Flat Tops / The 20th Century
John Hancock Center
Ornamental spires abound in New York, but Chicago has been a city of mostly flat tops for much of its high-rise history. In 1894, the city imposed a height restriction of 130 ft., – later raised to 200 ft. – that resulted in a generation of palazzo-like business blocks with a heavy cornice that visually capped the structure. After a period in the 1920s when the laws were changed, Chicago skyscrapers followed the example of Manhattan and sprouted many decorative towers.
In the post-WWII era, though, Chicago returned to its early rationalist roots that celebrated the clear expression of steel-frame construction and large expanses of window glass in paradigmatic structures by Mies van der Rohe and Skidmore, Owings & Merrill (SOM). As in New York, where in the mid-1950s, Lever House and the Seagram Building set the stage for decades of minimalist glass boxes, Chicago asserted the “engineer’s aesthetic” of innovative and efficient structure.
The collaboration of the partners in the Chicago office of SOM, architect Bruce Graham and structural engineer Fazlur Khan, created two influential skyscrapers of 100 and 108 stories that still define the city’s skyline: the John Hancock Center, which topped out in May 1968, and the Sears (now Willis) Tower, which opened in 1974 and reigned until 1996 as the world’s tallest building.
The John Hancock Center was in many ways a prophetic project. As seen in the elevation drawing below, the tapered tower presented a strongly expressive structural system in which diagonal braces acted as an enormous truss to tie together the steel columns. It was also a mixed-use building that combined the programs of office space on floors 13 to 41 and apartments from floors 45 to 92. On the 95th and 96th levels was a double-story height restaurant, as seen in the above photographs. The views of Lake Michigan and the street grid of the Illinois prairie from these heights, as well as from the observation deck on the 94th floor, have changed considerably since the 1970s, but the classic modernism of the John Hancock Center has not diminished.
WTC Antenna
These images of the erection of the antenna atop the North Tower of the original World Trade Center in 1979 were taken by Peter B. Kaplan, the fearless photographer who specializes in heights and construction views. The construction workers who erected the 360-foot antenna atop the WTC sometimes work above the clouds: one of Kaplan’s photographs captures the tops of the Woolworth and Empire State as the true “cloud piercers.”
Antennas provide a significant revenue source for building owners. The antenna atop Tower 1 replaced the transmitter from the Empire State Building for all New York TV channels. And in one measure of vertical height, according to the CTBUH, antennas count. Until its destruction on 9/11, the 1,741-foot height to the top of the antenna of Tower 1 made it the tallest in the world.
The original twin towers of the World Trade Center were flat-topped, with slightly different heights. The North Tower, the first to be completed, in 1971, was the taller of the two at 1,368 feet, so remained the world’s tallest building even after the South Tower was completed at 1,362 feet in 1973. Both were surpassed by the Sears Tower in 1974 at 1,451 feet.
Blueprint elevation of the John Hancock Center, Courtesy of SOM.
High Rise and Skyscrapers
In an essay on skyscraper history, the renowned architect Cesar Pelli drew a distinction between the terms high-rise and skyscraper:
In the 1960s and 70s, it was clear to all architects that a high-rise building was not a skyscraper. Buildings such as the Seagram Building were never called skyscrapers. On the other hand, we all knew that the Empire State and the Chrysler buildings were real skyscrapers. There were commonly recognized formal and ideological differences between skyscrapers and high-rise buildings.
…When we hear the word “skyscraper,” the image that immediately comes to our minds is most likely that of the Empire State Building or the Chrysler Building…. Both are point towers, with the vertical dimension completely dominant and with an almost identical image from all directions. Both buildings develop considerable upward thrust that culminates in a great crown that resolves and continues this thrust up into the sky itself…. Their silhouettes are easily recognizable, even when seen in the New York fog and from many miles away. These are works of architecture that glory in their optimism and celebrate in their verticality the extraordinary achievement of their construction.
Of these two skyscrapers, the Empire State Building has the purest image; a pre-eminent object filling its block in the center of Manhattan. The Chrysler Building shares its block with other buildings, thus it is not possible to perceive its entire height from all sides. The tower and its base are not of one piece and from nearby it is an unexceptional building. Yet when seen at some distance the tower becomes detached from its base and with its stainless steel crown it is all that a skyscraper ever wanted to be, exhilarating in its beauty, soaring and majestic against the sky.
The Chrysler, Empire State, and RCA buildings mark the maturity of an architectural search that started some sixty years earlier. This exploration produced several masterpieces and collectively gave form to Manhattan and several other American cities. The most complete and successful urban form was that of downtown Manhattan as it existed from the late twenties until the early fifties. The skyscrapers made Manhattan into one of the great city forms of the world with the most powerful and uplifting silhouette.
Skyscrapers,” Perspecta 18, The Yale Architectural Journal (1982)
Aerial view of the Empire State Building observatories, New York. Historic postcard collection of The Skyscraper Museum.
