Mackinac Bridge
Coordinates: 45 ° 48 '59 " N , 84 ° 43' 39" W.
| Mackinac Bridge | ||
|---|---|---|
 |
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| use | Road bridge, 4 lanes | |
| Convicted | Interstate 75 | |
| Crossing of | Mackinac Street | |
| place | Mackinaw City , St. Ignace | |
| construction | Suspension bridge | |
| overall length | 8038 m | |
| width | 20.7 m | |
| Longest span | 1158 m | |
| Clear height | 47 m | |
| start of building | May 7, 1954 | |
| completion | November 1, 1957 | |
| planner | David B. Steinman | |
| toll | toll | |
| location | ||
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The Mackinac Bridge ([ 'mækɪnɔː bɹɪdʒ ]), also known as "Mighty Mac" or "Big Mac" , is a four-lane road bridge with all entrances, eight kilometers long, which leads Interstate 75 over Mackinac Street . With the bridge, a direct fixed connection between the main area of the US state Michigan and its upper peninsula was created for the first time. It connects the small towns of Mackinaw City in the south and St. Ignace in the north.
The construction of a bridge had been discussed since the 1880s, but could only begin after many decades. The American engineer David B. Steinman designed the suspension bridge .
history
At the beginning of the 1880s, railway companies had started a ferry service on Mackinac Street, but this could not be maintained all year round. Soon after, the area became a popular holiday destination, especially nearby Mackinac Island with Mackinac National Park , created in 1875, which also includes Fort Mackinac .
After the inauguration of the Brooklyn Bridge in New York, residents began to imagine that a similar structure would be of practical use here. In 1884, a St. Ignace shopkeeper had run a newspaper ad with an artistically designed reprint of the Brooklyn Bridge and the headline: "Proposed Bridge over Mackinac Street . "
On July 1, 1888, at a meeting of the directors of the Mackinac Island Grand Hotel , Cornelius Vanderbilt II, a grandson of the famous entrepreneur Cornelius Vanderbilt , proposed building a bridge over the waterway to extend the hotel's holiday season. He was thinking of a bridge similar to the Forth Bridge built at that time in Scotland over the Firth of Forth . The proposal to build a bridge was also debated in the Michigan Parliament, inspired in part by the construction of the Brooklyn Bridge.
Despite the recognized need for a bridge, decades passed without a concrete plan. In 1920, Michigan State Highways Commissioner proposed the construction of a floating tunnel. At the invitation of Parliament, CE Fowler of New York City presented a plan for a series of dams and bridges, starting from Cheboygan , which is 17 miles southeast of Mackinaw City, across several islands to Saint Ignace.
In 1923, Parliament commissioned the State Highway Department to establish a permanent ferry service. Because this was very expensive until 1928, the then governor Fred W. Green commissioned a feasibility study for a bridge. The Department believed the idea was viable and estimated that the cost of a two- lane bridge plus one rail for rail traffic would not exceed $ 32,400,000.
Michigan Parliament established the Mackinac Straits Bridge Authority in 1934 to pursue the feasibility of the bridge and authorized it to sell bonds for the project. In the mid-1930s, the agency tried unsuccessfully to get funds from Washington, despite the approval of the US Army Corps of Engineers and then US President Franklin D. Roosevelt . Nevertheless, between 1936 and 1940, a route was established and drilling was carried out for a detailed geological study.
The project was delayed because of the Second World War . The authority founded in 1934 was abandoned by parliament in 1947, but later re-established (in June 1950). She was asked to contact three leading specialists for advice. According to the engineers' report of January 1951, the agency was hired on April 30, 1952 to sell $ 85 million in bonds for bridge construction. In 1953, a weak stock market led to another delay of a year before the bonds could be issued.
