Golden Gate Bridge, San Francisco
The Golden Gate Bridge is one of the longest suspension bridges in the world. Built across the Golden Gate or the opening of the San Francisco Bay into the Pacific Ocean, the Golden Gate Bridge is a symbol of great engineering feats. Connecting the city of San Francisco and California, the Golden Gate is a well-known civil engineering symbol not only in America, but throughout the world.
SPANNING THE GOLDEN GATE
- The entrance to San Francisco Bay was named the ‘Golden Gate’ by early settlers
- One mile wide gap
- 60 Mph wind tunnel like gusts of wind for much of the year
- 2.5 Million cubic feet of water per second pass through the Golden Gate
- $35 million in bonds
- $38 million in interest
- Bridge completely paid off on June 30, 1971
- Bridge completely paid for by tolls
- No state or federal money went into the bridge
- Bank of America extended a $5 million line of credit to the bridge project
- People put up their homes, farms and businesses as collateral for the bridge to generate the $35 million in bond issue
- Presently, bridge still operates only with money collected from tolls
The Golden Gate was designed and constructed under the supervision of chief engineer, Joseph Strauss. However, the team was composed of many experts from both cable suspension design experts and civil architects. When the construction of the gate began in January 1993 the estimated cost of building the whole bridge was more than $35 million dollars. A few of the important criterias planned for the bridge are listed below:
- Length of the span is one of the most important criteria for building a bridge, especially when the bridge is a suspension one, having a length of 1970 meters.
- Dead load is the total weight of the materials used in the construction of the bridge. For Golden Gate it was kept 21,300 pounds per lineal foot.
- Dynamic or wind load is the load by the wind force on the bridge. For Golden Gate it was kept 30 pounds per square foot for the cables and 50 pounds per square foot for the towers.
- Terrain is an important aspect while designing a bridge. It plays an important part in making the bridge foundation and for the ships that will be using the waterway beneath it. For this reason, special criteria were made for each tower, depending on the terrain on which it was made.
- Maintenance is one more important aspect as just constructing the bridge is not enough. It was thus decided that the vehicles using the Golden Bridge will have to pay a toll, which would ensure the bridge’s maintenance costs.
SAFETY UNDER STRAUSS
- 1st Mandatory use of hard hats
- Issuing of safety belts and tie off lines
- Dizzy riveters: respirators for burning paint fumes
- Riveters and ironworkers required to wear leather gloves
- Sun goggles, sun-block lotion
- Field hospital at south end staffed full time; treated 12,000 injuries by December, 1935.
- No deaths for the first 3 years and 8 months of project
- A safety net was placed 60 feet below the construction to prevent the fall of workers.
To offer sound measures of protection of the workers, 120,000 Safety net suspended 60 feet below the road surface. For the first time, a protection measure was adopted in a bridge project. Averaging 1 worker will be killed for every million dollars spent on construction, a toll of 36 men was expected. Yet the safety measures led to the death of only 12 men (9 of which were killed because of concrete collapse), but saved the fall of 19 other men (Half-Way to Hell Club).
- 1920: A feasibility Study recommends construction of the Golden Gate Bridge
- May 25, 1923: The State Legislature Passes the Golden Gate Bridge and Highway District Act of California into Law
- August 27, 1930: Joseph Strauss, Chief Engineer submits his final plans for the bridge
- November 4, 1930: $35 million bond issue approved by the six counties in the District, by a vote of 145,667 to 46.954
- January 5, 1933: Construction of the bridge begins
- February 1933: Anchorages are completed
- June 1935: The towers are completed
- March 1936: The suspension cables are completed
- April 1937: The deck surface is completed
- May 27, 1937: Bridge opens to pedestrians
- May 28, 1937: Bridge open to automobiles. The toll was 50 cents one way, $1 round trip and 5 cents surcharge if there were more than 3 passengers
- February 22, 1985: The one-billionth car crosses the bridge. Toll is $2 southbound on Friday and Saturday, $1 otherdays. No northbound toll
- May 28, 1987: Bridge closed to vehicles for its fiftieth birthday. An estimated 300,000 pedestrians jammed the bridge
- September 2 2008: Toll increased to $6 southbound. No northbound toll.
