The Golden Gate Bridge

The most famous bridge in the world, a symbol of San Francisco and American ingenuity.

The Golden Gate Bridge is one of the world’s most famous and admired structures. Spanning the picturesque Golden Gate Strait north of San Francisco, the bridge transforms the strait into a more beautiful and dramatic setting. This unique site and its bridge are a graceful and majestic entry into the San Francisco Bay, a breathtaking sight welcoming vessels from all over the Pacific.

Built between 1933 and 1937, the Golden Gate Bridge became a symbol of the City of San Francisco and American ingenuity and progress. It is one of the greatest bridges ever built and certainly the most celebrated of all. The suspension steel bridge structure is 6,450 ft (1,966 m) long (the entire bridge, approaches included, is 8,980 ft (2,737 m) long). It carries six traffic lanes and two pedestrian/bicycle lanes. The height of the bridge towers is 746 ft (227 m); (152 m (500 ft) above the deck); the navigation clearance is 220 ft (67 m).

The bridge links San Francisco with Marin County and the North Bay at the points of the shortest distance across the Bay entrance. Long considered an impossible dream by Californians, together with the San Francisco-Oakland Bay Bridge, the Golden Gate Bridge made the Bay Area into a unified economic entity, significantly contributing to its future development as a major financial and cultural center. The site provides the essential background for a great bridge.

The Golden Gate Bridge was designed by Joseph Strauss (Chief Engineer) and his task group of extraordinarily experienced design and consulting engineers: Charles Ellis, Leon Moisseiff, Othmar Ammann, and Charles Derleth Jr., together with the geologist Andrew C. Lawson. Working closely with the engineers, architect Irving Morrow was responsible for the aesthetic design, including the signature Art Deco style of the bridge towers and the bridge’s characteristic “International Orange” color.

In 1921, Strauss submitted an initial design for a bridge that would cross the Golden Gate Strait — a hybrid bridge with a suspension span supported by cantilever trusses extending from the bridge towers. The idea was considered “ugly” by the contemporary local press. However, Strauss’ design was ahead of its time. His idea was to significantly reduce the suspension span (preserving the longer clear span), something employed much later for “hybrid suspension” bridge structures with considerable savings vis-à-vis classic suspensions.

With the engineering team’s help, the bridge’s design evolved into the much simple and visually powerful structure that we admire today.

The engineers relied on recent advances in Suspension Bridge Design Theory for the design. They verified these 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. With a scaled-down force, one test simulated the actual 120 million pounds (54,000 metric tons) of vertical load that would be placed atop each full-sized tower by the main cables. (To visualize that much weight, picture a large ocean liner.) 

The geology of the south tower location was investigated before construction began. This tower was planned for construction over 1,100 feet (335 meters) offshore on serpentine rock. Consulting geologist, Andrew Lawson, oversaw a load test performed by placing weight equivalent to a fully loaded railroad boxcar on an area of serpentine rock only 20 inches (508 millimeters) square. The rock was more than strong enough.

The bridge’s central span held the long-span world record until 1964 when the Verrazano Narrows Bridge surpassed it by 18 meters. Invariably on the list of the greatest bridge structures, the Golden Gate Bridge is also included in ASCE’s list of Modern Wonders of the World. What makes this structure so unique and famous? Far from being “merely” utilitarian, the Golden Gate Bridge is one of the few structures significantly enhancing its environment. Combining the breathtaking setting and the elegant, powerful structure contributes to its appreciation as one of the greatest bridges ever built. The ever-changing conditions, from bright sunlight and shades to drifting fog often obscuring bridge elements, offer spectacular visual variations. Pedestrian accessibility on the bridge deck and at multiple viewpoints along the structure makes it a favorite site for photographers and tourists, adding to its popularity. The elegant lines of the suspension cable system, its long span, and the majestic towers create a familiar and fascinating picture. This structure, perfect in both functionality and design, is also an object of art. The bridge is a monument to the creative spirit and the eternal drive to surpass previous achievements. 

To fully appreciate this achievement, one must consider the project’s challenges and the state of design and construction in the 1930s. The strait is a 6,700 ft (2,042 m) wide opening in a mountain range, where the Bay connects to the Pacific Ocean, with its strong tides and currents, winds exceeding 60 mph, and grade up to 330 ft (100 m) below the water surface. These conditions required an enormous bridge span never achieved before. As Kevin Starr rightly noted, “Not since the Brooklyn Bridge was built more than half a century earlier had bridge-builders faced such a challenge.”

