Engineering the Design

Engineering the Design. . In 1921, engineer Joseph B. Strauss submitted a design for a bridge that would cross the Golden Gate Strait , a hybrid bridge with a suspension span supported at each end by cantilever trusses. By 1929, consulting engineers Leon S. Moisseiff and O.H. Ammann had persuaded Strauss to accept the more graceful all-suspension bridge design that we see today., Strauss assigned engineer Charles A. Ellis to work in collaboration with Moisseiff to perform the calculations needed to complete the design, which was complex and challenging work done without modern computers. The most common “calculator” that structural engineers used in that era was a slide rule, and drafting was done with pencil and paper on drawing boards., The engineers relied on recent advances in suspension bridge design theory. They verified those calculations with tests on a steel tower model of 1:56 scale (56 times smaller than one of the actual towers). The tests confirmed that the tower calculations were sound., The geology of the south tower location was investigated before construction could begin. The south tower was planned for construction more than 1,100 feet (335 meters) offshore on serpentine rock. Consulting geologist Andrew C. Lawson oversaw a load test performed by placing weight equivalent to a fully loaded railroad boxcar on a square shaped area of serpentine rock 20 inches by 20 inches (508 millimeters). The rock was more than strong enough., Captions: , Left: A model of one of the Bridge towers was loaded in a civil engineering testing machine at Princeton University in 1933. One test, with a scaled-down force, simulated the actual 120 million pounds (54 million kilograms) of vertical load that would be placed on the top of each full-sized tower by the main cables. (To visualize that much weight, picture a large ocean liner.) , Top right: This early design, dubbed as "ugly” by the local press, called for heavy-looking cantilevered structures jutting out from the towers. , Bottom right: The Bridge as it looks today.