.....the largest bridge of its kind ever attempted, with the tallest steel towers ever erected, the longest, largest cables ever spun, and the most enormous concrete anchorages. - Richa Singhi Bapat
On May 27, 1937, the vermilion suspension span was officially christened, and this year the famous bridge turns 75. To celebrate the power of this enduring landmark and learn how to appreciate it from all the right angles, lets know more about it. A dazzling feat of engineering, the Golden gate bridge is a counterpart to the statue of Liberty, greeting the world at America’s shore and proclaiming the nation’s lofty aspirations and great achievements.
Extraordinary natural obstacles stood in the way of the construction of the Golden Gate Bridge at the entrance to the San Francisco Bay. The site is within 20km of a major earthquake fault and directly fronts the Pacific Ocean. Seismic loads, tidal waves, ocean currents and gale force winds placed unprecedented demands on both substructure and superstructure. Yet it completed, its span of 1280m between towers, was larger than any of the suspension bridge in the world at that time. To truly experience the bridge, one has to walk across it as it’s in constant motion. One can hear it rumble and feel it bounce in response to changes in weather, temperature, and load. The seismic retrofitting, still under way aims to help the bridge survive an 8.3 quake. Joseph Strauss, the chief engineer, had only a modest amount of formal training but he acquired extensive practical knowledge of bridge design and construction by working for the New Jersey steel and iron Company before setting up his own bridge design practice. It could so easily have gone so
wrong. Early opponents of the Golden Gate Bridge derided Strauss’s initial design as an “upside down rat trap” that would desecrate the natural beauty of San Francisco Bay. In 1933, with construction poised to begin, some experts warned that no bridge can withstand the forbidding depths and currents of the narrow strait. Foes filed some 2300 lawsuits. The great depression spooked funders and even after the bonds had been secured, new designers were hired, and a graceful art deco span had started to take shape. The U.S. Navy wanted to paint the bridge in black and yellow stripes for safety reasons. The Navy thought the stripes would be more visible to the ships in the heavy fog. However, in the end, consulting Architect Irving Morrow held sway, arguing that the orange red primer already covering the steel would make a lovely and highly visible final coat. Such a construction project, on such a monumental scale, involving such an intricate interplay of construction technology and human skill, required an equally intricate interplay of labour and management at all levels of supervision and work. The steel towers each weighing around 22,200 tonnes and sustaining 61,500 tonnes force from the cables – soar 150m above the roadway, which is itself 75m above the water. Foundations for the towers reach 34m below mean sea level.
The founding of the San Francisco Tower pier was one of the most complex construction problems encountered. The site was in virtually open sea and a concrete fender had to be built to act as a cofferdam within which the pier could be built. Explosives were used to excavate the rock on which the pier was founded; these were dropped down a tube and timed to explode automatically. This was a hazardous process as the erection of the trestle almost stretched from the shore to pier was 335m away. Storms, tides and shipping accidents all delayed the progress. A 20m of concrete was placed under the water to form the base and then the water was pumped out leaving an oval cavity in which the labourers were set to work. Several tests were carried out to ensure the strength of the ocean floor. After they pronounced it satisfactory, the inspection shafts and chambers were pumped dry and concreted. The steel towers were the largest and highest in the world, making fabrication a challenge, 75 years back. Because of its huge size, this was done outdoors. It was then dismantled and sent by rail and ship through the Panama Canal and up the pacific coast. The Towers were set on 19 base plates, each 125mm thick. Each section was hoisted into place by a giant movable crane that climbed along with the tower. Rivets were heated by coal forges set on scaffolds outside the tower, and sent via a pneumatic tube to be fitted into a hole and forged into place. The work was stressful and dangerous; also the workers often worked in poorly ventilated semi-darkness area both inside and outside the tower.
The noise was terrific and many suffered from the fumes of the red lead paint given off when riveting the painted portion. The cables were installed by the Roebling Company. Each cable comprised over 25,000 wires in 61 strands, which were bunched together and compacted using hydraulic jacks. At the top of each tower a saddle was installed on giant bearings to allow flexibility as the tower and the cables responded to the changes in stress and temperature. Grooved bands were clamped around each cable along its length so that wire suspender ropes could be fitted. From these paired suspenders was hung the steel structure to support the roadway, which grew outwards from the towers. The project was completed early from the expected time. The final coat of the Orange paint was applied and the bridge opened on 27th May 1937. 200,000 pedestrians crossed it before taken over by vehicular traffic on 28th May 1937. It has always remained an icon in bridge designs all-over the world and the city of San Francisco celebrated its 75th birthday bash at Crissy Field on May 26-27 in honour of this Bridge of dreams.
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