World Trade Center Towers Design and Construction

From 911Encyclopedia
Jump to: navigation, search

Extensive WTC Image Collection at


Construction Video Footage Compiled By From Nate Flach:

Pictures from before 2001:


World Trade Center Site Maps

World Trade Center Blueprints

World Trade Center Construction Images

World Trade Center Fire Code Compliance

World Trade Center Fire Suppression System

The World Trade Center Twin Towers Were Designed For Jet Impacts

The World Trade Center Towers Design Exceeded Wind Load Codes

World Trade Center Floor Structure

World Trade Center Floor Trusses

World Trade Center Building Cores

World Trade Center Elevators

World Trade Center Concrete

World Trade Center Tenants

Perimeter Column Data

Core Column Data

Spandrel Data

World Trade Center Mass And Potential Energy


World Trade Center Radio Frequency (RF) Exposure Documentation

1,362-1,368ft (411m) tall, 70ft (21m) below grade, 208ft (63.5m) on each side, 90ft x 130ft (27m x 40m) core

The WTC actually consisted of [World Trade Center Complex seven buildings] , including one hotel, spread across 16 acres of land. The buildings were connected by an underground mall (the concourse).The Twin Towers (1 WTC, or the North Tower, and 2 WTC, or the South Tower) were the signature structures, containing 10.4 million square feet of office space. Both towers had 110 stories, were about 1,350 feet high, and were square; each wall measured 208 feet in length. On any given workday, up to 50,000 office workers occupied the towers, and 40,000 people passed through the complex. Each tower contained three central stairwells, which ran essentially from top to bottom, and 99 elevators. Generally, elevators originating in the lobby ran to "sky lobbies" on higher floors, where additional elevators carried passengers to the tops of the buildings.

The WTC towers, also known as WTC 1 and WTC 2, were the primary components of the seven-building World Trade Center complex. Each of the towers encompassed 110 stories above the Plaza level and seven levels below. WTC 1 (the north tower) had a roof height of 1,368 feet, briefly earning it the title of the world’s tallest building. WTC 2 (the south tower) was nearly as tall, with a roof height of 1,362 feet. WTC 1 also supported a 360-foot-tall television and radio transmission tower. Each building had a square floor plate, 207 feet 2 inches long on each side. Corners were chamfered 6 feet 11 inches. Nearly an acre of floor space was provided at each level. A rectangular service core, with overall dimensions of approximately 87 feet by 137 feet, was present at the center of each building, housing 3 exit stairways, 99 elevators, and 16 escalators.

WTC 1 and WTC 2 were similar, but not identical. WTC 1 was 6 feet taller than WTC 2 and also supported a 360-foot tall transmission tower. The service core in WTC 1 was oriented east to west, and the service core in WTC 2 was oriented north to south.

Floor construction typically consisted of 4 inches of lightweight concrete on 1-1/2-inch, 22-gauge non-composite steel deck. In the core area, slab thickness was 5 inches. Outside the central core, the floor deck was supported by a series of composite floor trusses that spanned between the central core and exterior wall.

The floor system design was not typical of open-web-joist floor systems. It was considerably more redundant and was well braced with transverse members.

Floors were designed for a uniform live load of 100 pounds per square foot (psf) over any 200-square-foor area with allowable live load reductions taken over larger areas. At building corners, this amounted to a uniform live load (unreduced) of 55 psf.

The core consisted of 5-inch concrete fill on metal deck supported by floor framing of rolled structural shapes, in turn supported by a combination of wide flange shape and box-section columns. Some of these columns were very large, with cross-sections measuring 14 inches wide by 36 inches deep. In upper stories, these rectangular box columns transitioned into heavy rolled wide flange shapes.

