SAN FRANCISCO – The engineers of the new San Francisco-Oakland Bay Bridge have planned for the long term.
工程師在設計舊金山與奧克蘭間新建的海灣大橋時，做了長遠的考慮。
At intervals inside the $6 billion span’s box girders – which have the closed-in feel of a submarine – are anchor blocks, called deadmen, cast into the structure. They are meant to be used perhaps in the next century, when the concrete girders start to sag. By running cables from deadman to deadman and tightening them, workers will be able to restore the girders to their original alignment.
造價60億美元的橋面盒式大梁感覺有如密閉的潛艦，其間是澆鑄融入結構體，又名錨墩的固定塊。它們可能是針對下個世紀設計的，屆時水泥大梁會開始下陷。以鋼索通過每個錨墩並拉緊，工程人員就可以讓大梁恢復原有的排列。
The deadmen are one sign that the new eastern span of the Bay Bridge is meant to last at least 150 years after its expected opening in 2013. (The existing eastern bridge will then be torn down.)
使用錨墩意味，新橋2013年啟用後，新建的大橋東側至少可以使用150年（現有的東側大橋屆時將拆除）。
At some point, the new span may have to survive a major earthquake, like the one that destroyed much of San Francisco in 1906 or the one that partly severed the Bay Bridge in 1989.
有那麼一天，新的大橋必須能在強震來襲後仍然屹立。1906年的一場強震使舊金山受到嚴重破壞，1989年的另一次強震則導致海灣大橋部分橋段崩坍。
Keeping the bridge intact in an earthquake is the engineers’ chief goal. So they designing flexible structures in which any potential damage would be limited to specific elements.
如何使大橋在地震來襲後仍保完整是工程師的主要考量。有鑑於此，他們設計了彈性結構體，可使潛在的損害侷限於特定部位。
“The flexibility of the system is such that it basically rides the earthquake,” said its lead designer, Marwan Nader, a vice president at the engineering firm T. Y. Lin International.
領導設計工作的林同棪工程顧問公司副總裁納德說：「基本上，整套系統的彈性經得起強震的劇烈搖晃。」
Another potential approach, making the bridge structures large enough, and rigid enough, to resist movement, was rejected.
另外一種可能的方法是，把大橋的結構體造得夠大夠堅固，以抵抗劇烈搖晃。這個構想遭到駁回。
The new design includes a 160-meter-tall suspension bridge tower made up of four steel shafts that should sway in a major earthquake, up to about 1.5 meters at the top. But the brunt of the force would be absorbed by connecting plates between the shafts, called shear links.
新的設計包括一座160公尺高的懸吊式橋塔。它以四根鋼軸構成，可在強震中搖晃，頂端最大搖動幅度可達1.5公尺。強震的搖晃力道主要由又稱切變環的軸間連結板塊吸收。
The bridge’s concrete piers are designed to sway as well, limiting damage to areas with extra steel reinforcing. And at joints along the entire span there are 18-meter sliding steel tubes, called hinge pipe beams, with sacrificial sections of weaker steel that should help spare the rest of the structure as it moves in a quake.
大橋的水泥橋墩同樣也會搖晃，把損害限制於以鋼材額外強化的部位。整座大橋的各個接合處另有長18公尺，又名鉸鏈梁管的滑動鋼管。理論上，較弱鋼材構成的部位可在大橋於強震中晃動時犧牲，以保住整個結構體的其他部位。
“At the seismic displacement that we anticipate, there will be damage,” Mr. Nader said. “But the damage is repairable and the bridge can be serviceable.”
納德說：「我們預期的地震移位過程一定會產生損害，不過這些損害可以修復，大橋也還可以使用。」
It was the Loma Prieta quake of 1989 that made this 3.5-kilometer replacement span replacement span necessary. The 6.9- magnitude quake caused part of the existing span to collapse, killing a motorist and closing the bridge for a month. That quake caused movement far greater than the 1930s-vintage bridge had been designed to handle. Most experts believe a stronger quake could cause a total collapse of the span.
