When the World Trade Center’s Twin Towers opened to the public in 1973, they were the tallest buildings in the world. Even before they became iconic features of the New York City skyline, they reflected America’s soaring ambition, innovation and technological prowess.
The towers' eye-popping statistics amply illustrate that ambition: They rose a quarter-mile in the sky. They contained 15 miles of elevator shafts and nearly 44,000 windows—which took 20 days to wash. From the South Tower observation deck on a clear day, visitors could see 45 miles. The Trade Center complex was so big, it had its own zip code.
But some of the same impressive architectural elements may have also helped worsen the tragedy on the fateful morning of September 11, 2001. Calling the project “the architecture of power,” Ada Louise Huxtable, an architecture critic for The New York Times offered a prescient warning when the towers were going up in 1966: “The trade-center towers could be the start of a new skyscraper age or the biggest tombstones in the world,” she wrote.
These facts and figures offer some perspective on the engineering and architectural feats that made the Twin Towers possible.
Time to build: 14 years (from formal proposal to finish)
David Rockefeller, grandson of the first billionaire in the U.S., had the idea to build a World Trade Center in the port district in Lower Manhattan in the 1950s. By 1960, city, state and business leaders came on board.
The Port Authority of New York and New Jersey presented a formal proposal to the two states’ governors in 1961, then hired an architect and cleared 14 blocks of the city’s historic grid. They broke ground in 1966.
Two or three stories went up weekly. The towers used 200,000 tons of steel and, according to the 9/11 Memorial & Museum, enough concrete to run a sidewalk between New York City and Washington, D.C.
The ambitious project overcame community opposition, design and construction setbacks, attempted sabotage by New York real estate rivals and major engineering challenges to open its doors in April 1973 while still under construction. The towers were completed in 1975.
Number of architectural design drafts: 105
After creating more than 100 design ideas with various combinations of buildings, architect Minoru Yamasaki’s team settled on a seven-building complex with a centerpiece of two identical 110-story towers. The towers' design featured a distinctive steel-cage exterior consisting of 59 precise, narrowly spaced slender steel columns per side.
Cost to build: more than $1 billion
According to The New York Times, the cost of building the towers ballooned to more than $1 billion, far beyond its original budget of $280 million. Project managers faced cost overruns as safety, wind and fire tests were conducted. And engineers embraced or created innovative construction techniques and new technologies to make the towers lighter and taller.
Rentable floor space: about one acre per floor
The Twin Towers’ innovative design, which placed structural load on the outside columns rather than inside pillars, facilitated the owners’ desire for a maximum amount of rentable space. With 10 million square feet of office space—more than Houston, Detroit or downtown Los Angeles had at the time, according to The New York Times—the World Trade Center came to be dubbed “a city within a city.”
Depth of the Twin Towers’ foundation: 70 feet
To build such tall towers on landfill that had piled up onto Lower Manhattan for centuries, the towers needed exceedingly strong foundations. So engineers dug a huge rectangular hole seven stories down into the soft soil to reach bedrock.
Using a technique developed by Italian builders in the 1940s, the towers’ builders used slurry, a mud-type material lighter than soil, to dig a 70-foot-deep trench and keep the surrounding soil from collapsing as they poured in concrete to form three-foot-thick walls, like a waterproof “bathtub.”
But it worked like a bathtub in reverse. It didn’t keep water in, but rather kept water from the Hudson River out—and away from the Trade Center complex. On 9/11, the crashing debris damaged the walls, but they mostly held up. If they hadn’t, engineers fear the Hudson River would have flooded the city’s subway system and drowned thousands of commuters.
Recommended for you
Extra land created by building the WTC: 23 acres
The 1.2 million cubic yards of soil dug up to build the “bathtub” were used to add 23 acres to Lower Manhattan—about a quarter of the area of a planned community of parks, apartment buildings, stores and restaurants nearby called Battery Park City that lines the Hudson River.
Twin Towers' elevator speed: 1,600 feet per minute
The Twin Towers had 198 elevators operating inside 15 miles of elevator shafts, and when they were installed, their motors were the largest in the world. The towers’ innovative elevator design mimicked the New York City subway, with express and local conveyances. That innovation lessened the amount of space the elevators took, leaving more rentable floor space. On 9/11, the tower’s elevator shafts became an efficient conduit for airplane fuel—and deadly fire.
Windspeed the towers could sustain: 80 m.p.h.
Engineers concluded in wind tunnel tests in 1964 that the towers could sustain a thrashing of 80-m.p.h. winds, the equivalent of a category 1-force hurricane. With this study, one of the first of its kind for a skyscraper, engineers tested how the towers’ innovative tubular structural design, lighter than the traditional masonry construction, would handle strong winds.
But they also realized that in the winds coming off the harbor, the towers could sway as much as 10 feet, making office space potentially tough to rent.
So the chief engineers developed viscoelastic dampers as part of the towers’ structural design. Some 11,000 of these shock absorbers were installed in each tower, diminishing the sway to about 12 inches side to side on windy days, according to the 9/11 Memorial & Museum.
Number of sprinklers in the towers: 3,700
Two months after the release of the blockbuster movie The Towering Inferno, a three-alarm blaze in the North Tower in 1975 raised concerns that the Twin Towers had no sprinklers.
That was common for skyscrapers at the time, and the Port Authority of New York and New Jersey, which owned the buildings, was exempt from the city’s fire safety codes. But facing pressure from state lawmakers and employees in the Center, Port Authority officials spent $45 million to install some 3,700 sprinklers in the two buildings during the 1980s.
But the sprinklers failed when they were needed the most. On 9/11, the attacking planes snapped the water intake system upon impact, so they didn’t work.
READ MORE: 9/11 Lost and Found: The Items Left Behind
Height of the tightrope walk between the towers: 1,350 feet
On the morning of August 7, 1974, French acrobat Philippe Petit walked the more than 130 feet between the Twin Towers on a high wire approximately one-quarter mile up in the air. Thousands of commuters stared up, gasping in amazement.
Exuding confidence in his 45-minute show, the tightrope artist laid down on the wire, knelt down on one knee, talked to seagulls and teased police officers waiting to arrest him. Using his 50-pound, 26-foot-long balancing pole, he crossed between the tallest buildings in the world eight times before stopping when it started to rain.
Initially critiqued as a “white elephant,” the new towers had difficulty attracting tenants in the early years. Petit’s show, followed by a skydiver jumping off the North Tower and a toymaker climbing up the wall of the South Tower, began to turn that around, making the towers seem more human in scale and more accessible to New Yorkers and tourists.
9/11: World Trade Center
Force of tremor when the towers fell: akin to 2.1 and 2.3 earthquakes
On September 11, 2001, seismologists in 13 stations in five states—including the furthest in Lisbon, New Hampshire 266 miles away—found that the collapse of the South Tower at 9:59 a.m. generated a tremor comparable to that of a small earthquake registering 2.1 on the Richter scale. Measurements for the North Tower collapse half an hour later: 2.5 on the Richter scale.