Construction of the World Trade Center

The construction
Construction
In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

of the World Trade Center
World Trade Center
The original World Trade Center was a complex with seven buildings featuring landmark twin towers in Lower Manhattan, New York City, United States. The complex opened on April 4, 1973, and was destroyed in 2001 during the September 11 attacks. The site is currently being rebuilt with five new...

was conceived as an urban renewal

Urban renewal

Urban renewal is a program of land redevelopment in areas of moderate to high density urban land use. Renewal has had both successes and failures. Its modern incarnation began in the late 19th century in developed nations and experienced an intense phase in the late 1940s – under the rubric of...

project, spearheaded by David Rockefeller

David Rockefeller

David Rockefeller, Sr. is the current patriarch of the Rockefeller family. He is the youngest and only surviving child of John D. Rockefeller, Jr. and Abby Aldrich Rockefeller, and the only surviving grandchild of oil tycoon John D. Rockefeller, founder of Standard Oil. His five siblings were...

, to help revitalize Lower Manhattan

Lower Manhattan

Lower Manhattan is the southernmost part of the island of Manhattan, the main island and center of business and government of the City of New York...

. The project was developed by the Port Authority of New York and New Jersey

Port Authority of New York and New Jersey

The Port Authority of New York and New Jersey is a bi-state port district, established in 1921 through an interstate compact, that runs most of the regional transportation infrastructure, including the bridges, tunnels, airports, and seaports, within the Port of New York and New Jersey...

, which hired architect Minoru Yamasaki

Minoru Yamasaki

was a Japanese-American architect, best known for his design of the twin towers of the World Trade Center, buildings 1 and 2. Yamasaki was one of the most prominent architects of the 20th century...

who came up with the specific idea for twin towers. After extensive negotiations, the New Jersey

New Jersey

New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

and New York State

New York

New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

governments, which oversee the Port Authority, agreed to support the World Trade Center project at the Radio Row site on the lower-west side of Manhattan

Manhattan

Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York...

. To make the agreement acceptable to New Jersey, the Port Authority agreed to take over the bankrupt Hudson & Manhattan Railroad (renamed as PATH

Port Authority Trans-Hudson

PATH, derived from Port Authority Trans-Hudson, is a rapid transit railroad linking Manhattan, New York City with Newark, Harrison, Hoboken and Jersey City in metropolitan northern New Jersey...

), which brought commuters from New Jersey to the Lower Manhattan site.

The towers were designed as framed tube structures

Tube (structure)

In structural engineering, the tube is the name given to the systems where in order to resist lateral loads a building is designed to act like a three-dimensional hollow tube, cantilevered perpendicular to the ground. The system was introduced by Fazlur Rahman Khan while at Skidmore, Owings and...

, which provided tenants with open floor plans, uninterrupted by columns or walls. This was accomplished using numerous closely spaced perimeter columns to provide much of the strength to the structure, along with gravity load shared with the core columns. The elevator

Elevator

An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...

system, which made use of sky lobbies and a system of express and local elevators, allowed substantial floor space to be freed up for use as office space by making the structural core smaller. The design and construction of the World Trade Center twin towers involved many other innovative techniques, such as the slurry wall

Slurry wall

A slurry wall is a technique used to build reinforced-concrete walls in areas of soft earth close to open water or with a high ground water table. This technique is typically used to build diaphragm walls surrounding tunnels and open cuts, and to lay foundations.A trench is excavated to create a...

for digging the foundation

Foundation (architecture)

A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...

, and wind tunnel

Wind tunnel

A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...

experiments. Construction of the World Trade Center's North Tower began in August 1968, and the South Tower in 1969. Extensive use of prefabricated components helped to speed up the construction process. The first tenants moved into the North Tower in December 1970 and into the South Tower in January 1972. Four other low-level buildings were constructed as part of the World Trade Center in the 1970s, and a seventh

7 World Trade Center

7 World Trade Center is a building in New York City located across from the World Trade Center site in Lower Manhattan. It is the second building to bear that name and address in that location. The original structure was completed in 1987 and was destroyed in the September 11 attacks...

building was constructed in the mid-1980s.

Planning

In 1942, Austin J. Tobin

Austin J. Tobin

Austin Joseph Tobin served as the executive director of the Port of New York Authority, the precursor to the Port Authority of New York and New Jersey, from 1942 until 1972...

became the Executive Director of the Port Authority

Port Authority of New York and New Jersey

, beginning a 30-year career during which he oversaw the planning and development of the World Trade Center. The concept of establishing a "world trade center" was conceived during the post-World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

period, when the United States thrived economically and international trade was increasing. In 1946, the New York State Legislature passed a bill that called for a "world trade center" to be established. The World Trade Corporation was founded, and a board was appointed by New York Governor Thomas E. Dewey to develop plans for the project. Architect John Eberson

John Eberson

John Eberson was an American architect best known for his movie palace designs in the atmospheric theatre fashion.Born in Czernowitz, Austro-Hungarian Empire , Eberson went to highschool in Dresden and studied electrical engineering in Vienna. He arrived in the United States in 1901 and at first...

and his son Drew devised a plan that included 21 buildings over a ten-block area, at an estimated cost of $150 million. In 1949, the World Trade Corporation was dissolved by the New York State Legislature, and plans for a "world trade center" were put on hold.

Original plans

During the post-war period, economic growth was concentrated in Midtown Manhattan

Midtown Manhattan

Midtown Manhattan, or simply Midtown, is an area of Manhattan, New York City home to world-famous commercial zones such as Rockefeller Center, Broadway, and Times Square...

, in part stimulated by the Rockefeller Center

Rockefeller Center

Rockefeller Center is a complex of 19 commercial buildings covering between 48th and 51st streets in New York City, United States. Built by the Rockefeller family, it is located in the center of Midtown Manhattan, spanning the area between Fifth Avenue and Sixth Avenue. It was declared a National...