David B. Steinman was commissioned to develop the project in January 1953. By the end of 1953, the financing was clarified and the necessary contracts were negotiated. Construction finally began on May 7, 1954. The American Bridge Company and United States Steel Corporation received a $ 44 million contract to build the steel superstructure. It took two and a half years to build and killed five workers. The newly built bridge was opened to traffic punctually on November 1, 1957. The inauguration took place from June 25 to 28, 1958. The hundred millionth crossing took place on June 25, 1998, exactly 40 years after the inauguration of the Mackinac Bridge.
The bridge's design was directly influenced by the catastrophic misconstruction of the first Tacoma Narrows Bridge , which collapsed in 1940 due to instability in high winds. Three years after the accident, Steinman published a theoretical analysis of stability problems in suspension bridges. He recommended using high, stiff deck girders in future constructions to avoid vibrations in the bridge deck, and an open framework to reduce wind resistance. Both techniques were included in the construction of the bridge.
description
Suspension bridge
The central structure of the crossing of Mackinac Street is a suspension bridge with spans of 1158 m (3800 ft ) in the main opening and 549 m (1800 ft) in each of the two side openings. In contrast to the usual situation, there are no anchor blocks at the outer ends of the side openings , but only pillars with cable saddles, via which the suspension cables are directed down to the anchor blocks 149 m away, in which they are fastened well below the deck girder. Steinman's original design was for a conventional suspension bridge. In order to save costs, he left the main opening unchanged, but lengthened the side openings and moved the position of the anchor blocks even further outwards so that these 36 m long, over 20 m wide concrete structures founded on natural rock could be built closer to the bank where the rock bottom was encountered earlier. When viewed from the side, the Mackinac Bridge, with its suspension cables drawn outwards in a wide curve, differs significantly from other suspension bridges such as Ammann's George Washington Bridge, with its short suspension cables stretched almost in a straight line to the anchor blocks.
The carriageway girder is a 11.6 m high and 20.7 m (68 ft) wide truss construction on which the 16.5 m (54 ft) wide four-lane carriageway is elevated as a separate structural element. These dimensions clearly show the departure from the design principles of the Tacoma Narrows Bridge. While this had a ratio of 1: 355 between its span of 853 m and the height of the deck girder of only 2.40 m, this figure is only 1: 100 for the Mackinac Bridge. In complex wind tunnel tests, a framework with a side view that is as permeable to wind as possible was developed. In addition, the two inner lanes and the middle sill were made of an open steel grille through which air can flow, so that any aeroelastic fluttering is prevented. The two outer lanes are each 3.7 m (12 ft) wide, the two inner lanes 3.4 m (11 ft). The walkway and railings are 0.9 m (3 ft) wide on each side.
The two pylons are 168.25 meters (552 ft) above the water level. They consist of slim steel shafts that are connected and stiffened by clad trusses.
The suspension cables each consist of 12,580 wires that were bundled and pressed into parallel wire ropes with a diameter of 62.23 cm using the air-jet spinning process .
The color choices of green (Steinman's preferred tone) for the girder and suspension cables and ivory for the pylons is said to go back to an advertisement in Fortune magazine from July 1954, in which an artist's depiction of the future bridge served as the background graphic.
The suspension bridge has a clearance height of 47 m (155 ft). It is thus slightly higher than the Ambassador Bridge . For this first bridge over waterways in the area of the Great Lakes above the Niagara River , a greater height was required than for the bridges on the east coast. There the Brooklyn Bridge had set the standard with a clearance height of 135 ft (41 m).
Access bridges and roads
In the south, a 1741 m long steel truss bridge , supported at regular intervals on concrete pillars, leads from the anchor block to the shore at Mackinaw City. There is a 180 m long bridge that crosses a large parking lot and North Hudson Avenue. It is a conventional reinforced concrete T - beam bridge on concrete pillars.
In the north, a 1097 m long truss bridge connects the anchor block there with a 1074 m long dam that leads to the shore. From there, a 1320 m long road leads over the toll booth to the center of the junction between Interstate 75 and US Highway 2 .
Length specifications
The frequently cited length of the bridge of 8,038 m or five miles is the distance from the center point of the junction to the north, including the toll road and the dam, to the abutment at the end of the concrete overhead bridge on North Hudson Avenue in Mackinaw City.
