7 World Trade Center (7 WTC) refers to two buildings that have existed at the same location within the World Trade Center site in Lower Manhattan, New York City. The current structure is the second building to bear that name and address in the World Trade Center complex. The original structure, part of the original World Trade Center, was completed in 1987 and was destroyed in the September 11 attacks in 2001. The current building opened in May 2006. Both buildings were developed by Larry Silverstein, who holds a ground lease for the site from the Port Authority of New York and New Jersey.

Where Did the Towers Go? Is a forensic study text book and is the only comprehensive investigation of what happened to the WTC on 9/11/01 in the public domain (i.e. Not classified). The National Institute of Standards and Technology (NIST) did not analyze the collapse of the World Trade Center towers, despite the fact their report is entitled, NIST NCSTAR 1-Final Report on the Collapse of the. Download where did the towers go evidence of directed free energy technology on 9 11 ebook free in PDF and EPUB Format. Where did the towers go evidence of directed free energy technology on 9 11 also available in docx and mobi. Read where did the towers go evidence of directed free energy technology on 9 11 online, read in mobile or Kindle. Where Did the Towers Go?: Evidence of Directed Free-Energy Technology on 9/11 Summary: This book is a forensic analysis of what effectively is a crime scene.

The original 7 World Trade Center was 47 stories tall, clad in red masonry, and occupied a trapezoidal footprint. An elevated walkway connected the building to the World Trade Center plaza. The building was situated above a Consolidated Edison power substation, which imposed unique structural design constraints. When the building opened in 1987, Silverstein had difficulties attracting tenants. Salomon Brothers signed a long-term lease in 1988, and became the main tenants of 7 WTC.

On September 11, 2001, the structure was damaged by debris when the nearby North Tower of the World Trade Center collapsed. The debris also ignited fires, which continued to burn throughout the afternoon on lower floors of the building. The building's internal fire suppression system lacked water pressure to fight the fires, and the building collapsed completely at 5:21:10 pm, according to FEMA,^[5]:23 while the 2008 NIST study placed the final collapse time at 5:20:52 pm.^{[6]:19, 21, 50–51} The collapse began when a critical internal column buckled and triggered structural failure throughout, which was first visible from the exterior with the crumbling of a rooftop penthouse structure at 5:20:33 pm. The collapse made the old 7 World Trade Center the first tall building known to have collapsed primarily due to uncontrolled fires,^[7] and at the time, the only steel skyscraper in the world to have collapsed due to fire.^[8]

Construction of the new 7 World Trade Center began in 2002, and was completed in 2006. The building is 52 stories tall (plus one underground floor), making it the 28th-tallest in New York.^[2][3][4] It is built on a smaller footprint than the original, and is bounded by Greenwich, Vesey, Washington, and Barclay Streets on the east, south, west, and north, respectively. A small park across Greenwich Street occupies space that was part of the original building's footprint. The current building's design emphasizes safety, with a reinforced concrete core, wider stairways, and thicker fireproofing of steel columns. It also incorporates numerous green design features. The building was the first commercial office building in New York City to receive the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) certification, where it won a gold rating. It was also one of the first projects accepted to be part of the Council's pilot program for Leadership in Energy and Environmental Design – Core and Shell Development (LEED-CS).^[9]

• 1Original building (1987–2001)
  • 1.39/11 and collapse
• 2New building

Original building (1987–2001)[edit]

Design and layout[edit]

The original 7 World Trade Center was a 47-story building, designed by Emery Roth & Sons, with a red granite facade. The building was 610 feet (190 m) tall, with a trapezoidal footprint that was 330 ft (100 m) long and 140 ft (43 m) wide.^[10][11]Tishman Realty & Construction managed construction of the building.^[10] The ground-breaking ceremony was hosted on October 2, 1984.^[12] The building opened in May 1987, becoming the seventh structure of the World Trade Center.^[13]

7 World Trade Center was constructed above a two-story Con Edisonsubstation that had been located on the site since 1967.^[14][12] The substation had a caisson foundation designed to carry the weight of a future building of 25 stories containing 600,000 sq ft (56,000 m²).^[15] The final design for 7 World Trade Center was for a much larger building than originally planned when the substation was built.^[16]:xxxviii The structural design of 7 World Trade Center therefore included a system of gravity column transfer trusses and girders, located between floors 5 and 7, to transfer loads to the smaller foundation.^[6]:5 Existing caissons installed in 1967 were used, along with new ones, to accommodate the building. The 5th floor functioned as a structural diaphragm, providing lateral stability and distribution of loads between the new and old caissons. Above the 7th floor, the building's structure was a typical tube-frame design, with columns in the core and on the perimeter, and lateral loads resisted by perimeter moment frames.^[15]

A shipping and receiving ramp, which served the entire World Trade Center complex, occupied the eastern quarter of the 7 World Trade Center footprint. The building was open below the 3rd floor, providing space for truck clearance on the shipping ramp.^[15] The spray-on fireproofing for structural steel elements was gypsum-based Monokote, which had a two-hour fire rating for steel beams, girders and trusses, and a three-hour rating for columns.^[5]:11

Mechanical equipment was installed on floors four through seven, including 12 transformers on the 5th floor. Several emergency generators installed in the building were used by the New York City Office of Emergency Management, Salomon Smith Barney, and other tenants.^[5]:13 In order to supply the generators, 24,000 gallons (91,000 L) of diesel fuel were stored below ground level.^[17] Diesel fuel distribution components were located at ground level, up to the ninth floor.^[18]:35 After the World Trade Center bombings of February 26, 1993, New York City mayor Rudy Giuliani decided to situate the emergency command center and associated fuel tanks at 7 World Trade Center. Although this decision was criticized in light of the events of 9/11, the fuel in the building is today not believed to have contributed to the collapse of the building.^[19][20]:2 The roof of the building included a small west penthouse and a larger east mechanical penthouse.^[14]

Each floor had 47,000 sq ft (4,400 m²) of rentable office space, which made the building's floor plans considerably larger than most office buildings in the City.^[21]In all, 7 World Trade Center had 1,868,000 sq ft (173,500 m²) of office space.^[5]:1 Two pedestrian bridges connected the main World Trade Center complex, across Vesey Street, to the third floor of 7 World Trade Center. The lobby of 7 World Trade Center held three murals by artist Al Held: The Third Circle, Pan North XII, and Vorces VII.^[22]

Tenants[edit]

In June 1986, before construction was completed, developer Larry Silverstein signed Drexel Burnham Lambert as a tenant to lease the entire 7 World Trade Center building for $3 billion over a term of 30 years.^[23]In December 1986, after the Boeskyinsider-trading scandal, Drexel Burnham Lambert canceled the lease, leaving Silverstein to find other tenants.^[24]Spicer & Oppenheim agreed to lease 14 percent of the space, but for more than a year, as Black Monday and other factors adversely affected the Lower Manhattan real estate market, Silverstein was unable to find tenants for the remaining space. By April 1988, he had lowered the rent and made other concessions.^[25]