, which was developed in the 1930s. Meanwhile, Lower Manhattan was left out of the economic boom

Boom and bust

A credit boom-bust cycle is an episode characterized by a sustained increase in several economics indicators followed by a sharp and rapid contraction. Commonly the boom is driven by a rapid expansion of credit to the private sector accompanied with rising prices of commodities and stock market index...

. One exception was the construction of One Chase Manhattan Plaza

One Chase Manhattan Plaza

One Chase Manhattan Plaza is a banking skyscraper located in the downtown Manhattan Financial District of New York City, between Pine, Liberty, Nassau, and William Streets. Construction on the building was completed in 1961...

in the Financial District

Financial District, Manhattan

The Financial District of New York City is a neighborhood on the southernmost section of the borough of Manhattan which comprises the offices and headquarters of many of the city's major financial institutions, including the New York Stock Exchange and the Federal Reserve Bank of New York...

by David Rockefeller

David Rockefeller

, who led urban renewal

Urban renewal

efforts in Lower Manhattan. In 1958, Rockefeller established the Downtown-Lower Manhattan Association (DLMA), which commissioned Skidmore, Owings and Merrill

Skidmore, Owings and Merrill

Skidmore, Owings and Merrill LLP is an American architectural and engineering firm that was formed in Chicago in 1936 by Louis Skidmore and Nathaniel Owings; in 1939 they were joined by John O. Merrill. They opened their first branch in New York City, New York in 1937. SOM is one of the largest...

to draw up plans for revitalizing Lower Manhattan

Lower Manhattan

Lower Manhattan is the southernmost part of the island of Manhattan, the main island and center of business and government of the City of New York...

. The plans, made public in 1960, called for a World Trade Center to be built on a 13 acres (52,609.2 m²) site along the East River

East River

The East River is a tidal strait in New York City. It connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates Long Island from the island of Manhattan and the Bronx on the North American mainland...

, from Old Slip to Fulton Street

Fulton Street (Manhattan)

Fulton Street is a busy street located in Lower Manhattan. It is in New York City's Financial District, a few blocks north of Wall Street. It runs from Church Street at the site of the World Trade Center to South Street, terminating in front of the South Street Seaport...

and between Water Street and South Street

South Street (Manhattan)

South Street is a street in Lower Manhattan, New York City, located immediately adjacent to the East River.It runs from Whitehall Street near the southern tip of Manhattan to Jackson Street near the Williamsburg Bridge. The Franklin D...

. The complex would include a 900-foot (275 m) long exhibition hall

Convention center

A convention center is a large building that is designed to hold a convention, where individuals and groups gather to promote and share common interests. Convention centers typically offer sufficient floor area to accommodate several thousand attendees...

, and a 50–70 story building, with some of its upper floors used as a hotel. Other amenities would include a theater, shops, and restaurants. The plan also called for a new securities

Security (finance)

A security is generally a fungible, negotiable financial instrument representing financial value. Securities are broadly categorized into:* debt securities ,* equity securities, e.g., common stocks; and,...

exchange building, which the Downtown-Lower Manhattan Association hoped would house the New York Stock Exchange

New York Stock Exchange

The New York Stock Exchange is a stock exchange located at 11 Wall Street in Lower Manhattan, New York City, USA. It is by far the world's largest stock exchange by market capitalization of its listed companies at 13.39 trillion as of Dec 2010...

.

David Rockefeller suggested that the Port Authority would be a logical choice for taking on the project, and argued that the Trade Center would provide great benefits in facilitating and increasing volume of international commerce coming through the Port of New York. Given the importance of New York City

New York City

New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

in global commerce, Port Authority director Austin J. Tobin remarked that the proposed project should be the World Trade Center, and not just a "world trade center." After a year-long review of the proposal, the Port Authority formally backed the project on 11 March 1961.

Agreement

The States of New York and New Jersey also needed to approve the project, given their control and oversight role of the Port Authority. Objections to the plan came from New Jersey Governor

Governor of New Jersey

The Office of the Governor of New Jersey is the executive branch for the U.S. state of New Jersey. The office of Governor is an elected position, for which elected officials serve four year terms. While individual politicians may serve as many terms as they can be elected to, Governors cannot be...

Robert B. Meyner

Robert Baumle Meyner of Phillipsburg, New Jersey was an American Democratic Party politician, who served as the 44th Governor of New Jersey, from 1954 to 1962...

, who resented that New York would be getting this $335 million project. Meanwhile, ridership on New Jersey's Hudson and Manhattan Railroad (H&M) had declined substantially from a high of 113 million riders in 1927 to 26 million in 1958, after new automobile tunnels and bridges opened across the Hudson River

Hudson River

The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...

. Toward the end of 1961, negotiations with outgoing New Jersey Governor Meyner regarding the World Trade Center project reached a stalemate. In December 1961, Tobin met with newly elected New Jersey Governor Richard J. Hughes

Richard J. Hughes

Richard Joseph Hughes was an American Democratic Party politician, who served as the 45th Governor of New Jersey from 1962 to 1970, and as Chief Justice of the New Jersey Supreme Court from 1973–1979...

, and made a proposal to shift the World Trade Center project to a west side site where the Hudson Terminal

Hudson Terminal

Hudson Terminal was an urban railway station on the Hudson and Manhattan Railroad in Lower Manhattan, New York City, and the office skyscraper built to serve the terminal.- Station :...

was located. In acquiring the Hudson & Manhattan Railroad, the Port Authority would also acquire the Hudson Terminal and other buildings which were deemed obsolete. On 22 January 1962, the two states reached an agreement to allow the Port Authority to take over the railroad and to build the World Trade Center on Manhattan's lower west side. The shift in location for the World Trade Center to a site more convenient to New Jersey, together with Port Authority acquisition of the H&M Railroad, brought New Jersey to agreement in support of the World Trade Center project.

