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

????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，安全性對于高層建筑來說更加重要，因為高層建筑有更多居住者，而且高層建筑本身的造價也更加不菲。此外，我也很清楚，如果經(jīng)過合理的設(shè)計和建造，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，可以承受強風(fēng)和地震時的極端側(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。美國國家標準與技術(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，全部疏散需要三個多小時。在這個過程中，14,000人，即全部居住者的28%，會因為樓梯間容量不足而喪生。

????NIST強調(diào)，時間是疏散的關(guān)鍵。而樓宇停機坪對疏散時間的影響微乎其微。直升機降落、登機和起飛都需要時間。而且，直升機每次僅能運送極少數(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，即便直升機可以接近世貿(mào)中心大廈的屋頂（樓宇停機坪已被停用），也無法降落。

????極少被用到的直升機停機坪，可能會增強人們的安全感，除此之外毫無用處。NIST的研究呼吁更明智地使用建筑設(shè)計實現(xiàn)安全性。這些設(shè)計要素包括：

????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假設(shè)將發(fā)生“分階段疏散”。當一層發(fā)生火災(zāi)時，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420英尺的非居住用摩天大樓，應(yīng)該設(shè)置三個電梯間，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、煤氣泄漏或其他類似事件時）。

????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).