The Mackinac Bridge in Michigan is a bridge of ideal, assured aerodynamic safety secured without sacrificing either economy or graceful proportions. The design of the bridge, with a main span of 3800 ft., was predetermined scientifically, in final form, without spending years in groping, cut-and-try experimentation, and without successive modification of design to overcome aerodynamic instability. It was the longest suspension bridge when built.
Facts and Figures:
- The total length of the bridge, including the approaches, is 8 km.
- The total quantity of concrete used in the substructure (anchorages, piers, and foundations) was around 336,000 cubic meters. Of this amount, 268,000 cubic meters was placed underwater.
- The total weight of the steel superstructure (cables, structural steel, and roadway) is 66,500 tons.
- Total 33 spans were constructed on 34 piers, out of which two main piers were carried down to the highest depth of 70 m.
The contract for the substructure was awarded for USD 25 million, while that for the superstructure at USD 45 million. This was the single-largest contract the United States Steel Corporation had ever received at that time. In fact, it was the single-largest contract in the history of bridge engineering.
The use of the prepakt method for placing concrete in the foundations for the Mackinac Bridge had created a new world record for underwater concrete placement from a single floating plant: 4800 cubic meters in 24 hours.
1. Need for the Bridge
The Strait of Mackinac is 4 miles wide, and it joins with Lake Michigan and Lake Huron. The Strait divides the state of Michigan into the 41,700-square-mile Lower Peninsula and the 16,500-square-mile Upper Peninsula.
The greater part of the population of the state is concentrated in the highly industrialized Lower Peninsula, but the Upper Peninsula possesses immense natural resources which, when further developed, will attract additional population and industrial activity.
The Upper Peninsula is 400 miles long and is nearly equal to the combined area of four New England states. The principal industries at that time were forestry, mining, agriculture, and recreation. Part of this area is popularly known as the Copper Country. The area is also known as a vacation paradise for drawing tourists and sportsmen from many states for hunting, fishing, camping, sailing, and winter sports.
The Mackinac Bridge replaced the existing state-operated highway ferry system in order to provide an all-year, all-weather, direct, time-saving connection between these two great peninsulas of Michigan. It is recognized that the project has contributed most to the development of the Upper Peninsula.
The 5-mile ferry crossing takes over an hour. The Mackinac Bridge has reduced the crossing time to just ten minutes. But, more importantly, the Mackinac Bridge has saved the time lost in waiting in line for the ferries. During summer, this waiting-in-line time could be around three to four hours. On holidays and during the deer-hunting season, cars had to wait in line for as long as 14 to 17 hours. The queues of waiting cars have extended along the highway as far back as 20 miles from the ferry.
2. Timeline of the Mackinac Bridge Construction
In 1920, the Michigan highway commissioner suggested a submerged floating tunnel for the Mackinac Straits crossing.
In 1928, the state highway department recommended a bridge, but the subsequent depression put a stop to the project.
In 1934, a bridge authority was created by the State Legislature. The authority retained three successive consultants, who presented respective diverse plans in 1934, 1935, and 1940. However, the ongoing World War II put an end to all the planning.
In 1950, the present Mackinac Bridge Authority was created by the Michigan State Legislature.
In 1951, the three-man Board of Consultants reported that construction of the bridge was feasible.
In January, 1953, the authority selected Glenn B. Woodruff as the design engineer. Within two months, preliminary contract plans and estimates of quantities were ready, and the substructure and superstructure contracts were negotiated and awarded for prompt commencement of construction.
Because of the delay caused by completing the financing of the project, it was not until the spring of 1954 that the contractors proceeded to order materials and to mobilize equipment. During the next few months, $5,000,000 worth of floating construction equipment was assembled and in place along the line of the bridge for the substructure contract. It was the largest and finest floating equipment ever assembled for a construction contract.