DESIGN AND SPECIFICATIONS
- Design of the Golden Gate Bridge consists of two main towers at the center.
- The weight of the entire bridge is supported by two cables that run parallel to each other on either side, passing through the two towers and fixed to the concrete structures at each end.
- The cables used in the bridge consist of 80,000 miles of wire and 27572 strands.
- Though the Golden Gate Bridge appears red in color its original color is an orange vermillion. The Bridge was painted with red lead primer initially and was then applied with a lead based coating. Later on in early nineties, the bridge was top coated with Acrylic paint.
- The Golden Gate Bridge is regarded as one of the most beautiful bridges in the world. It is also supposed to be known as one of the most photographed bridge in the world.
- With recent advances in technology, the Bridge was retrofitted with earthquake resistant mechanisms, which would allow it to resist the seismic activities in a better way. Today, the Golden Gate Bridge stands as a prominent American Landmark, which not only stands as a great civil engineering wonder, but also continues to carry the weight of thousands vehicles every day.
BRIDGE DESIGN AND CONSTRUCTION STATISTICS
Total length of Bridge including approaches from abutment to abutment: 1.7 miles = 8,981 ft = 2,737 m Total length of Bridge including the distance to the Toll Plaza: 9,150 ft = 2,788 m Length of suspension span including main span and side spans: 1.2 miles = 6,450 ft = 1,966 m Length of main span portion of suspended structure (distance between towers): 4,200 ft = 1,280 m Length of one side span: 1,125 ft = 343 m
Width of Bridge: 90 ft = 27 m Width of roadway between curbs: 62 ft = 19 m Width of sidewalk: 10 ft = 3 m
Clearance above mean higher high water: 220 ft = 67 m
Total weight of each anchorage: 60,000 tons = 54,400,000 kg Original combined weight of Bridge, anchorages, and approaches: 894,500 tons = 811,500,000 kg Total weight of Bridge, anchorages, and approaches (1937): 894,500 tons = 811,500,000 kg (1986): 887,000 tons = 804,700,00 kg Weight of Bridge, excluding anchorages and approaches, and including the suspended structure, main towers, piers and fenders, bottom lateral system and orthotropic redecking (1986): 419,800 tons = 380,800,000 kg
Swing equal to 3 station wagons end to end
Foundation comprised of 2 anchorage blocks, 64000 tons of concrete and steel, 1 for each of the 2 main suspension cables Running out 1100 feet into the golden gate from the fort, is the contractors’ working trestle, ending up at south is the San Francisco pier location. The construction of this pier practically enabled 1 of the most difficult foundation jobs ever attempted. The pier will support of the 2 746 foot steel towers. It was necessary to build the south tower offshore, but working so far into the open water of the gate, made the trestle a sitting target for disaster. Once by the sheer force of the raging pacific current the trestle was sliced into 2, washing 2 million cubic feet of water past the site every second. The disaster took 10 months of hard labor to repair. In building the San Francisco pier it was necessary to excavate down into the rock to a depth of over 100 feet below the surface of the golden gate, and over an area of an acre, this was done by the ‘Pilot Bomb’ method. Small bombs were inserted in guide tubes, carefully located over the spot desired; dropped and fired. This looses the rock sufficiently, so the cranes can get to work. Unlike the south tower twin, the north tower was erected on solid ground of the Marine county shore.
Trestle laid across the Gate
Guide Tube used in Pilot Bombing
Mountains of sand, gravel and cement were floated to the site in a non stop armada of barges, and then offloaded into enormous concrete mixing facilities, built specially for the project. The first challenge was to construct the 4 enormous anchorages (240 million pounds each), to which the bridge’s suspension cables would be grounded. Mixed concrete was pumped directly to the locations through movable shutes, where man could pack and vibrate the thick sludge to remove air pockets from the 12 foot thick blocks, laced with steel reinforcing bars. Once the work started, the pumping could not stop, or the materials would not set properly. Every hour a 120 cubic yards of concrete was pumped in a maze of steel frame.
