INTRODUCTION
The Golden Gate Bridge is recognized by the American Society of Civil Engineers as one of the 7 civil engineering wonders of the world. When it opened in 1937, the bridge had the longest main span of any suspension bridge in existence. It held that record until the Verrazano-Narrows Bridge was completed in 1964.
The Golden Gate Bridge links San Francisco with California’s Marin County. It spans the Golden Gate Strait (which is a narrow water passage) between San Francisco Bay and the Pacific Ocean.
Work on the Golden Gate Bridge began in 1933. Building the bridge was difficult and dangerous. The Golden Gate Strait is swept by 60-mile-per-hour winds, fierce ocean currents and tides, and for much of the year, the bridge is shrouded in thick fog.
The bridge was designed as a „bending bridge,“ capable of 21-foot sway and a 10-foot sag. It was built to withstand winds of up to 100 miles-per-hour as well as the earthquakes that threaten the area.
Workers who had to climb to great heights were put on special diets to prevent dizziness. A huge safety net was slung beneath the bridge. It saved the lives of 19 workers who fell off the bridge. In February 1937, disaster struck! A scaffold gave way and broke through the net, killing 10 men.
Work on the 5-lane Golden Gate Bridge continues. They have replaced much of the pedestrian hand railing and are now working to strengthen the bridge against earthquakes.
Golden Gate Bridge History
For many years before the Golden Gate Bridge was built, the only way to get across San Francisco Bay was by ferry, and by the early twentieth century the Bay was clogged with ferries. In the 1920s, engineer and bridge-builder Joseph Strauss became convinced that a bridge should be constructed across the Golden Gate.
Many groups opposed him, each for their own selfish reasons: the military, loggers, the railroads. The engineering challenge was also enormous – the Golden Gate Bridge area has winds of up to 60 miles per hour, and strong ocean currents sweep through a rugged canyon below the surface. If all that weren’t enough, it was the middle of the Great Depression, funds were scarce, and the San Francisco Bay Bridge was already under construction. In spite of everything, Strauss persisted, and San Francisco voters overwhelmingly approved $35 million in bonds to construct the Golden Gate Bridge.
The now-familiar art deco Golden Gate Bridge design and International Red color were chosen, and construction began in 1933. The Golden Gate Bridge project was completed in 1937. Strauss was a pioneer in building safety, with innovations including hard hats and daily sobriety tests. The Bay Bridge (which was being built at the same time) lost 24 lives while the Golden Gate Bridge lost only 12, an outstanding accomplishment in an era when one man was killed on most construction projects for every million spent.
DESIGN AND CONSTRUCTION
Political and legal delays resulted in nearly a decade between Joseph Strauss’ first design proposal for the Golden Gate Bridge — an unsightly railroad trestle that would have blocked views and sunsets — and his appointment to serve as chief engineer. His associates on the project included resident engineer Russell Cone, consultants O.H. Ammann and Leon Moisseiff, University of California, Berkeley professor of engineering Charles Derleth, Jr., and retired UC geology professor Andrew C. Lawson.
But it was chief assistant engineer Clifford Paine and architect Irving Morrow who actually deserve credit for the design and construction of the bridge that stands today. Morrow designed the towers with a subtlety of ornamentation and an understanding of height and perspective. He is also responsible for the offset bays and large curving walkways around the towers encouraging Bridge pedestrians to pause and enjoy the view.
Construction of the bridge began on January 5, 1933. Foundations were dug out of the hillsides to hold the deep-set anchorages that would support the concrete pylons. Strong tides and heavy swells hampered construction of the south pier, the keystone of the entire structure, which required excavating in hard rock 65 feet below the surface of the water.
The steel sections of the Bridge’s towers were fabricated in Bethlehem, Pennsylvania and sent by rail to East Coast seaports where they shipped via the Panama Canal to the McClintic-Marshall Corporation’s storage yards in Alameda. From Alameda the steel was taken by barge across the Bay to the Golden Gate construction site. The girders were hoisted by giant cranes and erected by gangs of men working in teams to bond the sections together with rivets and hot steel.
Strauss chose John A. Roebling and Sons of New Jersey, builders of the Brooklyn Bridge, to spin the cables. Because no derrick could lift cables as heavy as these would be, the Roebling Company spun the cables on site, from anchorages in San Francisco and Marin. Steel saddles provided a seat for the cables (and workers) as they passed over the tower tops. The crews spinning the cables used catwalks to travel from one side of the channel to the other. These catwalks of wire rope and redwood planking were the first structures to span the Golden Gate. Six months later, when the cabling was completed, the roadway was built. The roadway is comprised of poured concrete with copper expansion joints every fifty feet.
SUSPENSION STRUCTURES
DEFINITION
Suspension structures are those with horizontal planes
(road decks, roofs, and even floors) supported by cables (hangers) hung from the parabolic sag of large, high-strength steel cables.
STRUCTURAL CHARACTERISTICS
The strength of a suspended structure is derived from the parabolic form of the sagging high-strength cable. This parabolic form is designed so that its shape closely follows the exact form of the moment diagram(s). This creates a highly efficient structure. The sagging cable performs best under symmetric loading conditions because the cable may deform significantly as it attempts to adjust to an eccentric loading. As the cable adjusts to this load it shifts the rest of the structure. Th is adjustment causes secondary stresses in the horizontal surface and additional deformation. The parabolic curve of the cable is also susceptible to developing harmonics from eccentric or lateral loads such as wind. These increased harmonics can create great movement in a structure, sometimes enough to cause dramatic failure, as in the case of the Tacoma Narrows bridge. Rather extensive calculations must be made to determine the natural frequency of a suspension structure and to test the stiffness of its horizontal surface in order to prevent the structure from developing destructive harmonics.
CONSTRUCTION CHARACTERISTICS
The horizontal surface (bridge deck, etc.) is usually a simple or continuous beam, most commonly configured as a truss or box beam. The box beam is advantageous because it resists tensional forces well, although it provides a greater surface area subject to wind loading. The large curving cable consists of many, many smaller cables which are tightly spun together. As the cables are being spun together they are also stretched over the span and attached to the supports. After being attached the appropriate curve is created by tensioning the cable. This curve is formed without the real dead load of the structure, therefore the completed curve has a different shape than the one created during construction. Finally, the horizontal surface supported by the cable is hung piece by piece from the sagging cable.
TYPICAL MATERIALS