World Trade Center Towers 1 & 2 (Sept. 11, 2001)
Ryan English, BAE, Penn State, 2010


September 11, 2001, was far from an ordinary day for the men and women who worked in the World Trade Center Tower 1 and Tower 2 (WTC 1 and WTC 2). The World Trade Center Towers were located in the World Trade Center Plaza complex in New York City and were the tallest of the six buildings, topping out at 1,362 ft. for WTC 1 and 1,368 ft. for WTC 2. At 8:46 a.m. on September 11, 2001, the first of two hijacked commercial jetliners were flown into the WTC 1. At 9:03 a.m. the second jetliner was flown into WTC 2. The purpose of this report is to provide information on the collapse of the World Trade Center Tower 1 and Tower 2 due to the September 11, 2001, terrorist attacks.

Building Description

Groundbreaking for the towers was in 1966 with WTC 1's first tenants occupying its space in December 1970 and WTC 2 being occupied in January 1972. The two towers topped out at 110 stories high with WTC 1's roof level at 1,368 ft. above the concourse level, 6 ft. taller than WTC 2. WTC 1, also, supported a 260 ft. tall television and radio transmission antenna. Each tower had a square plan with side dimension of about 207 ft. Both towers had a service core area of approximately 135 ft. by 87 ft. The core of WTC 1 was orientated with the long axis of the core in the east-west direction, and WTC 2 was oriented in the north-south direction. The towers were divided into three zones by the skylobbies located on the 44th and 78th floors. These skylobbies were used to distribute passengers among express and local elevators. The project team is listed in table 1 below.

Table 1:Project Team.
Architectural Design
Minoru Yamasaki & Associates
Architect of Record
Emory Roth & Sons, P.C.
Structural Engineer of Record
Skilling, Helle, Christiansen, Robertson (SHCR)
Mechanical Engineers
Jaros, Banum & Bollers
Electrical Engineers
Joseph R. Loring & Associates
General Contractor
Tishman Construction Corporation
Engineering Dep. Port Authority New York

Structural System

Each tower was comprised of five structural systems: exterior walls, core, foundation, floor system, and hat trusses.
Figure 1: WTC1 and WTC 2 tower floor plan (NIST 2005, p7) Pending Approval

Each tower's face was comprised of fifty-nine 14 in. box columns spaced at 3 ft. 4 in. on center. Below the 7th floor, the columns were combined into group of three to form base columns which were spaced 10 ft. on center. The columns were interconnected to form a 10 ft. wide and 36 ft. tall panel. Panels were staggered so that only one third of the columns were spliced at any one story with the exception at the mechanical floors. Lateral loads of the buildings were resisted mainly by the exterior walls of a framed tube system. The interior core columns did not contribute to the lateral stiffness of the buildings. However, both the core and exterior columns were designed to support approximately equal amount of the gravitational loads. Core columns were comprised of two types: welded box columns and rolled wide flange shapes. The core columns were typically spliced at three story intervals. A column base plate distributed the column load to steel grillage which distributed the load to the reinforced concrete spread footings.

The floor system of the towers were designed for four main functions: support vertical gravitational loads, distribute wind loads, resist torsional motion, and lateral support to the columns. The floor inside the core and mechanical floors was framed with structural steel shapes with shear studs. Outside of the core, the typical floor was framed with steel bar trusses with a 1½ in. 22 gages and fluted metal deck with 4 in. thick lightweight concrete slab. The bar of the steel trusses extended above the top cord and into the slab to achieve composite action. Spans of the trusses were about 25 ft. in the short span and 60 ft. in the long span. Floor trusses were pre-assembled into 20 ft. wide floor panels. A pair of flat bars extended from the top cord of the trusses to the perimeter columns. (NIST 2005)