Empire State Building
Completed in 1931, the Empire State was the world’s first 100-story building – or rather, the equivalent of 102 stories, since the floors between the 86th floor observation deck and the small room at the apex of the mooring mast were only estimated levels.
The silver spire that gives the skyscraper its iconic silhouette and twenty percent additional height was a later addition to the design of the 1,050-foot, flat-topped office building illustrated in this architectural elevation. The height evolved from 80 to 85 floors, topped with an observation deck that was designed to be a major tourist attraction and revenue stream.
The main elevator banks serving the office floors from the lobby ended at the 80th floor, which was the “economic height” of the building – that, is, the most efficient and profitable number of stories. For floors 81 to 85 and the observation deck, visitors transferred to a separate, short shaft of elevators.
The addition of the 200-foot “mooring mast” – which brought the total height to 1,250 feet – was clearly intended to decisively claim the title of world’s tallest building. The Chrysler Building had topped out in October 1929 at 1,046 feet, surpassing 40 Wall Street at 927 feet. In December, newspapers reported that the Empire State would be topped with a stainless-steel spire that would serve as a landing station for dirigibles for long-distance business travelers.
Announcing this feature to the press, the President of the Empire State Building, former Governor Alfred E. Smith noted: “Thus the structure will be not only the tallest building in the world, but the first to be equipped for a future age of transportation that is now only a dream of pioneers in aviation.”
The mooring mast was constructed, but never used: no dirigible ever successfully docked there. The greatest success at the top of the Empire State has been the observation deck, which through the decades has seen tens of millions of tourists and today welcomes more than four million annually to enjoy the best views in the city.
As the example of the Empire State Building suggests, even developers designing a speculative property with a keen eye to cost and return see value in attention-getting height and design.
Empire State Building Mooring Mast
Shreve, Lamb, & Harmon, the architects of the Empire State Building, designed the mooring mast as a 200-foot tall, modernistic metal tower, flanked by stylized wings clamped tight to the ribbed shaft and topped with a small octagonal room that was to serve as the embarkation point to transfer onto a transatlantic airship.
As with the ornamental vertex of the Chrysler Building, most of the mast had no occupied floors, but was simply a steel skeleton clad in sheet metal of Nirosta stainless steel. The construction of the framework is documented in the Lewis Hine print of iron workers and in the scrapbook of progress photographs – one of the Museum’s most prized items of its collection.
The views from the rooftop outdoor observation deck and from the 102nd floor room – after the plan for mooring dirigibles was quickly abandoned – were unparalleled. At once high above the city, but also at its very center, the top of the Empire State became the vehicle from which to see New York and the object that represents it.
Architectural model on loan from SOM
Burj Khalifa wind tunnel model. On loan from RWDI Consulting Engineers
Drawing showing the sequential construction of the Burj Khalifa spire. Courtesy of Samsung C & T.
Excerpts from a video of William F. Baker, Partner in Charge of Structural and Civil Engineering at Skidmore, Owings & Merrill LLP, 2007.
Burj Khalifa
At 828 meters/ 2,717 feet, Burj Khalifa is the tallest building in the world in all three categories of the Council on Tall Buildings and Urban Habitat. Taller than two Empire State Buildings, it surpasses the current second-tallest structure, Shanghai Tower, by nearly 700 feet.
The altitude of its highest occupied floor is 1,918 feet, which means that the top 800 feet – 30 percent of the tower’s height – was designed, at great expense and through a remarkable feat of engineering and construction, purely as an aesthetic feature and to decisively assert its world-record height. Currently, though, the world’s next title-taker is in the early stages of construction in Jeddah, Saudi Arabia: although its final height has not been disclosed, the Kingdom Tower will reach or exceed 1,000 meters, or a kilometer in height.
Principally residential, the 163-story Burj Khalifa is a mixed-use building that reverses the normal planning logic: the lower stories are occupied by a boutique hotel, above which there are condominium apartments up to the level of the first observation deck on the 124th floor. The top 37 floors, those with the smallest floor plates, are reserved for elite corporate office suites, save for a second observation deck on the 148th floor.
Burj Khalifa is the centerpiece of Downtown Dubai, a 500-acre commercial and residential complex.
Burj Khalifa Wind Tunnel Model
The “Y” shape and extraordinary height of Burj Khalifa was a result of its innovative “buttressed core” structural system. (The tower’s structural engineer William Baker explains the system in a video on the screen across the gallery.) The form tapers as it rises, and the scalloped sections spiral as they step up the tower in ways that were determined by tests in a wind tunnel with models like this force-balance model which measures vibration.
Integrating wind-engineering principles designed to “confuse the wind” (also known as “vortex-shedding") had a significant impact on the shape and details of the tower. These included softening of the shape of the scalloped buttresses and the reversal in the direction of the stepped spiral to the prevailing wind.