In November 1988, Salomon Brothers withdrew from plans to build a large new complex at Columbus Circle in Midtown, and agreed to a 20-year lease for the top 19 floors of 7 World Trade Center.^[26] The building was extensively renovated in 1989 to accommodate the needs of Salomon Brothers. This led to the alternative naming of the building as the Salomon Brothers building.^[27] Most of three existing floors were removed as tenants continued to occupy other floors, and more than 350 tons (U.S.) of steel were added to construct three double-height trading floors. Nine diesel generators were installed on the 5th floor as part of a backup power station. 'Essentially, Salomon is constructing a building within a building – and it's an occupied building, which complicates the situation', said a district manager of Silverstein Properties. The unusual task was possible, said Larry Silverstein, because it was designed to allow for 'entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors.'^[27]

At the time of the September 11, 2001, attacks, Salomon Smith Barney was by far the largest tenant in 7 World Trade Center, occupying 1,202,900 sq ft (111,750 m²) (64 percent of the building) which included floors 28–45.^[5]:2[28] Other major tenants included ITT Hartford Insurance Group (122,590 sq ft/11,400 m²), American Express Bank International (106,117 sq ft/9,900 m²), Standard Chartered Bank (111,398 sq ft/10,350 m²), and the Securities and Exchange Commission (106,117 sq ft/9,850 m²).^[28] Smaller tenants included the Internal Revenue Service Regional Council (90,430 sq ft/8,400 m²) and the United States Secret Service (85,343 sq ft/7,900 m²).^[28] The smallest tenants included the New York City Office of Emergency Management,^[29]National Association of Insurance Commissioners, Federal Home Loan Bank, First State Management Group Inc., Provident Financial Management, and the Immigration and Naturalization Service.^[28] The Department of Defense (DOD) and Central Intelligence Agency (CIA) shared the 25th floor with the IRS.^[5]:2 (The clandestine CIA office was revealed only after the 9/11 attacks.)^[30] Floors 46–47 were mechanical floors, as were the bottom six floors and part of the seventh floor.^[5]:2[30]

9/11 and collapse[edit]

Collapse[edit]

As the North Tower collapsed on September 11, 2001, heavy debris hit 7 World Trade Center, damaging the south face of the building^{[31]:18 (PDF p. 22)} and starting fires that continued to burn throughout the afternoon.^{[6]:16, 18} The collapse also caused damage to the southwest corner between Floors 7 and 17 and on the south face between Floor 44 and the roof; other possible structural damage included a large vertical gash near the center of the south face between Floors 24 and 41.^[6]:17 The building was equipped with a sprinkler system, but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the fire pump to deliver water.^[32]:11 Additionally, water pressure was low, with little or no water to feed sprinklers.^[33]:23–30

After the North Tower collapsed, some firefighters entered 7 World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts.^[34] Over the course of the day, fires burned out of control on several floors of 7 World Trade Center, the flames visible on the east side of the building.^[35] During the afternoon, the fire was also seen on floors 6–10, 13–14, 19–22, and 29–30.^{[31]:24 (PDF p. 28)} In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon.^[7] At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might collapse.^[36] During the afternoon, firefighters also heard creaking sounds coming from the building.^[37] Around 3:30 pm, FDNY Chief Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel.^[38] The fire expanded the girders of the building, causing some to lose their structural integrity. This led column number 79, a critical column supporting a large part of the 13th floor, to buckle, causing the floors above it to collapse to the fifth floor; however, this could not be seen from outside the building. The structure also developed cracks in the facade just before the entire building started to fall.^[6]:21[39] According to FEMA, this collapse started at 5:20:33 pm EDT when the east mechanical penthouse started crumbling.^[5]:23[40] Differing times are given as to what time the building completely collapsed:^[40] at 5:21:10 pm EDT according to FEMA,^[5]:23 and at 5:20:52 pm EDT according to NIST.^{[6]:19, 21, 50–51} There were no casualties associated with the collapse.^[39] NIST found no evidence to support conspiracy theories such as the collapse being the result of explosives; it found that a combination of factors including physical damage, fire and the building's unusual construction set off a chain-reaction collapse.^[41]

Reports[edit]

In May 2002, the Federal Emergency Management Agency (FEMA) issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of Dr. W. Gene Corley, P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued burning unabated due to lack of water for sprinklers or manual firefighting. The report did not reach conclusions about the cause of the collapse and called for further investigation.^[20]:3

Subsequently, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse of the World Trade Center Twin Towers and 7 World Trade Center.^[7] The investigation, led by Dr S. Shyam Sunder, drew upon in-house technical expertise as well as the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE); the Society of Fire Protection Engineers (SFPE); the National Fire Protection Association (NFPA); the American Institute of Steel Construction (AISC); the Council on Tall Buildings and Urban Habitat (CTBUH); and the Structural Engineers Association of New York (SEAoNY).^[42]

The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the Twin Towers.^[7] In the meantime, NIST provided a preliminary report about 7 WTC in June 2004, and thereafter released occasional updates on the investigation.^[31] According to NIST, the investigation of 7 World Trade Center was delayed for a number of reasons, including that NIST staff who had been working on 7 World Trade Center were assigned full-time from June 2004 to September 2005 to work on the investigation of the collapse of the Twin Towers.^[43] In June 2007, Shyam Sunder explained,

We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible. The 7 WTC investigation is in some respects just as challenging, if not more so, than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers.^[44]

In November 2008, NIST released its final report on the causes of the collapse of 7 World Trade Center.^[6] This followed NIST's August 21, 2008, draft report which included a period for public comments,^[7] and was followed in 2012 by a peer-reviewed summary in the Journal of Structural Engineering.^[45] In its investigation, NIST utilized ANSYS to model events leading up to collapse initiation and LS-DYNA models to simulate the global response to the initiating events.^[46]:6–7 NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the Twin Towers or the transfer elements (trusses, girders, and cantilever overhangs). The lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on Floors 8 to 14. With a loss of lateral support across nine floors, column 79 buckled – pulling the east penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the remaining portion of the building above to fall downward as a single unit. The fires, which were fueled by office contents and burned for 7 hours, along with the lack of water, were the key reasons for the collapse.^[6]:21–22 Incidentally, this made the old 7 WTC the only steel skyscraper at the time to have collapsed from fire.^[8]

When 7 WTC collapsed, debris caused substantial damage and contamination to the Borough of Manhattan Community College's Fiterman Hall building, located adjacent at 30 West Broadway, to the extent that the building was not salvageable.^[47] A revised plan called for demolition in 2009 and completion of the new Fiterman Hall in 2012, at a cost of $325 million.^[48] The adjacent Verizon Building, an art deco building constructed in 1926, had extensive damage to its east facade from the collapse of 7 World Trade Center, though it was able to be restored at a cost of US$1.4 billion.^[49]