1989年的洛馬普雷塔地震使這座全長3.5公里的新橋有興建的必要。當年規模6.9的強震導致現有的部分橋面塌陷，除有一名汽車駕駛人罹難，當局且被迫關閉大橋一個月。那次強震導致建於1930年代的舊大橋移動幅度大於原設計的極限。多數專家認為，如果再出現強震，大橋可能全垮。
There is a strong likelihood of another earthquake in the Bay Area of magnitude 6.7 or larger before 2036, according to the United States Geological Survey.
美國地質測量所認為，舊金山灣區極可能在2036年之前出現另一次6.7或更大的強震。
Unlike more conventional suspension bridges, in which parallel cables are slung over towers and anchored at both ends in rock or concrete, the 624-meter suspension bridge has only a single tower and a single cable that is anchored to the road deck itself, looping from end to end and back again. (With a conventional design it would have been extremely difficult to create an anchorage on the bridge’s eastern end, in the middle of the bay.)
傳統吊橋的平行鋼索懸掛在橋塔上，並在兩岸的終點以岩石或水泥固定。不同的是，這座624公尺鋼懸吊橋只有一個橋塔與一條固定於橋面甲板的鋼索，從一端繞到另一端之後，再繞回（如果採傳統設計，勢必很難在大橋的東端，也就是海灣的中間製造一個固定點）。
The new bridge is the longest self-anchored suspension bridge in the world, with one side of the span longer than the other.
這座新大橋是舉世最長的自錨式吊橋，一側的墩距大於另一側。
In a self-anchored design, the deck has to be built first. “You have a kind of chicken-and-egg situation,” Mr. Nader said. “You need the deck to carry the compression so that the cable anchors into it, but the deck can’t carry itself until the cable is there to carry it. So you have to build a temporary system,” which needs to be seismically secure as well.
採用自錨式設計後，橋面甲板必須先造。納德說：「我們面臨雞與蛋孰先的問題。我們必須利用甲板產生擠壓作用，使鋼索得以固定於甲板，然而在鋼索就位並把它拉起之前，甲板不可能上升就位。如此一來，我們必須建造能夠經得起地震的臨時系統。」
The single tower created design problems. It’s “Just like a pole,” he said. “If you have a pole and the pole starts shaking, all the damage will occur at the bottom.”
單一橋塔產生設計上的問題。納德說：「它有如一根竿子。如果你有這麼根竿子，它開始搖晃，一切損害全會集中在底部。」
The solution, was to split the tower into four shafts and tie them together with the shear links.
解決辦法是，把橋塔分為4個軸，再以切變環把它們繫在一起。
The links are of a special grade of steel that deforms more easily. Their placement at points along the length of the tower affects how the shafts will move in a quake.
這些切變環以比較容易變形的特級鋼材製成。把它們安放在大橋沿線各定點之後，就可以影響軸在地震中移動的方式。
Mr. Nader said the shear links about two-thirds of the way up would be most damaged in a major earthquake. But the tower would still be structurally sound, he said, and the links would not have to be replaced immediately.
納德說，如果切變環安放在高度約三分之二的位置，可能在強震中受損最嚴重，不過橋塔的結構還是會保持完整，切變環也不必立即更換。
It’s like a fender bender, he said. “Your car is perfectly drivable, and it’s designed that way, with a bumper that can take the shock.
他說，這有如車子小擦撞。「你的車子還是完全可以開；設計上就是讓保險桿來承受撞擊的。」
“So you basically stop, just to make sure,” he went on. “You see everything’s O.K., and you can come in anytime you want to repair your bumper.”
「基本上，你為了確定沒事而停車，」他繼續說。「你確定一切OK，而保險桿則隨時可以更換。」
原文參照：
http://www.nytimes.com/2012/02/07/science/to-survive-a-quake-new-bay-bridge-span-will-offer-least-resistance.html
2012-02-21聯合報/G5版/UNITEDDAILYNEWS 陳世欽譯 原文參見紐時週報十版
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