Burj Khalifa was designed as an all reinforced-concrete building with high-performance concrete from the foundation level to level 156; above, it is topped with a structural steel braced frame from level 156 to the highest point of the tower. While several structural options were considered (including a composite system), high-performance concrete was selected as the primary structural material for the tower because of its mass, stiffness, high strength, moldability, continuity, and pumping ability. Speed of construction due to its advanced formwork system was key, as well as the advantages that concrete offered for residential buildings, because it insulates against sound between floors.
Construction The Burj Spire
It is one thing to draw a spire that reaches 828 meters into the air and to engineer the structure to support that height, but it is an added level of complication to figure out how it can be constructed.
Burj Khalifa is built of concrete with a jump-form system, up to level 156, where the tower is constructed of structural steel. As this drawing sequence from SOM illiterates, the steel was lifted and erected by a crane and derrick that was attached to the concrete core. The final 136 meters of the spire, a steel pipe weighing 350 tons, had to be assembled in sections within the tower, then jacked up into place. This method recalls the system used in the Chrysler Building in 1929, but at a much larger scale.
The following technical description of the erection of the spire prepared and executed by Samsung C & T, was provided by the construction project managers from Turner:
The spire at the Burj Khalifa is comprised of over twenty sections welded together to form a hollow steel structure over 110 meters long (inside the building and exposed). These steel sections were assembled within the main tower structure while it was being erected, and the completed spire pipe was then hydraulically jacked into final position over the course of eight separate lift cycles. The main construction sequence allowed for various tower structural members to be temporarily left out to accommodate the lifting equipment, which included three hydraulic strand jacks and a series of roller guides. The exposed portion of the spire’s exterior is equipped with stainless steel fins, which maintain the tower’s spiral geometry and optimize the wind flow across the structure.
In the video on the left Baker describes the structural system of the Burj Khalifa.
Makkah Royal Clock Tower
Great height is generally motivated by a desire for attention, but the building that is currently the third tallest on the planet is little known outside the Muslim world. As part of the King Abdullah Al Saud Endowment for the expansion of the Two Holy Mosques, the Makkah Clock Tower is the centerpiece of the Abraj Al Bait Complex of seven luxury hotels, including the Fairmont Hotel, housed within the tower that rises 601 meters to the tip of the crescent spire, and overlooks the Holy Mosque for Muslims performing Umrah and Hajj.
The Makkah Clock Tower was a separate commission from the high-rise hotel below. The engineers at the German firm SL Rasch, who had earlier worked on other aspects of accommodating an ever-increasing number of pilgrims to Makkah, were asked to create the world’s largest clock atop the building already under construction. Their work began 347 meters above the ground in a structure of steel weighing 12,000 tons. In addition to the clock’s precision mechanisms, inside the spire are also an astrological observatory, exhibition space, and observation deck. The golden crescent that crowns the entire structure contains a prayer room for the royal family and dignitaries. It is the highest occupied man-made space in the world.
The element of time is of great significance to Muslims. Four of the five pillars of the Islamic faith are regulated by time – prayer, almsgiving, fasting, and the Hajj. The first pillar is the proclamation of faith, which is displayed on the side panels of the tower. The gigantic height and immense size of the Makkah Clock Tower allows it to be easily visible from anywhere in Makkah, as well as from the approach roads to the Holy City.
The clock is the host of Makkah Time, an independent time signal spread worldwide unifying all Muslims. It introduces a second time standard besides Greenwich Mean Time so that the faithful can be aware of such markers as the rising sun in Makkah, when Ramadan starts, when fasting can be broken after sunset or when the five daily prayers are to be performed towards Makkah.
Pilgrims to The Holy Mosque and Kaaba
The Al-Masjid al-Haram Mosque in Makkah is built around the Kaaba, the cuboid shrine, considered the House of God, towards which all Muslims face for their prayers. During the Umrah, the pilgrims circumambulate the Kaaba seven times, in a form of worship known as the Tawaf. The Hajj is a five-day long pilgrimage that all able Muslims are obligated to perform at least once in their lifetime, with the Masjid al-Haram as the starting point.
The engineers from SL Rasch have been involved with several projects designed to accommodate the ever-increasing number of pilgrims during the Hajj, which now number nearly three million annually.
The Clock Tower
The clock housing is designed in the Islamic style, with a visitors’ platform below each of the clock faces. It culminates in a large crescent, made from glass and carbon fiber; decorated with 98 million mosaic pieces with 24-karat gold leaf and 2 million LEDs that illuminate the outer façade. The crescent, known as the Hilal, is the symbol of the Prophet and the acme of the Clock Tower. It houses the world’s highest prayer room at 592 meters.
The clock faces measure up to 43 meters and are the largest in the world. Each is driven by its own mechanism that supports both clock hands, which extend to 23 and 17 meters and weigh 12 tons, combined. The call to prayer resonates from the top of the clock through the most powerful sound system of its kind, and during the call to prayer 21,000 green and white lights illuminate the clock, thereby increasing its visibility in all areas of Makkah.