Files relating to numerous federal investigations had been housed in 7 World Trade Center. The Equal Employment Opportunity Commission estimated over 10,000 of its cases were affected.^[50] Investigative files in the Secret Service's largest field office were lost, with one Secret Service agent saying, 'All the evidence that we stored at 7 World Trade, in all our cases, went down with the building.'^[51] Copies of emails in connection with the WorldCom scandal that were later requested by the SEC from Salomon Brothers, a subsidiary of Citigroup housed in the building, were also destroyed.^[52]

The NIST report found no evidence supporting the conspiracy theories that 7 World Trade Center was brought down by controlled demolition. Specifically, the window breakage pattern and blast sounds that would have resulted from the use of explosives were not observed.^[6]:26–28 The suggestion that an incendiary material such as thermite was used instead of explosives was considered unlikely by NIST because of observations of the fire and the building's structural response to the fire, and because it is unlikely the necessary quantity of material could have been planted without discovery.^[7] Based on its investigation, NIST reiterated several recommendations it had made in its earlier report on the collapse of the Twin Towers.^[6]:63–73 It urged immediate action on a further recommendation: that fire resistance should be evaluated under the assumption that sprinklers are unavailable;^[6]:65–66 and that the effects of thermal expansion on floor support systems be considered.^{[6]:65, 69} Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations was that buildings should not collapse from fire even if sprinklers are unavailable.^[6]:63–73

New building[edit]

The new 7 World Trade Center has 52 stories and is 741 ft (226 m) tall.^[53] The building has 42 floors of leasable space, starting at the 11th floor, and a total of 1,700,000 sq ft (160,000 m²) of office space.^[54] The first ten floors house an electrical substation which provides power to much of Lower Manhattan. The office tower has a narrower footprint at ground level than did its predecessor, so the course of Greenwich Street could be restored to reunite TriBeCa and the Financial District. The original building, on the other hand, had bordered West Broadway on the east, necessitating the destruction of Greenwich Street between Barclay Street and the northern border of the World Trade Center superblock.^[55]

Design[edit]

David Childs of Skidmore, Owings and Merrill worked in conjunction with glass artist and designer James Carpenter to create a design that uses ultra-clear, low-iron glass to provide reflectivity and light, with stainless-steel spandrels behind the glass to help reflect sunlight.^[56] Stainless steel used in the building façade is molybdenum-containing Type 316, which provides improved resistance to corrosion.^[57] To enclose the power substation and improve its aesthetics, the base of the building has a curtain wall with stainless steellouvers that provide ventilation for the machinery.^[58] During the day, the curtain wall reflects light, while at night it is illuminated with blue LED lights.^[59] The curtain wall around the lobby uses heavily laminated, heat-strengthened glass that meets high standards for blast resistance.^[60] At night, a large cube of light above the lobby also emanates blue light, while during the day it provides white light to the lobby, and at dusk it transitions to violet and back to blue.^[61] Inside the main lobby, artist Jenny Holzer created a large light installation with glowing text moving across wide plastic panels.^[56] The entire wall, which is 65 ft (20 m) wide and 14 ft (4.3 m) tall, changes color according to the time of day. Holzer worked with Klara Silverstein, the wife of Larry Silverstein, to select poetry for the art installation. The wall is structurally fortified as a security measure.^[62]

The building is being promoted as the safest skyscraper in the U.S.^[63] According to Silverstein Properties, the owner of the building, it 'incorporate[s] a host of life-safety enhancements that will become the prototype for new high-rise construction.'^[64] The building has 2-foot-thick (0.61 m) reinforced-concrete and fireproofed elevator and stairway access shafts. The original building used only drywall to line these shafts.^[65] The stairways are wider than in the original building to permit faster egress.^[65]

7 World Trade Center is equipped with Otisdestination elevators.^[66] After pressing a destination floor number on a lobby keypad, passengers are grouped and directed to specific elevators that will stop at the selected floor (there are no buttons to press inside the elevators). This system is designed to reduce elevator waiting and travel times. The elevator system is integrated with the lobby turnstile and card reader system that identifies the floor on which a person works as he or she enters and can automatically call the elevator for that floor.^[67]

Nearly 30 percent of structural steel used in the building consists of recycled steel.^[68] Rainwater is collected and used for irrigation of the park and to cool the building.^[56] Along with other sustainable design features, the building is designed to allow in plenty of natural light, power is metered to tenants to encourage them to conserve energy, the heating steam is reused to generate some power for the building, and recycled materials are used for insulation and interior materials.^[69]

Construction[edit]

The construction phase of the new 7 World Trade Center began on May 7, 2002, with the installation of a fence around the construction site.^[1] Tishman Construction Corporation of New York began work at the new 7 World Trade Center in 2002, soon after the site was cleared of debris. Restoring the Con Ed electrical substation was an urgent priority to meet power demands of Lower Manhattan.^[55] Because 7 World Trade Center is separate from the main 16-acre (6.5 ha) World Trade Center site, Larry Silverstein required approval from only the Port Authority, and rebuilding was able to proceed quickly.^[70] Building Seven was not included in the original World Trade Center master plan by Daniel Libeskind, but was designed by Skidmore, Owings & Merrill under the leadership of David Childs, who largely redesigned One World Trade Center.

Once construction of the power substation was complete in October 2003, work proceeded on building the office tower. An unusual approach was used in constructing the building; erecting the steel frame before adding the concrete core. This approach allowed the construction schedule to be shortened by a few months.^[71] Construction was completed in 2006 at a cost of $700 million.^[56] Though Silverstein received $861 million from insurance on the old building, he owed more than $400 million on its mortgage.^[72] Costs to rebuild were covered by $475 million in Liberty Bonds, which provide tax-exempt financing to help stimulate rebuilding in Lower Manhattan and insurance money that remained after other expenses.^[73]

A 15,000 sq ft (1,400 m²) triangular park was created between the extended Greenwich Street and West Broadway by David Childs with Ken Smith and his colleague, Annie Weinmayr, of Ken Smith Landscape Architect. The park comprises an open central plaza with a fountain and flanking groves of sweetgum trees and boxwood shrubs.^[74] At the center of the fountain, sculptor Jeff Koons created Balloon Flower (Red), whose mirror-polished stainless steel represents a twisted balloon in the shape of a flower.^[75]

Opening[edit]

The building was officially opened at noon on May 23, 2006, with a free concert featuring Suzanne Vega, Citizen Cope, Bill Ware Vibes, Brazilian Girls, Ollabelle, Pharaoh's Daughter, Ronan Tynan (of the Irish Tenors), and special guest Lou Reed.^[76][77] Prior to opening, in March 2006, the new 7 World Trade Center frontage and lobby were used in scenes for the movie Perfect Stranger with Halle Berry and Bruce Willis.^[78]