Fire Protection System

The structural steel members in the towers were protected with fire resistive materials either sprayed fire-resistive materials (SFRM), gypsum wallboards, or a combination of the two. The types of SFRM used in the WTC towers are dry ingredients composed of a binder and insulation materials. Water is than added at the job site as the materials are sprayed onto the steel. The water mixes with the other materials and allows the SFRM to adhere weakly to the steel. Alcoa recommended the use of a sprayed material produced by U.S. Mineral Products, Co. known as BLAZE-SHIELD Type D, a asbestos-containing material. In 1970 New York City issued restrictions on the application of sprayed thermal insulation containing asbestos and BLAZE-SHIELD Type D was stop at the 38th floor of WTC 1. The BLAZE-SHIELD Type D SFRM was replaced with BLAZE-SHIELD Type DC/F, which contained mineral wool, glassy fibers, instead of asbestos fibers. Floor trusses and beams were covered with SFRM with the columns inside the core being covered with gypsum wallboard or a combination of gypsum wallboard and SFRM. Exterior columns were covered with SFRM on three faces and vermiculite plaster was applied to the inner face of the columns. The columns were required to have a 3 hour fire rating, and the floor system required a 2 hour rating. At the time of construction, all columns were required to have about 2 in. of spray-on fireproofing, and the beams and bar joist required ½ in. cover. In the years 1995 to 2001, the fire protection in the towers was upgraded. The new requirements were for any floor undergoing new construction or renovation. The floor trusses were to be protected with 1 ½ in. of sprayed mineral fiber fire-resistive material. No fire protection was applied to the underside of the metal deck. Both towers were protected by an automatic fire sprinkler system. (NIST 2005)

The Attack

The morning of September 11, 2001, was a day that was put into history. On that morning, two Boeing 767-200ER commercial jetliners were hijacked and deliberately flown into the WTC towers. American Airlines Flight 11, the first plane, departed Boston’s Logan International Airport at 7:59 a.m. Eastern Daylight Time (EDT). The plane was flown south and at 8:46 a.m. EDT crashed into the north face of the WTC 1 building. United Airlines Flight 175, the second plane, also, departed Boston at 8:14 a.m. EDT. At 9:03 a.m. EDT, the second plane was flown into the south face of the WTC 2 building. (Delatte, p 201) The events of September 11, 2001, can be seen in table 2.

Table 2: Time Line
Time (EDT)
7:59 am
American Airlines Flight 11 Departed (AA)
8:14 am
United Airlines Flight 175 Departed (UA)
8:46 am
AA Flight 11 crashed into WTC 1
9:03 am
UA Flight 175 crashed into WTC 2
9:59 am
WTC 2 Collapsed (56 minutes after the attack)
10:29 am
WTC 1 Collapsed (1 hours and 43 minutes after the attack)

Figure 4: South Face of WTC2, showing aircraft component location (NIST 2005, p39) Pending Approval

The Aircraft

Both hijacked planes that hit the towers were the Boeing 767-200ER twin-engine wide-body aircraft with a wingspan of 156 ft. 1 in. and a length of 159
Figure 3: Simulation of cumulative aircraft impact damage to WTC1. (NIST 2005, p23) Pending Approval
ft. 2 in. Each plane could carry a total of 181 passengers in its three-class setting layout and a maximum of 24,000 gal. of jet fuel at a typical cruising speed of 530 mph with a maximum carrying capacity of 395,000 lb. (Iwankiw)

American Airlines Flight 11 was much lighter with only about half of the full load of jet fuel, about 10,000 gal., and the passenger cabin was less than half full, 76 passengers with 11 crew members. The total aircraft
Figure 4: Simulation of cumulative aircraft impact damage to WTC2. (NIST 2005, p40) Pending Approval
weight was estimated wa

283,600 lb. The aircraft was flown almost straight into the north tower, a banked angle of approximately 25 degrees to the left and a descent angle of about 10 degrees at impact. The plane's nose hit the exterior of the tower at the 96th floor at a speed of about 440 mph which created a gash in the building that was over half the width of the building and extended from the 93rd floor to the 99th floor(Figure 2 and 3). (NIST 2005, p 20)

United Airlines Flight 175, like the American Airlines Flight, was much lighter with only 9,100 gal. of jet fuel and 51 passengers and 9 crew members on board. The plane was flown into the south face of WTC 2 about 23 ft. east of the center with a heading of about 15 degrees east of plan north. The nose of the plane struck at the 81st floor slab with a banked angle of 38 degrees to the left and a descent angle of about 6 degrees. With a steeper bank angle than that of American Airlines Flight 11, the plane created a gash that stretched over nine floors, from the 77th floor to the 85th floor (Figure 4 and 5). United Airlines Flight 175 hit the WTC 2 tower about 15 stories lower than that of American Airlines Flight 11 on WTC 1 tower. (NIST 2005, p 38)
Figure 2: South Face of WTC1, showing aircraft component location (NIST 2005, p22) Pending Approval



Both towers were subjected to three loading conditions that lead to the complete collapse of the buildings; the initial aircraft impact, the ignition and growth of fires on several floors, and a progressive sequence of failures. The aircraft destroyed an estimated 31-36 columns over four stories of WTC 1. It was clear that the core of the building suffered significant damage, but it was undetermined. With the building's high degree of redundancy, the immediate collapse was limited to the general area of impact. Once the columns were destroyed, the alternative load paths were formed through the trusses. The columns were probably near, but not over, their ultimate load capacity.