Controversy

Even once the agreement between the states of New Jersey, New York, and the Port Authority was finalized, the World Trade Center plan faced continued controversy. The site for the World Trade Center was the location of Radio Row

Radio Row

Radio Row is a nickname for an urban street or district specializing in the sale of radio and electronic equipment and parts. Radio Rows arose in many cities with the 1920s rise of broadcasting and declined after mid century.-New York:...

, which was home to hundreds of commercial and industrial tenants, property owners, small businesses, and approximately 100 residents. The World Trade Center plans involved evicting these business owners, some of whom fiercely protested the forced relocation. In June 1962, a group representing approximately 325 shops and 1,000 other affected small businesses filed an injunction

Injunction

An injunction is an equitable remedy in the form of a court order that requires a party to do or refrain from doing certain acts. A party that fails to comply with an injunction faces criminal or civil penalties and may have to pay damages or accept sanctions...

, challenging the Port Authority's power of eminent domain

Eminent domain

Eminent domain , compulsory purchase , resumption/compulsory acquisition , or expropriation is an action of the state to seize a citizen's private property, expropriate property, or seize a citizen's rights in property with due monetary compensation, but without the owner's consent...

. The dispute with local business owners worked its way through the court system, up to the New York State Court of Appeals

New York Court of Appeals

The New York Court of Appeals is the highest court in the U.S. state of New York. The Court of Appeals consists of seven judges: the Chief Judge and six associate judges who are appointed by the Governor to 14-year terms...

, which in April 1963 upheld the Port Authority's right of eminent domain, saying that the project had a "public purpose." On 12 November 1963, the United States Supreme Court

Supreme Court of the United States

The Supreme Court of the United States is the highest court in the United States. It has ultimate appellate jurisdiction over all state and federal courts, and original jurisdiction over a small range of cases...

refused to accept the case. Under the state law, the Port Authority was required to assist business owners in relocating, though many business owners regarded what the Port Authority offered as inadequate. Questions continued while the World Trade Center was constructed, as to whether the Port Authority really ought to take on the project, described by some as a "mistaken social priority."

Private real estate

Real estate

In general use, esp. North American, 'real estate' is taken to mean "Property consisting of land and the buildings on it, along with its natural resources such as crops, minerals, or water; immovable property of this nature; an interest vested in this; an item of real property; buildings or...

developers and members of the Real Estate Board of New York also expressed concerns about this much "subsidized" office space going on the open market, competing with the private sector when there was already a glut of vacancies. An especially vocal critic was Lawrence A. Wien, owner of the Empire State Building

Empire State Building

The Empire State Building is a 102-story landmark skyscraper and American cultural icon in New York City at the intersection of Fifth Avenue and West 34th Street. It has a roof height of 1,250 feet , and with its antenna spire included, it stands a total of 1,454 ft high. Its name is derived...

, which would lose its title of tallest building in the world. Wien organized a group of builders into a group called the "Committee for a Reasonable World Trade Center" to demand that the project be scaled down.

In January 1964, the Port Authority inked a deal with the State of New York to locate government offices at the World Trade Center. The Port Authority began signing commercial tenants in the spring and summer of 1964, including several banks. In 1965, the Port Authority signed the United States Customs Service

United States Customs Service

Until March 2003, the United States Customs Service was an agency of the U.S. federal government that collected import tariffs and performed other selected border security duties.Before it was rolled into form part of the U.S...

as a tenant.

A final obstacle for the Port Authority was getting approval from New York City Mayor

Mayor of New York City

The Mayor of the City of New York is head of the executive branch of New York City's government. The mayor's office administers all city services, public property, police and fire protection, most public agencies, and enforces all city and state laws within New York City.The budget overseen by the...

John Lindsay

John Vliet Lindsay was an American politician, lawyer and broadcaster who was a U.S. Congressman, Mayor of New York City, candidate for U.S...

and the New York City Council

New York City Council

The New York City Council is the lawmaking body of the City of New York. It has 51 members from 51 council districts throughout the five boroughs. The Council serves as a check against the mayor in a "strong" mayor-council government model. The council monitors performance of city agencies and...

, who raised concerns about the limited extent that the Port Authority involved the city in the negotiations and deliberations. Negotiations between The City of New York and the Port Authority were centered on tax issues. A final agreement was made on 3 August 1966, that the Port Authority would make annual payments to the City, in lieu of taxes, for the portion of the World Trade Center leased to private tenants. In subsequent years, the payments would rise as the real estate tax

Property tax

A property tax is an ad valorem levy on the value of property that the owner is required to pay. The tax is levied by the governing authority of the jurisdiction in which the property is located; it may be paid to a national government, a federated state or a municipality...

rate increased.

Design

On 20 September 1962, the Port Authority announced the selection of Minoru Yamasaki

Minoru Yamasaki

was a Japanese-American architect, best known for his design of the twin towers of the World Trade Center, buildings 1 and 2. Yamasaki was one of the most prominent architects of the 20th century...

as lead architect, and Emery Roth & Sons

Emery Roth

Emery Roth was an American architect who designed many of the definitive New York City hotels and apartment buildings of the 1920s and 30s, incorporating Beaux-Arts and Art Deco details...

as associate architects. Originally, Yamasaki submitted to the Port Authority a concept incorporating twin towers, but with each building only 80 stories tall. Yamasaki remarked that the "obvious alternative, a group of several large buildings, would have looked like a housing project."

