Architecture and Structural Analysis

Since the collapse of the World Trade Towers in New York on September 11, 2001, there has been greatly increased emphasis on studies of structural analysis and the design of architecture able to withstand acts of terrorism. Historically, structural analysis has also been employed in the construction of biochemical, petrochemical, and industrial plants, in order that they might withstand accidental chemical explosions.

Since 9/11, buildings, and the sites upon which they are constructed, have become increasingly designed to withstand and minimize the devastating effects of terrorist weapons and bomb blast loads. In addition, emphasis has been placed on retrofitting existing structures, in order to make them less vulnerable to terrorist attack. Much structural analysis research (also called blast mitigation research) has been done since 9/11, with the goal of understanding the mechanisms and factors that cause structural damage in a blast, minutely characterizing the sequence of structural responses during a blast, and quantifying the likely effects on the human inhabitants of the building in the event of a blast. There are a variety of means of gathering this data: extensive study of the aftereffects of actual terrorist events (such as the World Trade Towers, the Murrah Building in Oklahoma City, the Pentagon, and the Khobar Towers bombing in Saudi Arabia), assessment of existing architecture, creating controlled explosions in experimental structures, and the use of computer modeling sequences.

There has been much forensic, architectural, and structural analysis of the debris remaining after the collapse of the World Trade Towers, in part because there was a general expectation that the structures should have been able to withstand the impact of the aircraft. The Towers were built between the mid-1960s and the early 1970s, and were intended as a model of modern architecture. They utilized modular construction, and were comprised of very lightweight materials. The World Trade Towers were squared; the width of each face measured 64 meters (209.97 feet). They spanned 411 meters (1348.43 feet) above street level, and were placed on foundations that reached 21 meters (68.90 feet) below the ground. The height to width ratio of the Towers was 6.8. Each Tower weighed approximately 500,000 tons (1,000,000,000 pounds), and was built to stand firm against hurricane wind force of 225 kilometers per hour (139.81 miles per hour) and to withstand a wind load of 2 kilopascals (41.77 pounds per square foot) and a lateral wind load of 5,000 tons (10,000,000 pounds). In order to meet all of the structural integrity requirements, the architecture comprised a "perimeter tube" design, containing 244 exterior columns of 36 centimeter square (1.18 feet) steel box sections on 100 centimeter (3.28 feet) centers. Inside each perimeter tube was a 27-meter (88.58 feet) by 40-meter (131.23 feet) core, which was designed to support the weight of the Tower. The elevators, stairwells, mechanical risers, and the utilities were also housed within the core. The core was attached to the perimeter at each floor level by web joists that were 80 centimeters (2.62 feet) tall; these were covered with concrete slabs used to create the floors of the structures.

This type of building design is often referred to as an egg crate structure; it is actually about 95% air and 5% solid material. In comparison, most other buildings constructed during the same era contained massive columns seated on 5 meter (16.40 feet) centers, with enormous amounts of masonry designed to carry the brunt of the structural load.

The "airiness" of the Towers was the reason that the rubble and debris left by the buildings' collapse only rose a few stories above the ground. The strength of the structures resulted from their redundancy; that is, the architecture was such that if a few columns were destroyed, the load would redistribute among the remaining columns, with no appreciable loss of structural integrity.

Each Tower contained more than 1,000 times the mass of the aircraft that crashed into it, and had been constructed to be able to withstand steady wind loads of 30 times the weight of each plane. Had the blasts been confined only to impacts, with no resultant fires, the Towers could probably have remained standing. The Towers' collapse resulted from the fires caused by the explosion, and resultant ignition, of approximately 90,000 liquid gallons of jet fuel. Initially, it was hypothesized that the heat of the fire melted the steel girders in the structures, causing the Towers to collapse. After forensic investigation and structural analysis, that was found to be incorrect. The fire caused by the blast was characterized as "diffuse and fuel-rich," meaning that the fuel and air mixed together in unpredictable and uncontrolled ways after the blast, and there was more fuel than there was fire. In fuel-rich fires, the excess fuel is heated, but unburned. This was apparent in the Trade Tower fires because of the copious amounts of thick, black smoke given off; which was a by-product of incompletely burned fuel. The structural analysts estimated that the temperature during the fire was in range of 750°C–800°C (1382°F–1472°F), which is generally too low to melt steel (requiring a minimum of 1,500°C or 2732°F). Structural steel starts softening at about 425°C (797°F), and its strength is halved at 650°C (1202°F). However, even loss of half their structural steel strength should not have caused the Towers to collapse. The critical issue was the uncontrolled nature of the fire: it caused differential buckling and structural distortion in some areas, leading to the buildings' collapse.

As the structural analysts reconstructed the scenario, using both direct examination of rubble and structural steel, computer modeling, and simulation scenarios, the perimeter tubes of the World Trade Towers were able to withstand the initial impacts of the aircrafts, shifting loads from severed or damaged columns to those left standing. The extraordinary speed of the fire's spread generated very high heat, causing weakening, softening, twisting, and buckling of structural steel. The Towers' most vulnerable points were deemed by the analysts to be the angle clips connecting the floor joists between the core structure and the columns on the perimeter walls. As the joists fell away on the most seriously burned floors, the outer box columns began to bow outward. The outward bowing caused the affected floors to collapse and fall downward; it also caused the floors above them to implode downward, setting in motion a domino effect that caused the Towers to collapse vertically within approximately 10 seconds, hitting the ground at a velocity of about 200 kilometers (124.27 miles) per hour.

Overall, the analysts deemed the Towers did not contain structural defects; they were simply not created to withstand the intentional impact of airliners filled with incendiary jet fuel. Their conclusion: the buildings were impossible to save; rather than attempting to build terrorist impervious structures, it is more practical to focus on creating better emergency preparedness (communications, emergency response, and evacuation) systems.

see also Aircraft accident investigations; Building materials; Computer modeling; Endothermic reaction; Exothermic reactions; Explosives; Fire debris; First responders; NTSB (National Transportation Safety Board).