However, the fires proved fatal. Each aircraft carried approximately 10,000 gal. of jet fuel at the time of impact. Some of the fuel was used up in the huge fireball on impact while the rest of the fuel remained in the building to fuel the fires. The impact caused damage to the fire protection systems, both the active and passive systems. With the lack of fire protection, the fires were able to grow and cause significant damage to the structural system. The actual temperature of the fires in the building was unknown, but it is known that at about 700 degrees Fahrenheit, structural steel begins to lose its strength. After a significant amount time, the columns gave out which lead to a complete structural collapse. (Irfanoglu)


The aircraft destroyed an estimated 27-32 columns over five stories of the building. As with the other tower, some aircraft debris passed through the structure and may have caused major damage to the core columns. There were important differences between the aircrafts that crashed into WTC 2 and WTC 1. The aircraft that hit WTC 2 was traveling at a much higher speed which caused about 60% more energy to the structure than that of the aircraft that hit WTC 1. Furthermore, the impact on WTC 2 was about 20 stories lower than that on WTC 1 which means that the columns were carrying a much higher gravity load. These few important differences caused WTC 2, which was hit after WTC 1, to fall first. (Delatte p 203)

Conspiracy Theories

Floor Trusses were not responsible for the collapses:

The failure of the floor trusses was shown not to have had any significant role in the distruction of the towers. A review of photos and videos by a team of fire engineers from Hughas Associates found that the floors of the towers did survived the impact of the airplanes and only suffered localized damage from the crashes. The studies also found that the ensuing fires did not lead to the collapses of impact floors before the towers fell. In addition, computer modeling of the collapses showed that the failure of the columns alone explained the collapses. (Silverstein)

Fireproofing was inspected regularly:

There is speculation that the fireproofing in the twin towers had not been properly monitored. The maintenance records of the towers were destroyed in the collapses. However, inspection reports were located and analyzed. A study of the reports confirmed that the fireproofing on the structural steel was regularly examined. It was also found that the structural integrity inspection program conducted by the Port Authority represented a greater standard of care than is generally followed for high-rise office buildings in New York City. Futhermore, the analysis showed that the fireproofing was stripped from the steel aircraft impact. (Silverstein)

Why tower 2 fell first:

It was found that the plane that hit tower 1 hit the middle of the tower, while the plane that struck tower 2 hit the tower off center on a diagonal path and low on the building. This left tower 2 without one of its corner columns in the core of the building. Thus, tower 2 had less ability to redistribute the weight. In contrast, the core of tower 1 retained its corner columns and was better able to redistribute the weight. This allowed tower 1 to remain standing longer than tower 2. (Silverstein)

Figure 6: Reduction in young's modulus with temperature (Miamis). Pending Approval
Fire temperatures were lower than typical office fires:

A review of the fires showed that the jet fuel acted like a huge “match” that started the fires. But contrary to some speculation, the “match” burned out quickly and the fires did not spread faster and hotter. The analysis showed that the fires on the impacted floors did not get as hot or spread as fast as normal office fires, primarily because of the dust and debris. However, the average air temperature in the impacted floors ranged from 750 degrees F to 1300 degrees F, which is hot enough to eventually weaken the stripped fireproofed columns. Figures 6 and 7. (Silverstein)

Tower 2 did not causes or contribute to the collapses of tower 1:

Once the collapse was initiated in each of the towers, essentially all of the interior structure of the towers fell straight down because the perimeter columns and spandrels acted like a funnel. Some debris was thrown outward from the tower creating a lobe pattern of debris. Based on a review of photos and videos, the actual pattern of debris from each building collapse was able to be documented. Moreover, based on the information gathered, it was found that the collapse of tower 2 did not cause any significant structural damage to tower 1. Tower 1 stood out of the way of the falling walls of tower 2, and pieces of debris only scraped the surface of tower 1. (Silverstein)

