????40年來(lái)，洛杉磯的建筑條例一直要求，所有75英尺以上的建筑，要在消防部門(mén)批準(zhǔn)的位置設(shè)置緊急直升機(jī)降落設(shè)施。1974年通過(guò)該法律的目的，是讓摩天大樓更安全，這在很大程度上是受到了巴西一場(chǎng)慘重火災(zāi)的影響。但我們都知道，其實(shí)有許多更好的方法，可以讓美國(guó)銀行（U.S. Bank）大廈這樣的地標(biāo)性建筑更安全。最近，洛杉磯宣布終止這項(xiàng)要求摩天大樓必須設(shè)計(jì)為平頂?shù)恼撸覍?duì)此決定表示歡迎。這項(xiàng)政策對(duì)各地的高層建筑有相當(dāng)大的借鑒意義。

????作為一名城市規(guī)劃師和建筑師【成為教授之前，筆者曾在SOM-Chicago建筑事務(wù)所（原Skidmore, Owings & Merrill建筑事務(wù)所）擔(dān)任建筑師】，我很清楚，相比低層建筑，安全性對(duì)于高層建筑來(lái)說(shuō)更加重要，因?yàn)楦邔咏ㄖ懈嗑幼≌撸腋邔咏ㄖ旧淼脑靸r(jià)也更加不菲。此外，我也很清楚，如果經(jīng)過(guò)合理的設(shè)計(jì)和建造，摩天大樓在某些方面比中低層建筑更安全。摩天大樓的混凝土芯，可以承受強(qiáng)風(fēng)和地震時(shí)的極端側(cè)向力和橫向載荷。摩天大樓的防火安全設(shè)施包括灑水裝置，以及可以直接連接消防軟管的濕式和干式豎管。

????2001年世貿(mào)中心大廈倒塌之后，人們發(fā)現(xiàn)了高層建筑安全規(guī)定的不足。美國(guó)國(guó)家標(biāo)準(zhǔn)與技術(shù)研究所（NIST）得出的結(jié)論是，如果大樓擠滿人的情況下遭遇襲擊，全部疏散需要三個(gè)多小時(shí)。在這個(gè)過(guò)程中，14,000人，即全部居住者的28%，會(huì)因?yàn)闃翘蓍g容量不足而喪生。

????NIST強(qiáng)調(diào)，時(shí)間是疏散的關(guān)鍵。而樓宇停機(jī)坪對(duì)疏散時(shí)間的影響微乎其微。直升機(jī)降落、登機(jī)和起飛都需要時(shí)間。而且，直升機(jī)每次僅能運(yùn)送極少數(shù)摩天大樓居住者。研究顯示，由于大火產(chǎn)生的熱量和濃煙，即便直升機(jī)可以接近世貿(mào)中心大廈的屋頂（樓宇停機(jī)坪已被停用），也無(wú)法降落。

????極少被用到的直升機(jī)停機(jī)坪，可能會(huì)增強(qiáng)人們的安全感，除此之外毫無(wú)用處。NIST的研究呼吁更明智地使用建筑設(shè)計(jì)實(shí)現(xiàn)安全性。這些設(shè)計(jì)要素包括：

????假設(shè)整棟建筑都需要疏散。按照慣例，高層建筑的建造者會(huì)假設(shè)將發(fā)生“分階段疏散”。當(dāng)一層發(fā)生火災(zāi)時(shí)，居住者在可以安全返回之前，應(yīng)該先疏散到臨近樓層。世貿(mào)中心大廈倒塌證明，在緊急情況下，高層建筑的居住者更希望全部疏散。NIST建議，所有高度超過(guò)420英尺的非居住用摩天大樓，應(yīng)該設(shè)置三個(gè)電梯間，且耐火材料應(yīng)該能夠承受每平方英尺1,000磅的壓力（發(fā)生炸彈襲擊、煤氣泄漏或其他類(lèi)似事件時(shí)）。

????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).