Crane, removing rocks from water bed
Workers, packing and vibrating concrete
MAIN TOWER STATS
The Golden Gate Bridge has two main towers that support the two main cables.
Height of tower above water: 746 ft = 227 m
Height of tower above roadway: 500 ft = 152 m
Tower base dimension (each leg): 33 x 54 ft = 10 x 16 m
Load on each tower from main cables: 61,500 tons = 56,000,000 kg
Weight of both main towers: 44,000 tons = 40,200,000 kg
Transverse deflection of towers: 12.5 inches = 0.32 m
Longitudinal deflection of towers: shoreward: 22 in = 0.56 m and channelward: 18 in = 0.46 m
The south tower foundation depth below mean low water is: 110 ft = 34m
To build south tower pier to support the south tower, construction workers pumped 9.41 million gallons or 35.6 million liters of water out of the fender that was constructed first.
Each wire was specially designed for golden gate bridge from carbon and alloy steel, meeting exact characteristics. Samples were tested for elongation and strength exceeding 235,000 pounds per sq inch. In the end these flexible wires would be a part of a 3 foot thick cables, but it was necessary to weave these wires on site, 746 feet above the water surface, in windy conditions of faster than 45 miles an hr. A crane was used to lift the first few strands of the cable. Once they were in place, a mid-span work platform was lowered across the gap. The operation was not a complete success at first attempts.
SWIMMING THE CABLE
With these first few thin strands, the mighty cables will be bundled and draped over the steel tower super structures. On either shore, the wires connected to massive adjusting rods while 150 tons saddled over the towers, cradled the cables in place. A system of spinning wheel carriages transported 27,572 wires across the span, 6 wires at a time. Individual wires were placed by hand into a geometric pattern, radio transmissions instructed the opposing bases to tighten the stretch. On an 8 hour shift an alarming 1000 miles of wires were spun over the gate. This took a total of 191 days to complete. These wires were first bundled by hand, and then by a hydraulic press to get them bonded in a tight cocoon. Finally these wires were wrapped in a final wrapping of cable, and painted to make it water-proof.
MAIN CABLE STATS
- The Golden Gate Bridge has two main cables which pass over the tops of the two main towers and are secured at either end in giant anchorages.
- The main cables rest on top of the 746-foot main towers in huge steel castings called saddles.
- Diameter of one main cable including the exterior wrapping: 36 3/8 in. = .92 m
- Length of one main cable: 7,650 ft = 2,332 m
- Total length of galvanized steel wire used in both main cables: 80,000 mi = 129,000 km
- Number of galvanized steel wires in one main cable that are 0.192 inches in diameter: 27,572
- Number of bundles or strands of galvanized steel wire in one main cable: 61
- Weight of both main cables, suspender cables and accessories: 24,500 tons = 22,200,000 kg
- The galvanized steel wire comprising each main cable was laid by spinning the wire using a loom-type shuttle that moved back and forth as it laid the wire in place to form the cables. The spinning of the main cable wires was completed in 6 months and 9 days.
The galvanized steel wire used for the main cables is carbon steel with the following average chemical composition and physical properties:
SUSPENDER ROPE (VERTICAL ONE) STATS
The Golden Gate Bridge has 250 pairs of vertical suspender ropes that are spaced 50 feet apart across both sides of the Bridge. Each suspender rope is 2-11/16 inches in diameter. All of the ropes were replaced between 1972 and 1976, with the last rope replacement completed on May 4, 1976.
The vulnerability study showed that the Fort Point Arch, which spans a Civil War era landmark building, would be subject to uplift during a major earthquake. The steps to be taken in future to counter uplift during earthquake are:
- The arch bearings will be modified such that any possible arch lifting will be guided in a controlled manner.