To meet the Port Authority's requirement to build 10 million square feet (930,000 m²) of office space, the buildings would each need to be 110 stories tall. A major limiting factor in building heights is elevators; the taller the building, the more elevator

Elevator

An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...

s are needed to service the building, requiring more space-consuming elevator banks. Yamasaki and the engineers decided to use a new system that included sky lobbies, which are floors where people can switch from a large-capacity express elevator, which goes only to the sky lobbies, to a local elevator that goes to each floor in a section (the local elevators can be stacked within the same elevator shaft). Located on the 44th and 78th floors of each tower, the sky lobbies enabled the elevators to be used efficiently, while also increasing the amount of usable space on each floor from 62 to 75 percent by reducing the number of required elevator shafts. The World Trade Center towers were the second supertall buildings to use sky lobbies, after the John Hancock Center

John Hancock Center

John Hancock Center at 875 North Michigan Avenue in the Streeterville area of Chicago, Illinois, is a 100-story, 1,127-foot tall skyscraper, constructed under the supervision of Skidmore, Owings and Merrill, with chief designer Bruce Graham and structural engineer Fazlur Khan...

in Chicago

Chicago

Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...

. This system was inspired by the New York City Subway

New York City Subway

The New York City Subway is a rapid transit system owned by the City of New York and leased to the New York City Transit Authority, a subsidiary agency of the Metropolitan Transportation Authority and also known as MTA New York City Transit...

system, whose lines include local stations where local trains stop and express stations where all trains stop.

Yamasaki's design for the World Trade Center was unveiled to the public on 18 January 1964, with an eight-foot model. The towers had a square plan, approximately 207 feet (63 m) in dimension on each side. The buildings were designed with narrow office windows, only 18 inches (45 cm) wide, which reflected on Yamasaki's fear of heights

Acrophobia

Acrophobia is an extreme or irrational fear of heights. It belongs to a category of specific phobias, called space and motion discomfort that share both similar etiology and options for treatment.Most people experience a degree of natural fear when exposed to heights, especially if there is little...

and desire to make building occupants feel secure. Yamasaki's design called for the building facades to be sheathed in aluminum-alloy. In all, the World Trade Center complex contained six buildings within the 16 acres (64,749.8 m²) superblock.

The World Trade Center design brought criticism of its aesthetics from the American Institute of Architects

American Institute of Architects

The American Institute of Architects is a professional organization for architects in the United States. Headquartered in Washington, D.C., the AIA offers education, government advocacy, community redevelopment, and public outreach to support the architecture profession and improve its public image...

and other groups. Lewis Mumford

Lewis Mumford

Lewis Mumford was an American historian, philosopher of technology, and influential literary critic. Particularly noted for his study of cities and urban architecture, he had a broad career as a writer...

, author of The City in History
The City in History
The City in History: Its Origins, Its Transformations, and Its Prospects is a 1961 National Book Award winner by American historian Lewis Mumford.It was first published by Harcourt, Brace & World ....

and other works on urban planning

Urban planning

Urban planning incorporates areas such as economics, design, ecology, sociology, geography, law, political science, and statistics to guide and ensure the orderly development of settlements and communities....

, criticized the project and described it and other new skyscrapers as "just glass-and-metal filing cabinets." Television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

broadcasters raised concerns that the World Trade Center twin towers would cause interference in television reception for viewers in the New York City area

New York metropolitan area

The New York metropolitan area, also known as Greater New York, or the Tri-State area, is the region that composes of New York City and the surrounding region...

. In response to these concerns, the Port Authority offered to provide new television transmission facilities at the World Trade Center. The Linnaean Society of the American Museum of Natural History

American Museum of Natural History

The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...

also opposed the Trade Center project, citing hazards the buildings would impose on migrating bird

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

s.

The structural engineering firm Worthington, Skilling, Helle & Jackson worked to implement Yamasaki's design, developing the tube-frame structural system

Tube (structure)

used in the buildings. The Port Authority's Engineering Department served as foundation engineers

Geotechnical engineering

Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...

, Joseph R. Loring & Associates as electrical engineers

Electrical engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...

, and Jaros, Baum & Bolles as mechanical engineers

Mechanical engineering

Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...

. Tishman Realty & Construction Company was the general contractor

General contractor

A general contractor is responsible for the day-to-day oversight of a construction site, management of vendors and trades, and communication of information to involved parties throughout the course of a building project.-Description:...

on the World Trade Center project. Guy F. Tozzoli, director of the World Trade Department at the Port Authority, and the Port Authority's Chief Engineer, Rino M. Monti, oversaw the project.

Structural design

As an interstate agency, the Port Authority was not subject to local laws and regulations of the City of New York, including building code

Building code

A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

s. Nonetheless, the Port Authority required architects and structural engineers to follow the New York City building codes. At the time when the World Trade Center was planned, new building codes were being devised to replace the 1938 version that was still in place. The structural engineers ended up following draft versions of the new 1968 building codes, which incorporated "advanced techniques" in building design

Building design

Building design refers to the broadly based architectural, engineering and technical applications to the design of buildings. All building projects require the services of a building designer, typically a licensed architect or structural engineer...

.

The World Trade Center towers included many structural engineering

Structural engineering

Structural engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads. Structural engineering is usually considered a specialty within civil engineering, but it can also be studied in its own right....

innovations in skyscraper design and construction

Skyscraper design and construction

The design and construction of skyscrapers involves creating safe, habitable spaces in very tall buildings. The buildings must support their weight, resist wind and earthquakes, and protect occupants from fire. Yet they must also be conveniently accessible, even on the upper floors, and provide...

, which allowed the buildings to reach new heights and become the tallest in the world. Traditionally, skyscraper

Skyscraper

A skyscraper is a tall, continuously habitable building of many stories, often designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper...

s used a skeleton of columns distributed throughout the interior to support building loads, with interior columns disrupting the floor space. The tube-frame concept, earlier introduced by Fazlur Khan

Fazlur Khan

Fazlur Rahman Khan was a Bangladeshi born architect and structural engineer. He is a central figure behind the "Second Chicago School" of architecture, and is regarded as the "Father of tubular design for high-rises"...

