In the years since the terrorist attacks in New York City, individual engineers and committees of experts have studied the crumpling of the World Trade Center twin towers. By examining the building's destruction step-by-step, experts are learning how buildings fail and discovering ways to build stronger structures by answering the question: What caused the twin towers to fall?

Aircraft Impact

When hijacked commercial jets piloted by terrorists struck the twin towers, some 10,000 gallons (38 kiloliters) of jet fuel-fed an enormous fireball.﻿﻿ But the impact of the Boeing 767-200ER series aircraft and the burst of flames did not make the towers collapse right away. Like most buildings, the twin towers had a redundant design, which means that when one system fails, another carries the load.

Each of the twin towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. In this tubular design system, when some columns became damaged, others could still support the building.

"Following the impact, floor loads originally supported by the exterior columns in compression were successfully transferred to other load paths," wrote examiners for the official Federal Emergency Management Agency (FEMA) report. "Most of the load supported by the failed columns is believed to have transferred to adjacent perimeter columns through Vierendeel behavior of the exterior wall frame."

Belgian civil engineer Arthur Vierendeel (1852-1940) is known for inventing a vertical rectangular metal framework that shifts shear differently than diagonal triangular methods.

The impact of the aircraft and other flying objects:

Compromised the insulation that protected the steel from high heat Damaged the sprinkler system of the building Sliced and cut many of the interior columns and damaged others Shifted and redistributed the building load among columns that were not immediately damaged

The shift put some of the columns under "elevated states of stress."

Heat From Fires

Even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire. Fed by the spray of jet fuel, the heat became intense. It is no comfort to realize that each aircraft carried less than half of its full capacity of 23,980 U.S. gallons of fuel.

Jet fuel burns at 800 to 1,500 degrees Fahrenheit. This temperature is not hot enough to melt structural steel.﻿﻿ But engineers say that for the World Trade Center towers to collapse, its steel frames didn't need to melt—they just had to lose some of their structural strength from the intense heat. Steel will lose about half its strength at 1,200 Fahrenheit. Steel also becomes distorted and will buckle when the heat is not a uniform temperature. The exterior temperature was much cooler than the burning jet fuel inside. Videos of both buildings showed inward bowing of perimeter columns resulting from sagging of heated trusses on many floors.

Collapsing Floors

Most fires start in one area and then spread. Because the aircraft hit the buildings at an angle, the fires from impact covered several floors almost instantly. As the weakened floors began to bow and then collapse, they pancaked. This means that upper floors crashed down on lower floors with increasing weight and momentum, crushing each successive floor below.

"Once movement began, the entire portion of the building above the area of impact fell in a unit, pushing a cushion of air below it," wrote researchers of the official FEMA report. "As this cushion of air pushed through the impact area, the fires were fed by new oxygen and pushed outward, creating the illusion of a secondary explosion."

With the weight of the plunging floors' building force, the exterior walls buckled. Researchers estimate that the "air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 mph."﻿﻿ Loud booms were heard during the collapse. They were caused by airspeed fluctuations reaching the speed of sound.

Why They Flattened

Before the terrorist attack, the twin towers were 110 stories tall. Constructed of lightweight steel around a central core, the World Trade Center towers were about 95 percent air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.

Stephen Chernin/Getty Images

Strong Enough?

The twin towers were built between 1966 and 1973. No building constructed at that time would have been able to withstand the impact of the terrorist attacks in 2001. We can, however, learn from the collapse of the skyscrapers and take steps to construct safer buildings and minimize the number of casualties in future disasters.

When the twin towers were constructed, the builders were granted some exemptions from New York's building codes. The exemptions allowed the builders to use lightweight materials so the skyscrapers could achieve great heights. According to Charles Harris, author of "Engineering Ethics: Concepts and Cases," fewer people would have died on 9/11 if the twin towers had used the type of fireproofing required by older building codes.﻿﻿

Others say the architectural design actually saved lives. These skyscrapers were designed with redundancies—anticipating that a small plane could accidentally penetrate the skyscraper skin and the building would not fall from that type of accident.

Both buildings withstood the immediate impact of the two large aircraft bound for the West Coast on 9/11. The north tower was hit at 8:46 a.m. ET, between floors 94 and 98—it did not collapse until 10:29 a.m., which gave most people one hour and 43 minutes to evacuate. ﻿﻿﻿Even the south tower was able to stand for a remarkable 56 minutes after being hit at 9:03 a.m. ET. The second jet hit the south tower on lower floors, between floors 78 and 84, which structurally compromised the skyscraper earlier than the north tower. Most of the south tower occupants, however, began evacuating when the north tower was hit.

The towers could not have been designed any better or stronger. Nobody anticipated the deliberate actions of an aircraft filled with thousands of gallons of jet fuel.

9/11 Truth Movement

Conspiracy theories often accompany horrific and tragic events. Some occurrences in life are so shockingly incomprehensible that some people begin to doubt theories. They might reinterpret evidence and offer explanations based on their prior knowledge. Passionate people fabricate what becomes alternative logical reasoning. The clearinghouse for 9/11 conspiracies became 911Truth.org. The mission of the 9/11 Truth Movement is to reveal what it believes to be the United States' covert involvement in the attacks.

When the buildings collapsed, some thought it had all of the characteristics of a "controlled demolition." The scene in Lower Manhattan on 9/11 was nightmarish, and in the chaos, people drew on past experiences to determine what was happening. Some people believe that the twin towers were brought down by explosives, although others find no evidence for this belief. Writing in the Journal of Engineering Mechanics ASCE, researchers have shown "the allegations of controlled demolition to be absurd" and that the towers "failed due to gravity-driven progressive collapse triggered by the effects of fire."﻿﻿

Engineers examine evidence and create conclusions based on observations. On the other hand, the Movement seeks the "suppressed realities of September 11th" that will support their mission. Conspiracy theories tend to continue in spite of the evidence.

Legacy on Building

While architects strive to design safe buildings, developers don't always want to pay for over-redundancies to mitigate outcomes of events that are unlikely to happen. The legacy of 9/11 is that new construction in the United States must now adhere to more demanding building codes. Tall office buildings are required to have more durable fireproofing, extra emergency exits, and many other fire safety features. The events of 9/11 changed the way we build, at local, state, and international levels.