A deep subterranean structure was present beneath the WTC Plaza and the two towers. The western half of this substructure, bounded by West Street to the west and by the 1/9 subway line that extends approximately between West Broadway and Greenwich Street on the east, was 70 feet deep and contained six subterranean levels. The structure housed a shopping mall and building mechanical and electrical services, and it also provided a station for the PATH subway line and parking for the complex. fema.gov_p10

Floors within the substructure were of reinforced concrete flat-slab construction, supported by structural steel columns. Many of these steel columns also provided support for the structures located above the plaza level. fema.gov_p11

Tower foundations beneath the substructure consisted of massive spread footings, socketed into and bearing directly on the massive bedrock. Steel grillages, consisting of layers of orthogonally placed steel beams, were used to transfer the immense column loads, in bearing, to the reinforced concrete footings. fema.gov_p11

Design architecture was provided by Minoru Yamasaki & Associates, and Emery Roth & Sons served as the architect of record. Skilling, Helle, Christiansen, Robertson were the project structural engineers; Jaros, Baum & Bolles were the mechanical engineers; and Joseph R. Loring & Associates were the electrical engineers. The Port Authority provided design services for site utilities, foundations, basement retaining walls, and paving. Groundbreaking for construction was on August 5, 1966. Steel construction began in August 1968. First tenant occupancy of WTC 1 was in December 1970, and occupancy of WTC 2 began in January 1972. Ribbon cutting was on April 4, 1973.

“The structural analysis carried out by the firm of Worthington, Skilling, Helle & Jackson is the most complete and detailed of any ever made for any building structure. The preliminary calculations alone cover 1,200 pages and involve over 100 detailed drawings… The building as designed is sixteen times stiffer than a conventional structure. The design concept is so sound that the structural engineer has been able to be ultra-conservative in his design without adversely affecting the economics of the structure.” amazon.com_p134

James Glanz and Eric Lipton, City in the Sky: The rise and fall of the World Trade Center, Times Books, Henry Hold and Company, LLC, 2003, pages 134-136.

The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covers eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, is under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 (ENR 7/9/64 p. 36). The 1.8-million-sq-ft Seven World Trade Center, constructed in the mid-80s, also had a steel moment frame from the seventh story up (ENR 11/28/85 p. 30). Massive Assault Doomed Towers (9/12/01):

In 1974 the American Society of Civil Engineers recognized the World Trade Center as an exemplary civil engineering project by designating it as an Outstanding Civil Engineering Achievement (OCEA). Established in 1960, this prestigious award honors the project that best illustrates superior civil engineering skills and represents a significant contribution to civil engineering progress and society. Honoring an overall project rather than an individual, the award recognizes the contributions of many engineers.

The national award judged the WTC towers to be “the engineering project that demonstrates the greatest engineering skills and represents the greatest contribution to engineering progress and mankind.”

Angus K. Gillespie, Twin Towers: The Life of New York City's World Trade Center (New Brunswick, N.J.: Rutgers University Press 1999), 117

Like many modern structures and buildings, the WTC Towers were over-designed to withstand weight distribution in the event of structural damage. According to calculations made by the engineers who helped with the design of the Twin Towers, “all the columns on one side of a Tower could be cut, as well as the two corners and some of the columns on each adjacent side, and the building would still be strong enough to withstand a 100-mile-per-hour wind.” amazon.com_p133 Glanz and Lipton, City in the Sky, page 133.

As well, “Live loads on these columns can be increased more than 2,000% before failure occurs.”

How Columns Will Be Designed for 110-Story Buildings, Engineering News-Record, April 2, 1964: 48-49.

"Skilling—a recognized expert in tall buildings—doesn't think a single 200-pound car bomb would topple or do major structural damage to a Trade Center tower. The supporting columns are closely spaced and even if several were disabled, the others would carry the load. ‘However,’ he added, "I'm not saying that properly applied explosives—shaped explosives—of that magnitude could not do a tremendous amount of damage." Although Skilling is not an explosives expert, he says @@there are people who do know enough about building demolition to bring a structure like the Trade Center down.@@ ‘I would imagine that if you took the top expert in that type of work and gave him the assignment of bringing these buildings down with explosives, I would bet that he could do it."