, was a major innovation, allowing open floor plans and more space to rent. The buildings used high-strength, load-bearing perimeter steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure. There were 59 perimeter columns, narrowly spaced, on each side of the buildings. In all, the perimeter walls of the towers were 210 feet (64 m) on each side, and the corners were bevel

Bevel

A beveled edge refers to an edge of a structure that is not perpendicular to the faces of the piece. The words bevel and chamfer overlap in usage; in general usage they are often interchanged, while in technical usage they may sometimes be differentiated as shown in the image at right.-Cutting...

ed. The perimeter columns were designed to provide support for virtually all lateral loads (such as wind loads) and to share the gravity loads with the core columns. Structural analysis of major portions of the World Trade Center were computed on an IBM 1620

IBM 1620

The IBM 1620 was announced by IBM on October 21, 1959, and marketed as an inexpensive "scientific computer". After a total production of about two thousand machines, it was withdrawn on November 19, 1970...

.
The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected by spandrel plates. The perimeter columns had a square cross section, 14 inches (36 cm) on a side, and were constructed of welded steel plate. The thickness of the plates and grade of structural steel

Structural steel

Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties...

varied over the height of the tower, ranging from 36,000 to 100,000 pounds per square inch (260 to 670 MPa). The strength of the steel and thickness of the steel plates decreased with height because they were required to support lesser amounts of building mass on higher floors. The tube-frame design required 40 percent less structural steel than conventional building designs. From the 7th floor to the ground level, and down to the foundation, the columns were spaced 10 feet (3 m) apart. All columns were placed on bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

, which, unlike that in Midtown Manhattan, where the bedrock is shallow, is at 65–85 feet (20–26 m) below the surface.

The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop. The modular pieces were typically 52 inches (1.3 m) deep, and extended for two full floors and half of two more floors. Adjacent modules were bolted together, with the splices occurring at mid-span of the columns and spandrels. The spandrel plates were located at each floor, transmitting shear stress

Shear stress

A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...

between columns, allowing them to work together in resisting lateral loads. The joints between modules were staggered vertically, so the column splices between adjacent modules were not at the same floor.

The building's core housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces. The core of each tower was a rectangular area 87 by 135 feet (27 by 41 m), and contained 47 steel columns running from the bedrock to the top of the tower. The columns tapered after the 66th floor, and consisted of welded box-sections at lower floors and rolled wide-flange sections at upper floors. The structural core in 1 WTC was oriented with the long axis east to west, while that of 2 WTC was oriented north to south. All elevators were located in the core. Each building had three stairwells, also in the core, except on the mechanical floor

Mechanical floor

A mechanical floor, mechanical penthouse, or mechanical level is a storey of a high-rise building that is dedicated to mechanical and electronics equipment. "Mechanical" is the most commonly used term, but words such as utility, technical, service, and plant are also used...

s where they were located outside the core.

The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their own weight, as well as live loads, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls. The floors consisted of 4 inches (10.2 cm) thick lightweight concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

slabs laid on a fluted steel deck with shear connections for composite action

Composite construction

Composite construction is a generic term to describe any building construction involving multiple dissimilar materials. Composite construction is often used in building aircraft, watercraft, and building construction. There are several reasons to use composite materials including increased...

. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses had a span of 60 feet (18.3 m) in the long-span areas and 35 feet (10.7 m) in the short span area. The trusses connected to the perimeter at alternate columns, and were on 6 in 8 in (2.03 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 the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic

Viscoelasticity

Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...

dampers, which helped reduce the amount of sway felt by building occupants.

Hat truss

Truss

In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...

es (or "outrigger truss") located from the 107th floor to the top of the buildings were designed to support a tall communication antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

on top of each building. Only 1 WTC (north tower) actually had an antenna fitted, which was added in 1978. The truss system consisted of six trusses along the long axis of the core and four along the short axis. This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower.

Wind effects

The tube frame design using steel core and perimeter columns protected with sprayed-on fire resistant material created a relatively lightweight structure that would sway more in response to the wind, compared to traditional structures such as the Empire State Building

Empire State Building

that have thick, heavy masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

for fireproofing

Fireproofing

Fireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...

of steel structural elements. During the design process, wind tunnel

Wind tunnel

tests were done at Colorado State University

Colorado State University

Colorado State University is a public research university located in Fort Collins, Colorado. The university is the state's land grant university, and the flagship university of the Colorado State University System.The enrollment is approximately 29,932 students, including resident and...

and at the National Physical Laboratory

National Physical Laboratory, UK

The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

to establish design wind pressures that the World Trade Center towers could be subjected to and structural response to those forces. Experiments were also done to evaluate how much sway occupants could tolerate. Subjects were recruited for "free eye exams," while the real purpose of the experiment was to subject them to simulated building sway and find out how much they could comfortably tolerate. Many subjects did not respond well, experiencing dizziness

Dizziness

Dizziness refers to an impairment in spatial perception and stability. The term is somewhat imprecise. It can be used to mean vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness....

and other ill effects. One of the chief engineers Leslie Robertson worked with Canadian engineer Alan G. Davenport to develop viscoelastic dampers

Damping

In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...

to absorb some of the sway. These viscoelastic dampers, used throughout the structures at the joints between floor trusses and perimeter columns, along with some other structural modifications reduced the building sway to an acceptable level.

Aircraft impact

The structural engineers on the project also 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

B-25 Empire State Building crash

The B-25 Empire State Building crash was a 1945 aircraft accident in which a B-25 Mitchell piloted in thick fog crashed into the Empire State Building...

into the 79th floor of the Empire State Building

Empire State Building

. A year later, another airplane nearly crashed into the 40 Wall Street

40 Wall Street

40 Wall Street is a 70-story skyscraper in New York City. Originally known as the Bank of Manhattan Trust building, and also known as Manhattan Company Building, it was later known by its street address when its founding tenant merged to form the Chase Manhattan Bank and today is known as the...

building, and there was another near-miss at the Empire State Building. In designing the World Trade Center, Leslie Robertson considered the scenario of the impact of a jet airliner, the Boeing 707

Boeing 707

The Boeing 707 is a four-engine narrow-body commercial passenger jet airliner developed by Boeing in the early 1950s. Its name is most commonly pronounced as "Seven Oh Seven". The first airline to operate the 707 was Pan American World Airways, inaugurating the type's first commercial flight on...