Since the building opened, several unleased upper floors have been used for events such as charity lunches, fashion shows, and black-tie galas. Silverstein Properties allowed space in the new building to be used for these events as a means to draw people to see the building.^[79] From September 8 to October 7, 2006, the work of photographer Jonathan Hyman was displayed in 'An American Landscape', a free exhibit hosted by the World Trade Center Memorial Foundation at 7 World Trade Center. The photographs captured the response of people in New York City and across the United States after the September 11, 2001, attacks. The exhibit took place on the 45th floor while the space remained available for lease.^[80]

By March 2007, 60 percent of the building had been leased.^[81] In September 2006, Moody's signed a 20-year lease to rent 15 floors of 7 World Trade Center.^[82] Other tenants that had signed leases in 7 World Trade Center, as of May 2007, included ABN AMRO,^[83]Ameriprise Financial Inc.,^[84][77] law firm WilmerHale, Darby & Darby P.C.,^[85] Mansueto Ventures LLC, business publisher of Fast Company and Inc.,^[86] and the New York Academy of Sciences.^[87][77]

The space occupied by Mansueto Ventures has been designed to use the maximum amount of natural light and has an open floor plan.^[88] The space used by the New York Academy of Sciences on the 40th floor, designed by H³ Hardy Collaboration Architecture, works with the parallelogram shape of the building. Keeping with the green design of the building, the NYAS uses recycled materials in many of the office furnishings, has zoned heating and cooling, and lights that detect motion, coming on automatically only when people are present, and adjust according to incoming sunlight.^[89]

Silverstein Properties also has offices and the Silver Suites executive office suites^[90] in 7 World Trade Center, along with office space used by the architectural and engineering firms working on 1 World Trade Center, 150 Greenwich Street, 175 Greenwich Street, and 200 Greenwich Street.^[91] The building became fully leased in September 2011 when MSCI was reported to have leased a 125,000 square feet (11,600 m²) space on the top floor.^[92]

See also[edit]

References[edit]

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External links[edit]

• 7 World Trade Center on CTBUH Skyscraper Center

Coordinates: 40°42′41.12″N74°00′44.00″W / 40.7114222°N 74.0122222°W

The Twin Towers of New York City's World Trade Centercollapsed after being deliberately struck by two commercial passenger jets during the September 11 attacks. A total of four commercial aircraft were hijacked by al-Qaedaterrorists on 9/11 and two of those were crashed into the Twin Towers.^[1]American Airlines Flight 11 was crashed into the north side of the North Tower (One World Trade Center) and United Airlines Flight 175 was crashed into the south side of the South Tower (Two World Trade Center).^[2] The collapse of the Twin Towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures. The South Tower collapsed at 9:59 am, less than an hour after being hit, followed by the North Tower at 10:28 am. Later that day, the nearby Seven World Trade Center collapsed at 5:21 pm from fires that had started when the North Tower collapsed.^[3] As a result of the attacks to the towers, a total of 2,763 people died including 2,192 civilians, 343 firefighters, and 71 law enforcement officers as well as all the passengers and crew on the airplanes, including 147 civilians and the 10 hijackers.^[4]

Immediately following the attacks, a building performance study team of engineering specialists was formed by the Structural Engineering Institute of the American Society of Civil Engineers and the Federal Emergency Management Agency. The building performance study team issued its report in May 2002, finding that the aircraft impacts caused, 'extensive structural damage, including localized collapse' and that the resulting fires, 'further weakened the steel-framed structures, eventually leading to total collapse'.^[5]

In September 2005, the National Institute of Standards and Technology, which also consulted outside engineering entities, produced a more detailed engineering investigation into the collapse. The investigators did not find anything substandard in the design of the twin towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in buildings in the past. They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns that 'led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.'^[6]

The cleanup of the World Trade Center site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, One World Trade Center, which was opened in November 2014. As of January 2018, five new buildings had been erected on the site; the last one, Two World Trade Center, is scheduled for completion in 2022.

• 1September 11, 2001
• 2Towers' collapse
• 3Mechanics of Twin Towers' collapse
• 4Investigations
• 5Aftermath
• 7References

September 11, 2001

Aircraft impacts and resultant fires

During the September 11th attacks, four teams of al-Qaeda terrorists hijacked four different jetliners. Two of these jetliners, both Boeing 767's, were hijacked after takeoff from Boston's Logan International Airport. In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly 440 miles per hour (710 km/h) into the north facade of the North Tower (WTC 1) at 8:46 am, impacting between the 93rd and 99th floors. Seventeen minutes later, United Airlines Flight 175 approached from the southwest, over New York Harbor, and crashed into the South Tower (WTC 2) south facade at 9:03 am between the 77th and 85th floors at 540 miles per hour (870 km/h).^[7] In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the resulting explosions in each tower ignited 10,000 US gallons (38,000 L) of jet fuel along with office contents. According to NIST estimates, Flight 11 was carrying around 10,000 US gallons (38,000 L) when it hit the North Tower. Up to 1,500 US gallons (5,700 L) was instantly consumed in the initial fireball and a similar amount was consumed in the fireball outside the building. Approximately 7,000 US gallons (26,000 L) burnt inside the office spaces igniting combustibles. Flight 175 was carrying around 9,100 US gallons (34,000 L) when it hit the South Tower. Up to 1,500 US gallons (5,700 L) was instantly consumed in the initial fireball and up to 2,275 US gallons (8,610 L) was consumed in the fireball outside the building. More than 5,325 US gallons (20,160 L) was burnt in the office spaces. NIST estimated that each floor of both buildings contained around four pounds per square foot (60 tons per floor) of combustibles.^[8]

Jet fuel from the impact traveled down at least one elevator shaft and exploded on the 78th floor of the North Tower, as well as in the main lobby.^[9] The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half.^[10] It has been suggested^{[by whom?]} that the fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely.^[11][12] The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above.^[13] The final fires at the World Trade Center complex were extinguished on December 20, exactly 100 days after the attacks.^[14]

Emergency response and evacuation

Almost all the deaths in the Twin Towers occurred in the zones above the points of aircraft impact. As the North Tower had been struck directly midway into the structure, the three stairways in the tower core were all damaged or blocked by debris preventing escape to lower floors. In the South Tower, the impact was slightly off center to the central section of the tower and stairway A in the northwest portion of the central core was only partially blocked, and 14 to 18 civilians managed to escape from the point of aircraft impact and the floors above that. The exact numbers of who perished and where in some cases is not precisely known, however the National Institute of Standards and Technology report indicated that a total of 1,402 civilians perished at or above the impact point in the North Tower with hundreds estimated to have been killed at the moment of impact. In the South Tower, 614 civilians perished at the impacted floors and the floors above that. Less than 200 of the civilian fatalities occurred in the floors below the impact points but all 147 civilian passengers and crew on the two aircraft as well as all 10 terrorists perished, along with at least 18 people on the ground and in adjacent structures.^[15]

All told, emergency personnel killed in the collapse included 343 New York City Fire Department (FDNY); and 71 law enforcement officers including 23 members of the New York City Police Department (NYPD), 37 members of the Port Authority Police Department (PAPD), five members of the New York State Office of Tax Enforcement (OTE), three officers of the New York State Office of Court Administration (OCA), one fire marshal of the New York City Fire Department (FDNY) who had sworn law enforcement powers (and was also among the 343 FDNY members killed), one member of the Federal Bureau of Investigation (FBI), and one member of the United States Secret Service (USSS). The total death toll for civilian and non-civilians is estimated to be 2,606 persons.

