 1984 it undertook a restoration and renovation program. The tower was reinforced in places, 1,560 tons (1,320 tonnes) of excrescences were removed, and the elevators were replaced. It requires regular maintenance, including painting every seven years. The Eiffel Tower continues to be a prime tourist attraction, with over 6 million visitors annually. Each of the viewing platforms is accessible

and Eiffel’s office has been opened to tourists. The exclusive Le Jules Vernes restaurant occupies the second level. During the Paris millennium celebrations of 2000, the tower was covered with thousands of small lights that nightly illuminated the gracious “iron lady” of Paris.

Durham Cathedral | England

Durham Cathedral

England

Durham Cathedral, built principally between 1093 and 1133 to house the relics of the Northumbrian evangelist St. Cuthbert of Lindisfarne and the Venerable Bede, is the finest example of Early Norman architecture in England. Its significance in the development of Western architecture lies in the use of rib-and-panel vaulting, the pointed arch, and flying buttresses in the gallery roofs—all prophetic of the elegant structural system that we now know as the Gothic.

The cathedral stands in a hairpin bend of the River Wear in County Durham. William I (the Conqueror) selected the naturally defensive site, and by 1072 a castle was commenced on the neck of the steep-sided peninsula to defend the northern region of Norman Britain against the Scots. In 1091 an earlier Saxon church was demolished, and two years later work commenced upon the great building dedicated to Christ and the Virgin Mary. It was to form part of the Benedictine monastery that had been started about a decade before, and the whole precinct soon became the seat of the powerful feudal prince-bishops of Durham. Early in the twelfth century the peninsula was encircled by a wall, much of which survives.

Serious attempts to build “in the Roman manner,” with semicircular stone arches, vaults, and domes—its architecture has been categorized as Romanesque—date from the second half of the eleventh century. The earliest examples saw barrel (or wagon) vaults used in such churches as Santiago de Compostela, Spain (begun 1078), and St. Sernin, Toulouse (begun 1080). These roofs exerted continuous sideways thrust on the side walls, creating the need to build those walls thicker (to prevent overturning); windows were small, in case they diminished the strength of the walls. Sometimes the walls were braced with arches above their piers. Experiments were also made with the Roman cross or groin vault, in which the church was divided into square bays, each of which was covered with a ceiling made by intersecting two barrel vaults at right angles. Although the groin vault transmitted the loads to the walls at equidistant points (thus allowing for thinner side walls with more and larger openings, braced at intervals with massive piers), most of the stress in the vault itself was at its weakest part: the groin. The system can be seen in parts of Durham and in Speyer Cathedral, Germany (originally 1030–1065).

Instead of groin vaults, the nave and choir (ca. 1104) of Durham Cathedral are covered using a revolutionary technique: the bays are framed by lateral, transverse, and diagonal beams or “ribs”—forerunner of the steel- or concrete-framed buildings of modern times—with panels of stone spanning the much smaller areas between them. The most exciting innovation among several at Durham, these are the first known examples of pointed ribbed vaults. The ribs carry their own weight and that of the stone roof to collection points above the piers, and the complex dynamic nature of the loads is thus cleverly resolved. It seems that the northern Italian clerics behind the development of Norman Christianity knew something of ribbed-vault construction, which the invaders took to England. Some sources believe that Lombard experiments may—and only may—have been as early as 1080, but there are certainly no examples on such a large scale as Durham, which therefore preempts by almost a century the key to the dramatic Gothic constructional system.

The church consists of a western galilee, or Lady Chapel; an aisled nave with two western towers; transepts flanking a taller tower above the crossing; and an aisled chancel (which was reduced in length during the thirteenth century). The eight bays of the nave are divided by piers disguised as clusters of columns, alternating with massive circular columns. The same articulation can be found in the choir and transepts. On the face of each pier is a tall shaft rising from the floor that appears to carry the slightly pointed transverse arches that support the vault, nearly 80 feet (24 meters) above. At the triforium (second level), each arch of the arcade is subdivided into two, and on the clerestory (the highest level), arches are supported by a pair of freestanding columns. The nave vault is laterally braced by quadrant arches—heralding the flying buttress of Gothic architecture—concealed in the triforium galleries. The substructures of the 218-foot-high (65.4-meter) central tower and much of the transepts were begun before 1096. The 155-foot-high (47-meter) vault of the crossing, not completed until the fifteenth century, is carried by four huge arches. The original roof of the choir was replaced by the present vault around 1250.

Like many medieval churches, Durham Cathedral has undergone alterations and additions (and, on occasion, what passed for restoration) through almost nine centuries. None has diminished the first impression of overwhelming power and stability experienced by the modern visitor when entering this “fortress of God” at the frontier of the Normans’ domain.

Dover Castle

Dover Castle

Kent, England

The science of medieval warfare and the design of castle architecture developed side by side until the latter reached its highest degree of sophistication in the almost impregnable concentric castle, exemplified in the royal castle at Dover, known as the “key of England,” the first castle of its kind in western Europe. On a clear day the French coast, 21 miles (37 kilometers) across the English Channel, can be seen from the ramparts above the famous white cliffs of Dover, Europe’s historical gateway to Britain.

In 55 b.c. Julius Caesar landed his reconnaissance force nearby, and following a full-scale invasion in a.d. 43, the Romans built a walled town, Dubris (from which Dover is derived). They built an 80-foot-high (25-meter) flint pharos (lighthouse) on the nearby 375-foot (114-meter) Castle Hill, the site of an Iron Age earthworks that had existed long before. It was inevitable that the commanding position would continue to be used for defense. In the fifth century the Angles and Saxons came in the wake of the Roman withdrawal and founded a fortified town on the hill, employing the ancient defenses. Once Christianized, they built the church of St. Mary-in-Castro (St. Mary in the Fortress) as a chapel for the castle garrison and adapted the Roman lighthouse as part of its bell tower.

William I (the Conqueror) also recognized the strategic value of Dover. He instructed his half brother,

Odo of Bayeux, should the Norman invasion succeed, to land there with building materials for a castle. It took just eight days in 1066 to construct the fortress—probably a motte and bailey—within the Anglo-Saxon earthworks. Nothing of it remains. The motte was an earth mound crowned with a wooden keep and guarded by a wooden palisade; the bailey was a defensible area, also with a palisade and connected to the motte by a bridge. All was surrounded by a ditch. The earliest stone castles were organized in the same way.

Castles multiplied in Britain after the Conquest, responding to the internal tensions created by the feudal system. Dover continued to be strategically important in an international context, a “royal castle” that was not for a feudal baron but for the defense of the realm. Its evolution into a finely tuned concentric castle was a response to changes in medieval military technology and the science of war. Little is known of its earlier defensive works, but extensive rebuilding was undertaken after 1168. Most work was carried out in the 1180s under the supervision of King Henry II’s chief architect, a master mason known only as Maurice. Richard I (the Lionhearted) almost completed it in 1189–1190, and his brother John extended the outer curtain wall at the north side so that the outer bailey had been enlarged to include most of the hilltop. The “completed” castle dates from about 1200. Repairs and extensions were necessary after a siege by rebel barons and their French allies in 1216, during which, despite the collapse of the east tower, it was successfully defended by a force of only 140 knights and men-at-arms. By 1256 Dover Castle reached its maximum strength and size, its outer walls then extending to the cliff’s edge.

Dover Castle, Kent, England; architect(s) unknown, ca. 1168–1200. View showing concentric curtain walls and keep.

Concentric castles comprised a carefully designed keep that was the last line of defense, surrounded by a curtain wall that enclosed a large bailey. Sometimes there was a second, slightly lower curtain wall

(as at Dover) or even a third. Most functions were served by buildings in the bailey. Dover’s daunting keep—the largest in England—was almost 100 feet (30 meters) square and 95 feet (29 meters) high; in places its walls were 21 feet (6.5 meters) thick. It was defended by an inner curtain wall with fourteen projecting “mural towers”—the first in England—which allowed archers to shoot toward any point at the base. The outer curtain wall at Dover was nearly 1 mile (1.6 kilometers) in circumference, with 20 similar towers. Each wall was interrupted only by fortified gatehouses with barbicans. When gunpowder was introduced into the country in the fourteenth century, cannon were developed that could shoot missiles 3 miles (5 kilometers). Given the thickness of its walls, that was of little consequence to Dover Castle. It has been involved in almost every conflict since the Middle Ages. Small wonder it has been called England’s greatest castle.

