Secondly, the tube design made for great real estate. With the support structure moved to the sides and center of the building, there was no need to space bulky columns throughout each floor. The vertical support columns at the core of the building went all the way down below the bottom floor, through the basement structure, to the spread footing structure below ground. In the spread footing design, each support column rests directly on a cast-iron plate, which sits on top of a grillage.
The grillage is basically a stack of horizontal steel beams, lined side by side in two or more layers see diagram below. The grillage rests on a thick concrete pad poured on the solid bedrock deep underground. This pyramid shape distributes the concentrated weight from the columns over a wide, solid surface. With the steel in place, the entire structure was covered with concrete. Near the base of each tower, at the plaza level, the narrowly spaced perimeter support columns rested on "column trees.
Each of these columns rested on additional, smaller support footings in the foundation. To stand up to the horizontal force of wind, skyscrapers need the right combination of stability and flexibility. They have to be rigid enough that the wind can't push them too far from side to side, but flexible enough that they can give a little, absorbing some of the wind energy.
The WTC crew ran extensive tests to find out just how much sway they could allow without disturbing the building occupants. They put structural models in wind tunnels and even lured unsuspecting test subjects to movable rooms hooked up to heavy hydraulics. In the end, they designed the towers so they could sway about 3 feet in either direction. To minimize the sway sensation, they installed about 10, visco-elastic dampers between support columns and floor trusses throughout the building.
The special visco-elastic material in these dampers could move somewhat, but it would snap back to its original shape. In other words, it could give a little and then return to its initial position, absorbing much of the shock of the building's swaying motion. In addition to the support structure of the buildings, the WTC crew had to consider how people would actually get around the towers.
Elevator systems have always been a difficult balancing act for skyscraper designers. As you build upward, increasing the available space and therefore occupancy of a building, you need more elevators to handle the extra people. But adding more elevators running to the top floor reduces the available floor space somewhat, and therefore total occupancy which reduces the revenue potential. It's tricky getting all the numbers to work out, and it functionally limits the size of the skyscraper.
Before the WTC, architects were hesitant to build higher than 80 stories, largely due to the elevator problem. The WTC crew proposed a completely different system for the huge towers. Instead of building enough elevators to move everybody from the ground floor to their destination, they decided to split the trip to the upper floors between multiple elevators.
If people wanted to get from the ground to the top floor, they would need to jump from elevator to elevator, in the same way you might switch cars on a subway system. First, they would take an express elevator from the main lobby directly to a sky-lobby on the 78th floor.
From there, they could go to their destination floor directly. To keep things orderly, all the person elevators had doors on each side -- you would enter on one side, move to the front, and exit on the other side.
This way, the passengers could keep their place in line all the way up. Essentially, each tower functioned as three buildings stacked on top of one another. The system turned out to be a great success -- with 99 elevators total per tower, each serving only specific floors, occupants could get around quickly and easily. Most super skyscrapers built after the WTC used the same basic system.
Before the Port Authority could build up, to erect the massive towers, they had to build down to establish the buildings' foundations. Massive skyscrapers need to rest on bedrock , the solid rock underneath the ground's soil, or they won't be able to stand up.
To get to this level, the crew has to dig up a huge mass of dirt as the first stage in construction. At the WTC site, the bedrock is between 55 feet and 80 feet m down. Digging to this level is no simple task, obviously, but it's par for the course in skyscraper construction. The WTC crew faced an additional, atypical challenge, however. The build site was immediately adjacent to the Hudson River, and only a few feet down, the soil was completely saturated -- if the crew started digging, the excavation site would be flooded.
Draining the Hudson River would have been a logistical nightmare. Among other things, it would have compromised the stability of other buildings along the shore.
Instead, the Port Authority decided to use the unconventional "slurry trench method," previously employed mainly in subway construction. The process was pretty simple, at least conceptually. The crew used excavating machinery to dig 3-foot-wide trenches down to bedrock level. As they dug, they piped in a slurry made of water and an expansive clay called bentonite. The bentonite slurry material would expand along the sides of the trench, blocking the groundwater. Once they finished a foot 6.
Then they poured in concrete from the bottom of the trench while pumping the slurry out through the top. In this way, they built solid, steel-reinforced concrete walls underground. This box, commonly referred to as a "bathtub," formed a water-tight perimeter wall for the two towers' foundation structure. With the bathtub in place, the construction crew could start digging down to the bedrock to lay the buildings' foundation support.
The only problem was that the soil inside the bathtub was the primary support means holding the walls in place -- remove the dirt inside, and the weight of the dirt and water outside would push the walls inward. To keep the walls in place while they built up the foundation, the crew had to run underground tiebacks , cables extending from the perimeter walls to rock surrounding the bathtub.
This provided temporary support until the crew could finish a support structure inside the bathtub. With the perimeter walls secured in place, the crew could begin excavating the foundation site. They ended up digging up more than 1 million cubic yards of fill, which they dumped in the Hudson, extending the shore. The excavation actually added 28 acres of prime New York real estate, forming what is now Battery Park City. When they had dug down to the bedrock, they blasted away large pits for the towers' support structure and set about building the massive foundation structure for the buildings above.
Additionally, the basement structure had seven levels of usable space, which housed parking decks, stores and subway stations.