Towers' collapse

South Tower collapse

As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to 9-1-1 dispatchers. At 9:37 a.m., an occupant on the 105th floor of the South Tower reported that floors beneath him 'in the 90-something floor' had collapsed.^[16] The New York City Police Department aviation unit also relayed information about the deteriorating condition of the buildings to police commanders.^[17] Only 14 people escaped from above the impact zone of the South Tower after it was hit (including Stanley Praimnath, who saw the plane coming at him), and only four from the floors above it. They escaped via Stairwell A, the only stairwell which had been left intact after the impact. Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded. Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact.^[18] At 9:52 a.m., the NYPD aviation unit reported over the radio that 'large pieces may be falling from the top of WTC 2. Large pieces are hanging up there'.^[16] With the warnings, the NYPD issued orders for its officers to evacuate. During the emergency response, there was minimal communication between the NYPD and the New York City Fire Department (FDNY), and overwhelmed 9-1-1 dispatchers did not pass along information to FDNY commanders on-scene. At 9:59 a.m., the South Tower collapsed, 56 minutes after being struck.

North Tower collapse

After the South Tower collapsed, NYPD helicopters relayed information about the deteriorating conditions of the North Tower. At 10:20 a.m., the NYPD aviation unit reported that 'the top of the tower might be leaning', and a minute later reported that the North Tower, 'is buckling on the southwest corner and leaning to the south'. At 10:28 a.m., the aviation unit reported that 'the roof is going to come down very shortly'^[16] and indeed, the North Tower collapsed immediately thereafter, at 10:28 a.m., after burning for 102 minutes.

After the South Tower collapsed, FDNY commanders issued orders for firefighters in the North Tower to evacuate. Due to radio communications problems, firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed.^[19] 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings.^[20][21][22] No one was able to escape the North Tower from the impact zone or above, as all stairwells and elevator shafts on those floors were destroyed or blocked.^[23]After the collapse, light dust reached as far as the Empire State Building, located 2.93 miles (4.72 km) away.

Building 7 collapse

As the North Tower collapsed, heavy debris hit 7 World Trade Center, causing damage to the south face of the building^[24] and starting fires that continued to burn throughout the afternoon.^[25] Structural damage occurred to the southwest corner between Floors 7 and 17 and on the south face between Floor 44 and the roof; other possible structural damage includes a large vertical gash near the center of the south face between Floors 24 and 41.^[25] The building was equipped with a sprinkler system, but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the fire pump to deliver water. Also, water pressure was low, with little or no water to feed sprinklers.^[26][27]

Some firefighters entered 7 World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts.^[28] Fires burned into the afternoon on the 11th and 12th floors of 7 World Trade Center, the flames visible on the east side of the building.^[29][30] During the afternoon, fire was also seen on floors 6–10, 13–14, 19–22, and 29–30.^[24] In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon.^[31] At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might cave to one side or 'collapse'.^[32] During the afternoon, firefighters also heard creaking sounds coming from the building and issued uncertain reports about damage in the basement.^[33] Around 3:30 pm FDNY Chief Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel.^[34] At 5:20:33 pm EDT on September 11, 2001, 7 World Trade Center started to collapse, with the crumble of the east mechanical penthouse, while at 5:21:10 pm EDT the entire building collapsed completely.^[35][36] There were no casualties associated with the collapse.

When 7 World Trade Center collapsed, debris caused substantial damage and contamination to the Borough of Manhattan Community College's Fiterman Hall building, located adjacent at 30 West Broadway, to the extent that the building was not salvageable. In August 2007, Fiterman Hall was scheduled for dismantling.^[37] A revised plan called for demolition in 2009 and completion of the new Fiterman Hall in 2012, at a cost of $325 million.^[38][39] The building was finally demolished in November 2009 and construction of its replacement began on December 1, 2009.^[40] The adjacent Verizon Building, an Art Deco building constructed in 1926, had extensive damage to its east facade from the collapse of 7 World Trade Center, though it was successfully restored at a cost of US$1.4 billion.^[41]

Other buildings

Many of the surrounding buildings were also either damaged or destroyed as the towers fell. 5 WTC endured a large fire and a partial collapse of its steel structure and was torn down. Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott World Trade Center (Marriott Hotel 3 WTC), South Plaza (4 WTC), and U.S. Customs (6 WTC). The World Financial Center buildings, 90 West Street, and 130 Cedar Street suffered fires. The Deutsche Bank Building, the Verizon Building, and World Financial Center 3 had impact damage from the towers' collapse, as did 90 West Street. One Liberty Plaza survived structurally intact but sustained surface damage including shattered windows. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank Building, which was covered in a large black 'shroud' after September 11 to cover the building's damage, was deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.^[42][43] Many works of art were destroyed in the collapse.

Mechanics of Twin Towers' collapse

Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides. In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air from the upper levels.

During the collapse, large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside, pushed by the increasing pile of rubble. The result was that the walls peeled off and separated away from the buildings by a large distance (about 500 feet in some cases), hitting other neighboring buildings. Some connections broke as the bolts snapped, leaving many panels randomly scattered.^[44] The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 seconds. The lower portions of both buildings' cores (60 stories of WTC 1 and 40 stories of WTC 2) remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed.^[45]

While the buildings were designed to support enormous static loads, they provided little resistance to the moving mass of the sections above the floors where the collapses initiated. Structural systems respond very differently to static and dynamic loads, and since the motion of the falling portion began as a free fall through the height of at least one story (roughly three meters or 10 feet), the structure beneath them was unable to stop the collapses once they began. Indeed, a fall of only half a meter (about 20 inches) would have been enough to release the necessary energy to begin an unstoppable collapse.^[46]

Collapse initiation

After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating. The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel. As a result, the core columns were slowly being crushed, sustaining plastic and creep deformation from the weight of floors above. As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards. In the case of 2 WTC, this caused the eastern face to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse. In the case of 1 WTC, the south wall later buckled in the same way, and with similar consequences.^[47]