- Pylons located at either end of the arch are made of lightly reinforced concrete. They are about 76m in height and need extensive retrofitting. Thick steel plates embracing the concrete walls of pylons will be covered with cast-in-place reinforced concrete that preserves the aesthetics of the original construction.
STEPS TAKEN TO COUNTER THE UPLIFT DURING EARTHQUAKE ARE:
- A battery of energy-absorbing devices was installed in order to reduce possible movements between the pylons and the arch.
- The foundation of pylon S1 was enlarged to 16m x 42m x 7m and 90 of 37-Ø15.2 mm DYWIDAG Multistrand Tendons were placed in both directions on two levels of the foundation. Additional, 32 tiedown anchors of 45-Ø15.2mm epoxy coated multistrand where also installed.
- The pylon S2 foundation was also reinforced with 24 tiedown anchors of 45 -Ø15.2mm epoxy coated strand and several passive Ø32 mm DYWIDAG THREADBAR ® Tendons grade 835/1030.
- The South Anchorage Housing was also inadequately reinforced. The upper section of towers 2 and 3 has been tied vertically and horizontally with high strength DYWIDAG THREADBAR ® Tendons grade 835/1030. Shear walls were placed between the bents.Their foundations were filled with concrete and tied down with DYWIDAG Multistrand Anchors consisting of 7-19 Ø15.2 mm epoxy coated strands.
- The south abutment and the foundations of bents S6 through S10 were reinforced with concrete and 36 mm DYWIDAG THREADBAR ®Tendons placed in both directions.
GOLDEN GATE BRIDGE PAINT
- The Golden Gate Bridge's paint color is orange vermillion, also called international orange. Architect Irving Morrow selected the color because it blends with the bridge's setting
- The bridge was fully painted when it was first built and then touched up for the next 27 years. In 1965, the original paint was removed because of corrosion and replaced with an inorganic zinc silicate primer and an acrylic emulsion top coat, a project that took 30 years.
- A crew of 38 painters continually touches up the bridge with paint, along with 17 ironworkers who replace corroding steel and rivets.
Golden Gate Bridge, 1937. Cable “saddle” on top of one of the towers
SAN FRANCISCO APPROACH – TWO-WAY TOLL COLLECTION
SAN FRANCISCO APPROACH – TWO-WAY TOLL COLLECTION
GOLDEN GATE BRIDGE FACTS - TRAFFICE
- Average crossings: About 41 million per year, counting both north- and southbound crossings, compared to 33 million crossing the first year it was open
- Fewest crossings: January, 1982, during a storm which closed U. S. 101 north of the bridge. On January 6, only 3,921 southbound vehicles passed the toll gates
- Most crossings: October 27, 1989, a few days after the Loma Prieta earthquake, when the Bay Bridge was closed. 162,414 vehicles (counting those going both directions) crossed the bridge that day
- Total crossings: Through October 30, 2002, the Golden Gate Bridge Highway District reported 1,754,094,967 vehicles had crossed the bridge
- Closures: The bridge has been closed three times for weather, for gusting winds over 70 mph. It closed briefly for visits by President Franklin D. Roosevelt and French President Charles DeGaulle. It was also closed on its fiftieth birthday
- Riveting crew held unofficial speed contests; they sometimes threw red-hot rivets distances of 70 feet or more
- 80,000 miles of pencil diameter steel cable in bridge
- Steel for the bridge was made in Pennsylvania and ‘shipped’ to California via the Panama Canal
- $75,000 paid in tolls each day
- A gold rivet was hammered into place to celebrate the completion of the bridge
- ‘International Orange’
- Blind woman and guide dog the first pedestrians ever to cross the bridge
- 1.25 billion cars have crossed bridge since ’37
- Bridge ‘Firsts’ First person to cross on stilts First person to cross sticking tongue out all the way First wooden hat to cross the bridge
- Each anchorage weighs as much as 17,000 elephants