Figure 7: Reduction in yield stress with temperature (Miamis). Pending Approval


Structures are designed for certain loads and hazards, and the structural engineers must decide on what loading condiction the building will be designed for. Most commercial buildings are not designed for meteorite impact, nuclear blast, or military attacks just to name a few. However, there are four major hazards that every building must be able to resist: gravity, wind, earthquake, and fire. The probability of extreme loading is not predictable, but most designers are starting to consider extreme loading beyond design load. There are no building designers that can ever design for every possible hazard and loads on a building. However, it is important for the designers to identify hazards that need to be address for the building. (Magnusson)

Other recommendation based on the investigating team study and findings of the collapse of the WTC 1 and WTC 2 (Delatte):
• Structural framing systems need redundancy and/or robustness to form alternative load paths when building damages occur.
• Fireproofing needs to adhere to structural members after impact and fire conditions that deform steel members.
• Critical structural framing components need to be analyzed under impact loads and fire loads to improve design capabilities and performances.
• Sprinklers in buildings should require a reliable and redundant water supply.
• Egress systems should be evaluated for redundancy and robustness in providing egress when building damages occur.


Delatte, N. J. (2009). “The World Trade Center Attacks.” Beyond Failure: Forensic Case Studies For Civil Engineers, American Society of Civil Engineers (ASCE), Reston, VA, 195-206.
-Book. An overview of the design, construction, and attack on the world trade center towers.

Irfanoglu, A. and Hoffmann, C. M. (2008). “Engineering Perspective of the Collapse of WTC-I” Journal of Performance of Constructed Facilities, ASCE, Vol. 22, No 1, pp. 62-67.
-Journal Article. This article is a report on a simulated performance study of the World Trade Center Tower 1 during the attack on September 11, 2001. Also discuss the possible damage to the structural core and its behavior under structural and thermal loading.

Iwankiw, N., P.E., Ph.D., Griffis, L.G., P.E., (October 5, 2004) “Comparison of Structural Performance of Multi-Story Buildings Under Extreme Events” American Institute of Steel Construction, Inc.(AISC).
-Report. This report compare and assess the structural performance of major multi-story building collapses due to exposure of blast, impact, and fire from terrorist attacks.

Magnusson, J., P.E., (2004). “Learning from Structures Subjected to Loads Extremely Beyond Design” Modern Steel Construction, March 2004.
-Journal Article. This article is about building loads that goes beyond the typical design loads for a building.

Miamis, A., Ifanoglu, A. and Sozen, M. A. (2009). “Dominant Factor in the Collapse of WTC-I”. Journal of Performance of Constructed Facilities, ASCE, Vol.22, No 4, pp. 203-208.
-Journal Article. In this article they discuss the effect of elevated temperatures on the mechanical properties of structural steel. In addition they discuss the damage caused by the aircraft and its effect on the fire protection on the core structural farming.

NIST. (2005). "Design, Construction, and maintenance of Structural and Life Safety Systems" National Institute of Standards and Technology. NCSTAR 1
-Report. This report describe in detail the design, construction, and maintenance of the structural of the World Trade Center Towers before the attacked on September 11, 2001.

NIST. (2005). "Final Report on the Collapse of World Trade Center Towers," National Institute of Standards and Technology. NCSTAR 1-1
-Report. This is the official report of the collapse of the World Trade Center Towers.

NIST. (2005). "Passive Fire Protection," National Institute of Standards and Technology. NCSTAR 1-6A
-Report. This is the report of the passive fire protection of the World Trade Center Towers.

Silverstein Properties, Inc., (2003). “Why they stood-and how they fell” Modern Steel Construction, January 2003.
-Journal Article. This article is an in-depth look at the performance of the structure of the World Trade Center Towers sustained from the attacked. Also discuss how the towers prolonged the collapse and how they collapsed.

Additional Resources

Eagar, T. W. and Musso C., (2001) “Why Did the World Trade Center Collapse? Science, Engineering, and Speculation” Journal of Materials. December 2001. pp. 8-11.
-Journal Article. This article takes a look at the airplane impact, the fire, and the collapse of the World Trade Center towers.