, which might be lost in the fog, seeking to land at JFK

John F. Kennedy International Airport

John F. Kennedy International Airport is an international airport located in the borough of Queens in New York City, about southeast of Lower Manhattan. It is the busiest international air passenger gateway to the United States, handling more international traffic than any other airport in North...

or at Newark

Newark Liberty International Airport

Newark Liberty International Airport , first named Newark Metropolitan Airport and later Newark International Airport, is an international airport within the city limits of both Newark and Elizabeth, New Jersey, United States...

airports. The National Institute of Standards and Technology

National Institute of Standards and Technology

The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

(NIST) found a three page white paper

White paper

A white paper is an authoritative report or guide that helps solve a problem. White papers are used to educate readers and help people make decisions, and are often requested and used in politics, policy, business, and technical fields. In commercial use, the term has also come to refer to...

that mentioned another aircraft impact analysis, involving impact of a jet at 600 mi/h, was indeed considered, but the original documentation of the study was lost when Port Authority offices were destroyed in the collapse of the World Trade Center.

Fire protection

Sprayed-fire resistant material

Fireproofing

s (SFRMs) were used to protect some structural steel elements in the towers, including all floor trusses and beams. Gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

wallboard in combination with SFRMs, or in some cases gypsum wallboard alone, was used to protect core columns. Vermiculite

Vermiculite

Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...

plaster was used on the interior-side and SFRMs on the other three sides of the perimeter columns for fire protection. The 1968 New York City building codes were more lenient in some aspects of fire protection, such as allowing three exit stairwells in the World Trade Center towers, instead of six as required under older building codes.

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.

More fireproofing was added after a fire in February 1975 that spread to six floors before being extinguished. After the 1993 bombing

1993 World Trade Center bombing

The 1993 World Trade Center bombing occurred on February 26, 1993, when a truck bomb was detonated below the North Tower of the World Trade Center in New York City. The 1,336 lb urea nitrate–hydrogen gas enhanced device was intended to knock the North Tower into the South Tower , bringing...

, 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 WTC 1, including all the floors affected by the aircraft impact and fires on September 11, and on 13 floors in WTC 2, although only three of these floors (77,78, and 85) were directly affected by the aircraft impact.

The 1968 New York City building codes did not require sprinkler

Fire sprinkler

A fire sprinkler system is an active fire protection measure, consisting of a water supply system, providing adequate pressure and flowrate to a water distribution piping system, onto which fire sprinklers are connected...

s for high-rise buildings, except for underground spaces. In accordance with building codes, sprinklers were originally installed only in the underground parking structures of the World Trade Center. Following a major fire in February 1975, the Port Authority decided to start installing sprinklers throughout the buildings. By 1993, nearly all of 2 WTC and 85 percent of 1 WTC had sprinklers installed, and the entire complex was retrofitted by 2001.

Construction

In March 1965, the Port Authority began acquiring property at the World Trade Center site. The Ajax Wrecking and Lumber Corporation was hired for the demolition work, which began on 12 March 1966 to clear the site for construction of the World Trade Center.

Groundbreaking was on 5 August 1966, marking the beginning of construction of the World Trade Center's foundations. The site of the World Trade Center was located on landfill

Landfill

A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...

, with the bedrock located 65 feet (19.8 m) below grade. In order to construct the World Trade Center, it was necessary to build the "bathtub," with the slurry wall

Slurry wall

along the West Street

West Side Highway

The West Side Highway is a mostly surface section of New York State Route 9A that runs from West 72nd Street along the Hudson River to the southern tip of Manhattan. It replaced the West Side Elevated Highway, built between 1929 and 1951, which was shut down in 1973 due to neglect and lack of...

side of the site, to keep water from the Hudson River

Hudson River

out. This method was used in place of conventional dewatering methods because lowering the groundwater table would cause large settlements

Groundwater-related subsidence

Groundwater-related subsidence is the subsidence of land resulting from groundwater extraction, and a major problem in the developing world as major metropolises swell without adequate regulation and enforcement, as well as a being a common problem in the developed world...

of nearby buildings not built on deep foundation

Deep foundation

A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a...

s. The slurry method involves digging a trench

Trench

A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide , and by being narrow compared to their length ....

, and as excavation proceeds, filling the space with a "slurry" mixture, composed of bentonite

Bentonite

Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

which plugs holes and keeps water out. When the trench was dug out, a steel cage was inserted, with concrete poured in, forcing the "slurry" out. The "slurry" method was devised by Port Authority chief engineer John M. Kyle, Jr. Towards the end of 1966, work began on building the slurry wall, led by Montreal

Montreal

Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...

-based Icanda, a subsidiary of an Italian engineering firm, Impresa Costruzioni Opere Specializzate (I.C.O.S.). It took fourteen months for the slurry wall to be completed, which was necessary before excavation of material from the interior of the site could begin. The original Hudson Tubes

Downtown Hudson Tubes

The Downtown Hudson Tubes are a pair of tunnels that carry PATH trains under the Hudson River between New York City and Jersey City in the United States. In lower Manhattan the trains travel to and from the World Trade Center station. In Jersey City the trains stop at the Exchange Place station...

, which carried PATH trains into Hudson Terminal, remained in service as elevated tunnels until 1971 when a new PATH station

World Trade Center (PATH station)

The World Trade Center PATH station originally opened on July 19, 1909 as the Hudson Terminal. When the Hudson Terminal was torn down to make way for the World Trade Center, a new station was built, which opened in 1971...

was built.