Total progressive collapse

The collapse of the World Trade Center has been called 'the most infamous paradigm' of progressive collapse.^[48] After the collapse initiated, it proceeded through two phases. During the crush-down phase, the upper block destroyed the structure below in a progressive and accelerating series of column failures. After falling through the distance of a single story, the block impacted the columns of the story below, which then buckled, allowing the block to fall through the distance of that story. This process continued until the upper block reached the ground and the crush-up phase began. Here, the columns also buckled successively, one story at a time, starting from the bottom of the upper section. As each story buckled, the weight of the remaining block pushed down from above until the entire section had been crushed.^[48]

Structural design

The towers were designed as 'framed tube' structures, which provided tenants with open floor plans uninterrupted by columns or walls. Numerous, closely spaced perimeter columns provided much of the strength to the structure, along with gravity load shared with the steel box columns of the core. Above the tenth floor, there were 59 perimeter columns along each face of the building, and there were 47 heavier columns in the core. All of the elevators and stairwells were located in the core, leaving a large column-free space between it and the perimeter that was bridged by prefabricated floor trusses.^[7]

The floors consisted of 4-inch-thick (10 cm) lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors with shear connections to the concrete slab for composite action.^[7] The trusses had a span of 60 feet (18 m) in the long-span areas and 35 feet (11 m) in the short-span area.^[7] The trusses connected to the perimeter at alternate columns, and were therefore on 6.8-foot (2.1 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to interior box columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants.

The towers also incorporated a 'hat truss' or 'outrigger truss' located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis. This truss system allowed optimized load redistribution of floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment frames to transfer sway into compression on the core, which also mostly supported the transmission tower.

Safety concerns regarding aircraft impacts

The structural engineers working on the World Trade Center considered the possibility that an aircraft could crash into the building. In July 1945, a B-25 bomber that was lost in the fog had crashed into the 79th floor of the Empire State Building. A year later, a C-45F Expeditor^[49] crashed into the 40 Wall Street building,^[50] and there was a near-hit at the Empire State Building.^[51]Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, has since said he personally considered the scenario of the impact of a Boeing 707 or another jet airliner, which might be lost in the fog and flying at relatively low speeds while seeking to land at either JFK Airport or Newark Airport. However, in an interview with the BBC, Robertson claimed that, 'with the 707, the fuel load was not considered in the design, I don't know how it could have been considered.'^[51][52]

During their investigation into the collapse, the National Institute of Standards and Technology (NIST) found a three-page white paper that mentioned aircraft-impact of a Boeing 707 at 600 miles per hour (970 km/h). The original documentation of the study, which was part of the building's 1,200-page structural analysis, was lost when the records in the Port Authority of New York and New Jersey's offices were destroyed in the collapse of the 1 WTC; the copy was lost in 7 WTC.^[53] In 1993, John Skilling, lead structural engineer for the WTC, recalled doing the analysis, and remarked, 'Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed', he said. 'The building structure would still be there.'^[54] In its report, NIST stated that the technical ability to perform a rigorous simulation of aircraft impact and ensuing fires is a recent development, and that the technical capability for such analysis would have been quite limited in the 1960s.^{[45][note 1]}

Fireproofing

In April 1970, the New York City Department of Air Resources ordered contractors building the World Trade Center to stop the spraying of asbestos as an insulating material.^[55] Fireproofing was incorporated in the original construction and more was added after a fire in 1975 that spread to six floors before being extinguished. After the 1993 bombing, inspections found fireproofing to be deficient. The Port Authority was in the process of replacing it, but replacement had been completed on only 18 floors in 1 WTC, including all the floors affected by the aircraft impact and fires,^[56] and on 13 floors in 2 WTC, although only three of these floors (77, 78, and 85) were directly affected by the aircraft impact.^{[57][note 2]}

Investigations

Initial opinions and analysis

In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse. Abdolhassan Astaneh-Asl, a structural engineering professor at the University of California at Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become 'soft and mushy', and eventually they were unable to support the structure above. Astaneh-Asl also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that, once the initial structural failure occurred, progressive collapse of the entire structure was inevitable.^[60]Cesar Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked, 'no building is prepared for this kind of stress.'^[61]

On September 13, 2001, Zdeněk Bažant, professor of civil engineering and materials science at Northwestern University, circulated a draft paper with results of a simple analysis of the World Trade Center collapse. Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation before losing their carrying capacity and buckling. Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred. Bažant later published an expanded version of this analysis.^[62] Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who also described a collapse mechanism on September 21, 2001.^[63] They later contributed to an MIT collection of papers on the WTC collapses edited by Eduardo Kausel called The Towers Lost and Beyond.^[64]

Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. While there are clear procedures for the investigation of aircraft accidents, no agency had been appointed in advance to investigate building collapses.^[65] A team was quickly assembled by the Structural Engineers Institute of the American Society of Civil Engineers, led by W. Gene Corley, Senior Vice President of CTLGroup. It also involved the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers.^[66] ASCE ultimately invited FEMA to join the investigation, which was completed under the auspices of the latter.^[66]

The investigation was criticized by some engineers and lawmakers in the U.S. It had little funding, no authority to demand evidence, and limited access to the WTC site. One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings' steel components.^[67] Indeed, when NIST published its final report, it noted 'the scarcity of physical evidence' that it had had at its disposal to investigate the collapses. Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.^[68]

FEMA published its report in May 2002. While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, 2002 (a year after the disaster), there was growing public pressure for a more thorough investigation.^[69] Congress passed the National Construction Safety Team bill in October 2002, giving NIST the authority to conduct an investigation of the World Trade Center collapses.^[70]

FEMA building performance study

FEMA suggested that fires in conjunction with damage resulting from the aircraft impacts were the key to the collapse of the towers. Thomas Eagar, Professor of Materials Engineering and Engineering Systems at MIT, described the fires as 'the most misunderstood part of the WTC collapse'. This is because the fires were originally said to have 'melted' the floors and columns.^[71]Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires.^[72] As Eagar said, 'The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.'^[73] This led Eagar, FEMA and others to focus on what appeared to be the weakest point of the structures, namely, the points at which the floors were attached to the building frame.^[74]

The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings’ contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.^[75]

NIST report

After the FEMA report had been published, and following pressure from technical experts, industry leaders and families of victims, the Commerce Department's National Institute of Standards and Technology conducted a three-year, $16 million investigation into the structural failure and progressive collapse of several WTC complex structures.^[76] The study included in-house technical expertise, along with assistance from several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers, Society of Fire Protection Engineers, National Fire Protection Association, American Institute of Steel Construction, Simpson Gumpertz & Heger Inc., Council on Tall Buildings and Urban Habitat, and the Structural Engineers Association of New York.

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The scope of the NIST investigation was focused on identifying 'the sequence of events' that triggered the collapse, and did not include detailed analysis of the collapse mechanism itself (after the point at which events made the collapse inevitable).^[77][78][79] In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these using the software program Fire Dynamics Simulator. NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed. These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models. So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point.