Between Early 1984 and October 1985:

The Office of Special Planning (OSP), a unit set up by the New York Port Authority to assess the security of its facilities against terrorist attacks, spends four to six months studying the World Trade Center. It examines the center’s design through looking at photographs, blueprints, and plans. It brings in experts such as the builders of the center, plus experts in sabotage and explosives, and has them walk through the WTC to identify any areas of vulnerability…@@”O’Sullivan consults ‘one of the trade center’s original structural engineers, Les Robertson, on whether the towers would collapse because of a bomb or a collision with a slow-moving airplane.’ He is told there is "little likelihood of a collapse no matter how the building was attacked."@@ amazon.com_p227

Glanz and Lipton, City in the Sky, Page 227. See also

One of these hypothetical examples was put to the test in the 1993 WTC bombing. This attack prompted more discussions about the safety of the WTC towers. In response to these concerns, WTC building designer John Skilling explained that they “looked at every possible thing we could think of that could happen to the buildings, @@even to the extent of an airplane hitting the side@@… A previous analysis carried out early in 1964, calculated that the towers would handle the impact of a 707 traveling at 600 mph without collapsing.”


Thomas Eagar, an MIT professor of materials engineering, says that @@the impact of the airplanes would not have been significant@@, because "the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure" (Eagar and Musso, 2001, pp. 8-11).

North Tower (WTC1) Occupants

South Tower (WTC2) Occupants


Karl Koch in his book, “Men of Steel had this to say about the panels on page 375-376:

“…… everywhere I looked I could see familiar things: the three-column exterior panels still held together by their spandrels, the treetops that were so big each one needed its own rail flatbed. I looked for any signs of a floor panel, just one of the 6,800 I knew was in here, but I couldn’t identify a single one.” Men of Steel p. 376

This was 5 days after the towers fell Carl wrote this in his book as he went there himself!

(Speaking of the floor panels in conversation at “Fresh Kills” landfill with one of his former employees Tom Petrizzo):

“They must have disintegrated. Because they did not get here and I handled this from day fuckin one” Tom Petrizzp said. “None? Well, that’s impossible. There were six thousand eight hundred of them.” Karl said. (Men of Steel; p. 375

Seven World Trade Center, New York, Fabrication and Construction Aspects:

Using the available [WTC 1,2 Structural Data Sources sources] , in particular chapter 2 of the NIST document NIST NCSTAR 1-3: Mechanical and Metallurgical Analysis of Structural Steel , a set of basic structural data on WTC towers 1 and 2 has been gathered and is used in the [Model of Typical Story for WTC 1,2 model] of a "typical" story of those buildings. A typical story is defined as one that is above the 7th story and is not a mechanical floor or in the hat truss area. For these floors, the basic structural elements and the associated dimensions are given in the following table.


|building width| 63.14 |

|building depth| 63.14 |

|core width| 41.8 |

|core depth| 26.52 |

|perimeter column count| 236 (59 per side)|

|core column count| 47 |

|story height| 3.6576 |

|perimeter column spacing| 1.02 |

|number of long span trusses| 60 (30 per side) |

|number of short span trusses| 28 (14 per side) |

|truss spacing| 2.04 |

|number of transverse trusses| 8 |

|transverse truss spacing| 4.08 |

|All lengths are in meters.|c

The long span trusses connect the perimeter columns to the core along the sides of the buildings that have the greatest distance from the perimeter to the core. The short span trusses are along the other two sides. The transverse trusses are also called the bridging trusses and are orthogonal to the long and short span trusses. In the table above, the transverse trusses refer only to those that are orthogonal to the long span trusses. If the transverse trusses orthogonal to the short span trusses are determined to be structurally significant, they will be added to the model.

The links below give additional structural information that will be required in the modeling effort about each of the main elements of the model:





































"The towers represented the first use of dampening devices on a building: each held about 10,000 viscoelastic damping units."

Structural Engineer 2002-01-01 - - IN STRUCTURAL HISTORY THE WORLD TRADE CENTER (link)

Structural Engineer 2002-08-01 - - What we learned: Building performance study of the WTC collapse (link)





Costruzione del WTC (1972) - Filmati Originali:

Personal tools