Changes to artillery were not the main reason for the demise of castles; rather, the feudal system gave place to centralized government and the power of the monarch. In Tudor times, the design of castles was to alter dramatically. As a royal castle, with an eye on the Spanish, Dover was heavily fortified with cannon in the reign of Elizabeth I. It continued to function well beyond that: it was “modernized” during the Napoleonic Wars. Caves were excavated to hide troops waiting in ambush should the French invade. The towers were truncated—some say vandalized—to serve as gun platforms. The caves were again used as headquarters of the Dover Patrol in World War I and as bomb shelters and a hospital in World War II. The castle remained in the hands of the British army until 1958; five years later it was put in the custody of the Department of the Environment (now English Heritage) as a national monument. Conservation work continues.

Dome of the Rock (Qubbat As-Sakhrah)

Dome of the Rock (Qubbat As-Sakhrah)

Jerusalem, Israel

Jerusalem is a city holy to Judaism, Christianity, and Islam. At its center, the rocky outcrop known as Mount Moriah was the site of three successive Jewish temples, then a sanctuary of the Roman god Jupiter, before it was capped by the Arabic Dome of the Rock, which was for a short while Islam’s most important sacred site. During the Crusades it was commandeered as a Christian shrine before returning to Islamic hands. Today it is at the very core of bitter dispute between Palestinians and Israelis. Although sometimes referred to as the Mosque of Omar, the Dome of the Rock is in fact not a mosque. Nevertheless, as the oldest extant Islamic monument, it served as a model for architecture and other artistic endeavors across three continents for a millennium.

About 1000 b.c. King David of Israel captured the Jebusite town of Urusalim. He renamed it Jerusalem, established his capital there, and chose Mount Moriah—already held sacred as the place where Abraham was prepared to sacrifice his son Isaac—as the site of a future temple. Solomon’s Temple was completed in 957 b.c., only to be destroyed by the Babylonians in 586. The Second Temple was completed by 515 and enlarged and refurbished by Herod the Great (reigned 37–34 b.c.). It was leveled by the Roman legions of Titus in a.d. 70 and has never been rebuilt. The Roman emperor Constantine (reigned a.d. 306–337) decriminalized Christianity in 313. Soon afterward his mother Helena visited Jerusalem, where, according to mythology, she identified the locations associated with Christ, generating a tradition of Christian pilgrimages that continued until the invading Persians destroyed all the churches in 614.

Twenty-four years later Jerusalem was captured by Caliph Umar Ibn al-Khattab, who renamed it Al-Quds (The Holy). Umar cleared the accumulated debris on top of Mount Moriah (Haram al-Sharif) and had a small wooden mosque built on the vast rectangular platform of the demolished Jewish temples.

Dome of the Rock (Qubbat As-Sakhrah), Jerusalem, Israel; architect(s) unknown, 688–692. Restored 1992–1994.

The Dome of the Rock was built between a.d. 688 and 692 for the tenth caliph, Abd al-Malik ibn Marwan. It is an elaborate canopy encircling the bare rock summit of the mount, the sakhra from which Mohammed was miraculously carried through the heavens into the very presence of Allah to receive the tenets of the faith. There is a tradition that, by building the dome, Abd al-Malik was attempting to transfer the Islamic hajj (pilgrimage) to Jerusalem from Mecca, where his rival, Abdullah ibn al-Zubayr, had rebuilt the Kaaba in 684. It is also possible that Abd al-Malik wished to make some tangible statement about Islam’s superiority over Judaism and Christianity, a motive suggested by the form of his building. The Dome of the Rock is more Roman or Byzantine than Islamic, and the caliph’s Byzantine Christian architects employed architectural language understood by Muslims and Christians alike. Because Islamic architecture had not yet established a tradition, they referred to the best Byzantine models, and the congruence in plan and decoration between the Dome of the Rock and the centrally planned church of San Vitale (525–548) at Ravenna, Italy, is not coincidental.

The 60-foot-diameter (18-meter), timber-framed double dome, covered internally with colored and gilded stucco and originally roofed with lead covered in gold, rises 115 feet (35 meters) over the holy rock. It is carried on a tall drum, originally faced with glass mosaics, that rests in turn upon a circular

arcade of twelve Corinthian marble columns, set in threes between four large rectangular piers. At the top of the drum, sixteen colored glass windows light the central space. Surrounding the circle is an octagonal, marble-flagged, 30-foot-high (9-meter) ambulatory of twenty-four piers and columns, reached from outside through four doorways with porticoes facing the cardinal directions. The ambulatory is screened from the sanctuary by half-height walls. The columns and most of the capitals were quarried from older buildings. The marble-faced outer walls of the building also describe an octagon; each side is about 60 feet (18 meters) long. Inside and outside, the Dome of the Rock was enriched with marble columns and facings and floral patterns of mosaic. The total effect must have been awesome: “thousands of lights … supplemented the meagre illumination from the windows, making the mosaics glitter like a diadem crowning a multitude of columns and marble-faced piers around the sombre mass of the black rock surmounted, by the soaring void of the dome” (Ettinghausen and Grabar 1994, 30).

The tolerant Arabian caliphs allowed pilgrims of other faiths access to Jerusalem. Not so the Egyptian Fatimid caliphs who gained control of the city in 969, destroying all the synagogues and churches. In 1071 the Seljuk Turks closed the pilgrimage routes, provoking the Crusades and resulting in the European seizure of Jerusalem in 1099. The Dome of the Rock was converted to Templum Domini, a Christian shrine. The Muslims recaptured the city in 1187, and Jerusalem remained under Islamic control until the nineteenth century.

Although the building has survived in much of its original form, changes have occurred over the centuries. Repairs were made under Caliph al-Mamun (reigned 813–833), and the dome was replaced in the twelfth century; before the successive restorations, its curve was probably slightly horseshoe shaped. More recently, its lead roof has been replaced with aluminum. The glass mosaics that covered the drum of the dome and the exterior walls above the sill line were replaced by ceramic tiles in 1554, when the lower windows were also replaced. In modern times, restorations were carried out in 1924 and 1959–1964. The most recent took place between 1992 and 1994; financed by the late King Hussein of Jordan, it included gilding the dome with 5,000 gold plates and cost U.S.$8 million.

Ditherington Flax Mill

Ditherington Flax Mill

Shrewsbury, England

The Industrial Revolution gave rise to a new building type: the factory, where a managed workforce could operate machines that were driven by steam power. The advent of machines also created a demand for iron to be produced on a large scale; in addition to being used to build machines, it soon became apparent that iron could be used to construct industrial buildings. The forerunner was the prefabricated cast-iron bridge at Coalbrookdale, England, of 1775–1779. But the factories, especially textile mills, involved problems other than the structural ones. Because they handled large quantities of cotton, flax, and wool, and because their wooden floors were quickly saturated with the oil used to lubricate the machines, they presented a fire hazard. The earliest textile mills had timber floor and roof framing and solid masonry external walls. Cast iron was non-combustible, and it was believed that it offered, as well as greater strength, a measure of fire resistance. Designed in 1795 and built the following year by the

engineer Charles Bage of the milling firm of Bennion, Bage, and Marshall, the Ditherington Flax Mill, in the Shropshire town of Shrewsbury, was the world’s first iron-framed building, the predecessor of most modern factories and even office blocks.

Ditherington was the largest flax mill of its day and one of the largest textile mills of any kind in Britain. The five-story building has conventional load-bearing masonry external walls with very large windows. Internally, it is divided into four bays by three rows of slender, cruciform-section, cast-iron columns, extending for eighteen bays on a north-south axis. Each bay measures about 10 feet (3 meters) square, and the average ceiling height is about 11 feet (3.4 meters). The columns support cast-iron beams spanned by the brick vaults that form the floor above.