James Quintiere, professor of fire protection engineering at the University of Maryland, called the spoliation of the steel 'a gross error' that NIST should have openly criticized.^[80] He also noted that the report lacked a timeline and physical evidence to support its conclusions.^[81] Some engineers have suggested that understanding of the collapse mechanism could be improved by developing an animated sequence of the collapses based on a global dynamic model, and comparing it with the video evidence of the actual collapses.^[82] The NIST report for WTC 7 concluded that no blast sounds were heard on audio and video footage, or were reported by witnesses.^[83]

7 World Trade Center

In May 2002, FEMA issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of Dr. W. Gene Corley, P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued unabated due to lack of water for sprinklers or manual firefighting. The report did not reach conclusions about the cause of the collapse and called for further investigation.^{[citation needed]}

In response to FEMA's concerns, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse of the World Trade Center twin towers and 7 World Trade Center.^[84] The investigation, led by Dr S. Shyam Sunder, drew not only upon in-house technical expertise, but also upon the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE), the Society of Fire Protection Engineers (SFPE), the National Fire Protection Association (NFPA), the American Institute of Steel Construction (AISC), the Council on Tall Buildings and Urban Habitat (CTBUH), and the Structural Engineers Association of New York (SEAoNY).^[85]

The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the collapse of the World Trade Center twin towers.^[31] In the meantime, NIST provided a preliminary report about 7 World Trade Center in June 2004, and thereafter released occasional updates on the investigation.^[24] According to NIST, the investigation of 7 World Trade Center was delayed for a number of reasons, including that NIST staff who had been working on 7 World Trade Center were assigned full-time from June 2004 to September 2005 to work on the investigation of the collapse of the twin towers.^[45] In June 2007, Shyam Sunder explained, 'We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible. The 7 WTC investigation is in some respects just as challenging, if not more so, than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers.'^[86]

In November 2008, NIST released its final report on the causes of the collapse of 7 World Trade Center.^[25] This followed their August 21, 2008 draft report which included a period for public comments.^[31] In its investigation, NIST utilized ANSYS to model events leading up to collapse initiation and LS-DYNA models to simulate the global response to the initiating events.^[87] NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the twin towers, nor did the transfer elements (trusses, girders, and cantilever overhangs). But the lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near Column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on Floors 8 to 14. With a loss of lateral support across nine floors, Column 79 soon buckled – pulling the East penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the entire building above to fall downward as a single unit. From collapse timing measurements taken from a video of the north face of the building, NIST observed that the building's exterior facade fell at free fall acceleration through a distance of approximately 8 stories (32 meters, or 105 feet), noting 'the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent.'^[88] The fires, fueled by office contents, along with the lack of water, were the key reasons for the collapse.^[25]

The collapse of the old 7 World Trade Center is remarkable because it was the first known instance of a tall building collapsing primarily as a result of uncontrolled fires.^[31] Based on its investigation, NIST reiterated several recommendations it had made in its earlier report on the collapse of the twin towers, and urged immediate action on a further recommendation: that fire resistance should be evaluated under the assumption that sprinklers are unavailable; and that the effects of thermal expansion on floor support systems be considered. Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations is that buildings should not collapse from fire even if sprinklers are unavailable.^[25]

Other investigations

In 2003, Asif Usmani, Professor of Structural Engineering at University of Edinburgh, published a paper with two colleagues. They provisionally concluded the fires alone, without any damage from the airplanes, could have been enough to bring down the buildings. In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.^[89] When the NIST report was published, Barbara Lane, with the UK engineering firm Arup, criticized its conclusion that the loss of fire proofing was a necessary factor in causing the collapses; 'We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact.'^[90]Jose L Torero, formerly of the BRE Centre for Fire Safety Engineering at the University of Edinburgh, pursued further research into the potentially catastrophic effects of fire on real-scale buildings.^[91][92][93]

Aftermath

Cleanup

The cleanup was a massive operation coordinated by the City of New York Department of Design and Construction. On September 22, a preliminary cleanup plan was delivered by Controlled Demolition, Inc. (CDI) of Phoenix, Maryland.^[94] Costing hundreds of millions of dollars, it involved round-the-clock operations with many contractors and subcontractors.^[95] By early November, with a third of the debris removed, officials began to reduce the number of firefighters and police officers assigned to recovering the remains of victims, in order to prioritize the removal of debris. This caused confrontations with firefighters.^[96] Despite efforts to extinguish the blaze, the large pile of debris burned for three months, until the majority of the rubble was finally removed from the site.^[97][98] In 2007, the demolition of the surrounding damaged buildings was still ongoing as new construction proceeded on the World Trade Center's replacement, 1 World Trade Center.

Health effects

The collapse of the World Trade Center produced enormous clouds of dust that covered Manhattan for days. On September 18, 2001, the United States Environmental Protection Agency (EPA) assured the public that the air in Manhattan was 'safe to breathe'.^[99] In 2003 the EPA's inspector general found that the agency did not at that time have sufficient data to make such a statement. Dust from the collapse seriously reduced air quality and is likely the cause of many respiratory illnesses in lower Manhattan. Asbestosis is such an illness, and asbestos would have been present in the dust.^[100] Significant long term medical and psychological effects have been found among first responders including elevated levels of asthma, sinusitis, gastroesophageal reflux disease and posttraumatic stress disorder.^[101]

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Health effects also extended to residents, students, and office workers of Lower Manhattan and nearby Chinatown.^[102] Several deaths have been linked to the toxic dust, and the victims' names will be included in the World Trade Center memorial.^[103] More than 18,000 people have suffered from illnesses from the dust.^[104]

See also

References

Explanatory notes

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1. ^The three-page white paper titled Salient points with regard to the structural design of The World Trade Center towers described an analysis of a Boeing 707 weighing 336,000 pounds (152 t) and carrying 23,000 US gallons (87 m³) of fuel striking the 80th floor of the buildings at 600 miles per hour (970 km/h). It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design, but this study is in line with remarks made by John Skilling following the 1993 WTC bombing. Without original documentation for either study, NIST said any further comments would amount to speculation.—NIST 2005. pp. 305–307
2. ^Despite reports that both towers had asbestos fireproofing to their 64th floors^[58] and that the fireproofing was being replaced due to its asbestos content, in fact the builders had been informed of a proposed ban on using asbestos/vermiculite fireproofing during construction and had ceased using it. By this time, only the fireproofing of the lower 40 floors of the north tower had been completed, and more than half of this was later replaced before the building was completed.^[59] NIST concluded that the thickness of the fireproofing was 0.75 inches (1.9 cm) in the North Tower and 1.5 inches (3.8 cm) in the South Tower. NIST noted that upon reviewing the building records, it could not determine how this thickness was arrived at. NIST further concluded that the aircraft impact removed a significant portion of the fireproofing, contributing to the buildings' collapse, though many engineers, including one of NIST's top advisers, strongly disputed that claim, instead saying the fireproofing was simply not thick enough.

