A Feat of Construction Eengineering
Possibly the most famous bridge in the United States of America, the Golden Gate Bridge spans a mile across the Golden Gate strait. The historically famous bridge connects San Francisco city to Main county and carries two highways with it - Route 1 and Route 101. One of the reasons this picturesque bridge is so famous because at the time of construction it was the longest suspension bridge in the world. Its construction began in January 1933 and it was open for public use in April 1937. It is 8981 feet long in total or 2.7 kilometers. At that time, this was a marvelous feat of construction engineering.
Today we will look at how this feat was achieved.
The Golden Gate strait, across which flies the Golden Gate Bridge, is the main drainage of 60% of rain and snowfall on the state of California. Being only a mile wide, that massive volume of water causes strong tides. Along comes frequent winds and fog. The Pacific Ocean lying on the other end of the strait also supplies a significant amount of salt air. But the biggest headache probably is the San Andreas fault line - an earthquake origin located only 7 miles away under the sea. All of these stack up on the difficulty of building the bridge. Not to mention, the that-time impossibility to span a mile of water.
Financial and Demographic Challenges
Before the 1900s visualizing a bridge across a mile of tidal water would have been considered a dream only. But with the modern advances in construction engineering technologies in the starting decades of the previous century, this dream started to look like a possibility. However, people still called the plan “the bridge that couldn’t be built.” And that was not only for the technical issues, but there were also financial problems as well. As you may know, in 1929 America was hit by the Great Depression. With that amount of economic hardship, it was a madman’s idea to spend so much money on a construction project that may not even work! An early estimate of the bridge, in 1916, was put at $100 million. That sort of money was impossible to gather.
Not only that. Many people were vehemently against the project as well. The concern faced opposition and even litigation from many sources. The Navy thought the bridge will stop ship traffic, or it may be a potential security vulnerability. Ferry services sued the project because it would damage their business. Unions demanded favors about the usage of labors and permits. The Humboldt county rose up in fury against the Bridge and the traffic it would generate.
Fortunately, other, cheaper plans were available, and the brave people of six counties in northern California came forward. They voted to finance the building of the Golden Gate Bridge. In 1930, they put together $35 million for the project in spite of the hardships of the time. Manpower was at least something the country never lacked for, and with skill, determination, and imagination, constructors and engineers rushed forward to plan “the bridge that couldn’t be built.”
Mr. Joseph Strauss, a germen-based engineer, is called the father of the Golden Gate Bridge, as he was the Chief Engineer of the project. He proposed two double cantilever spans linked by a central suspension segment. However, because of recent advancements in metallurgy, a suspension bridge design was considered the most practical with these particular challenges. However, Strauss himself had not much expertise in that particular type of construction, so he agreed to take consultation from various other projects. The final graceful suspension design was the brainchild of Leon Moissoff who also built the Manhattan Bridge. The architect and decor chief of the project was Irving Morrow, who is responsible for the famous orange color of the bridge, despite the Navy’s suggestion of a yellow and black striped outlook.
Basically, the plan was like this. The Golden Gate Bridge is one tower on each side, each 746 feet high. Each abutment is placed over a foundation embedded on the seafloor. The actual bridge hangs from the two towers by two main supporting cables which have never been changed from 1937. These cables and suspender ropes from them hang the main span of trusses, on top of which the roads are built.
The Golden Gate Bridge remained the longest Suspension Bridge, until 1964. From one abutment to another, the main span is 8981 feet or 2737 meters long. It is now the second longest suspension bridge span.
As per the suspension-bridge idea, the weight of the roadway is slung from two towers at either end of the channel, each of which is 746 feet (227 meters) high. Until 1993, that held the record for being the world’s tallest bridge as well. The span itself clears 67 meters or 220 feet above the water. The two towers have a honeycomb lattice cellular design which makes them lightweight yet strong, like tree trunks. Between them, the towers required 1.2 million rivets to make this cellular design. Each tower consumed about 40000 metric tons of steel.
The towers stand upon concrete piers on both sides. The north pier’s foundations extend 100 feet below the surface and 35 feet into bedrock. Even then, it was questioned whether the structure can withhold a major earthquake. However, it was relatively easier to build. But the south pier was a different story altogether. It connects to a shelf of bedrock 110 feet below the surface of the water, and the sea bed is sloped there. With the major difficulties, it took an extra year to build only this pier and foundation. To build this, a huge oval-shaped concrete barrier or fender had to be constructed first around the location and then pump out the water out of this. When done, it earned the nickname of “a bathtub for the South Tower.” Divers played a critical role in this.
The roadway that is supported by triangular-frame trass hangs from vertical suspension ropes which deliver the load to two main cables from tower to tower. Each cable is 36.5 inches wide and is made of 27572 strands of galvanized steel wire each a pencil thick. The cables hang from saddles on top of the towers. Each cable alone weighs 12000 tons. Too heavy to carry up and slung in place, instead they were spun in place using a method called cable spinning. It involved securing one end of one pencil-thick steel wire at one side of the bridge, carry it over both saddles on both towers, securing it at the other anchorage and then sending it back the same way. This spinning marked a milestone of efficiency when it was finished in only six months.
The triangles in them easily mark out the trusses on each side of the roadway deck, which hold up the weight of the road, the cars on it, and other assorted buildings on top of the roadway. They also stiffen the bridge against the strong winds in this region. For a reference, the immensely powerful towers still sway 12 feet at the top due to the wind! The trusses span 50 feet between the suspender ropes. They may look delicate and thin from a distance, but they are 25 feet deep. They had to be built through a careful routine so as not to disturb the load-bearing harmony of the two suspender cables. The final paving for the roadway was finished just a month before the opening of the bridge in April 1937.
Strauss was adamant on the point that the brave workers who have come forward to take up such a difficult challenge should not get harmed. It is to be noted that at that time heavy construction much more of a dangerous task than today; each million spent generally consumed a human life. However, Joseph Strauss meant to change all that.
The most well-known safety feature used by Strauss is the Safety Net. Slung beneath the bridge, it saved 19 lives, though it took $130000 to make. Additionally, Strauss made the workers wear special protective goggles to protect from glare, special cream to protect from the salty air. Also, the workers were told to consume only certain dietary to reduce dizziness.
To be noted that eleven men still died in the Golden Gate Bridge project even after all the safeguards in two accidents.
At last, at 6 am on the 27th of May, 1937, the bridge was opened for pedestrian use. At least 18000 people were waiting to be the first ones to access the bridge despite a lot of superstition, fear, and negativity spread by the opposition. Over that day only, at least fifteen thousand people crossed the bridge, each paying 25 cents for the privilege. The next day (28 May) it was opened for vehicles. 500 US Navy aircraft did a fly-by salute to Josef Strauss on the inauguration.
Joseph Strauss said this bridge will last forever, but nothing really does. To make sure the Golden Gate Bridge remains secure and to strengthen it further, multiple upgrades have been applied to it since its construction. These included bracings to make it sway less and replacing some of the road deck sections with lighter but stronger construction. As weathering took its toll, the vertical suspension ropes were replaced one by one. Seismic resistant upgrades were applied as well to endure earthquakes. Finally, with the modern digital era, a fleet of motion sensors are fitted on the bridge to monitor constantly how the bridge is reacting to transport load, wind, water current, hear/cold, and other parameters.
- Quantity Takeoff Software
- PileGroup is an exclusive
- New construction against design
- Tekla formwork placing tools
- Powerful construction program