Putting the Twin Towers up was a major logistical challenge, in addition to a mind-boggling engineering problem. The buildings required a massive amount of steel -- some , tons total -- but the construction site only had room for a little bit at any one time.
In order to keep construction moving without taking up too much construction site space, the Port Authority had to institute "just in time steel delivery. In this system, all the steel was transported from the manufacturers to a giant railroad yard in New Jersey.
Every major piece of steel was marked with a long ID number, indicating where and when it would be used. According to the construction schedule, the Port Authority would ship the steel pieces from the yard to the site exactly when it was needed -- smaller pieces went by truck and larger pieces by tugboat.
The construction process worked from the inside out. First, the crew built the steel framework of the inner core to a particular height, and then assembled the perimeter wall around it. The perimeter structure was actually formed from pre-fabricated sections of vertical columns attached to horizontal beams called spandrels.
The prefabricated sections were about 10 feet 3 m wide, either two or three stories high, and weighed about 22 tons. The floor structure was then installed between the outer perimeter wall and the inner core. The floors also came in pre-assembled sections, consisting of inch-deep cm trusses topped with a corrugated metal surface.
To finish each floor, the crew would pour concrete over the metal surface and top it off with tile. The floor sections included pre-assembled ducts for phone lines and electrical cable, to make things easier for the electricians who would come in later.
After the steel structure was in place, the crew attached the outer "skin" to the perimeter -- anodized aluminum , pre-cut into large panels. This continued, section by section, as the towers climbed higher and higher. The crew lifted the steel sections into place using four large cranes four per tower , mounted to long steel structures fitted inside the tube structure.
The cranes could actually lift themselves higher, using heavy hydraulics, as the floors were finished. While the crew kept building upward, other workers started to flesh out the floors below, down to installing blinds and painting the walls.
A number of businesses actually moved into their new WTC offices years before the towers officially opened. In addition to utilizing an unconventional structural design, the Twin Towers were also an aesthetic departure from the older buildings in New York.
Most of the city's skyscrapers have a "wedding cake" shape, with larger sections on the bottom tapering off into smaller sections on top. This was partly due to the prevailing architectural style in the first half of the 20th century, but it was also a result of New York's zoning restrictions. In order to ensure that walls of skyscrapers didn't block all light from reaching the street, the city passed a resolution in dictating that all skyscrapers would have an overall pyramid shape.
A new resolution in shifted the focus, regulating overall height rather than shape. The new restrictions dictated a maximum number of floors, based on the district of the building and the total area of the plot. He also designed One WTC. Located at the heart of the new Downtown, 4 WTC offers direct access to virtually every city subway and some of the best new shops and restaurants in New York.
Column-free corners and foot perimeter column spans enhance interior design options, increase the amount of natural light, and offer uninterrupted views from every window.
In , he assumed the post of assistant professor of architecture at Washington University in St. Louis, where he was awarded his first commission: the design of Steinberg Hall on the university's Danforth Campus. He personally takes a leadership role in all commissions, from design inception through to completion — including construction supervision. With a structure that is exposed and visible — the design for 3 World Trade Center stands out at the center of the new WTC.
The defining aspect of 3 WTC is its load-sharing system of diamond-shaped bracing, which helps to articulate the building's east-west configuration.
This allows unimpeded degree panoramic views of New York. The tower consists of a reinforced concrete core with steel structure outside the core, and clad in an external structural steel frame. The three-story high lobby provides visitors a "big picture window" of the Memorial park outside.
Upper level floors straddle those beneath in a podium building formation, lending the tower a distinct interlocking nature and facilitating the high occupancy of the office floors.
The design of 2 WTC is derived from its urban context at the meeting point between two very different neighborhoods: the Financial District with its modernist skyscrapers and TriBeCa with its lofts and roof gardens. The design combines the unique qualities of each, melding high-rise with low-rise and modern with historical.
The needs and requirements of its tenants are concentrated into seven separate building volumes, each tailored to their unique activities. The stacking creates 38, sf 3, sm of outdoor terraces full of lush greenery and unprecedented views of the surrounding cityscape, extending life and social interaction outdoors.
Building the foundation of the twin towers required digging 70 feet to the bedrock and excavating more than 1 million cubic yards of dirt. To avoid flooding the site, workers dug a 3,foot-long, three-foot-wide trench around the perimeter of the site comprised of more than foot-long sections and filled it with a slurry made from water and bentonite, an absorbent type of clay.
Because the slurry was denser than the dirt that surrounded it, it prevented the dirt from filling the trench. Steel cages some seven stories high and weighing 25 tons each were then lowered inside the trench panels and concrete poured around it, forcing the lighter slurry up and out. Traditional skyscrapers owed their stability to a system of large vertical columns running through each floor at intervals of feet, with the exterior walls providing little support on their own.
The builders of the World Trade Center put extensive research into the effect of wind on the towers, commissioning one of the earliest wind tunnel studies for a skyscraper and performing perceptual tests disguised as eye exams on unsuspecting subjects to figure how much the building could sway in high winds without people noticing. With this cutting-edge shock absorbing system in place, the towers were designed to be able to sway up to three feet in either direction on a windy day.
New York City, circa v iew from the Hudson River of the first tower of the World Trade Center under construction, with 'kangaroo' cranes. In all, workers used some , tons of steel to construct the twin towers of the World Trade Center.
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