Analysis and Lessons on the Collapse of the Twin Towers of the World Trade Center in New York City

Analysis and Lessons Learned about the Collapse of the Twin Towers of the World Trade Center in New York City Zou1, Tang Jiamin2, and Ji Jing2's status analyzes the causes of the collapse, and puts forward some suggestions on how to improve the high-rise building on the premise that the building will collapse. Rescue efficiency advice in the fire.

1 Structural system 11 Box-shaped lattice drum structure In the construction of the twin towers, high-strength steel has replaced reinforced concrete and a unique box-shaped lattice structure has been adopted, ie, a dense steel column has been installed on the outer surface of the building. To form the outer frame tube. This design allows the Twin Towers to withstand the 13K 000kN wind pressure in Manhattan and uses only half as much steel as traditional frame structures.

In the structural design, the outer wall of the tower is used as a bearing wall to bear most of the horizontal loads and half of the vertical loads. The tower's core barrel is used to withstand the other half of the vertical load and accommodate elevator shafts, stairwells and equipment rooms.

The external walls of the Twin Towers are made up of densely packed columns and wall panels. Among them, the column spacing is about 1m, and the wallboard is set at each floor height. Each grid assembly is assembled from 3 columns. On the floor, the prefabricated grids are welded together to form a light and rigid plate.

12 Floor System The towers of the twin towers are supported by light steel trusses with a height of 813 mm. The top surface of the truss is welded with steel plates, which are used to form prefabricated floors. The dimensions of each steel plate are 182mx4m. These steel plates are first lifted into place after the surface is prefabricated. When the truss is in place, a layer of 100mm thick concrete is poured on it.

13 Resistance to Impact Loads During the design phase, the designer did consider the possibility of an aircraft crashing into the Twin Towers due to a crash. According to a 1945 plane crashed into the Empire W.:Zou (1963~), male, Ph.D., associate professor, mainly engaged in research on construction technology and construction management.

Building experience, they designed the Twin Towers to withstand the impact of the Boeing 707, the largest commercial aircraft of the time, on towers at low speeds. 14 Features of the structural system Innovative mode: Rigid/hollow cylinders formed from dense steel columns and floor trusses spanning 2 cm to the central core tube; the steel columns use a silver aluminum alloy as the protective layer, and the column width 18 The façade of the building consists of prefabricated steel grids that act as wind bracing to resist all overturning loads; wind loads will only be transmitted along the external sash of the building and will not be transmitted along the floor to the central core; from a span of 20m. The floor made of prefabricated steel trusses acts as a cross brace for the reinforcement of the outer wall; besides supporting the floor cover, the truss can also be connected to the surrounding steel columns and core barrels, thereby preventing the steel columns from destabilizing to the outside.

15 Weakness of the structural system The weakness of the box-shaped lattice structure is that the steel column can be destroyed by an impact, and the lightweight truss-type floor has poor fire resistance. Because the truss has a small self-weight, it heats up rapidly in the fire. When there is no good protection, the trusses are prone to bending and deformation. Due to the design, if any one of the connection points between the outer wall surface and the column or between the inner truss and the column is damaged, a major unit in the structure will be displaced and the entire structure will collapse.

2 Fire protection measures Fireproof material is sprayed on the surface of each column before being lifted into place. This fireproof material contains a dry brittle material made of mineral fibers. At the same time, the facade of the building can be used as a protective layer for steel columns.

3 Causes of Collapse 11 During the terrorist attacks, the fire was caused by the explosion of the aircraft fuel tank. Boeing 767 airliner hitting the North Tower is about 4 As mentioned earlier, the impact load of a Boeing 707 aircraft has been considered in the design of the Twin Towers. However, the Boeing 767 aircraft in this accident has a much larger capacity and size than the Boeing 707, and its flight speed is much faster than expected. The plane and its wings are like a missile with a speed of about 760km/h and a weight of 150t. The missile, which has a total length of 47.5m from the tip of one wing to the tip of the other wing, penetrates the façade of the 4th to 5th floor easily.

W丨18 had commented that the energy released by such explosions was approximately 35 times the energy generated by the aircraft’s impact on the building. Hydrocarbon fires are different from any conventional building fire. It is generally believed that the temperature of hydrocarbon fires can reach 1000°C to 1200°C in a matter of seconds. 31 Fire Hazards The raging fire generated by aviation fuel can raise the temperature to 1000°C in a matter of seconds. Between 1200°C, the heat generated causes severe damage to the exposed steel in the twin towers. At high temperatures, the lightweight truss floor system begins to sag and deform. As a result, the bolt connection between the truss and the column begins to fail. Under the action of pulling force, the connection of the bolt nodes on the column also loses its effect, and the bolt fixing parts of the 3rd to 5th layer of the continuous end anchor bolts are loosened, resulting in the instability of other complete external box shaped columns. Later, due to the failure of the column, the floor above the impact surface collapsed. Because it cannot withstand the loads imposed on it, these 10 to 30 floors fall, one after the other, like dominoes. A lot of rubble is like a giant hammer piston hitting a box-shaped lattice column. As the scattered crushed stones hit the outer surface of the box column, the rest of the floor was destroyed.

