如何让摩天大楼更安全？

    40年来，洛杉矶的建筑条例一直要求，所有75英尺以上的建筑，要在消防部门批准的位置设置紧急直升机降落设施。1974年通过该法律的目的，是让摩天大楼更安全，这在很大程度上是受到了巴西一场惨重火灾的影响。但我们都知道，其实有许多更好的方法，可以让美国银行（U.S. Bank）大厦这样的地标性建筑更安全。最近，洛杉矶宣布终止这项要求摩天大楼必须设计为平顶的政策，我对此决定表示欢迎。这项政策对各地的高层建筑有相当大的借鉴意义。

    作为一名城市规划师和建筑师【成为教授之前，笔者曾在SOM-Chicago建筑事务所（原Skidmore, Owings & Merrill建筑事务所）担任建筑师】，我很清楚，相比低层建筑，安全性对于高层建筑来说更加重要，因为高层建筑有更多居住者，而且高层建筑本身的造价也更加不菲。此外，我也很清楚，如果经过合理的设计和建造，摩天大楼在某些方面比中低层建筑更安全。摩天大楼的混凝土芯，可以承受强风和地震时的极端侧向力和横向载荷。摩天大楼的防火安全设施包括洒水装置，以及可以直接连接消防软管的湿式和干式竖管。

    2001年世贸中心大厦倒塌之后，人们发现了高层建筑安全规定的不足。美国国家标准与技术研究所（NIST）得出的结论是，如果大楼挤满人的情况下遭遇袭击，全部疏散需要三个多小时。在这个过程中，14,000人，即全部居住者的28%，会因为楼梯间容量不足而丧生。

    NIST强调，时间是疏散的关键。而楼宇停机坪对疏散时间的影响微乎其微。直升机降落、登机和起飞都需要时间。而且，直升机每次仅能运送极少数摩天大楼居住者。研究显示，由于大火产生的热量和浓烟，即便直升机可以接近世贸中心大厦的屋顶（楼宇停机坪已被停用），也无法降落。

    极少被用到的直升机停机坪，可能会增强人们的安全感，除此之外毫无用处。NIST的研究呼吁更明智地使用建筑设计实现安全性。这些设计要素包括：

    假设整栋建筑都需要疏散。按照惯例，高层建筑的建造者会假设将发生“分阶段疏散”。当一层发生火灾时，居住者在可以安全返回之前，应该先疏散到临近楼层。世贸中心大厦倒塌证明，在紧急情况下，高层建筑的居住者更希望全部疏散。NIST建议，所有高度超过420英尺的非居住用摩天大楼，应该设置三个电梯间，且耐火材料应该能够承受每平方英尺1,000磅的压力（发生炸弹袭击、煤气泄漏或其他类似事件时）。

    For 40 years, Los Angeles’ building code has required all buildings 75 feet and taller to have a rooftop emergency helicopter landing facility in a location approved by the fire chief. The idea in 1974, when the law was passed, was to make skyscrapers safer, in part as a reaction to a catastrophic fire in Brazil. But we know now there are better ways to make structures like the landmark U.S. Bank tower safe. I, for one, am cheering for the recently announced end of a policy requiring flat-topped buildings in Los Angeles. It’s a policy that holds lessons for tall buildings everywhere.

    As an urban planner and architect (before becoming a professor, I was an architect at SOM-Chicago, the former Skidmore, Owings & Merrill), I know safety is more critical in tall buildings than in low-rise structures because tall buildings host a greater number of inhabitants and are themselves expensive investments. I also know that, if appropriately designed and built, skyscrapers are safer in many respects than low-rise and mid-rise buildings. They have concrete cores that are designed to withstand the extreme lateral forces and loads that occur during high winds and earthquakes. Fire safety systems in skyscrapers include sprinklers and wet and dry standpipes, to which firefighting hoses can be connected.

    Codes for tall building safety were found to be deficient following the World Trade Center collapse in 2001. The National Institute of Standards and Technology concluded it would have taken more than three hours to evacuate the buildings if they had been full of people at the time of the attacks. In the process, 14,000 people – 28% of the occupants – would have died because of insufficient stairwell capacity.

    NIST stressed that time is of the essence in evacuation. And helipads have a very small impact on evacuation times. Helicopters take time to land, load people, and take off. They only take a small number of a skyscraper’s occupants each time. Research indicated that if the World Trade Center rooftops had been accessible (the helipad fell in disuse), helicopters couldn’t have landed because of the heat and smoke.

    Our rarely used helipads may enhance the perception of safety but do little else. The NIST study called for a smarter strategy of using building design for safety. Among the key elements:

    Assume that the full building will evacuate. Conventionally, builders of high-rises have assumed “staged evacuations” will occur. During a fire on one floor, occupants were supposed to evacuate to adjacent floors until it was safe to return. After the World Trade Center collapse, it became clear a tall building’s occupants would likely want to evacuate all at once in an emergency situation. NIST recommends that all non-residential skyscrapers that exceed 420 feet in height have three stairwells and fireproofing capable of withstanding a pressure of 1,000 pounds per square foot (in the event of a bomb, gas breakout, or something similar).