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.
My American Experience
From the Empire State Buiding to the carvings on Mount Rushmore, the Golden Gate Bridge, and the Space Needle, the U.S. has dozens of impressive, important, and iconic structures. Which ones have you been to? Which has had the biggest impact?