Construction work began on the North Tower in August 1968 with construction beginning on the South Tower by January 1969. In January 1967, $74 million in contracts were awarded to the Pacific Car and Foundry Company

PACCAR

PACCAR Inc is the third largest manufacturer of heavy-duty trucks in the world , and has substantial manufacture in light and medium vehicles through its various subsidiaries.-History:...

, Laclede Steel Company, Granite City Steel Company

U.S. Steel

The United States Steel Corporation , more commonly known as U.S. Steel, is an integrated steel producer with major production operations in the United States, Canada, and Central Europe. The company is the world's tenth largest steel producer ranked by sales...

, and Karl Koch Erecting Company

Karl Koch Steel Consulting

Karl Koch Steel Consulting is a steel construction firm, based in Oyster Bay, New York. The company was established in the 1950s as Karl Koch Erecting Company, Inc., of Carteret, New Jersey. The Karl Koch Erecting Company was hired by the Port Authority of New York and New Jersey as one of the...

to supply steel

Steel

for the project. The Port Authority chose to use many different steel suppliers, bidding on smaller portions of steel, rather than buy larger amounts from a single source such as Bethlehem Steel

Bethlehem Steel

The Bethlehem Steel Corporation , based in Bethlehem, Pennsylvania, was once the second-largest steel producer in the United States, after Pittsburgh, Pennsylvania-based U.S. Steel. After a decline in the U.S...

or U.S. Steel

U.S. Steel

as a cost-saving measure. Karl Koch was also hired to do all the work of erecting the steel, and a contract for work on the aluminum facade was awarded to the Aluminum Company of America

Alcoa

Alcoa Inc. is the world's third largest producer of aluminum, behind Rio Tinto Alcan and Rusal. From its operational headquarters in Pittsburgh, Pennsylvania, Alcoa conducts operations in 31 countries...

. Tishman Realty & Construction

Tishman Realty & Construction

Tishman Realty & Construction is an American corporation that owns and operates Tishman Construction Corporation and Tishman Hotel & Realty.-Tishman Construction Corporation:...

was hired in February 1967 to oversee construction of the project.

Extensive use of prefabricated parts for the perimeter framing and floor truss systems helped speed up the construction process and reduce costs, while providing greater quality control

Quality control

Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...

. Steel components were freighted into a Penn Central

Penn Central Transportation

The Penn Central Transportation Company, commonly abbreviated to Penn Central, was an American railroad company that operated from 1968 until 1976. It was created by the merger on February 1, 1968, of the Pennsylvania Railroad and the New York Central Railroad...

yard in Jersey City

Jersey City, New Jersey

Jersey City is the seat of Hudson County, New Jersey, United States.Part of the New York metropolitan area, Jersey City lies between the Hudson River and Upper New York Bay across from Lower Manhattan and the Hackensack River and Newark Bay...

. From there, they were brought in early morning hours through the Holland Tunnel

Holland Tunnel

The Holland Tunnel is a highway tunnel under the Hudson River connecting the island of Manhattan in New York City with Jersey City, New Jersey at Interstate 78 on the mainland. Unusual for an American public works project, it is not named for a government official, politician, or local hero or...

to the construction site, and lifted into place by a crane

Crane (machine)

A crane is a type of machine, generally equipped with a hoist, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of...

. Larger pieces were brought to the construction site by tugboat

Tugboat

A tugboat is a boat that maneuvers vessels by pushing or towing them. Tugs move vessels that either should not move themselves, such as ships in a crowded harbor or a narrow canal,or those that cannot move by themselves, such as barges, disabled ships, or oil platforms. Tugboats are powerful for...

s. A special type of crane, suitable for constructing such tall buildings, that used hydraulics

Hydraulics

Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...

to lift components and provided its own power was used in construction of the World Trade Center. The Favco Standard 2700 Crane, manufactured by Favelle Mort Ltd. of New South Wales

New South Wales

New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

, Australia was informally called a "kangaroo

Kangaroo

A kangaroo is a marsupial from the family Macropodidae . In common use the term is used to describe the largest species from this family, especially those of the genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern Grey Kangaroo and Western Grey Kangaroo. Kangaroos are endemic to the country...

crane."

In 1970, tugboat workers went on strike

Strike action

Strike action, also called labour strike, on strike, greve , or simply strike, is a work stoppage caused by the mass refusal of employees to work. A strike usually takes place in response to employee grievances. Strikes became important during the industrial revolution, when mass labour became...

, halting the transport of material to the construction site. The Port Authority attempted other means of transporting material, including via helicopter

Helicopter

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

. When this method was tried, the helicopter lost its load of steel into the Kill Van Kull

Kill Van Kull

The Kill Van Kull is a tidal strait between Staten Island, New York and Bayonne, New Jersey in the United States. Approximately long and wide, it connects Newark Bay with Upper New York Bay. The Robbins Reef Light marks the eastern end of the Kill, Bergen Point its western end...

. Some other mishaps occurred during the construction process, including disruption of telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

service in Lower Manhattan when telephone cables were crushed by pile driver

Pile driver

A pile driver is a mechanical device used to drive piles into soil to provide foundation support for buildings or other structures. The term is also used in reference to members of the construction crew that work with pile-driving rigs....

s. On 16 March 1970, an explosion injured six workers when a truck hit a propane

Propane

Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

tank. In all, 60 workers were killed in construction accidents while the World Trade Center was being built.

The topping out

Topping out

In building construction, topping out is a ceremony held when the last beam is placed at the top of a building. The term may also refer to the overall completion of the building's structure, or an intermediate point, such as when the roof is dried in...

ceremony of 1 WTC (North Tower) took place on 23 December 1970, with 2 WTC's ceremony (South Tower) occurring later on 19 July 1971. The first tenants moved into the North Tower in December 1970, and into the South Tower in January 1972. The buildings were dedicated on 4 April 1973; Tobin, who had resigned the year before, was absent from the ceremonies.