32 Fire Protection Problem When testing the waste steel of the Petronas Twin Towers, it was found that although there was a protective layer on each part of the steel, a complete fireproof material was hardly found on the steel. This shows that the powerful airflow caused by the impact blows away all the fireproof materials on the steel components. The fireproof material has no effect. Once the fireproof material is blown away, it means that the steel is vulnerable to the heat generated by the combustion of the aircraft fuel. However, the fire alone is not enough to melt the steel. After analysis, due to the steel being bent, it eventually lost the strength of the supporting load.

According to investigations, several fire protection materials have fallen off in the lower layers of the North Tower of the Twin Towers in the World Trade Center. Some of the trusses used to maintain stability have only little or no fire protection material. The structure does not reach full protection. Morse points out that at the core barrel, the fireproof material is simply sprayed on the loose rust of the steel components in the structure. This can also explain the previously mentioned problem of shedding of fireproof materials. He also pointed out that if the fireproof materials are well sprayed and maintained, the twin towers can last longer before they collapse.

33 The state of the structure in the fire The entire length of the aircraft from one end to the other is approximately 2/1 of the entire floor area of ​​the Twin Towers. Not only is the lattice structure around the building severely damaged, but the inner core barrel is also damaged. As the aircraft broke the elevator shaft and the stairs, it was impossible for people on the buildings above the impact plane to escape. Therefore, the collapse further highlights the weakness of the box-shaped lattice structure. Compared with traditional buildings, the first weakness of the Petronas Twin Towers is its load bearing column. In addition, due to the fact that the twin towers are actually open floor layouts, there is a lack of a central solid cylinder. Usually, when a building uses a central cylinder or internal column support, it will build a wall that can first act as a squeezing section “to withstand the initial impact. This can be/buffered” the impact, thus reducing the central The severity of damage to the barrel or internal column. However, the box lattice lattice structure of the Twin Towers does not have such protection.

However, the weakness of the box-shaped lattice structure cannot be used as the sole reason for the collapse of the twin towers. Wilson mentioned that some international authoritative experts agree that in this disaster, the Twin Towers of the World Trade Center showed good structural performance. From the live video footage, it can be seen that although the Twin Towers suffered large-scale damage, its structure The indeterminateness and ductility of the people give people the opportunity to escape. If it is a reinforced concrete or prestressed concrete structure, collapse will take more lives. Because of the use of reinforced concrete or prestressed concrete structures, the spacing between the columns will be greater, leading to early collapse. In this disaster, the North Tower maintained 103 minutes before it collapsed and South Tower maintained 57mh, which has reached the duration of the design requirements (taking into account the impact of hydrocarbon fires).

4 Suggestions and Lessons Although the designers have taken into account this fire, its power is still beyond people's normal estimates. It is generally believed that if it is not economical to construct a building that can protect itself and withstand such terrorist attacks, it is not feasible. Therefore, suggestions should be made on the premise that the building will collapse, including suggestions for reducing the death toll, improving early warning and communication systems, and increasing and improving the orderly evacuation procedures.

41 Prior planning Leibowitz pointed out that the emphasis on equipment safety is an extremely important building requiring fire safety planning and on-site plans, including emergency plans for the worst case. However, in the September 11th incident, there was no prior planning. Pre-planning is in a simple and easy-to-understand form for every detail (including water supply, ventilation, communications, elevators, etc.) and firefighting or investigation and fire commanders’ general information, as well as general information on hazardous materials and buildings. The fire department's strategy, etc.)

42 Contingency measures During this disaster, all possible escape routes and exits were locked or blocked, including doors to the roof, which resulted in a death toll of 3,000 people. There are also problems with the design of stairwells. In the Twin Towers, the stairs are all tightly clustered in the middle of the building. Therefore, when a fire occurs on one of the stairs, it may affect the use of other stairs. It turns out that the impact of the plane made the stairs unusable.

However, in fact, there is still a stairwell that is not affected by the impact of the aircraft. This is the stairwell outside the core of the building. It is a passage for transportation machinery. Unfortunately, most people do not know this passage and only 18 people escape from this stairwell.

Therefore, emergency measures at the design stage also require detailed consideration during construction. Emergency measures and indicators for important safety positioning designs, such as site layout, access and visibility, exit design, and fire door dimensions, should be considered during the design phase of the building.

3 Day-to-day inspection, maintenance and personnel training The building's fire detection, warning and communication devices all require daily inspections. Tenants of office buildings need to undertake these routine maintenance tasks for their units, including emergency systems. Similarly, in order to ensure that these systems are ready at all times, they are regularly tested, updated, and maintained. Every building needs to develop a corresponding emergency evacuation plan. It also requires trained staff to manage and assist in evacuation. These staff must be very familiar with the main evacuation routes. At the same time, they must also have access to backup channels in case the main access is locked or cannot be used. Building users need to be trained in evacuation procedures and fire evacuation exercises are conducted twice a year.

4 The person in charge of the strict inspection of the building by the responsible person shall actively cooperate with the safety evacuation of the user during the emergency. Inspectors from groups such as fire brigades, building owners and property committees can conduct spot checks to ensure that owners comply with the regulations.

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