The Golden Gate Bridge connects San Francisco Bay to Marine County and is seen as a symbol of power and progress of the United States of America. It is a type of suspension bridge with a tower at each end, and a deck supported by cables attached to the towers.
During the summers, a thick blanket of fog gets accumulated around the bridge concealing the complete span with just its two orange towers visibly rising above the fog blanket, giving an appearance of two golden ladders ascending towards the sky. It seems to represent a golden gate towards the heaven.
The bridge was opened to the public on 27 May, 1937, and was celebrated as Pedestrian Day with more than 200,000 people crossing over it. A similar gathering of around 300,000 people was observed to mark the 50th anniversary of the bridge that unfortunately led to a deflection in the middle portion by 7 feet, thereby flattening the iconic arch of the bridge.
1. Structural System of the Golden Gate Bridge
The suspension bridge consists of two towers, a deck, cables, anchorage, and piers. The anchors secure the cables, which carry the load acting on the deck. Each tower puts tension on the cables to keep the deck suspended. The tension creates a downward force on the towers that is absorbed by the piers.
1.1 Piers and Towers
Two towers of the bridge were constructed on each end of the San Francisco Bay. The towers hold the cables of the bridge that support the deck and transfer the weight of the bridge to the towers.
A compression force created by the cables is resisted by towers through piers that are made of reinforced concrete and rest on the bedrock under the San Francisco Bay. These piers transfer the compressive force to the rocks.
1.2 Cables
The two main cables of the bridge are hanged between the towers. The cables form a U-shaped curve and are made of 27,572 individual pieces of steel wires. Further, the steel ropes are suspended from the main cables to connect to the deck. Gravity pulls the deck, creating a tension force in the main cable to resist this load.
1.3 Anchorage Block
The tension force created due to the dead load of the deck has to be offset to prevent the collapse of the bridge. To offset this force, one anchorage block was provided on each side of the San Francisco bay. This anchorage provides a tight grip to the main cables. The end of the cables is supported by an anchorage on both sides of the bridge.
Further, the anchorages are connected to bedrock with rods, steel plates, and eye bars. Each anchorage consists of 60 eye bars (metal rods with holes at their tips). The wires of the main cables are fastened to the holes in the eye bars. This system keeps the cables from sagging under the weight of the bridge deck.
2. Loads Considered for Design of Golden Gate Bridge
The bridge was designed for dead load, live load, and wind load. These loads are discussed below:
2.1 Dead load
Dead load of any structure is due to the weight of that structure. Generally, metal beams are used in a suspension bridge. However, to reduce the dead load of the golden gate bridge, designers used metal cables as they are lighter than metal beams.
2.2 Live load
The primary live load for the Golden Gate Bridge includes cars, trucks, and people crossing the bridge. The increase in the traffic may cause deflection in the bridge's deck because the increasing weight can create more tension in the cables, therefore, the position of the deck may change. The engineers designed the bridge with a maximum deflection of 3.3 m in the worst possible scenario.
2.3 Wind load
The bridge was designed for two types of wind loads. The first one is a static wind load, which acts on the bridge when the wind blows across the bridge from side to side. To prevent the bridge from the static wind load effect, engineers considered flexible cables that allow the bridge to swing during windy conditions. It allows the bridge to move 8.4 m from side to side and remain steady.
Another load is the dynamic wind load, which acts on the bridge when the wind blows up and down over it. Such wind loads can cause buckling and twisting of the deck. To prevent the bridge from dynamic wind load effect, engineers provided truss in the lower portion of the deck to make it stiffer. Therefore, the movement of a stiff deck under dynamic conditions is significantly less than an ordinary deck.
3. Construction of Golden Gate Bridge
The construction of the bridge comprises construction of various structural elements discussed below:
3.1 Construction of Anchorage Block
San Francisco bay has a major fault line due to which earthquakes are common in the area. The anchorage blocks were designed in such a way that they do not slip during an earthquake. For this purpose, three interlocking concrete blocks were constructed.
