For months, workers have been dismantling the eastern span of the Bay Bridge, the almost 2-mile-long stretch that runs from Yerba Buena Island to Oakland. An 800-foot-long piece of the cantilever section has been removed. In February, Caltrans announced that it was planning to use hundreds of small explosive charges to blow up the largest pier holding up that section and that the entire eastern span would be gone by 2018.

The Bay Bridge is often overshadowed by its more famous sister 2 miles to the northwest. But before half the bridge disappears forever, it’s worth recalling the epic story of how the magnificent structure — at the time, by far the longest bridge in the world — was built in the first place.

As Richard Dillon writes in “High Steel: Building the Bridges Across San Francisco Bay,” San Franciscans began dreaming of a bridge that would connect the windy, isolated peninsula with the “contra costa” — the East Bay — almost from the city’s founding. In 1869 Emperor Norton, the city’s most beloved lunatic, decreed that three bridges be built — one to Oakland, another to Sausalito and a final bridge to, for reasons only the Protector of Mexico understood, the Farallon Islands. Although Norton’s proclamation ended “Whereof fail not under pain of death!” none of the bridges were built.

Until the 1920s, experts regarded building a bridge across the bay — a stretch of water nearly three times longer than that crossed by the then-longest bridge in the world, the 1890 Firth of Forth bridge in Scotland — as unfeasible. It was not until 1928 that the idea for a bay bridge was taken seriously, when President Herbert Hoover and California Gov. C.C. Young appointed a commission to explore the feasibility of building one.

Biggest challenge

The commission green-lighted the bridge. It would be planned and built by the state, with a loan from the federal government. The massive volume of ferry traffic across the bay gave Uncle Sam confidence the bonds would be repaid.

The biggest challenge was the western span, between San Francisco and Yerba Buena Island. This section would have to cross 2 miles of deep water — too long for a suspension bridge. But engineers discovered that there was a fortuitous ridge of bedrock midway between the island and the Embarcadero. If they could figure out a way to build an island on that ridge, they could build two suspension bridges, each a mile long.

But building this island would be no easy feat. It would have to be anchored to the bedrock so it could withstand the unthinkable tension generated by the bridge’s cables. And the bedrock was 200 feet below the bay’s surface. No such structure had ever been built.

The bridge’s engineer, Charles Purcell, went to the world’s foremost expert on deep-water foundations, Daniel Moran. Moran designed a massive steel caisson, or box, open at the top and containing 55 steel cylinders, each 15 feet in diameter. This caisson was then towed out into the bay and gradually filled with cement. As it sank into the bay, its sides were built up and the steel cylinders also lengthened at the top by welding.

This enormous box, half the size of a city block, was also a monstrous digging tool. Its bottom consisted of sharp edges, 17 feet high, that plunged into the bay mud, while huge clamshell buckets were dropped through the uncapped cylinders, scooping up almost three-and-a-half tons of mud at a time. When the caisson finally hit bedrock, steel rods leveled it and concrete was poured through the cylinders to a height of 30 feet, gluing the metal box to the rock. Then the man-made island was built up 200 feet above water. Now it was strong enough to withstand the 40,000 tons of pressure that would be transmitted through the cables.

The center anchorage, or Moran’s Pier as it is sometimes called, contains more concrete than was used in the Empire State Building and is larger than the Great Pyramid of Giza. It is one of the engineering marvels of the world.

Four mighty steel towers were built on smaller piers to hold the cables that supported the western roadway. Each could hold 65 million tons of weight. The tallest rose 519 feet above the water. Each tower represented a construction job equal to building a 60-story skyscraper.

Cantilever span

The eastern span of the bridge — the part now being demolished — also presented a daunting challenge. It had to cross a long stretch of even deeper water with no bedrock, making it prohibitively expensive to build an anchorage for a suspension bridge. So Purcell built a cantilever span, 1,400 feet long and 191 feet above the water. It was the largest and heaviest such span in the United States. The main underwater pier that held it up, which Caltrans now plans to blow up, is the deepest bridge pier in the world.

If one man was essential to the successful completion of the bridge, it was chief diver Bill Reed. As Harold Gilliam writes in “San Francisco Bay,” “Reed got $15,000 a year plus a dollar a foot for every dive — and he well deserved it.” It was too dark to see, so Reed worked by feel: “His fingers were the 'eyes’ of the engineers.”

Gilliam recounts that when the caisson for the pier closest to Yerba Buena was lowered through the mud, “it began to emulate the tower of Pisa, tilting precariously out of line. A million dollars’ worth of caisson teetered in the balance. Reed descended to the bottom of the leaning concrete, felt his way carefully along the higher edge, and found it hung up on a large boulder. He carefully planted dynamite, which disintegrated the rock, and the caisson swung slowly back into position.”

Bridge workers had minimal safety equipment and often no nets. Twenty-eight men died during the construction.

The bridge opened to traffic on Nov. 12, 1936, in the depths of the Great Depression. There were five days of celebrations, and a million people attended the parade. In a portent of things to come, the opening caused what The Chronicle called “the greatest traffic jam in the history of San Francisco.”

Almost 80 years later, it remains one of the world’s great bridges.

Bridge of distinction

Facts and figures about the Bay Bridge:

Its length from end of the western approach to end of the eastern approach is 8.25 miles.

It rests on 51 piers, seven on land and 44 in the water. (Many bridges have only one or two piers.)

The bore tunnel through Yerba Buena Island that connects the east and west spans was and still is the largest bore tunnel in the world.

The structural steel used to build the bridge equaled 6.7 percent of the total U.S. output for 1933.

Its cable wire could encircle the Earth nearly 3 times.

It cost $77 million, the equivalent of $1.19 billion today.

Gary Kamiya is the author of the best-selling book “Cool Gray City of Love: 49 Views of San Francisco,” awarded the 2013 Northern California Book Award in creative nonfiction. All the material in Portals of the Past is original for The San Francisco Chronicle. E-mail: metro@sfchronicle.com

Trivia Time

Last week’s trivia question: Which San Francisco building is connected to the first shots of the Civil War?

Answer: Fort Point. It’s similar in design to Fort Sumter, the Union fort shelled by the Confederacy to open the Civil War.

This week’s trivia question: Where is Irish Hill?

Editor’s note