The nearby warehouse and cross mill, also iron framed, were built soon after. In 1846 Professor Eaton Hodgkinson published Experimental Researches on the Strength … of Cast Iron, a definitive work that established a design methodology for cast-iron structures; together with Sir William Fairbairn he made a major contribution to the theory of nineteenth-century bridge construction. Cast iron is not fireproof; in fact, it fails structurally and rather dramatically at relatively low temperatures. Consequently, the designers of later iron-framed buildings found ways to protect the columns, often by encasing them in non-load-bearing masonry.

The Ditherington Flax Mill survives, reasonably intact. In 1886 the mill ceased operations, and the building was vacant for ten years. For another century, probably because it had large expanses of open floor space, it was converted to maltings for a brewery. It was empty again from 1987, when the brewery closed down, and has been quite badly vandalized since. In the mid-1990s proposals were put in hand for the refurbishment of all the buildings on the site, with the help of a grant from English Heritage. The project included the creation of shops, restaurants, a heritage information center, leisure facilities and offices, an art gallery, and some housing. In March 2000 Advantage West Midlands announced a £2.8 million (U.S.$4.1 million) grant for the restoration of the mill.

Deltaworks | The Netherlands

Deltaworks

The Netherlands

The Deltaworks comprises a series of audacious engineering projects that effectively shorten the coastline of the southwest Netherlands by about 440 miles (700 kilometers), seal outlets to the sea, and reinforce the country’s water defenses. Taking more than forty years to complete, the works involved the construction of huge primary dams totaling 20 miles (30 kilometers) in length, in four sea inlets between the Western Scheldt and the New Waterway, Rotterdam.

The Netherlands is located in the broad deltas of the Rhine, Maas, and Scheldt, and the small country’s history and geography have been greatly influenced by a continuous struggle against the rivers and the sea. Through the coincidence of several events in 1953, the southwestern provinces suffered huge floods in which nearly 2,000 people died and thousands of homes were destroyed. The central government quickly reacted, and the Ministry of Transport, Public Works, and Water Management set up the Delta Committee to devise measures to avert a future disaster. The plan informed the Delta Act of 1958, but its implementation, placed in the hands of a complex instrumentality known as Delta Service, took over four decades to complete.

The major elements of the plan were achieved in the following order: the Hollandse IJssel storm flood barrier (1954–1958), the Zandkreekdam (1957–1960); the Veerse Gatdam (1958–1961); the Grevelingendam (1958–1965); the Volkerakdam (1955–1977); the Haringvlietdam (1956–1972); the Brouwersdam (1963–1972); and the Oosterschelde storm flood barrier (1967–1986). The vast scope of the Deltaworks cannot be fully described here, but it may be measured by a brief overview of the largest, most difficult, and most expensive phase: the Oosterschelde (Eastern Scheldt) storm, flood barrier, immodestly referred to by its builders as “the eighth world wonder.”

It was originally intended to close off the Oosterschelde with a permanent dam, and work started in 1967. By 1973 joining das between parts of the coast had closed 3 miles (4.8 kilometers)—more than half—of the river mouth, and three sluices had been built. Then, in response to public protests, it was decided to construct a storm flood barrier instead of completely closing the estuary. Huge concrete pylons standing on the river bottom would support gates that could close to resist storm surges; a concrete roadway would cross the structure. The government signed a contract with the consortium De Oosterschelde Stormvloedkering Bouwkombinatie in 1977. A 3,000-yard-long (2.78-kilometer) access bridge was built to the 50-foot-deep (15-meter)

construction docks needed to fabricate the massive pylons. Commenced in April 1979, the first was finished early in 1983. In the meantime, work began on the sliding gates. Fifty-foot-deep foundations were prepared to support the pylons, and a special dredge was designed to secure the estuary floor against uneven scouring. By the end of 1982, the river bottom was secured by vast mats laid by purpose-designed vessels. All was ready for placing the pylons.

The construction docks were flooded and the pylons, each weighing 21,600 tons (18,300 tonnes) and between 100 and 135 feet (30 and 40 meters) high, were floated into position, then sunk to the prepared floor. Sixty-five pylons formed the spine of the barrier: sixteen in the northern opening, seventeen in the central, and thirty-two in the southern. They were connected by prefabricated elements, and the sliding gates, each 150 feet (45 meters) long and weighing 1,440 tons (1,220 tonnes), were then installed, a task that took a little under two years to complete. Then followed the fixing of each of the sixty-two 3,000-ton (2,270-tonne) precast concrete elements that carried the roadway across the barrier. The Stormvloedkering Oosterschelde was officially opened on 4 October 1986. It cost about a sixth of the 11 billion guilder (U.S.$5.5 billion) total of the Deltaworks.

The danger of overflowing rivers in the winter and early spring also threatens large parts of the Netherlands. Several inland engineering works—the Philipsdam (1976–1987); the Oesterdam (1977–1988); the Markiezaatskade (1980–1983); and the Bathse Spuikanaal and Spuisluis (1980–1987)—were adjuncts to the primary dams of the Deltaworks.

Holland’s struggle against the water continues. Despite the pleas of regional and local water authorities for river dike reinforcement, the national government concentrated its funding for forty years upon the Deltaworks. Moreover, conservationists oppose any dike improvements that would spoil the landscape. The Boertien Commission was established early in the 1990s to address potential problems, and it produced the Great Rivers Delta Plan, which involved reinforcing nearly 190 miles (300 kilometers) of river dikes and embankments. The first phase was completed by the end of 1996; the second, covering another 280 miles (450 kilometers), was finished by 2001. But that will not solve the problem; if nothing else is done, the next generation of Hollanders will have to raise the dikes again. Climate changes, deforestation, urbanization, and drainage in their upper reaches mean that the river systems will carry increasingly large peak volumes. Cooperative policy and water management must be integrated internationally, from the sources to the deltas.

Deal Castle Kent, England

Deal Castle

Kent, England

Deal Castle, built in 1539–1540 to stand guard over the town of the same name on the Kent coast of southeast England, is a fine example of a new building type, created in response to major changes in politics and the technology of warfare. With others at Walmer and Sandown, it epitomized Henry VIII’s new forts

by its assured and concentrated use of the design elements common to all. Deal is the largest, most impressive, and most complicated of the so-called Device forts. It probably looks just as was intended: crouching in wait low above the beach, stocky, powerful, and seemingly impregnable.

In the turbulent years that followed Henry VIII’s accession in 1509 he twice made war on France, the second time as an ally of the Holy Roman Emperor. Charles V of Spain. When he realized that France’s defeat would give Spain too much power, Henry changed sides, joining France and the pope against the empire. England was financially ruined by the campaigns of 1527–1528, and six years later, Henry’s divorce from Catherine of Aragon led to a break with the Catholic Church, isolating him from most of Europe. He tried to drive a diplomatic wedge between France and Spain, but in 1538 they signed a truce, arousing Henry’s fear of a joint invasion. He urgently launched an ambitious defense program. Using funds plundered from the monasteries by his religious “reforms,” in 1539 Henry initiated a chain of about thirty forts and batteries to defend England’s major ports and repel the expected invasion fleet. They included ten Device forts: Portland, Pendennis, and St. Mawes in southwest England; Hurst, Calshott, and Sandgate around the Solent; and Camber, Walmer, Sandown, and Deal on the southeast coast.

The nature of warfare was changing, and the sophisticated defense systems of medieval castles had become obsolete. Built to resist mechanical artillery, they now had to withstand, missiles shot with gunpowder. The clumsy bombards of the fifteenth century could be fired only a few times an hour. But by the early sixteenth century cast-iron cannonballs had replaced stone; powder quality had improved; and ordnance was generally smaller, reliable, and accurate. In 1386, Bodiam Castle in Sussex was among the first to replace archers’ loopholes with cannon and gun ports. The decline of feudalism also had its effect: enemies were more likely to be foreign than envious neighbor barons.

