Material Marvels: The Golden Gateway
From its completion in 1937 to the construction of the Verrazano-Narrows Bridge in New York in 1964, the Golden Gate Bridge had the longest main span in the world. It now stands as an iconic landmark in San Francisco. Kathryn Allen reports.
In May 1937, the Golden Gate Bridge, USA, opened to pedestrian and vehicular traffic, linking San Francisco to Marin County.
Prior to this, ferries transported people across the Golden Gate strait, which connects San Francisco Bay to the Pacific Ocean. The strait varies in width from 1.6–4.8km, reaching depths of 90m, and experiencing strong currents.
In the mid-to-late 19th and early 20th centuries, the ferries became increasingly busy due to population growth – driven by the gold rush and, later, industrial expansion resulting from the opening of the Panama Canal – and an increase in car ownership. This led to calls for a bridge to be built across the strait from railroad executive Charles Crocker and structural engineer and editor of the San Francisco Call Bulletin James H Wilkins in 1872 and 1916 respectively. City officials enlisted civil engineer Michael O’Shaughnessy in 1919 to assess the feasibility of the project, which had previously been viewed as impossible due to the strait’s width, the height needed to allow ships to pass underneath, and the consequent expenditure.
While most quotes put the cost of a bridge at more than US$100m, structural engineer Joseph Baermann Strauss suggested it could be done for US$25–30m. In 1921, he proposed a cantilever-suspension hybrid span for US$27m. In the same year, Strauss hired structural engineer and professor at the University of Illinois, USA, Charles A Ellis to oversee bridge design and construction at Strauss Engineering Corporation.
The Golden Gate Bridge and Highway District, incorporating the counties of San Francisco, Marin, Sonoma, Del Norte, and sections of Mendocino and Napa, was created in 1928 to oversee the construction and financing of the bridge. Despite the Great Depression, a US$35m municipal bond was arranged to finance the project. After completion, the last of the bonds were retired in 1971, with the cost and interest paid from bridge tolls.
However, at the planning stage, opposition came from locals who disliked the proposed bond measure, as well as from ferry operators, fearing lost business, shipping associations, who claimed the bridge would disrupt navigation and industry, and environmentalists.
Hybrid to full suspension
Strauss was appointed Chief Engineer in 1929, with his initial design having been made public in 1922 by O’Shaughnessy, according to the Golden Gate Bridge Highway and Transportation District – current operators of the bridge. Leon S Moisseiff, a suspension bridge engineer, worked as a consultant on the project and, despite finding Strauss’ design feasible, suggested a non-hybrid, suspension bridge design. Moisseiff and Ellis collaborated on the calculations needed for the final design, using a stainless steel tower model, 56 times smaller than the planned bridge, to test their calculations. However, in a paper published by the American Society of Civil Engineers, titled Joseph B Strauss, Charles A Ellis, and the Golden Gate Bridge: Justice at Last, Strauss, after firing Ellis in 1931, is reported to have afforded Ellis no credit for the final design.
Before construction could begin, the geology of the towers’ planned locations had to be assessed. Consulting Geologist Allan E Sedgwick, in his 1931 report Foundations of the Golden Gate Bridge, notes that the north pier would rest on solid diabase or basalt, capable of withstanding stress imposed by the bridge. The south pier is described to rest on ‘serpentine derived from peridotite’, which is relatively soft, leading Sedgwick to recommend that foundations and abutments be sealed with cement.
Sedgwick also notes that no evidence of tectonic plate faults running through the bridge was found, with the San Andreas Fault lying six miles west of Fort Point, on the south side of the strait. However, he advised caution as two other faults lie nearer to the bridge site, east of the San Andreas Fault. He suggested allowances should be made in the bridge design to resist stress from seismic activity.
Considering these recommendations, the bridge was designed to withstand lateral forces during earthquakes and strong winds. In his 1937 Report of the Chief Engineer to the Board of Directors of The Golden Gate Bridge and Highway District, Strauss claims ‘Although no one can predict just how a flexible shaft of this character will respond to an earthquake, some conclusions can be drawn as to its stability under these forces.’ He adds, ‘In the completed structure, the transverse deflection of the towers under the design wind load is more than 10 times any expected movement at the pier tops, and the stresses from transverse wind will be more than double the stresses due to transverse earthquake forces, it isfurther stated in the report. Due to the great flexibility of the towers in the longitudinal direction, stresses from longitudinal earthquake forces (5% gravity) will not exceed 50% of the longitudinal wind stresses.’
However, following the Loma Prieta earthquake in 1989, which caused significant damage to areas of San Francisco, a seismic retrofit programme began. This included installing isolators on the approaches to the bridge to reduce shaking, strengthening the foundations of the two concrete south pylons and enabling them to absorb energy via a rocking motion, and using seismic expansion joints.
Andrew Whittaker, Professor of Civil Engineering and Director of the University of Buffalo’s Multidisciplinary Centre for Earthquake Engineering Research, USA, explained how seismic isolators function. ‘The introduction of seismic isolators changes the dynamic characteristics of a bridge, and in doing so significantly reduce the accelerations experienced by the supported superstructure.’ The isolators, positioned between the structure and the ground, are flexible in the horizontal direction, reducing the rigidity of the structure and therefore its ability to absorb vibrations transferred to it.
Halfway to hell
However, initial construction began on 5 January 1933, and despite hindrance from strong winds and sea swells, north and south anchorages, the Marin pier and tower, and San Francisco pier and tower were completed by 1935. The Report of the Chief Engineer lists the grades of rolled steel used as silicon, rivet and carbon, with the latter including co