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  3. The authors attribute each tower’s collapse to three separate but related "loading events". 【F1】The first event was a Boeing air

The authors attribute each tower’s collapse to three separate but related "loading events". 【F1】The first event was a Boeing air

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问题     The authors attribute each tower’s collapse to three separate but related "loading events". 【F1】The first event was a Boeing aircraft hitting the building, cutting through the exterior structure and creating a fireball that immediately consumed some of the estimated 10, 000 gallons(38 kiloliters) of jet fuel. The high-rises, structural systems were sufficiently redundant. However, that this major damage by itself did not cause the collapse. According to the report, "most of the load supported by the failed columns is believed to have been transferred to adjacent perimeter columns through Vierendeel behavior of the exterior wall frame".
    【F2】The second event was the continuing fire, fed both by the remaining jet fuel and the office contents of furniture and paper. 【F3】This fire heated and weakened the structural systems, adding stress to the damaged structure. Meanwhile, the sprinklers were not operating as designed. "Even if these systems had not been compromised by the impacts, " says the report, "they would likely have been ineffective... 【F4】the initial flash fires of jet fuel would have opened so many sprinkler heads that the systems would have quickly depressurized and been unable to effectively deliver water to the large area of fire involvement. "
    The third event was a progressive collapse: 【F5】"As the large mass of the collapsing floors above accelerated and impacted the floors below, it caused an immediate progressive series of floor failures, punching each in turn onto the floor below, accelerating as the sequence progressed. Freestanding exterior walls...buckled at the bolted column splice connections and also collapsed. "
【F4】
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答案飞机燃油引起的大火使众多的喷头开始工作，致使灭火系统水压骤然下降，难以有效地将水输送到大范围的火场。
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