Finished late in 1540 Deal, Walmer, and Sandown completed the metamorphosis from medieval castle to modern artillery emplacement. Each of these squat, powerful-looking “castles in the Downs”—they were still called castles—comprised rounded bastions radiating from a circular keep. Their thick walls were curved to deflect cannonballs, and their many gun ports were widely splayed for easy traverse. There were three tiers of cannon for long-range offense and two tiers of defensive armaments. Built by an army of workmen at a total cost of £27,000—1,000 years’ pay for an artillery officer—and joined by earthen bulwarks (since vanished), they formed a defensive cluster along a vulnerable 2-mile (3.2-kilometer) stretch of coast. Sandown has succumbed to coastal erosion, and Walmer has been converted to a residence for the Warden of the Cinque Ports. Only Deal, overlooking the low-lying marshlands, has been conserved.

Henry VIII’s sexual notoriety has overshadowed his considerable abilities as a scholar, poet, and statesman. He took an interest in military engineering and personally amended the proposals for his forts, and the “device” (that is, the design) of Deal Castle has been attributed to him. The temptation to compare the concentric plan to the Tudor rose (as many have done), although alluring, must be resisted. Built with stone quarried from a nearby Carmelite priory, the castle’s architectural form was primarily constrained by serious military purpose: to pack the maximum firepower into the most compact possible structure.

Six semicircular bastions, with curved parapets and bristling with gun emplacements, radiate in two tiers from a central, cylindrical barracks-keep; the configuration is repeated in the surrounding moat. The upper tier abuts the tower; the lower forms the curtain wall. The concentric layout allowed ordnance to be effectively positioned and fired simultaneously without impeding each other. Almost 200 openings penetrate the massive walls at five levels, including 119 cannon ports and embrasures. The remaining loopholes and casemates, mostly at the lower levels, were for arquebuses and pistols. Gun positions within the bastions were vented to clear the smoke and gases. It is easy to imagine the withering salvo afforded by such purposeful design, but it has been suggested that Henry was unable to find enough cannon to fully equip his fortresses.

Because architects usually build upon what they know, Deal, simply because it had evolved from the

medieval castle, also employed traditional defenses. The entrance was at second-floor level and approached by a drawbridge across the moat; attackers then faced a portcullis, beyond which there were heavy, iron-studded oak doors. The gatehouse ceiling was penetrated by five “murder holes” (gun slots for small arms), and a cannon protected an inner door. In the manner of earlier keeps, the central tower was self-sufficient: its basement had supply and ammunition stores and a well. The garrison was quartered at ground level, with a mess hall with fireplace and bake ovens. The upper story housed, rather more comfortably, the captain of the guard.

The anticipated Catholic assault never came. Although Deal was again readied in 1588, this time to repulse the Spanish Armada, once more no invasion eventuated. Late in the English civil war the fortress was held briefly by the Royalists, but they surrendered after a sustained bombardment. In the eighteenth century Deal’s parapets were altered (some say disastrously) in unfulfilled expectation of attacks during the French Revolution, and again during the Napoleonic Wars. No shot was fired in anger until the German bombing of 1941. Since 1984 Deal Castle has been in the care of the Department of the Environment (now English Heritage).

De Stijl

Founded in Leiden, the Netherlands, in 1916, the group known as De Stijl was Europe’s most important theoretical movement in art and architecture until the mid-1920s, when leadership passed to Germany.

In 1916 the architect J. J. P. Oud met the critic and painter Theo van Doesburg and soon introduced him to another young architect, Jan Wils. First forming De Sphinx artist’s club in Leiden, the three founded, with the railwayman-philosopher Anthony Kok and the painters Piet Mondrian, Bart van der Leck, and expatriate Hungarian Vilmos Huszár, the group known as De Stijl. Others joined them: the fiery Communist Robert van ’t Hoff and the Belgian sculptor Georges Vantongerloo (both in 1917); the furniture

designer Gerrit Rietveld (1918); the architect Cor van Eesteren (1922); and the painter César Domela (1924). Later arrivals were balanced by departures.

The first manifesto was issued in November 1918, though not all the members signed it. Therefore, De Stijl should never be thought of as a group in the sense that, say, the Pre-Raphaelites or the Impressionists were groups. The members never reached unity of purpose; there were no meetings; and membership seems to have lain in contributing to De Stijl, a polemical journal jealously conducted by van Doesburg. He stretched and frayed their fragile ties by personality issues, and the whole fabric unraveled as members withdrew one by one, unable to work with him. Van der Leck lasted only until 1918; Wils and van ’t Hoff left in 1919; Oud and Vantongerloo two years later; and Mondrian in 1925. Others briefly established links with van Doesburg, but after 1925 only he was left to continue the magazine, by then published only spasmodically. He died in 1931.

Many De Stijl members were influenced by Theosophical doctrine and, subscribing to a holistic worldview “in which the geometric [was] the essence of the real,” they sought unity within the arts and between art and society. Perhaps because its mysticism, religion, and philosophy offered a palliative for the problems of burgeoning capitalism, Theosophy appealed to many in the industrializing world at the fin de siècle. Socialism was an important factor at the time of De Stijl’s birth and for some members social issues were all. They so concerned van ’t Hoff that, unwilling to work for middle-class clients, he soon forsook architecture altogether. Seeking an appropriate architecture, the others explored Constructivism, temporarily preached Neoplasticism, and generated what Oud called Cubism, but theory seldom extended to architectural realities. The few realized projects were spectacular: van Doesburg’s Café Aubette, Strasbourg (1926–1927, with Jean Arp and Sophie Taeuber-Arp), carried “painting into architecture, theory into practice.”

Rietveld’s Schröder house demonstrated De Stijl ideas and became an icon of European Modernism. In 1921, Rietveld began to collaborate with the interior designer Truus Schröder-Schrader. The tiny house in Utrecht (1924) that he designed for her expresses, more than anything else undertaken by the group, the principles valued by De Stijl. Earlier, Rietveld had collaborated with his De Stijl colleagues on fragments of schemes and unrealized projects. What they had been able to only dream of or explore in scale models, Rietveld built as his first complete architectural work.

The division among Dutch architects on religious and political grounds prevented wider acceptance of De Stijl’s ideas within the Netherlands. De Stijl became an international journal (or rather, by van Doesburg’s duplicity, an illusion of one), and through its pages and his personal preaching he shared with Europe the message of an architectural climax. De Stijl was moribund when van Doesburg died in 1931, but for a moment or two, through it, the Dutch had supplied a lot of theoretical and rather less practical input to modern architecture. Not least, by commenting upon his work to a wide audience, they provided a gateway for Frank Lloyd Wright’s “peaceful penetration of Europe.” In 1936 Alfred Barr of the New York Museum of Modern Art perceptively remarked that De Stijl had overshadowed German architecture and art in the mid-1920s. Moreover, had van Doesburg’s attempted insinuation into the Dessau Bauhaus succeeded, that critically important school of architecture and design would have been turned toward Russian Constructivism.

De Re Aedificatora

Leon Battista Alberti’s theoretical treatise on architecture, titled De Re Aedificatoria (About Buildings), was dedicated in 1452 but not published until 1485. What qualifies it as an architectural feat? It changed the understanding and practice of architecture in much of Europe and continued to influence developments there and in the New World for about 400 years. Although he was gathering the ideas for the book, Alberti (1404–1472) was not an architect but a Catholic priest.

Alberti was born in Genoa, the illegitimate child of Lorenzo, an exiled Florentine from a family of bankers. When he was about ten years old, Battista (he added “Leon” later) entered a boarding school in Padua to receive a basic classical education. Several years of legal studies at the University of Bologna led to a doctorate in church law in 1428, after which he went to Florence. He soon began writing. His first published anthology of poems, Il cavallo (The Horse) of 1431, was quickly followed by Della famiglia (About the Family)—the first of many philosophical dialogues—and La tranquillità (Composure), a collection of essays, short stories, and plays, both in 1432. By then he was employed as a secretary in the Papal Chancery in Rome and was about to undertake a lives of the saints and martyrs, written, as was fashionable, in classical Latin. Living in Rome opened Alberti’s eyes to classicism, although the city was to remain neglected for another fifteen years. In 1434 he wrote a study about urban design entitled Descriptio urbis Romae (Description of the City of Rome), in which he first explored the classical notion that beauty existed in harmony, achievable through mathematical rules.