On July 10, actual excavation was commenced for the subaqueous foundations. Over 750 men were engaged in the work at the site, working 20 to 24 hours a day as the winter ice conditions at the Straits limit the normal working season to only eight months.
3. Geologic Factors Affecting Design and Construction
The geologic factors influencing the construction design included (1) effects of wind and currents on piers, towers, and supporting cables (2) effects of ice pressure and ice push (3) thickness and character of unconsolidated glacial deposits flooring the Straits (4) geological structure of the bedrock in which the footings would be emplaced.
3.1 Tides and Currents
The average volume of water flowing through the Straits is small, and currents thus produced are negligible. Maximum currents result either from seiches (gravity-wave oscillations of the lake surface caused by resonant coupling with a fast-moving atmospheric pressure-jump line) or from protracted high wind velocity in any given direction.
Brief observations in 1939 indicated a maximum current velocity of 1.9 miles per hour for water moving through the Straits. It was believed that an absolute maximum velocity of 4 miles per hour might reasonably be expected, except for occasional seiches.
The highest sustained wind velocity recorded prior to construction was 78 miles per hour. Wind-tunnel tests on a sectional scale model of the bridge indicated that at a sustained wind velocity of 100 miles per hour and for zero-angle attack, the wind force or drag would be 670 pounds per lineal foot of bridge, and the upward vertical lift would be 50 pounds per square foot of bridge.
The bridge was therefore designed with a safety factor equal to 960 pounds per lineal foot of bridge, which corresponds to a sustained wind velocity of 120 miles per hour.
The average maximum ice thickness in the Straits is about 18 inches, with a maximum recorded deep-water thickness of 30 inches. Massive ice floes, driven into the Straits from Lake Huron by winds piles to a height of 50 feet or more.
The foundations were therefore designed to sustain vertical dead and live (assumed as 2,000 pounds per lineal foot) loads at bearing pressures of 15 tons per square foot, considered very conservative for the rock involved as based on loading tests.
Superstructure was designed for wind pressure of 50 pounds per square foot and the very severe assumption of ice 4 feet thick with a crushing strength of 400 pounds per square inch was assumed for pier design. Still, the net bearing pressure increased to 25 tons per square foot, which was a conservative value.
3.4 Geologic Structure
The Mackinac Straits lie near the northern rim of the Michigan Basin. Rock strata strike eastward nearly parallel to the axis of the Straits and dip southward toward the center of the structural basin. The rate of dip of the beds increases with stratigraphic depth, but it was believed that, in general, the average dip is 50-65 feet per mile.
The southward dip of the St. Ignace formation was calculated as 52 feet per mile and the Bois Blanc formation as 55 feet per mile. These regional dips are somewhat greater than average for the Michigan Basin and are probably the result of greater subsidence at the center than around the margins of the basin during Silurian and Devonian times.
The rock units involved in the bridge construction included Upper and Lower Silurian, Lower and Middle Devonian shale, limestone, dolomitic limestone, dolomite, chert, and thin evaporites.
4. Foundation Details of the Mackinac Bridge
Because of the unusual brecciated formation, the engineers doubted the rock underlying the Straits would not support the weight of the bridge. Exhaustive geological studies, laboratory compression tests, and in-situ load tests on the rock underwater at the site established that the rock under the Straits can safely support more than 60 tons per square foot. This capacity was four times more than the resultant due to the maximum possible load applied on the structure.
Maximum load included the combination of dead load, live load, wind load, and ice load. However, the open caisson foundation was designed for 15 tons per square foot. Thus, the rocks were able to provide safety way beyond what the engineers had thought of.
The maximum ice pressure ever obtained at the Straits field was around 21,000 pounds per lineal foot of pier width. Also, the greatest ice pressure producible in the laboratory under controlled conditions for theoretically maximum pressure was 23,000 pounds per lineal foot. However, the Mackinac Bridge was designed for five times more than the theoretically assumed pressure. The piers were designed for a hypothetical, impossible ice pressure of 115,000 pounds per lineal foot.