Citations

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2. ^'Security Council Condemns, 'In Strongest Terms', Terrorist Attacks on the United States'. United Nations. September 12, 2001. Retrieved September 11, 2006. The Security Council today, following what it called yesterday’s 'horrifying terrorist attacks' in New York, Washington, D.C., and Pennsylvania, unequivocally condemned those acts, and expressed its deepest sympathy and condolences to the victims and their families and to the people and Government of the United States.
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6. ^'NIST Response to the World Trade Center Disaster'(PDF). Retrieved April 23, 2008.
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56. ^NCSTAR 1–6, p lxxi
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72. ^Eagar & Musso 2001, p. 10: 'The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000 °C—hardly sufficient to melt steel at 1,500 °C.'
73. ^Eagar & Musso 2001, p. 9: 'The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.'
74. ^Eagar & Musso 2001, p. 11: 'It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure.'
75. ^FEMA(403) Executive Summary, p. 2.
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Bibliography

• Dwyer, Jim; Kevin Flynn (2004). 102 Minutes: The Untold Story of the Fight to Survive Inside the Twin Towers. Times Books. ISBN978-0-8050-7682-0. OCLC156884550.
• Corley, G.; Hamburger, R.; McAllister, T. (2002), 'Executive Summary', in FEMA Report 403 (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations and Recommendations, Federal Emergency Management Agency, retrieved November 25, 2012
• National Institute of Standards and Technology, Technology Administration (2006). 'NIST and the World Trade Center'. NIST building and fire safety investigation. U.S. Department of Commerce. Retrieved May 2, 2006.
• Wilkinson, Tim (2006). 'World Trade Center – Some Engineering Aspects'. School of Civil Engineering, The University of Sydney. Retrieved May 2, 2006.
• McAllister, Therese; W. Gene Corley; et al. (2002). 'World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations (FEMA 403)'(PDF). Federal Emergency Management Agency. Retrieved May 2, 2006.
• Eagar, T. W.; Musso, C. (2001), 'Why did the world trade center collapse? Science, engineering, and speculation'(PDF), JOM, 53 (12): 8, Bibcode:2001JOM....53l...8E, doi:10.1007/s11837-001-0003-1, archived from the original(PDF) on May 10, 2012, retrieved November 29, 2012
• Bažant, Zdeněk P.; Yong Zhou (2001). 'Why Did the World Trade Center Collapse? – Simple Analysis'(PDF). Journal of Engineering Mechanics. Retrieved September 11, 2006.
• Clifton, G. Charles (2001). 'Collapse of the World Trade Centre Towers'. CAD Digest. TenLinks, Inc. Archived from the original on April 27, 2006. Retrieved May 2, 2006.
• Edgar, Dr. Thomas; NOVA (2002). 'The Collapse: An Engineer's Perspective'. Why the Towers Fell. WGBH Educational Foundation. Retrieved May 2, 2006.
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• NOVA online (2002). 'The structure of metal'. Why the Towers Fell. WGBH Educational Foundation. Retrieved May 2, 2006.
• Kean, Thomas H. (2004). 'Eleventh Public Hearing'. Hearings. National Commission on Terrorist Attacks Upon the United States. Retrieved May 2, 2006.
• Seth Kramer, Daniel Miller (2002). America Rebuilds: A Year at Ground Zero (Television series). United States: PBS home video.
• 'Answers to Frequently Asked Questions (August 30, 2006)'. NIST Federal Building and Fire Safety Investigation of the World Trade Center Disaster. National Institute of Standards and Technology. 2006. Retrieved April 21, 2008.

Further reading

• Corley, G.; Hamburger, R.; McAllister, T. (2002), 'Executive Summary'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• McAllister, T.; Barnett, J.; Gross, J.; et al. (2002), 'Chapter 1. Introduction'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Hamburger, R.; Baker, W.; Barnett, J.; et al. (2002), 'Chapter 2. WTC 1 and WTC 2'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Baker, W. (2002), 'Chapter 3. WTC 3'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Barnett, J.; Gewain, R.; Gilsanz, R.; et al. (2002), 'Chapter 4. WTC 4, 5, and 6'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Gilsanz, R.; DePaola, E. M.; Marrion, C.; et al. (2002), 'Chapter 5. WTC 7'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Smilowitz, R.; Hapij, A.; Smilow, J. (2002), 'Chapter 6. Bankers Trust Building'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• McAllister, T.; Biggs, D.; DePaola, E. M.; et al. (2002), 'Chapter 7. Peripheral Buildings'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Milke, J.; Kodur, V.; Marrion, C. (2002), 'Appendix A. Overview of Fire Protection in Buildings'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Fisher, J.; Iwankiw, N. (2002), 'Appendix B. Structural Steel and Steel Connections'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Barnett, J.; Biederman, R. R.; Sisson, Jr., R. D. (2002), 'Appendix C. Limited Metallurgical Examination'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Gilsanz, R.; Massa, A. (2002), 'Appendix D. WTC Steel Data Collection'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• McAllister, T., ed. (2002), 'Appendix E. Aircraft Information'(PDF), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• DePaola, E. M. (2002), 'Appendix F. Structural Engineers Emergency Response Plan'(PDF), in McAllister, T. (ed.), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, ASCE/FEMA
• Eagar, T. W.; Musso, C. (2001). 'Why did the world trade center collapse? Science, engineering, and speculation'(PDF). JOM. 53 (12): 8. Bibcode:2001JOM....53l...8E. doi:10.1007/s11837-001-0003-1. Archived from the original(PDF) on May 10, 2012.
• Banovic, S. W.; Foecke, T.; Luecke, W. E.; et al. (2007), 'The role of metallurgy in the NIST investigation of the World Trade Center towers collapse', JOM, 59 (11): 22–30, Bibcode:2007JOM....59k..22B, doi:10.1007/s11837-007-0136-y
• Hammett, Kingsley (November 2001). 'WHY THE WORLD TRADE CENTER COLLAPSED: An Architect's Perspective'. Designer/Builder. Retrieved October 24, 2015.

External links

Where Did The Twin Towers Go

• NIST and the World Trade Center The National Institute of Standards and Technology's page on the collapse of the WTC. Contains most recent developments in investigations and FAQs.
• Video: The Collapse of World Trade Center 7: Why the Building Fell (NIST)
• World Trade Center – Some Engineering Aspects Early suggestion by University of Sydney engineering instructor about how the towers might have collapsed.
• Bill Biggart's Final Exposures contains a photo of the WTC Marriott severely damaged by the collapse of 2 WTC immediately before the collapse of 1 WTC in which the photographer was killed.
• World Trade Center StudiesAbdolhassan Astaneh-Asl (Principal Investigator)

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