Alberti’s future lay not in the law but in the church. Taking holy orders, he would eventually become a canon of the Metropolitan Church of Florence in 1447. Other clerical offices and their benefits followed: abbot of San Sovino, Pisa, Gangalandi Priory, Florence, and the rectory of Borgo San Lorenzo in Mugello. In 1436 he completed his first major book, written in classical Latin, that touched upon architecture: De pictura (About Painting) was an attempt to bring system to perspective and set down rules for the painter to achieve concord with cosmic harmony. An Italian translation appeared in the same year.

From about 1434 Alberti traveled through northern Italy in the retinue of Pope Eugenius IV, visiting Florence, Bologna, and Ferrara, where, in 1438, under the patronage of Marchese Leonello, he began a more careful study of classical architecture, delving into the ten-part book De Architectura, written by one Marcus Vitruvius Pollio around 20 b.c. Alberti returned to Rome six years later and extended that study among the ancient buildings. When Nicholas V succeeded to the papacy in 1447, Alberti was appointed inspector of monuments, an office he held

until 1455. De Re Aedificatoria, written in classical Latin and structured in ten parts like Vitruvius’s De Architectura, was completed in 1452. Vitruvius’s book was its principal source and model, but Alberti also drew upon Plato, Pythagoras, and the Christian fathers; his own archeological studies; and, importantly, the consensus of contemporary architectural thought. Vitruvius had summarized the architectural practice of his day; Alberti went further to lay down universal rules.

As Italian society and fashions changed, from around 1420 the mason-architect had begun to be displaced, first by the artist-architect and then the courtier-artist-architect. With training in neither building nor art, Alberti wrote a book about the art of building that completed the metamorphosis of the architect into a dilettante-scholar; that made “design distinct from matter,” as he put it, and turned the art of architecture into an academic pursuit in which creativity and design skill could be honed to perfection simply by obeying a set of rules. Intuition was replaced with measurable absolutes. It gave architectural design a thoroughly developed theory of harmony and proportion and made it simple—at least in theory. According to some sources, the last Latin edition was a folio version in Bologna, of 1782. Translations and many derivative works found their way through western Europe.

Book I of De Re Aedificatoria defined design, set down the criteria for good architecture (convenience, stability, and delight), and discussed the basis of composition and proportion. Book II dealt with matters of professional practice and building materials. Book III addressed practical building construction. Book IV covered many aspects of civic design, and Book V dealt with plans for various building types. The next book explored the esthetic dimension of architecture, defining beauty as “a harmony of all the parts in whatsoever subject it appears, fitted together with such proportion and connection, that nothing could be added, diminished or altered, but for the worse.” It also included a section on mechanical and technical details. Alberti’s strong attachment to antiquity was revealed in Books VII and VIII, that took up the subjects of ornament in religious buildings and Roman urban design, respectively. In Book IX the axiomatic principle underlying Renaissance architecture was restated: that beauty is an innate property of things, achieved by following cosmic rules. Then there was an assortment of chapters about mostly practical issues. Book X descended to the pragmatic: water supply, engineering, repairing cracks, and even how to get rid of fleas.

Alberti applied his theories in only a few buildings, mostly unfinished renovations or extensions. They included the facades of the Church of San Francesco (otherwise known as Tempio Malatestiano) of 1450, in Rimini; the facades of the Palazzo Rucellai (1446–1451) and Santa Maria Novella (1458–1471), both in Florence; and San Sebastiano (1459) and Sant’Andrea (1470–1472), both in Mantua. His biographer Giorgio Vasari wrote in 1550, “His writings possess such force that it is commonly supposed that he surpassed all those who were actually his superiors in art” and added, “He was a person of the most courteous and praiseworthy manners … generous and kind to all.”

Curtain walls

Curtain walls

Traditionally, the wall of a building served both structural and environmental purposes. That is, it carried to the ground the weight of the building and its contents and, while admitting air and light through openings, protected the interior from extremes of weather, noise, and other undesirable intrusions. The introduction of structures in which the loads are carried by beams and columns liberated the wall from load bearing, allowing it to function solely as an environmental filter—a relatively thin, light curtain, so to speak. This was first seen in the later medieval cathedrals with their vast stained-glass windows, but it would not be widely developed until the nineteenth century, with the advent of metal-framed architecture and, subsequently, reinforced concrete. The metal-and-glass membrane supported by the building frame, known as the curtain wall, is principally associated with multistory office buildings after about 1880.

Seagram Building, New York City; Ludwig Mies van der Rohe, architect, 1954–1958. Exterior, photographed in 1997.

Although the first skyscrapers, such as the Rookery (1885–1886) and Monadnock Building (1889–1891), both in Chicago and both designed by architects Burnham and Root, had thick conventional load-bearing walls, the twin economic necessities of getting buildings up quickly and optimizing the quantity and quality of interior space soon led to buildings whose outer walls consisted almost entirely of windows supported by perimeter columns and beams. This was a first step toward the development of a true curtain wall, that is, a continuous wall in front of the structural frame. The earliest example was Albert Kahn’s Packard Motor Car Forge Shop in Detroit (1905). A curtain of glass in steel frames allowed more space

and light in the factory, just as it would in an office tower, and Kahn again employed it for the Brown-Lipe-Chapin gear factory (1908) and the T-model Ford assembly plant in Highland Park, Michigan (1908–1909). This rational industrial architecture drew the admiration of Europe and was emulated in Peter Behrens’s A. E. G. Turbine Factory (1909–1910) in Berlin and Gropius and Meyer’s Fagus Works in Alfeld-an-der-Leine, Germany, of 1911.

It is widely accepted that the first office block with a curtain wall was Willis Jefferson Polk’s eight-story Hallidie Building (1917–1918) in San Francisco. Although it was cluttered in places with florid cast-iron ornament, the street facade, suspended 3 feet 3 inches (1 meter) in front of the structure by brackets fixed to cantilevered floor slabs, presented an unbroken skin of glass. Elsewhere, others dreamed of crystal prisms in which the building’s whole external membrane was glass: the serried towers of H. Th. Wijdeveld’s Amsterdam 2000 (1919–1920) and Le Corbusier’s Ville Contemporaine (1922) and—probably best known—the skyscrapers Ludwig Mies van der Rohe projected between 1919 and 1923. But dreams and visions they remained, because the technology was not yet available to turn them to reality. One exception was the A. O. Smith Research Building in Milwaukee (1928–1930) by Holabird and Root, the first multistory structure with a full curtain wall (rather than a single facade) of large sheets of plate glass supported on aluminum frames.

Spin-offs from defense technologies after World War II paved the way for tall curtain wall buildings. Important among them was cost reduction in the production of aluminum, whose corrosion resistance could be improved by a process known as anodizing. This lightweight metal could be extruded into the complicated profiles needed to frame the glass and strengthen the wall against wind loads. Reliable cold-setting synthetic rubber sealants had also become more widely available. These advances were combined with more efficient sheet glass manufacture, especially polished cast glass and, after 1952, the much flatter float glass. Wall elements could be fabricated off-site to exacting tolerances and then transported, assembled, fixed, and glazed with none of the “wet” processes that impede building contracts. Relevant engineering developments included reverse-cycle air-conditioning—available since 1928—and fluorescent lighting, first demonstrated at the 1938 Chicago World’s Fair. All these technologies were exploited in Pietro Belluschi’s twelve-story Equitable Building in Portland, Oregon (1944–1948), described by one historian as “an ethereal tower of sea green glass and aluminum.” Another writer asserts that it “set styles for hundreds that came after.”

The thirty-nine-story United Nations Secretariat Building in New York City followed in 1947–1952. The final design was developed from a proposal by Le Corbusier, and Wallace Harrison acted as executive architect in consultation with him. The curtain walls of the Secretariat Building’s east and west facades are all glass, cantilevered 27 inches (80 centimeters) from the line of the perimeter columns; black-painted glass spandrels hide the between-floor spaces. The blue-green tinted windows are of “Thermopane,” a special glass that absorbs radiant heat, preventing it from reaching the interior, thus reducing the load on the air-conditioning system. The only breaks in the sheer curtain wall are full-width air-conditioning intake grilles at four levels. Because of its innovation, and no doubt because of its associations, the U.N. Secretariat, together with Mies van der Rohe’s Lake Shore Drive Apartments (1951) in Chicago and Skidmore, Owings, and Merrill’s Lever House (1952) on Park Avenue, New York, contributed to the universal standard for high-rise buildings.

