Today is the 75th anniversary of one of the greatest days in Bay Area history - the day the San Francisco-Oakland Bay Bridge opened for business.

When it opened, it was the longest bridge in the world, and it remains one of the busiest. For 75 years, the Bay Bridge has been the main street of the Bay Area. "Today and forever," San Francisco Mayor Angelo J. Rossi said in the keynote speech on Nov. 12, 1936, "we are welded with a span of steel."

The construction of the Bay Bridge, which began in the summer of 1933, was considered one of the engineering wonders of the world.

Stretching 8.4 miles from San Francisco to Oakland, it was the longest bridge ever attempted. It is actually four bridges in one - an East Bay approach span, a cantilever bridge between the approach span and Yerba Buena Island, and two suspension spans between Yerba Buena and San Francisco.

Because the technology of the time could not produce a single suspension bridge linking the island to the city, engineers built a giant concrete anchorage in the middle of the bay and connected two identical bridges to it. It was an engineering solution that E.R. "Mike" Foley, a former chief bridge engineer, called "absolutely unique."

The connection is so seamless that most drivers hardly notice the anchorage.

The big tunnel

To connect the Oakland side of the bridge with the San Francisco side, the engineers drilled an enormous tunnel through Yerba Buena Island. It, too, was a wonder - if not the longest tunnel in the world, then the largest.

The original construction of the Bay Bridge took three years and four months and employed thousands of men. They worked without nets on the towers, the cables and the bridge deck hundreds of feet above the bay waters. Between 24 and 28 men - no one is sure of the exact number - were killed during construction.

"It was an inherently dangerous job," said a report on the history of the bridge.

The bridge cost $77 million to build. It was built in the depths of the Great Depression and financed by a federal loan. It opened with a four-day celebration - "a dazzling array of events," the papers called it - in San Francisco and Oakland.

"The opening of the San Francisco-Oakland Bay Bridge will direct the eyes of the world on the San Francisco Bay Region," said Earl Lee Kelly, state director of public works.

'Crowning achievement'

The bridge was hailed by The Chronicle as "the crowning achievement of California history ... the fulfillment of a cherished dream ... the greatest wonder of the Western world."

President Franklin D. Roosevelt pressed an electric key in the White House on Nov. 12, a light on the bridge flashed to green, and Gov. Frank Merriam used a blowtorch to cut a symbolic steel chain.

More than 60,000 cars and trucks crossed the bridge on the first day; the first car across belonged to a couple from Nebraska who somehow got on the bridge before the official opening - even before the governor's motorcade.

"Nobody stopped us," said William Acklie, of Norfolk, Neb., who never even heard of the Bay Bridge until the day before it opened.

Even the first dog across the bridge made the news: His name was Sonny Boy, and he carried a "Vote for Roosevelt" sign in his mouth. The first hitchhiker was also duly noted, as well as the first truck, and the first mishap, a stall, only seconds after the tollgates opened.

Pacific fleet, parade

The opening was accompanied by a huge air show, but the arrival of the Pacific fleet - 10 battleships and eight cruisers - almost stole the show.

There was a parade in San Francisco on Nov. 14, with 60,000 marchers watched by a million people - "the greatest gathering in the city's history," The Chronicle said.

By the second or third day, so many people wanted to drive over the bridge that the first of the bridge traffic jams ensued - the worst, the papers said, in the history of the Bay Area.

The bridge has seen many changes since then - electric trains were added on the lower deck in 1941 and removed in 1958, and after it was damaged in the 1989 Loma Prieta earthquake, the west spans were strengthened and the eastern half of the bridge is being replaced.

Added protection

Perhaps the least known modification to the bridge came in 1990, when ironworkers, concerned that the Bay Bridge needed a little extra protection, welded the figure of a small troll, a mythical creature that is thought to protect bridges, on a girder on the cantilever portion of the bridge, not far from Yerba Buena Island.

The Golden Gate Bridge, which opened in May 1937, has always been the most glamorous of the region's bridges, but today the Bay Bridge is the workhorse. It carries 280,000 vehicles a day - more than twice as many as the Golden Gate, which carries 106,000.

The bridge's 75th anniversary was acknowledged Thursday by a simple observance; Caltrans held a media event on Treasure Island to cut a birthday cake, offer a few speeches, and showcase an interpretative exhibit. But the exhibit is more focused on the bridge's future - the new eastern span, now under construction - than its past.

The new span, which has taken nine years to construct and cost more than $6.3 billion, will open in 2013. And then, when a complete seismic makeover of the Bay Bridge is complete, it will be the time to celebrate.