Spinning the Cables
The Golden Gate Bridge's builders introduced many innovations, but perhaps the most impressive was the precise and efficient technique they used to construct the massive cables.
Flexible and Strong
The bridge's designers carefully calculated the graceful dip of the suspension cables between the two towers to carry the needed weight. The cables had to be flexible enough to bend up to 27 feet laterally, in the Gate's formidable winds, and strong enough to support the structure of the bridge. The planned cables would be so long and strong that they would need to be fabricated in place.
A Famous Firm
The well-known engineering company of John Roebling and Sons oversaw the cable construction. Roebling had built many of the world's longest bridges -- including the Brooklyn Bridge, 52 years earlier. The firm had devised the most efficient strength-to-rigidity ratio for cables. It had also developed a technique of spinning cables on-site. The Roebling crew's work on the Golden Gate Bridge continued a tradition of innovation.
Parallel Wire Construction
Cable spinning began in October 1935. To create the cables, Roebling developed a method called parallel wire construction. The innovative technique enabled a cable of any length and thickness to be formed by binding together thin wires. It promised to give engineers the freedom to build a bridge of infinite length.
Loop De Loop
To spin the cables, 80,000 miles of steel wire less than 0.196 inch in diameter were bound in 1,600-pound spools and attached to the bridge's anchorages. A fixture within the anchorages called a strand shoe was used to secure the "dead wire" while a spinning wheel, or sheave, pulled a "live wire" across the bridge. Once it reached the opposite shore of the Gate, the live wire was secured onto the strand shoe, and the wheel returned with another loop of wire to begin the process again.
Largest Cables Ever
Hundreds of wires, each roughly the diameter of a pencil, were bound together into strands. Hydraulic jacks then bundled and compressed 61 strands to make a cable. Each of the two main cables is just over three feet in diameter, 7,659 feet long and contains 27,572 parallel wires. The Golden Gate uses the largest bridge cables ever made -- long enough to encircle the world more than three times at the equator.
Creating a Balance
One wire at a time, the cables for the Golden Gate bridge were spun from tower to tower, anchorage to anchorage. The spinning was tedious; not only did it take time for the spinning wheel to travel the mile between the two shores, but the work had to be performed in a precise sequence, in order to create the balance needed for the cables to absorb the proper amount of wind pressure as specified in Charles Ellis' design.
Speeding Up the Process
The construction budget was tight. Roebling and Sons was contracted to finish spinning cables within fourteen months. To make their deadline, the Roebling crew designed a "split tram," a second spinning wheel which met the first wheel in the middle of the bridge. This development sped progress. Eventually, Roebling devised a system to spin six wires simultaneously — color coded to prevent confusion. Six wires at a time had the spinning wheels guiding as much as 1,000 miles of wire across the span in an eight-hour shift. When the weather was good, the wheels took just six and a half minutes to travel halfway across the span.
Ahead of Schedule
On May 20, 1936, the spinning wheel was festooned with flags as it pulled the last wire across the bridge. Thanks to extraordinary technological innovations, Roebling finished spinning the cables eight months ahead of schedule, an impressive four times faster than had been anticipated.
Mother Nature set up one of the most formidable obstacles for the Golden Gate Bridge's builders. In January 1933, work began on the bridge's south tower, with foundations sunk deep into the ocean floor. Divers were hired to perform intricate procedures in the strait's violent currents, putting their equipment, skills and bravery to the test.
The narrow strait between Marin County and San Francisco is one of the world's most tumultuous bodies of water. Up to 335 feet deep and only a mile and a quarter wide, the Golden Gate is the largest California coastal opening -- a portal into which the Pacific Ocean surges. Powerful currents also flow in the opposite direction, as water from many of Northern California's freshwater rivers and streams rushes into San Francisco Bay. This freshwater flow collides with the incoming Pacific, creating complex and violent currents. At 2.3 million cubic feet per second, these currents pump one-sixth of the volume of the San Francisco Bay through the Gate and into the Pacific Ocean every day.
An Audacious Plan
To build the bridge, workers would have to erect a pier more than 1100 feet out in the middle of the Gate — the first bridge support ever constructed in the open ocean. Chief engineer Joseph Strauss' bold plan called for workers to first build a giant fender to protect the pier from stray, fog-bound ships. The fender would enclose a football-field-sized area from which water would be pumped out. The concrete tower foundation would be laid inside. Once this was completed, water was to be pumped back into the 40-foot-thick concrete walls of the fender, in order to strengthen the fender against the current.
90 Feet Down
Divers were crucial to the plan. They guided beams, panels, blasting tubes and 40-ton steel forms into position and secured them, striving all the while to avoid being swept away in the current. Workers shot timed black powder bombs deep into bedrock through the blasting tubes, often with such power that dozens of fish would be thrown out of the water and onto the south shore. Divers sometimes ventured as deep as 90 feet below the surface to remove detonation debris. They smoothed the floor's surface using underwater hoses that exerted 500 pounds of hydraulic pressure. To add to the difficulty, divers worked blindly, forced to feel their way due to murky water, fast-changing currents and bulky diving suits.
Work inside the fender was the riskiest. At any moment, its walls could collapse from contact with a stray ship lost in the fog, or from the intense pressure exerted by the currents. "We were down damn near 50 feet, and every time you go down 29 feet you double your atmospheric pressure," recalled diver Bob Patching. "Well, that's strong enough it can hold you smack against a wall, and you can't move."
Racing the Clock
The Gate's changing currents only afforded workers narrow windows of dive time. The men were restricted to submerging for four twenty-minute periods per day. With the construction team's tight schedule, divers were often forced to surface before having sufficient time to decompress, increasing the likelihood that they would develop caisson disease, a nitrogen deficiency also known as "the bends."
Despite the danger, men flocked to the underwater work. In Depression-era America, any steady, well-paid job was a godsend.
A Solid Foundation
The divers' efforts ended in success. On December 3, 1934, chief diver Chris Hansen descended into an inspection well with the pier job superintendent, Jack Graham, and resident engineer Russell Cone. A hundred and seven feet down, they inspected the bedrock and the foundations, congratulating themselves on a job well done. A few days later, Berkeley geologist Andrew Lawson made the descent, and reported that "the rock of the entire area is compact, strong serpentine remarkably free from seams... When struck with a hammer, it rings like steel."