The latter building, a twenty-four-story, green-tinted glass and stainless steel tower, designed by Gordon Bunshaft, marked a change of direction in American corporate architecture and in the way New Yorkers built. In keeping with the wishes of a client who made household cleaning products, Bunshaft produced an immaculate, clean-lined tower. The architectural critic Lewis Mumford called it “an impeccable achievement.” The top three floors are reserved for mechanical services. A mobile gantry carries a window cleaners’ platform that serves all faces of the building; such devices became standard for the curtain wall office buildings that followed. Lever House was the first skyscraper to exploit the allowable plot ratios in city planning regulations. By

occupying only a quarter of the site, it allowed much more natural light to enter the offices than conventional stepped-back skyscrapers that covered the whole allotment. Lever House is a New York historic landmark, and in November 1999 a $10.7 million contract was let to renovate its curtain walls, designed by Skidmore, Owings, and Merrill under the supervision of the New York City Historical Society.

That leads us to the inherent problems in curtain wall construction, for all of its advantages. In forty-five years, the pristine facades failed in a number of ways—water penetration and consequent damage, corrosion, and broken glass panels. Since their inception, curtain wall systems have been continually revised, most changes geared toward reducing weight while retaining strength. Stiffened sheet aluminum, enameled steel laminated with insulation, and later even thin sheets of stone were used for spandrel panels. The design of joints—problem spots for leaks—was improved and more durable sealants were invented. More recently, the availability of reliable adhesives has allowed architects to indulge in so-called “fish tank” joints between glass panels, doing away with framing bars. Glass technology has also been refined. Double glazing, first manufactured in the 1940s, improves both the sound and thermal insulation of curtain walls. Heat-absorbing glass, already available in the 1950s, evolved in the following decade into reflective glass with thin metallic coatings, also used to reduce heat gain within buildings. In 1984 heat mirror glass was developed; when combined with double glazing, its insulating value approaches that of masonry, but the esthetic effect seems to be a denial of the form of the building: all it does is reflect what’s around it.

Given that the two significant advantages of curtain wall construction are the reduction of weight and speed of erection, it might be concluded that it costs less than conventional work. That is not necessarily true, because its behavior as an environmental filter, especially in relation to heat flow, may result in higher air-conditioning costs. Often, the preciousness of the architect’s detailing increases costs, as evidenced by Mies van der Rohe’s bronze-and-brown-glass Seagram Building (1954–1958) in New York City. It cost $36 million, approximately twice as much as office towers normally did.

The tall glass prism was the major contribution of the United States to the so-called International Style of modern architecture. But its glorious day passed with the rise of postmodernism, and the crystal towers that Frank Lloyd Wright dismissed as “glass boxes on stilts” were replaced with less anonymous designs. Even Philip Johnson, Mies van der Rohe’s most ardent disciple, forsook the minimalist forms of curtain-wall architecture in favor of a more congenial architecture.

Crystal Palace | London, England

Crystal Palace

London, England

The Crystal Palace, a vast demountable building designed by Joseph Paxton for the Great Exhibition of 1851 in Hyde Park, London, was in many ways crucial in the development of architecture: it was the pinnacle of innovative metal structure, it revealed the exciting potential of efficient prefabrication, and it was an early demonstration of the modern doctrine that beauty can exist in the clear expression of materials and function. Altogether, it was one of the most noteworthy buildings of the nineteenth century.

The idea for a Great Exhibition came from the Society for the Encouragement of Arts, Manufactures, and Commerce, and was given impetus by Henry Cole, then an assistant keeper in the Public Records Office. His wide interests extended to the publication of The Journal of Design that encouraged artists to design for industrialized mass production and urged manufacturers to employ them. That, he believed, would raise the quality of everyday articles. Cole was elected to the society’s council in 1846, and the following year, with others, he successfully solicited Queen Victoria’s consort, Prince Albert of Saxe-Coburg-Gotha, to accept the role of its president. Under Royal Charter, and spurred by the success of French industrial expositions since 1844, the society held Exhibitions of Art Manufactures from 1847 through 1849.

After visiting the exclusively French exhibition in Paris in 1849, Cole realized that an international show would inform British industry of progress (and commercial competition) elsewhere in the world. Prince Albert, convinced that “that great end to which all history points—the realization of the unity of mankind” was imminent, caught the vision. The Royal Commission for the Exhibition of 1851 was established to expedite a self-financing “large [exhibition] embracing foreign productions.” It was envisioned as “a new starting-point from which all nations will be able to direct their further exertions,” but it was at the same time an expression of British nationalism. Britain had led the world into the Industrial Revolution, and her outlook was smug, to say the least. The Great Exhibition would provide a vehicle to flaunt her industrial, military, and economic superiority and justify her colonialism.

The show was to have a display area of 700,000 square feet (66,000 square meters), much bigger than anything the French had managed. That was too large even for the intended venue in the courtyard of Somerset House, so it was decided to locate it in Hyde Park. An open competition for the design of a building for the “Great Exhibition of the Works of All Nations” attracted 245 entries from 233 architects, including 38 from abroad. The Commissioners’ Building Committee liked none of them; besides, it was unlikely that any could have been completed on

time. Having prepared its own plan for a large dome standing on a brick drum, the committee called for bids. The result was alarming: building materials alone would have devoured at least half of the available funds of £230,000. Anyway, the design was generally considered ugly, especially by the architects whose proposals bad been rejected.

Fox and Henderson and Company, a firm of contractors, engineers, and ironmasters, tendered a price for an alternative, based on a design by the gardener Joseph Paxton. In 1826 Paxton had been appointed head landscape gardener at Chatsworth, the Derbyshire estate of the sixth Duke of Devonshire. He built large conservatories there, including one in 1886–1840 for the giant water lily, Victoria regia. Paxton claimed that his design for the Great Exhibition building was inspired by the structure of that lily, whose cross ribs strengthened the main radial ribs.

Learning that the invited architects had been turned down, Paxton had sketched out his proposal on a sheet of blotting paper—romantic tradition says it was during a train journey—and through a lucky meeting with a mutual friend he was able to show it to Cole. The idea was simple: a modular structure of a single cross section, built from prefabricated metal components, could be repeated ad infinitum to produce a building of any size. Paxton promised Cole that he would have detailed designs ready within a fortnight. In fact, they were completed in nine days and passed to Fox and Henderson on 22 June 1850. By then, the provision of a building was becoming urgent. Paxton’s proposal had the desirable advantage of rapid construction; moreover, unlike the other schemes, it could later be demounted to leave Hyde Park relatively undisturbed. The commission accepted it; the only modification asked for was a vaulted transept so the building could contain without damage the large elm trees on the site.

The Crystal Palace, as it was soon dubbed, was a single space, 1,851 feet long and 456 wide (554 by 136 meters), rising by 20-foot (6-meter) increments across flanking tiered galleries to a 66-foot-high (20-meter) central nave. It was intersected in the middle by a 108-foot-high (32-meter) vaulted transept. The building covered 19 acres (7.6 hectares) of Hyde Park. A filigree of 330 slender, cast-iron columns and arcades supported its clear glass walls and roofs and the wrought-iron beams that carried the galleries, alternately 24 feet (7.2 meters) and 48 feet wide.

Due largely to Paxton’s consummate organizational skills, Fox and Henderson accomplished its construction between September 1850 and January 1851. The Birmingham glassmaking firm of Chance Brothers supplied almost 294,000 panes, which were fixed in a specially designed roof-glazing system based on economical 49-inch-wide (1.25-meter) sheets that determined the module for the entire design. Building work oil-site consisted mostly of assembling the 3,920 tons (3,556 tonnes) of cast-iron components that came from ninety different foundries throughout Britain, often cast less than a day before they were fixed. The accuracy obtained through prefabrication and the mechanical fixing dramatically reduced the proportion of nonproductive labor common to traditional construction methods. Cast-iron columns were strength-tested, and on-site milling and machine painting included miles of timber-glazing bars. The building was decorated in red, green, and blue, and the columns were brightened with yellow stripes. The Crystal Palace established internationally a style and a standard for exhibition pavilions, next at Cork (1852), then at Dublin and New York (both in 1853), and Munich (1854).