We should also remember that in the 1930s, there were no computers, software, or even electronic calculators. The engineers had to rely on hand calculations and slide rules alone. There were no mobile cranes, welding, or high-strength bolts; all connections had to be done with rivets – a particularly challenging method, considering the difficult atmospheric conditions over open waves. Despite such challenges, the engineers and builders accomplished the task, building a bold and efficient bridge concept. On more than one occasion, accidents delayed the construction. A few months after construction began in 1933, a ship traveling westward in thick fog crashed into the just-completed access trestle to the San Francisco tower fender and destroyed a large part of it; later the same year, strong storms twice destroyed part of the access trestle. These accidents delayed progress on the bridge by five months.

Despite all obstacles, the structure was completed ahead of schedule in less than 4 ½ years for $33.7 million, $1.3 million below budget! The cost is equivalent to $710 million in 2023. However, in 2019 it was estimated that building the same bridge today would cost about $1,640 million.

The Golden Gate Bridge is a high-level engineering achievement with a new record-long span at a very challenging site, a structure many experts had considered impossible to build. The main span’s 4,200 ft (1,280 m) length exceeds the spans of two bridges connecting Europe and Asia at the Bosphorus Strait (with respective 3580 ft (1,090 m) and 3524 ft (1,074 m) spans).

The 66,043-metric-ton steel structure (including anchorages) requires periodic inspections and permanent maintenance to ensure its safe operation. During its long years of service, the bridge has been retrofitted several times:

• In 1953–1954, a lateral bracing system was added between the stiffening trusses to increase the lateral and torsional resistance of the bridge.
• n 1973–1976, all suspender cables were replaced.
• In 1982–1986, the original reinforced concrete deck was replaced with a stronger and lighter steel orthotropic deck, a replacement done in stages during night hours without closing traffic. Orthotropic deck consists of a structural steel plate stiffened with ribs.
• In 1980–1982, the North and South approach structures were seismically reinforced.

On October 17, 1989, the Loma Prieta Earthquake hit the San Francisco Bay Area with a 7.1 magnitude, with 15 seconds duration. The earthquake caused 68 deaths, at least 3,700 injuries, and an estimated loss of $6-7 billion. Although the Golden Gate Bridge suffered no observed damage from the Loma Prieta Earthquake, since the epicenter was located some 60 miles to the south, the earthquake served as a reminder of the area’s susceptibility to seismic activity and initiated an extensive seismic retrofit program.

• In 1996, three more retrofitting projects commenced. The first two—on the Marin (north) approach viaduct and the San Francisco (south) approach viaduct—are already completed (1997–2008), so the Bridge is expected to safely withstand an earthquake over 7.0 in magnitude. However, it may still experience damage in major seismic events that would require traffic closures. The final retrofit Phases 3A and 3B will further strengthen the Bridge against earthquakes or other disasters by reinforcing the main and side spans of the Bridge, both towers and the south tower pier, with work scheduled to start in late 2024 and finish in 2029.

These efforts reflect natural concerns over seismic safety in the area. In the mid-1990s, the U.S. Geological Survey (USGS) estimated a 62% probability of at least one magnitude 6.7 or greater earthquake capable of causing widespread damage, impacting the San Francisco Bay region before 2031. More recently, USGS estimated a 72% probability of a magnitude 6.7 or greater earthquake in the Bay Area before 2043.

Because of the significant strength of the steel-wire main cables, the load-carrying capacity of the structure does not need reinforcement and remains as originally designed. One unexpected load test occurred in 1987 when too many people assembled on the bridge deck during the 50th-anniversary celebration of the bridge. The celebration attracted 750,000 to 1,000,000 visitors, and the crowd on the bridge was about 300,000 people, causing the bridge’s center span to flatten out under the weight. This unexpected load caused the main structure to deflect, consuming all designed camber of the main span and temporarily transforming it into a concave line. The pedestrian “crowd” load exceeded by 50% the bridge design live load of 7.25 t/m’ (6 traffic + 2 pedestrian lanes). This super load caused a larger-than-designed deflection, but the structure successfully resisted the overload; the required tensile strength for the two main cables is 224,000 kips (101,900 metric tons) (for the total load above), while the capacity is 262,000 kips (119,000 metric tons), i.e., there was still a safety factor of 1.17 vs. factored load and 2.44 safety factor vs. nominal load demand.

