Engineers hard at work disassembling San Francisco’s old Bay Bridge

SAN FRANCISCO — The new Bay Bridge linking San Francisco to Oakland is an engineering marvel that has been $6.5 billion in the making. With just a single tower, the sweeping span is strong as well as beautiful, designed to withstand an 8.5 magnitude earthquake.

But building the new bridge also means taking down the old San Francisco–Oakland Bay Bridge, which has been a steadfast servant of at least 240,000 people a day for seven decades.

And that is a big, dangerous job. Disassembling something that enormous is as hard as building it — or at least that is what the engineers working on the task are rapidly discovering.

There are three phases of disassembling the bridge — the westernmost span where it touches Treasure Island in the middle of San Francisco Bay will come down first, and California Engineering/Silverado, the contractor, is just getting started on that part.

The stretch that comes to rest in Oakland will fall next. And last to go will be the base of the supports from the mud line down to the bottom of the bay. Each phase is a standalone project worth just shy of $100 million to the companies bidding for the contract — and they’ll get at least $10 million back from the steel they haul away and resell.

The bridge is a series of joints and long segments held either apart in tension or together in compression. The main span of the bridge, however, is in both states at once. The span is under compression at the top and in tension at the bottom.

Like a shelf heavy with books, the span wants to sag. Simply severing it would cause the span to collapse inward. So four jacks capable of holding 400 tons each had to pull the two towers apart to take pressure off the central span. And this week, they’re cutting it in half.

Movement is the real danger. San Francisco has already been extremely lucky. The new bridge opened before an earthquake could strike, which would have subjected the old bridge to forces that most engineers estimate it could not have withstood. The 1989 Loma Prieta earthquake, a mere 6.9 magnitude shake, broke loose a 50-foot section of the upper deck, swallowing cars in the process.

Still, an outdated bridge in an earthquake is less dangerous than a partially disassembled outdated bridge in an earthquake.

Maddeningly, that is a situation that San Franciscans will have to live with for a while. “We had to get all the traffic over onto the new bridge, and now we just need to get the old bridge down as fast as we can,” said Andrew Gordon, spokesman for the project. Proximity, he pointed out, is the problem. “The western end is the real problem, because over there you can reach out and touch the new bridge.” In the event of an earthquake, the old would undoubtedly bang into the new.

So the engineers are working as quickly as possible, but working fast in engineering terms is three to five years. So San Franciscans are left to hope that the bridge they relied on for 78 years can stay upright for just a little longer.