The Great Exhibition opened on 1 May 1851, with more than 13,000 exhibits from around the world. By the time it closed six months later, over 6.2 million people had visited it. Despite popular insistence that the building should remain, it was scheduled for dismantling. A consortium bought it and it was, under Paxton’s supervision, reerected in a modified form in a park designed by him at Sydenham Hill, southeast London. Reopened by Queen Victoria in June 1854, the Crystal Palace became a national center for exhibits of industry, art, architecture, and natural history, all held under the auspices of the Crystal Palace Company. Sporting events took place in the park from about 1857 and for twenty years after 1895 it became the venue for Football Association Cup finals. Motor racing followed in 1936.

In November of that year, the Crystal Palace was destroyed by fire. Only one terrace of the original park now survives, and even that is under threat. The Crystal Palace Partnership, with representatives of five London boroughs and private-sector groups, is undertaking a £150 million regeneration scheme for Crystal Palace Park that includes its “restoration,” a concert platform, modernization of the National Sports Centre, and a so-called new Crystal Palace on the surviving 12-acre (4.8-hectare) terrace. The latter, an insensitive proposal for a utilitarian building housing a twenty-screen cinema multiplex with restaurants, bars, and rooftop parking for a thousand cars, provoked local residents to launch the Crystal Palace Campaign in May 1997. A challenge to the scheme is being mounted in the High Court on the grounds that the Crystal Palace Act of 1990 provides that any building on the site should be “in the style and spirit of the former Crystal Palace.”

Confederation Bridge, Prince Edward Island Canada

Confederation Bridge, Prince Edward Island

Canada

The 8-mile-long (12.9-kilometer) Confederation Bridge, which crosses the Northumberland Strait between Jourimain Island, New Brunswick, and Borden-Carleton on Prince Edward Island, is the longest bridge over ice-covered water in the world. Its daring conception, the quality of its engineering, and the logistics of its realization are among the factors that make it one of the great constructional feats of the twentieth century. The project is also environmentally, politically, and culturally significant.

Prince Edward Island, on Canada’s Atlantic coast, is the nation’s smallest province, with a population of around 130,000. It lies in the Gulf of St. Lawrence at an average of 15 miles (24 kilometers) across the strait from mainland New Brunswick and Nova Scotia. The strait freezes for up to three months every year, and links with the island historically were expensive, freight and passengers having to be moved by ferry. In 1912 the Canadian government decided to build a railcar ferry to run between Borden-Carleton and Cape Tormentine, New Brunswick, and the Prince Edward Irland was commissioned in 1917. In the first year she made only 506 round-trips. In 1938, as a response to wider automobile ownership, a car deck was added, and the vessel continued to operate until 1969. The subsequent decades saw improvements to the service, and new ferries now make the seventy-five-minute crossing at hour-and-a-half intervals. Prince Edward Island has become a vacation resort and by the beginning of the 1990s tourism had joined commercial fishing and agriculture as a mainstay of its economy.

Between 1982 and 1986 several consortia approached Public Works Canada (PWC) with proposals for a privately financed permanent link between the island and the mainland. Three were for bridges (the first estimated at Can$640 million), one for a tunnel, and another for a combined causeway-tunnel-bridge link. In December 1986, the central government instructed PWC to commission feasibility studies of fixed-link alternatives. By June 1987 twelve expressions of interest were in hand, and the acceptance of Strait Crossing’s proposal was announced in

December 1992. Strait Crossing Development (SCD), a consortium of Janin Atlas, Ballast Nedam Canada, and Strait Crossing, was established to develop, finance, build, and operate the Confederation Bridge.

The proposal, put before the island population in a plebiscite the following January, was generally supported, but lobster fishermen and conservationists raised concerns that led to protracted delays. Their conservation measures won for the contractors the Canadian Construction Association’s 1994 Environmental Achievement Award. Working with the Canadian Wildlife Service, SCD provided nesting platforms for endangered osprey in Cape Jourimain National Wildlife Area. The consortium also initiated a Lobster Habitat Enhancement Program, using dredged material to establish new lobster grounds in three formerly nonproductive locations. Construction work commenced in mid-July 1995.

The shore-to-shore Confederation Bridge consists of three parts. The 1,980-foot (0.6-kilometer) east approach from Borden-Carleton and the 4,290-foot (1.3-kilometer) west approach from Jourimain Island, New Brunswick, join the 6.9-mile (11-kilometer) main bridge across the narrowest part of the Northumberland Strait. Its two-lane carriageway rises from 120 feet (40 meters) to 180 feet (60 meters) above the water at the central navigation span. The bridge takes about ten minutes to cross at the design speed of 50 mph (80 kph).

Engineers designed for a 100-year life, taking into account the combined severe effects of wind, waves, and ice. In part, this was achieved by using concrete up to 60 percent stronger than normal in construction. The concrete employed in the 60-foot-diameter (20-meter) ice shields, designed to break up the ice flow at the pier bases, was more than twice normal strength. Because climatic conditions limited on-site construction to six months of the year, the bridge was designed to be assembled in the summers from posttensioned concrete components precast during the winters. The parts of the approach bridges were cast at a staging facility in Bayfield, New Brunswick, transported by land or water to the site, and assembled by a twin launching truss with a traveling gantry crane. Another staging facility was set up in Borden-Carleton to precast the 175 main bridge components. Some weigh as much as 8,000 tons (8,128 tonnes); the main box girders are 570 feet (190 meters) long, yet designed to be joined with tolerances of less than 1 inch (2.54 centimeters).

In August 1995 a purpose-built floating crane, the Svanen, began placing the components of the east approach bridge, completing it in November; the west approach was built the following spring. The main bridge followed, and by August 1996 the navigation span was the last to be placed. On 19 November the structure was complete: sixty-five reinforced concrete piers, founded on bedrock, supported the 8-mile (12.9-kilometer) superstructure which curves gracefully across Northumberland Strait. During the next six months, the finishing work—the polymer-modified asphalt cement road surface, traffic signals, emergency call boxes, weather monitoring equipment, closed-circuit television cameras, and toll booths—was carried out, and the bridge was opened on 31 May 1997. The estimated direct construction cost was Can$730 million.

Colosseum (Flavian Amphitheater) Rome

Colosseum (Flavian Amphitheater)

Rome

The Flavian Amphitheater, now in ruins, towers over the southeast end of the Roman Forum, between the Esquiline and Palatine Hills. Its popular name, the Colosseum, was derived from the nearby colossal (120-foot-high, or 37.2-meter) bronze statue of Nero, long since vanished. The most ambitious example of a new building type associated with urbanization, the Colosseum was an architectural feat, even by Roman standards. Its size is awesome, but the logistics of moving crowds to and from their seats was also a major achievement.

The earliest amphitheater on the site was built in timber for the pontifex maximus Gaius Scribonius

Curio in 59 b.c.; that; was replaced about thirty years later by a stone-and-timber version for Augustus Octavian Caesar, the first emperor. The Colosseum was commissioned in a.d. 69 by Vespasian, whose son Titus dedicated it in a.d. 80. The highest part of that structure was also timber, and not rebuilt in stone until after a.d. 223. It seems that the first three ranges of seats were completed in Vespasian’s reign, that Titus added two more ranges, and that Domitian completed the building around 300. Although early sources claim that the Colosseum seated 87,000 spectators, modern scholarship puts the figure closer to 50,000. Other Italian amphitheaters at Capua, Verona, and Tarragona are of similar size. The vast Colosseum, elliptical in plan, measured 620 by 510 feet (189 by 156 meters), covering nearly 6 acres (about 2.4 hectares). Its general height was 160 feet (49 meters).

