- Associated Press - Monday, May 21, 2012

SAN FRANCISCO — The Golden Gate Bridge was heralded as an engineering marvel when it opened in 1937. It was the world’s longest suspension span and had been built across a strait that critics said was too treacherous to be bridged.

But as the iconic span approaches its 75th anniversary over Memorial Day weekend, the generations of engineers who have overseen it all these years say keeping it up and open has been something of a marvel unto itself.

Crews had to install a bracing system after high winds lashed and twisted the span in the 1950s, raising fears it would collapse. Years later, they had to replace vertical cables when they were found to have corroded in the bridge’s damp, foggy climate, potentially destabilizing the span.

The bridge, which rises majestically above a Civil War era fort on the San Francisco side and arches across to the Marin County headlands on the north side, is currently in the midst of a seismic upgrade that has seen many of its key structures replaced or modified. Plans for a moveable barrier to separate north and southbound traffic and a net system to prevent suicides are also moving forward.

“When [one of the bridge’s designers] made his final speech during opening day ceremonies in 1937, he said, ‘I present to you a bridge that will last forever,’ ” said Daniel Mohn, the bridge’s former chief engineer, who co-authored a book about the span. “What he should have said is, ‘I present to you a bridge that will last forever if properly maintained.’ “

The wind would continue to vex engineers years after the bridge’s completion. In 1951, it was closed for several hours when wind gusts approached 70 mph and caused the bridge to flutter.

It was twisting so badly, Mr. Mohn recalled during a recent phone interview, that the light standards at the center of the span were striking the main cables.

“It sure almost destroyed the Golden Gate Bridge,” he said. The Tacoma Narrows Bridge in Washington — a suspension bridge whose designer also worked on the Golden Gate — had twisted and snapped in about 40 mph winds a little more than a decade earlier. That 1940 collapse was captured on film.

Officials decided to add lateral bracing that made the trusses more stable and reduced the chances of the bridge going into a potentially catastrophic twisting motion.

The bridge would be able to withstand winds of 70 mph today although the goal is to eventually increase its tolerance to 100 mph, according to Ewa Bauer, the bridge’s current chief engineer.

The wind is not the only element to take its toll on the span. The damp, foggy air has also kept its painters and engineers busy.

“You couldn’t have put the bridge in a more corrosive atmosphere than in the middle of the Golden Gate with that salt fog coming in,” Mr. Mohn said.

Engineers discovered in the 1970s that the bridge’s suspender ropes — the vertical cables that connect the deck to the main cables — had corroded, some so badly that they could be picked apart with a pocket knife.

All of the cables were replaced in the mid-1970s.

LOAD COMMENTS ()

 

Click to Read More

Click to Hide