Source: AFP. Besides the twin towers, a storey third tower, known as Tower Seven, was also collapsed after seven hours on the same day. The then published reports had claimed that building fires were growing and spreading throughout the multiple floors, which became the ultimate reason for the collapse of the buildings.
Before the official investigative report was published, some conspiracy theorists argued that the third tower was "brought down" in a controlled demolition as it was not hit by a plane.
Some theorists continue to argue even after the government refuted the claims constantly. The building was utterly important due to the presence of the Secret Service, the CIA, the Department of Defense and the Office of Emergency Management, which was supposed to coordinate any response to terrorist attacks.
This permitted windows more than one-half meter wide. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story.
Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high. The egg-crate construction made a redundant structure i. Prior to the World Trade Center with its lightweight perimeter tube design, most tall buildings contained huge columns on 5 m centers and contained massive amounts of masonry carrying some of the structural load.
The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1, times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising.
The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage. While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, 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.
The ensuing fire was clearly the principal cause of the collapse Figure 4. The fire is the most misunderstood part of the WTC collapse. Even today, the media report and many scientists believe that the steel melted.
It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true. Part of the problem is that people including engineers often confuse temperature and heat.
While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density or mass. Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material.
One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles.
Thus, the fact that there were 90, L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel. In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame.
A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet.
This is what occurs in a jet engine, and this is the flame type that generates the most intense heat. In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame. A fireplace flame is a diffuse flame burning in air, as was the WTC fire.
Diffuse flames generate the lowest heat intensities of the three flame types. If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined. This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen. The reason is that every molecule of oxygen releases the heat of formation of a molecule of carbon monoxide and a molecule of water.
If pure oxygen is used, this heat only needs to heat two molecules carbon monoxide and water , while with air, these two molecules must be heated plus four molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times as many molecules must be heated when air is used. But it is very difficult to reach this maximum temperature with a diffuse flame.
There is nothing to ensure that the fuel and air in a diffuse flame are mixed in the best ratio. Typically, diffuse flames are fuel rich, meaning that the excess fuel molecules, which are unburned, must also be heated. This fuel-rich diffuse flame can drop the temperature by up to a factor of two again.
Some reports suggest that the aluminum from the aircraft ignited, creating very high temperatures. While it is possible to ignite aluminum under special conditions, such conditions are not commonly attained in a hydrocarbon-based diffuse flame. 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:.
Singapore offers warning. Second COP26 draft agreement softens language on coal and fossil fuel reduction. This sleepy country town is a secretive centre for Big Pharma drug production. Government releases its modelling underpinning the net zero emissions target. Perth Airport 'missed the mark' in move to recognise traditional owners on boarding gates.
A princess and a rugby royal are working to counter vaccine misinformation in the Pacific. Aussies highlight positives from poor T20 World Cup build-up after reaching final. Australian-first study paves way for mothers returning to elite sport. When Suzanne awoke from cosmetic surgery, she yelled at her doctor: 'What have you done? I can't breathe'. Court hears alleged murder victim's house was unusually clean and smelt like bleach. Massive funnel-web nicknamed Megaspider donated to venom-milking facility.
Child rescued from alleged porn operation, as man faces 43 charges.
0コメント