The steel bridge structure consisting of the suspension bridge and the two truss access bridges is 5464 m (17926 ft) long.
The suspension bridge is 2626 m (8614 ft) long including the anchor blocks and 2554 m (8380 ft) between the anchor blocks. Their spans of 549 m + 1158 m + 549 m result in a spanned length of 2256 m.
Record length between anchorages
The span of the bridge between the pylons of 1158 m is significantly shorter than that of the Golden Gate Bridge with 1280 m. However, in order to be able to present a record, the length including the anchor blocks of 2,626 m (8,614 ft) was chosen, in order to be able to describe the Mackinac Bridge as the "world's longest bridge between the anchorages", which is longer than the Golden Gate Bridge and also longer than that of the western suspension bridge of the San Francisco-Oakland Bay Bridge , which is equipped with an additional anchorage in the middle.
The Mackinac Bridge is still the longest bridge between two anchorages in the Western Hemisphere , but has fallen behind worldwide. In 1998 she had to give up the top spot to the Japanese Akashi Kaikyō Bridge , which holds the record with a total length of 3911 m between the anchors with a span of 1991 m. With its span (1158 m; 3800 ft ), the bridge is currently the fifteenth longest suspension bridge in the world and the third longest in the USA after the Golden Gate Bridge and the Verrazano Narrows Bridge .
Others
The bridge is subject to a toll. The speed is limited to 72 km / h (45 mph). It is not approved for cyclists or pedestrians. However, pedestrians, cyclists and their bikes can be transported over the bridge in a motor vehicle for a fee.
In October 2013, 330,610 vehicles drove over the bridge.
Two lanes are closed to Mackinac Bridge Walk participants every Labor Day .
In the 2004 science fiction film I, Robot by Alex Proyas, the Mackinac Bridge is a ruined bridge. There is no more water there in the movie as the entire area will be a landfill in 2035. US Robotics then builds a storage complex with containers for robots at the landfill.
The bridge was added to the List of Historic Civil Engineering Landmarks by the American Society of Civil Engineers in 2010 .
The bridge is mentioned in the flag oath of Michigan . This is officially set out in Public Act No. 165 of 1972:
"I pledge allegiance to the flag of Michigan, and to the state for which it stands, two beautiful peninsulas united by a bridge of steel, where equal opportunity and justice to all is our ideal."
"I swear allegiance to the flag of Michigan and the state it stands for, two beautiful peninsulas united by a steel bridge where equal opportunities for all and justice are our ideal."
Individual evidence
- ^ Straits of Mackinac on the MDOT - Michigan Department of Transportation website
- ↑ A detailed description of the immediate prehistory and the construction of the bridge can be found in Richard Scott: In the wake of Tacoma, suspension bridges and the quest for aerodynamic stability . ASCE Press, Reston, Va. 2001, ISBN 0-7844-0542-5 , pp. 109-117
- ↑ Clearly called cable bent piers in American descriptions
- ↑ Richard Scott: In the wake of Tacoma . P. 113
- ↑ About the Bridge ( Memento of 22 December 2009 at the Internet Archive ) on the website of the Mackinac Bridge Authority
- ↑ a b c Facts & Figures on the Mackinac Bridge Authority website
- ↑ Richard Scott: In the wake of Tacoma. P. 114
- ↑ Various measurements in the sources cannot be reconciled with one another and / or are incorrect.
- ↑ Strictly speaking, the length between the anchor blocks would be only 2554 m (8380 ft)
- ^ Fare Schedule ( Memento from November 15, 2013 in the Internet Archive )
- ↑ Monthly Traffic Statistics ( Memento from December 5, 2013 in the Internet Archive )
- ↑ Act 165 of 1972 Pledge to State Flag in Michigan Compiled Laws
Web links
- Official site .
- Page with information and photos on historic bridges.org
- Oldest site devoted to the Mighty Mac
- Webcam
- Mackinac Bridge on bernd-nebel.de