As per the current codes, the safe foundation pressure is achieved by dividing the maximum possible ice pressure by a factor of 2.5. However, during the design of Mackinac Bridge, the factor was considered as 4. Thus, the overall factor of safety of piers against ice pressure was 20, which is very safe. This is why the Mackinac Bridge is called the safest bridge in the United States.
For further safety against any possibility of ice damage, the concrete of the piers was protected by steel sheet piling, steel caissons, and armor plate.
The massiveness of the foundations and the resulting stability against the most severe wind reactions, ice pressure, or any other conceivable loads or forces are represented by the following figures:
- The diameter of each pier is 116 feet and the concrete filled inside the piers weighs more than 145 kilo-tons.
- The steel embedded inside the concrete piers of the open caisson foundation weighs around 30 kilo-tons.
- Thus, the total weight of the piers is around 175 kilo-tons, which is massive to resist any kind of load combination.
The two main cables of the Mackinac Bridge were providing 30 kilo-tons of tensile force on the anchorage block of the bridge. However, the weight of concrete in the anchorage block was more than 170 kilo-tons, which was providing a factor of safety of 5.5.
5. Aerodynamics Stability of the Superstructure of the Mackinac Bridge
By scientific design, utilizing the new knowledge of suspension bridge aerodynamics, the Mackinac Bridge has been made the most stable suspension bridge, aerodynamically, that has ever been designed.
The main span at Mackinac is a suspension bridge, which is inherently the safest possible type of bridge. The stiffening trusses are 38 feet deep, or 1/100th of the span length. This ratio is 68 percent greater than the ratio of the Golden Gate Bridge. Even without this generously high depth ratio, the Mackinac suspension bridge span would have more than ample aerodynamic stability.
This result was achieved, not by spending millions of dollars on building the structure (in weight and stiffness) to resist the effects, but by scientific design of the cross-section to eliminate the cause of aerodynamic instability. The vertical and torsional aerodynamic forces tending to produce oscillations are eliminated.
An important feature contributing to the high degree of aerodynamic stability was the provision of wide-open spaces between the stiffening trusses and the outer edges of the roadway. The trusses are spaced 68 feet apart, and the roadway is only 48 feet wide, leaving open spaces 10 feet wide on each side for the full length of the suspension bridge.
For further perfection of the aerodynamic stability, the equivalent of a wide longitudinal opening was provided in the middle of the roadway. The two outer lanes, each 12 feet wide, were made solid, and the two inner lanes and the center mall (24 feet of width) were made of open-grid construction.
In addition to the design features, yielding of steel assured aerodynamic stability because maximum torsional stability has been secured by providing two systems of lateral bracing, in the planes of the top and bottom chords, respectively.
No modification of the design had been found necessary or desirable. The wind-tunnel tests conclusively showed that the Mackinac Bridge design has:
- Complete and absolute aerodynamic stability against vertical oscillations at all wind velocities and all angles of attack
- Complete and absolute aerodynamic stability against torsional oscillations at all wind velocities and all angles of attack
- Complete and absolute aerodynamic stability against coupled oscillations (combining vertical and torsional) at all wind velocities and all angles of attack
The total length of the Mackinac bridge is 8 km, and the height of bridge is 168 m.
The total weight of the Mackinac bridge is approximately 1.5 million tons.
The construction of the Mackinac bridge started on 7th May 1954.
D.B. Steinman was the design engineer of the Mackinac Bridge.
The Mackinac Bridge was constructed at a cost of USD 100 million.
The Mackinac Bridge was constructed to reduce the travel time between upper peninsula and lower peninsula of Michigan state. To cross the Mackinac Strait, the ferry rides used to take more than 1 hour. However, after the construction of the Mackinac Bridge, the crossing time was reduced to 10 minutes.
Open well foundation was used to construct the piers of the Mackinac Bridge.
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