Description, Technical, and Statistical Data

The main bridge has a typical suspension bridge structure with two majestic towers and an elegant, simple suspension cable shape on one central-main span and two side spans over a continuous steel stiffener girder. The stiffener girder comprises two steel trusses, laterally braced in between, carrying the bridge deck. Originally the deck was reinforced concrete, replaced in 1982-1986 by a steel orthotropic deck system. The towers are 227 m tall from the water level or 152 m above the deck. They are steel-framed two-leg structures with art-deco-enhanced horizontal frame connections. Between both legs, each tower has four horizontal frame connections above the deck and two “X” braces below the deck.

• The length of the central main span of the bridge is 4,200 ft (1,280 m); the two side spans are 1,125 ft (343 m) each, with a total suspension length of 6,450 ft (1,966 m). The bridge length is 8,980 ft (2,737 m) between abutments.
• Width of the bridge deck: 90 ft; road width between curbs: 62 ft. 
• Vertical clearance above water level: 220 ft (67 m).
• Main cables length: 7,650 ft (2,332 m); main cable maximum sag: 144 m; cable diameters: 36˝ 3/8 (0.92 m) with the wrapping. Each main cable has 27,572 galvanized steel wires with a diameter of 0.192˝.
• Main cables weight: 24,500 t (22,200 m. tons).
• Strength characteristics of the cable steel wire: tensile strength, Fu = 235,600 psi; yield strength, 182,600 psi.
• There are 250 pairs of suspenders arranged 50 ft (15.2 m) apart, each with a diameter of 2 11/16˝. The original suspenders were replaced in 1972–1976.
• Weight of all cables, main and suspender, and accessories: 24,500 tons (22,200 metric tons).
• Total weight of each anchorage: 60,000 tons (54,400 metric tons).
• Total weight of the bridge, excluding anchorages and approaches: 419,800 tons (380,800 metric tons). This includes a reduction of 12,300 tons (11,158 metric tons) in weight from the re-decking in 1986.
• Maximum calculated deflections at midspan: downward, 10.8 ft (3.3. m); upward, 5.8 ft (1.8 m); transverse deflection: 27.7 ft (8.4 m).
• Deflections at the top of towers: transverse, 12.5 in (0.32 m); longitudinal, 22 in (0.56 m).
• Live load capacity: 4,000 lbs. (1,814.4 kg) per lineal foot.
• Load on each tower from main cables: 61,500 tons (56,000 metric tons).

Bethlehem Steel manufactured the steel structures in Trenton (New Jersey), Sparrows Point (Maryland), Bethlehem, Pottstown, and Steelton (Pennsylvania). They were transported by sea through the Panama Canal to the construction site in San Francisco. The cables for the bridge were produced and supplied by John A. Roebling’s Sons Company in Trenton, New Jersey.

Humankind has built bridges since the days of early civilizations. Several of them reflect the eternal aspiration to surpass previous achievements in construction. Some, like the Golden Gate Bridge, are landmarks that became symbols of cities, countries, and human progress. The American Society of Civil Engineers rightly recognizes the bridge as one of the Wonders of the Modern World.

To this day, the Golden Gate Bridge remains a symbol of San Francisco. It is what the Eiffel Tower is for Paris and what the Statue of Liberty is for New York. Admiring this iconic structure, we should honor its designers and builders, engineers, construction companies and workers, those who have updated and retrofitted it, and those who maintain it daily. This amazing structure inspires and motivates new generations of engineers and builders to higher structural and bridge engineering achievements. The Golden Gate Bridge has inspired generations of engineers to build larger, taller, and stronger structures. It symbolized the Art of American Bridge engineering and contributed to the leading role of American bridge designers and builders for most of the 20th century. This 86-year-old achievement should also motivate our engineers, builders, and relevant authorities to deliver more high-performing and efficient bridge structures to maintain our country’s transportation functionality and safety at the highest level.■ 

References

Golden Gate Bridge, the official site for the bridge, www.goldengatebridge.org
Golden Gate Bridge, Wikipedia, en.wikipedia.org/wiki/Golden Gate Bridge
Golden Gate Bridge brochure. Golden Gate Bridge, Highway and Transportation District
Starr, K., Golden Gate. The Life and Times of America’s Greatest Bridge. Bloomsbury Press, New York, 2010
Mladjov, R., Long Span Bridges, and the Art of American Bridge Engineering, SEAOC Convention, 2009
Mladjov, R. Structurally Sound, A Golden Achievement Modern Steel Construction, 2017
USGS, June 13, 2016, Earthquake Outlook for the San Francisco Bay Region 2014–2043
Data from the Golden Gate Bridge, Highway and Transportation District official website.
American Society of Civil Engineering, http://www.asce.org/Content.aspx?id=2147487305