The anchorage block was constructed above the concrete block. First, the steel girders were constructed above the concrete block to strengthen the anchorage blocks, and then eye bars were fixed inside the anchorage block mold, followed by the pouring of concrete. After the construction of the anchorage block, only eye bars were visible above the block.
3.2 Construction of North Pier
- Firstly, a dry workspace around the pier area was created to construct a foundation pier in the water, for which engineers had to construct a cofferdam.
- The cofferdam was constructed using a tall metal frame filled with stones.
- The frame was lowered above the site where the north pier would be constructed, then the frame was covered by watertight sheet piles. Further, the water was pumped to create a dry workspace.
- Drilling machines and explosives were used for the excavation because the bedrock base was located 10 m below the bay's water surface.
- Reinforced concrete was used for the construction of the pier.
- Finally, after the completion, the north pier reached 13 m above the bay's water surface.
3.3 Construction of South Pier
Construction of the south pier was complicated because the bedrock base was located 34 m below the seawater surface. It was the first time anyone had attempted to build bridge support in deep, open water.
- Firstly, an oval-shaped fender was constructed around the south pier area.
- To construct a fender, divers placed the explosives on the floor of the bay. The blast was required to reach the base of the bedrock through the bay floor.
- After that, high-pressure hoses were used to remove the pieces of broken material. Watertight molds and funnels were placed under the water to form the fender. Further, through funnels, the water inside the mold was removed and concrete was poured.
- After the fender's construction, the water inside the fender area was pumped out and an area the size of a football field was created.
- Excavation was carried out inside the fender to reach 34 m below the sea surface.
- Reinforced concrete was used for the construction of the pier.
- Finally, after the completion, the south pier reached 13 m above the seawater surface.
3.4 Tower Construction
After the construction of piers, a steel tower of 227 m height was erected over it. To construct such a high tower, engineers constructed scaffolds around the tower. Steel pieces were riveted at the site and struts and crossbars were used to join the two legs of the tower, which were 27 m apart.
The architect of the tower decided to create a fluting pattern that ran up and down on the side of the tower through riveting. As a result, such a pattern created a feeling of movement of tower as the sun rays reflected back from them.
3.5 Construction of Cables
After constructing the tower, two main massively long cables were constructed along with the two towers. For this purpose, steel wire was attached to an eyebar on an anchor and was carried over both the towers via the spinning wheel, and connected it to the eyebar on the opposite side of the bay.
It was a slow process and to speed it up, engineers used six spinning wheels at a time with color-coded wires. The construction of the cables was completed within six months.
Grooved metal bands were provided along the length of the main cables. The purpose of those groove bands was to support the 500 suspender ropes. Further, these suspender ropes connected the bridge deck with the two main cables. The suspender ropes were spaced 15 m apart from each other.
3.6 Construction of Deck
The deck of the Golden Gate Bridge is composed of steel beams supported by the trusses and hanged through suspender ropes. For this purpose, a cradle was created by bolting two pieces of suspender ropes together. The deck was hanged through the cradle.
The installation of deck was carried out from both the ends of the tower towards the center. As a matter of fact, this was done to spread the load evenly on the tower. A total number of 747 sections were installed. Once the deck was constructed, engineers paved the roadway above it.
FAQs
It was built to connect the San Francisco Bay to Marin County, otherwise the ferry ride from San Francisco bay used to take 27 minutes.
During the summers, a thick blanket of fog gets accumulated around the bridge concealing the complete span with just its two orange towers visibly rising above the fog blanket, giving an appearance of two golden ladders ascending towards the sky. It seems to represent a golden gate towards the heaven.
The design engineers of the bridge were Joseph Strauss and Charles Ellis. Irving Morrow was the architect.
After its completion, the Golden Gate Bridge was the world's longest suspension bridge. It remained the longest suspension bridge in the world for nearly 30 years. The span of the golden gate bridge is 1,280 m.
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