Building the World Trade Center involved excavating 1200000 cubic yards (917,465.8 m³) of material. Rather than transporting this material at great costs out to sea or to landfills in New Jersey

New Jersey

, the fill material was used to expand the Manhattan

Manhattan

shoreline across West Street. Work to demolish the pier

Pier

A pier is a raised structure, including bridge and building supports and walkways, over water, typically supported by widely spread piles or pillars...

s began on 5 January 1967, including Pier 7 to Pier 11 which were all constructed around 1910. The demolition work moved forward, despite conflicts between David Rockefeller

David Rockefeller

, Governor Nelson Rockefeller

Nelson Rockefeller

Nelson Aldrich Rockefeller was the 41st Vice President of the United States , serving under President Gerald Ford, and the 49th Governor of New York , as well as serving the Roosevelt, Truman and Eisenhower administrations in a variety of positions...

, and Mayor John Lindsay

John Lindsay

John Vliet Lindsay was an American politician, lawyer and broadcaster who was a U.S. Congressman, Mayor of New York City, candidate for U.S...

regarding plans for Battery Park City

Battery Park City, Manhattan

Battery Park City is a planned community at the southwestern tip of lower Manhattan in New York City, United States. The land upon which it stands was created by land reclamation on the Hudson River using 1.2 million cubic yards of soil and rocks excavated during the construction of the World...

. Landfill material from the World Trade Center

World Trade Center

The original World Trade Center was a complex with seven buildings featuring landmark twin towers in Lower Manhattan, New York City, United States. The complex opened on April 4, 1973, and was destroyed in 2001 during the September 11 attacks. The site is currently being rebuilt with five new...

was used to add land, and a cellular cofferdam

Cofferdam

A cofferdam is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out, creating a dry work environment for the major work to proceed...

was constructed to retain the material. The result was a 700 feet (213.4 m) extension into the Hudson River, running six blocks or 1484 feet (452.3 m). This land was a "gift" to New York City, allowing more tax

Tax

To tax is to impose a financial charge or other levy upon a taxpayer by a state or the functional equivalent of a state such that failure to pay is punishable by law. Taxes are also imposed by many subnational entities...

-generating developments in Battery Park City.

The original estimates put forth by the Port Authority had the costs for construction of the World Trade Center at $350 million—an optimistic figure. In December 1966, the Port Authority announced increased cost estimates, bringing the estimated total to $575 million. This announcement brought criticism of the project from private real estate developers, The New York Times
The New York Times
The New York Times is an American daily newspaper founded and continuously published in New York City since 1851. The New York Times has won 106 Pulitzer Prizes, the most of any news organization...

, and others in New York City. The critics charged that the Port Authority figure was an unrealistically low estimate, and they estimated the project would end up costing $750 million. When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million. The project was financed through tax-exempt

Tax exemption

Various tax systems grant a tax exemption to certain organizations, persons, income, property or other items taxable under the system. Tax exemption may also refer to a personal allowance or specific monetary exemption which may be claimed by an individual to reduce taxable income under some...

bond

Bond (finance)

In finance, a bond is a debt security, in which the authorized issuer owes the holders a debt and, depending on the terms of the bond, is obliged to pay interest to use and/or to repay the principal at a later date, termed maturity...

s issued by the Port Authority.

Other buildings

The World Trade Center complex included four other smaller buildings constructed during the 1970s. 3 World Trade Center was a 22-story building, which was home to the Marriott World Trade Center. It was designed by Skidmore, Owings and Merrill

Skidmore, Owings and Merrill

in 1978–79. 4 World Trade Center, 5 World Trade Center, and 6 World Trade Center were all 8–9 story buildings that were designed by the same team as the Twin Towers, including Minoru Yamasaki, Emery Roth & Sons, and Skilling, Helle, Christiansen, Robertson. 7 World Trade Center

7 World Trade Center

was built in the mid-1980s, just north of the main World Trade Center site. The 47-story building was designed by Emery, Roth & Sons, and constructed on top of a Con Edison power substation

Electrical substation

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

Modifications

Over time, numerous structural modifications were made to suit the needs of tenants in the Twin Towers. Modifications were made in accordance with the Port Authority's Tenant Alteration Review Manual and were reviewed by the Port Authority to ensure the changes did not compromise structural integrity of the buildings. In many instances, openings were cut in the floors to accommodate new stairways to connect tenant floors. Some steel beams in the core were reinforced and strengthened to accommodate heavy live loads, such as large amounts of heavy files that tenants had on their floors.

Repairs to structural elements on the lower levels of 1 WTC were made following the 1993 bombing

1993 World Trade Center bombing

. The greatest damage occurred on levels B1 and B2, with significant structural damage also on level B3. Primary structural columns were not damaged, but secondary steel members experienced some damage. Floors that were blown out needed to be repaired to restore the structural support they provided to columns. The slurry wall was in peril following the bombing and loss of the floor slabs which provided lateral support to counteract pressure from Hudson River water on the other side. The refrigeration

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

plant on sublevel B5, which provided air conditioning

Air conditioning

An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

to the entire World Trade Center complex, was heavily damaged and replaced with a temporary system for the summer of 1993. The fire alarm system for the entire complex needed to be replaced, after critical wiring and signaling in the original system was destroyed in the 1993 bombing. Installation of the new system took years to complete, and replacement of some components was still underway at the time of the September 11, 2001 attacks

September 11, 2001 attacks

The September 11 attacks The September 11 attacks The September 11 attacks (also referred to as September 11, September 11th or 9/119/11 is pronounced "nine eleven". The slash is not part of the pronunciation...

External links

New York: A Documentary Film - The Center of the World - Building the World Trade Center, an 18-minute film, with construction footage, produced by the Port Authority in 1983
World Trade Center - Skyscraper Museum
NIST and the World Trade Center
New York's World Trade Center - A Living Archive - by author, Eric Darton
The Death of New York's Radio Row, by Syd Steinhard - Downtown Express
RebuildGroundZero.org, a community project based on Ground Zero's rebuilding.

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