Pictures taken inside the stairwell show office workers in a devastatingly slow descent, at times stopped and pressed flat against the walls to allow firefighters carrying well over 20 kilos in heavy equipment to get up the stairs. Evacuees included hundreds of people with physical disabilities, some in wheelchairs, being carried down by their co-workers through the narrow stairs. Survivors said even as the descent moved at a snail's pace, people did not push.
Below the points of impact, the stairs saved thousands of people, with about 14, occupants of the lower floors making it out alive. The death toll would likely have been much higher that day, but for one crucial factor: The time of attack.
Investigators estimate the buildings were only half full when the first plane struck at am. Had the attack taken place later, the overcrowding they saw in the stairwells during the evacuation would have been catastrophic, with thousands more trapped as the towers collapsed.
There were no video cameras in the stairwells and radio communications had broken down, leaving them unable to coordinate rescue efforts or receive warnings to evacuate.
Accounts from survivors paint a grim picture of how the vacuum of information inside the towers compounded the death toll. Even with the sole stairwell in the south tower remaining passable, the occupants above the point of impact didn't get the information they needed to make the right choice.
When emergency operator recordings were released of distress calls that day, large numbers of victims had called for help from their mobile phones, only to be told to stay put and "defend-in-place". Professor Corbett believes that if information from survivors who had made it down the stairs had been relayed to those still in the building with a quick call to their phones, many more may have made it out alive. The US approved 23 building and fire code modifications in , following investigations into the World Trade Centre disaster.
They included measures to improve fire resistance in building materials, to reinforce structures against collapse, and add blast-resistant walls to elevator and stairwell shafts — all designed to help buildings stay intact long enough to get people out. High-rise buildings were required to improve radio coverage systems to ensure emergency crews can communicate with each other inside, and with personnel outside.
A requirement for an extra stairwell did get through, but only in buildings above metres, more than 40 storeys high. The width of stairways would be increased by 50 per cent, but only in a building code that does not cover most of the new high rise buildings being built across the United States today, including New York City. While ICC codes are broadly adopted around the US, they are a minimum standard for building and fire codes and it's up to states and local jurisdictions to decide what to enforce.
Karl Fippinger said the ICC will continue pushing the building industry to go above minimum safety codes. We acknowledge Aboriginal and Torres Strait Islander peoples as the First Australians and Traditional Custodians of the lands where we live, learn, and work.
The simplest of escape routes were the stairwells. More on:. Top Stories Government releases its modelling underpinning the net zero emissions target. Share sensitive information only on official, secure websites.
The collapse of the WTC buildings following the terrorist attacks on September 11, , was one of the worst building disasters in US history, killing 2, people, including emergency responders. NIST responded to calls from Congress and the public to carry out a federal investigation of why the buildings collapsed, the evacuation of building occupants and the emergency response. More than professionals and technical subject matter experts, including 85 NIST staff members, answered the call and participated in the investigation.
As part of the investigation, the NIST team gathered every bit of evidence they could find. Activities included:. Throughout the process, NIST held 23 public meetings and provided multiple opportunities for the public to review and comment on drafts of the reports. Read first-hand accounts of what it was like to work on the WTC investigation in our blog series.
Start with Shyam Sunder's reflections on the investigation and its legacy, and then keep reading for insights on everything from what it was like to catalogue the images and video to how changed one researcher The WTC investigation has had a significant legacy. In the reports, NIST made 31 recommendations for improvements to building and fire codes, standards, and practices based on the WTC investigation.
While the federal government has no regulatory authority for building and fire codes, many U. In addition, NIST scientists conducted tests of steel from the WTC buildings to measure their mechanical properties at normal and elevated temperatures. These tests led to the development and validation of performance criteria for fire resistive steel.
This type of steel, which was not available at the time the WTC was built, would not necessarily prevent a building from collapsing during a fire, but it would give occupants more time to escape. This effort was at the time — and still is — the largest forensic identification effort ever undertaken. This effort was particularly challenging because in many cases the DNA of the victims was severely degraded by exposure to intense heat from burning jet fuel, as well as moisture and decay in the weeks and months following the attack.
Simensen believes that it is overwhelmingly likely that the two aircraft were trapped inside an insulating layer of building debris within the skyscrapers. This leads him to believe that it was the aircraft hulls rather than the buildings themselves that absorbed most of the heat from the burning aircraft fuel.
The SINTEF scientist believes that the heat melted the aluminium of the aircraft hulls, and the core of his theory is that molten aluminium then found its way downwards within the buildings through staircases and gaps in the floor -- and that the flowing aluminium underwent a chemical reaction with water from the sprinklers in the floors below.
Simensen continues: "I regard it as extremely likely that it was these explosions that made the skyscrapers collapse by tearing out part of the internal structure, and that this caused the uppermost floors of the buildings to fall and crush the lower parts.
In other words, I believe that these were the explosions that were heard by people in the vicinity and that have since given life to the conspiracy theories that explosives had been placed in the skyscrapers.
One lesson is that we could develop means of rapidly emptying sprinkler systems in the floors under the point of impact. Another possibility would be to fire in a rocket carrying a fire-retardant that would overlie the aircraft body and prevent the metal alloy from becoming overheated. Neighbouring buildings were bombarded by flying debris when the towers collapsed. The storey skyscraper called 7 World Trade Center also caught fire and collapsed several hours later at The official report on the causes of the collapse of the three buildings was drawn up by a commission appointed by the federal government and has since been supported by other publications.
The report came to the conclusion that the collapse was caused by heating and failure of structural steel beams in the centre of the buildings. Let us start with what I think must have happened when the planes struck the two towers. They came in at high speed and at a low angle. The only similar phenomenon that we have any knowledge of is meteors that hit the Earth.
What we know is that these drag material with them on their way through the soil layer. The whole surface, including all its pores, is covered by the material that they carry along.
The innermost layer melts and turns into a glass coating on the surface of the meteor. Much of this material was plaster, a material with extremely poor heat conduction capacity. All this debris probably formed a shield that kept the heat close to the aircraft and protected the rest of the building. The disintegrated aircraft probably came to a stop near the centre of the buildings. The materials along the track of the collision must also have burned.
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