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 copper-bearing carbon steel. The wire cables – consisting of spun galvanised carbon steel wire – were in place by the end of 1936.
During the construction of the bridge’s main span, which would become the road and walkway, Strauss added a safety net. Suspended beneath the main span, where steel construction for the road was taking place, the net extended either side of the bridge and prevented 19 men from falling into the sea – giving way to the formation of a club for those who had cheated death, the Halfway to Hell Club.
But, according to Building the Golden Gate Bridge: A Workers’ Oral History, written by Harvey Schwartz, curator of the Oral History Collection at the International Longshore and Warehouse Union Library, San Francisco, and published by the University of Washington Press, the net failed on 17 February 1937. Timber scaffolding, which was supporting 12 men, gave way and broke through the net. Only two of the men survived, taking the project’s total death count up to 11. According to the Golden Gate Bridge Highway and Transportation District, the first man to die was Kermit Moore on 21 October 1936.
Strauss notes in his Report of the Chief Engineer that the net was costly and, ‘because it was new and unusual it met with resistance’. But, he points out that ‘in a field of activity, where one man killed per each million dollars expended has been axiomatic’, the low rate of fatalities up until 17 February 1937 established ‘a new all-time record.’ Strauss also claimed the net allowed the men to work faster and more efficiently as they felt better protected and moved more freely.
The completed bridge, which opened ahead of schedule in 1937 and cost US$35m, stretches 2.7km (including approaches from abutment to abutment) and is 27m wide, rising 67m above the mean higher high water mark. Until the completion of the Verrazano-Narrows Bridge in New York, USA, in 1964, the Golden Gate Bridge had the longest main span in the world at 1.3km (main span of suspended structure). Its towers stand 152m above the road, weighing a combined 44,400 tonnes. Steel castings hold the 2.3km-long main cables at the top of the towers.
The bridge is painted to protect the steel from corrosion or rusting caused by salt in the air and fog. According to the Golden Gate Bridge Highway and Transportation District, the steel from Bethlehem Steel foundries had been coated in a red lead primer when manufactured. In 1935, Consulting Architect Irving F Morrow wrote The Golden Gate Bridge: Report on Colour and Lighting, having undertaken studies to find the bridge’s ideal colour, believing that ‘poorly chosen colour may actually create disharmony between the structure and the site.’
Taking into account the bridge’s size, its surroundings – including the frequent fog – and the effectiveness of the red lead coating, Morrow concluded that orange vermillion – also known as international orange – was the best option. This complemented the warm land tones, stood out against the sea and sky, and was clearly visible to passing ships. Prior to Morrow’s study, the US Navy had proposed that the bridge be painted with yellow and black stripes. Luckily, Morrow won out. Golden Gate Bridge International Orange is now mixed to the bridge’s requirements, and the bridge is painted on an on-going basis as needed, following inspections of steel condition.
Along with the distinct colour, other features aim to improve safety in bad weather. Fog horns have been positioned on the bridge since its opening, located at the middle of the main span and on the south tower pier. These, along with navigational and warning lights, guide ships through the strait. To improve safety on the bridge’s road, which has operated a reversible lanes system since 1963, a moveable, steel and concrete barrier separating the northbound and southbound lanes was implemented in 2015, designed to eliminate the risk of head-on collisions.
Despite a relatively low death toll during construction, the bridge has sadly become a popular place for suicides, with 39 people jumping to their deaths and 184 people talked down from the bridge in 2016 alone. The Golden Gate Bridge Highway and Transportation District began a project to implement a suicide deterrent system in 2006, with construction of a stainless steel net beginning in April 2017. Completion is due in 2021 and the net will extend about 6m over the water, angled upwards on the outside edge.
First and foremost, the bridge has become a popular tourist attraction and signature landmark of San Francisco. Tourists can walk the span, or pay the toll and drive across the golden gateway.
To read Report of the Chief Engineer to the Board of Directors of The Golden Gate Bridge and Highway District visit bit.ly/2DYB98o