I believe there is no better way to understand the relationship between tension and compression than to analyze how a suspension bridge works. Perhaps there is no better suspension bridge to analyze than the Golden Gate Bridge in San Francisco, CA. There are plenty of books and manuals that define...
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I believe there is no better way to understand the relationship between tension and compression than to analyze how a suspension bridge works. Perhaps there is no better suspension bridge to analyze than the Golden Gate Bridge in San Francisco, CA.
There are plenty of books and manuals that define tension and compression, but do firefighters truly understand the tension/compression relationship and why structural engineers choose either tension or compression to move stress from one structural component to the next? This article will describe how the Golden Gate Bridge uses tension and compression to transfer live and dead loads — cars, trucks, concrete, steel, rain, wind, people and concrete — to the bottom of San Francisco Bay as compression. We begin with some Golden Gate Bridge background, followed with a photographic tour that will describe how the bridge uses tension and compression to resist gravity.
The Golden Gate Bridge
Engineered by Joseph B. Strauss, Golden Gate Bridge construction started in 1933 and was completed in 1937 at a cost of $27 million — five months late and $1.3 million under budget. (Strauss said, "It took two decades and 200 million words to convince people that the bridge was feasible.") Eleven workers were killed during construction; at least 1,200 people have jumped from the bridge. From abutment to abutment, the Golden Gate Bridge stretches 1.7 miles (8,981 feet). The total length of the suspended span sections is 1.2 miles (6,450 feet). The middle span suspended between the towers stretches 4,200 feet. The length of one side span is 1,125 feet. The highest clearance above the water is 220 feet. The height of the towers above the water is 746 feet. The height of the towers above the roadway is 500 feet. The width of the bridge is 90 feet. The width of the roadway between curbs is 62 feet. The sidewalk is 10 feet wide. Each leg of each tower base measures 33 by 54 feet.
As of 1986, the total combined dead load of the Golden Gate (bridge, anchorages and approaches) was 887,000 tons. The live-load capacity was calculated to be 4,000 pounds per lineal foot. The bridge deck (roadway, sidewalks and curbs) weighs 3,830 pounds per lineal foot. The concrete paving weighs 6,470 pounds per lineal foot. The bridge deck is suspended by a series of vertical cables.
To prevent aeroelastic flutter (dynamic oscillations), the bridge deck was stiffened by using a series of steel trusses that weigh 33,000 pounds per lineal foot. (The open-web trusses also let wind pass through.) For example, the design wind pressures were 30 pounds per square foot on the bridge deck and suspension cables, and 50 pounds per square foot on the towers. Additional rigidity is provided by diagonal bracing between the parallel chord trusses; the diagonal bracing adds 600 pounds per lineal foot.
The bridge has two main cables that pass over the tips of the two main towers and are secured at each end by gigantic on-shore anchorages. The total weight of each on-shore anchorage is 60,000 tons. The south anchorage is on the San Francisco shore; across the bay, the north anchorage is on the Marin County shore near Sausalito. More than one million tons of concrete was used to complete the anchorages.
The main cables rest atop the columns of each main tower in huge steel castings called saddles. The diameter of one main cable is 36 and 3/8ths inches. The length of one main cable is 7,650 feet. Each main cable is comprised of 80,000 miles of galvanized steel wires. Each main cable contains 61 bundles, or strands, of galvanized steel wire. The 61 bundled strands are comprised of 27,572 wires; the diameter of each wire is 0.192 inches. The spinning of the main cable wires took six months and nine days to complete. The (tested) tensile strength of the main cables in 235,000 pounds per square inch; the yield strength of the main cables is 182,600 pounds per square inch. The bridge deck and roadway are suspended by 250 pairs of vertical suspension cables (referred to as stringers or hangers). The diameter of each vertical cable measures two and 11/16th inches. Including the vertical cables and connection hardware, both main cables weigh 24,000 tons.