The structural skeleton of the Colosseum was made of travertine limestone, quarried at Tivoli in the hills near Rome and transported to the site along a specially built road. Travertine blocks, some of them 5 feet high and 10 feet long (1.5 by 3 meters), were fixed together with metal cramps to form concentric elliptical walls. These were linked with radiating tufa walls carrying complex rising vaults of brick-faced concrete, in which volcanic stone such as pumice was used to reduce the weight. The vaults carried the tiers of seats. The Colosseum was built to house extravagant spectacles that took place in an arena measuring 280 by 175 feet (86 by 54 meters). Apart from a number of minor entrances to the arena, there were four principal gates at the ends of the axes, directly joined by passages to the exterior. A 15-foot-high (4.5-meter) walls probably faced with marble, defined the arena and provided a measure of protection for the spectators. The floor of the arena was made of heavy planks, strewn with sand for the purpose of soaking up the blood of gladiators, prisoners of war, and wild animals that died in their thousands. Such emperors as Caligula and Nero even ordered cinnabar and borax to replace the sand. A labyrinth of chambers beneath the floor possibly housed the participants in the games, and there were complicated machines and hoists to lift men, beasts, and theatrical sets into the arena, adding to the spectacle. Sometimes the entire floor was removed and the arena flooded by a system of pipes so that galleys could be pitted against each other in mock naval battles.

The terrace on top of the surrounding wall was wide enough to contain two or three rows of movable seats. Undoubtedly the best in the house, they were reserved for senators, magistrates, the vestal virgins, and other important people. The emperor and his immediate retinue occupied an elevated cubiculum. Upon entering the Colosseum through numbered arches corresponding to their ticket numbers, other visitors climbed sloping ramps to the gradus (bleachers), which were divided into stories and allocated according to gender and social class. The first fourteen rows of marble seats were covered with cushions and set aside for the equestrian order. Above them a horizontal space defined the second range, where a third class of spectators, the populus, was seated. Still further up were the wooden benches for the common people. The open gallery at the very top was the only part of the amphitheaters from which women were permitted to watch. There were exceptions, of course. When the games were over, the crowd could quickly disperse through no fewer than sixty-four strategically placed exits, aptly known as vomitoria.

The external wall of the Colosseum was divided into four stories, reflecting the circulation corridors within. Its eighty arches, most of which provided access to the interior, were framed by superimposed orders of pilasters (nonstructural columns): Tuscan on the ground floor, Ionic above them, and Corinthian at the top. The fourth story, also embellished with Corinthian pilasters, had stone brackets for the wooden masts from which an awning (velarium) was suspended across the interior to shield spectators from the sun while they watched the slaughter below. Many of the visible parts of the building were enriched with moldings, ornament, facings of marble or polished stone, and statuary. Fountains of scented water were provided for refreshment.

The Flavian Amphitheater was damaged several times by lightning strikes and repaired as often, so that games continued spasmodically until the sixth century, despite the opposition of the church and some Christian emperors. The last recorded slaughter of wild beasts was in the reign of Theodoric (a.d. 454–526), since when it has been used sometimes as a fortress and (to its detriment) as a quarry. Renaissance palaces in Rome, such as the Cancellaria and the Farnese, and churches including Saint Peter’s Basilica, were built with columns plundered from the ancient monument. Various popes made efforts to preserve it, and in 1750 Pope Benedict XIV consecrated it to the martyrs who died there. Surprisingly, and despite popular belief, it was not the main venue for the execution of Christians. In 1996 a U.S.$25 million restoration of the Colosseum was launched. After the cellars were drained, fallen masonry replaced, bushes and weeds cleared from the arena, and the structure repaired and cleaned, the greatest amphitheater was reopened in July 2000 with a season of Greek plays.

CN (Canadian National) Tower | Toronto, Canada

CN (Canadian National) Tower

Toronto, Canada

The CN Tower, next to the city hall on Front Street, Toronto, stands on the shore of Lake Ontario. It transmits television and FM radio for more than twenty broadcasters, as well as serving various other communications purposes. Including the masts, it is the tallest freestanding structure in the world; the top of the transmission antenna is over 1,815 feet (553 meters) high. But at the beginning of the twenty-first century, as technically demanding as it is, height alone does not constitute an architectural feat. The twin Petronas Towers in Kuala Lumpur, Malaysia, currently rank as the world’s tallest buildings, at 1,483 feet (454 meters). Others are proposed that will exceed that, including the 1,660-foot (508-meter) Taipei Financial Center on Taiwan, to be completed in August 2002, and the 2,100-foot (642-meter) Russia Tower in Moscow; at 2,755 feet (843 meters), the Millennium Tower in Tokyo will dwarf them all. The CN Tower is remarkable architecture because of its construction technique. For about a year, concrete, mixed and tested on-site to ensure consistent quality, was poured around the clock into a “slip form” that gradually decreased in diameter, to create the elegantly tapered contour of the post tensioned hollow structure.

Slip forming is a rapid construction technique based on extrusion. It employs a self-raising formwork that continually moves upward as the concrete is being placed, at a rate that gives the concrete time to set before being exposed as the formwork rises on a ring of hydraulic jacks, developing enough strength to support the work above. Continuous slip forming obviously speeds up the construction process while enabling excellent quality control, optimizing labor, and reducing the cost of building plant and scaffolding. It also results in monolithic, seamless structures. Developed in North America in the 1920s—The Granary at Logan Square in Philadelphia (1925) was one of the first examples in the United States—it has been widely used to build grain silos, building service cores, and (normally) any tall structures with a consistent cross section.

Early in the 1970s the number of multistory office blocks in downtown Toronto increased significantly, with a consequent interference with television and radio reception in large parts of the city. Toronto needed an antenna taller than any existing office block, indeed, of any that was anticipated, and the CN Tower was proposed to meet that need. The project was initiated in 1972 by the Canadian National Railway, which commissioned John Andrews Architects, working in collaboration with Webb Zerafa Menkes Housden Architects of Toronto. The structural engineering consultant was Roger R. Nicolet of Montreal; the mechanical and electrical engineers were Ellard-Wilson Associates Ltd. of Toronto; and the manager-contractor was Foundation Building Construction.

The original design proposed three concrete towers linked by structural bridges, but that was developed into a single tower with three hollow “legs.” As well as serving as electrical and mechanical service ducts, the hollow columns provided the necessary degree of flexibility for such a tall structure. Construction started in February 1973, and in four months a Y-shaped, 22-foot-thick (6.7-meter) reinforced concrete base was founded on the bedrock 50 feet (15 meters) beneath the city. The continuous slip-form process then began. When the tower reached 1,100 feet (336 meters), a seven-story “SkyPod,” fabricated on the ground, was raised into position

and anchored by twelve steel-and-timber brackets that were slowly pushed up the tower by forty-five hydraulic jacks. The concrete-walled SkyPod, reached by four high-speed, glass-fronted elevators, houses a 400-seat revolving restaurant, a nightclub, and indoor and outdoor observation decks. Later, a 2.5-inch-thick (6.4-centimeter) glass floor was installed. Beneath the SkyPod, delicate microwave dishes and other broadcasting equipment are protected by an annular radome. The concrete tower continues to the Space Deck at 1,465 feet (447 meters)—an observation gallery that on a clear day provides a view with 100-mile (160-kilometer) visibility. A Sikorsky Skycrane helicopter lifted the tower’s 335-foot (100-meter) communications mast in forty sections, each of about 7 tons (6.4 tonnes), and they were bolted together in place. The mast, erected in three weeks, was covered by fiberglass-reinforced sheathing. The maximum sway experienced at the very top in 120-mph (190-kph) winds with 200-mph (320-kph) gusts is 3.5 feet (1.07 meters).

CN (Canadian National) Tower, Toronto, Canada. John Andrews and Webb Zerafa Menkes Housden Architects; Roger R. Nicolet, structural engineer, 1972–1975. View from Lake Ontario.

The CN Tower was completed in June 1975 and officially opened on 1 October. It cost Can$57 million and took about 1,550 workers forty months to construct. It is nearly twice the height of the Eiffel Tower and more than three times as tall as the Washington Monument. Soaring above Toronto, it is struck by lightning about seventy-five times every year.

In 1995 Canada National passed ownership to a public company, the Canada Lands Company. In June 1998, the CN Tower officially opened a 75,000-square-foot (7,100-square-meter) expansion including an entertainment center, shopping facilities, and restaurants.