The Mackinac Bridge in Michigan is a remarkable example of a bridge that achieves ideal, assured aerodynamic safety while maintaining economy and graceful proportions. The bridge’s design, featuring a main span of 3800 ft., was scientifically predetermined in its final form, without the need for years of groping, cut-and-try experimentation or successive modifications to overcome aerodynamic instability. Notably, the Mackinac Bridge was the longest suspension bridge at the time of its construction, showcasing its innovative and pioneering design.
Facts and Figures:
The bridge, including its approaches, has a total length of 8 km. The substructure, which consists of anchorages, piers, and foundations, required approximately 336,000 cubic meters of concrete, with 268,000 cubic meters of it being placed underwater. The steel superstructure, including cables, structural steel, and roadway, has a total weight of 66,500 tons. A total of 33 spans were constructed on 34 piers, with two main piers reaching a depth of 70 m.
Mackinac Bridge
The United States Steel Corporation was awarded the contract for the substructure of the Mackinac Bridge for a staggering USD 25 million, making it the single-largest contract in the history of the company at that time. Additionally, the contract for the superstructure amounted to USD 45 million, further solidifying the project’s immense scale and significance in the field of bridge engineering.
The construction of the Mackinac Bridge also broke a world record for underwater concrete placement using the innovative prepakt method. In just 24 hours, a single floating plant successfully placed an astounding 4800 cubic meters of concrete in the bridge’s foundations underwater, setting a new benchmark for underwater construction. This remarkable achievement highlights the cutting-edge technology and engineering expertise employed in the construction of the Mackinac Bridge, further cementing its status as an iconic engineering marvel.
1. Need for the Bridge
The state of Michigan is divided by the Strait of Mackinac, which is 4 miles wide and connects Lake Michigan and Lake Huron. The Lower Peninsula, spanning 41,700 square miles, is densely populated and highly industrialized, while the Upper Peninsula, covering 16,500 square miles, is rich in natural resources and has the potential for attracting more population and industrial activity with further development.
The Upper Peninsula stretches for 400 miles, equivalent to the combined area of four New England states. Its main industries at the time were forestry, mining, agriculture, and recreation, and it was particularly known as the Copper Country. The region also attracted tourists and sportsmen from various states for activities such as hunting, fishing, camping, sailing, and winter sports, making it a popular vacation destination.
The Mackinac Bridge was built to replace the existing state-operated highway ferry system, providing a direct and time-saving connection between the two peninsulas of Michigan throughout the year, regardless of weather conditions. This project is widely recognized for its contribution to the development of the Upper Peninsula, improving accessibility and connectivity in the region.
Mackinac Bridge constructed to connect lower peninsula and upper peninsula Michigan state
The crossing time for the 5-mile ferry route can take over an hour, and during the busy summer season, waiting in line for the ferry can add another three to four hours to the total travel time. In peak times such as holidays and deer-hunting season, cars have been known to wait in line for as long as 14 to 17 hours, causing queues of waiting cars to extend along the highway for up to 20 miles from the ferry terminal. However, the introduction of the Mackinac Bridge has greatly reduced the crossing time to just ten minutes, saving travelers the time previously lost in waiting in line for the ferries. This has significantly improved the efficiency and convenience of crossing the 5-mile stretch, especially during peak seasons, making travel much faster and smoother for commuters and tourists alike.
2. Timeline of the Mackinac Bridge Construction
In 1920, the idea of a submerged floating tunnel for crossing the Mackinac Straits was suggested by the Michigan highway commissioner. However, in 1928, the state highway department recommended a bridge instead. Unfortunately, due to the subsequent depression, the project was halted. In 1934, a bridge authority was created by the State Legislature, but the planning was interrupted by World War II in 1940.
In 1950, a new Mackinac Bridge Authority was established by the Michigan State Legislature. In 1951, a three-man Board of Consultants confirmed the feasibility of constructing the bridge. In January 1953, Glenn B. Woodruff was chosen as the design engineer by the authority, and preliminary contract plans and estimates were completed within two months. Substructure and superstructure contracts were then awarded to contractors for prompt commencement of construction.
However, delays caused by financing issues pushed the actual ordering of materials and mobilization of equipment to the spring of 1954. Over the next few months, a staggering $5,000,000 worth of floating construction equipment was assembled and positioned along the bridge’s construction line, making it the largest and most sophisticated floating equipment ever assembled for a construction contract.
Construction of the Mackinac Bridge
Excavation for the subaqueous foundations began on July 10, with a large workforce of over 750 men engaged in the work at the site. Due to the winter ice conditions at the Straits, which limit the normal working season to only eight months, the workers were putting in long hours of 20 to 24 hours a day to make progress.
3. Geologic Factors Affecting Design and Construction
The design of the construction was influenced by various geologic factors. Firstly, the effects of wind and currents on piers, towers, and supporting cables were taken into consideration. This was important because these structures needed to be able to withstand the force of these natural elements in order to remain stable and functional.
Secondly, the effects of ice pressure and ice push were also considered. This was because the construction was being built in an area where ice was a common occurrence, and the pressure and push of the ice needed to be accounted for in order to prevent any damage or disruption to the structure.
The thickness and character of the unconsolidated glacial deposits flooring the Straits were also taken into account during the design process. This was important because the construction needed to be able to support its own weight and withstand any potential movements or shifts in the ground beneath it.
Finally, the geological structure of the bedrock in which the footings would be emplaced was also considered. This was because the footings needed to be placed in a stable and secure position in order to support the weight of the entire structure. Therefore, the geological structure of the bedrock was carefully analyzed and taken into consideration during the design process.
3.1 Tides and Currents
Based on the information provided, the Straits appear to have a relatively low average volume of water flow, and currents are not typically significant. However, maximum currents can occur due to seiches or prolonged high wind speeds in a particular direction.
In 1939, brief observations suggested that the maximum current velocity through the Straits was 1.9 miles per hour. It was also believed that, except for occasional seiches, an absolute maximum velocity of 4 miles per hour might be expected.
It’s important to note that this information is based on observations from 1939 and may not reflect current conditions. It’s possible that further studies or more recent data may provide different insights into the currents and water flow in the Straits.
3.2 Wind
The highest sustained wind velocity that had been recorded prior to the construction of a certain bridge was 78 miles per hour. To ensure the safety of the bridge, wind-tunnel tests were conducted on a sectional scale model. The tests showed that at a sustained wind velocity of 100 miles per hour and with zero-angle attack, the wind force or drag acting on the bridge would be 670 pounds per lineal foot, while the upward vertical lift would be 50 pounds per square foot of bridge.
To account for potential wind velocities beyond 100 miles per hour, the bridge was designed with a safety factor equal to 960 pounds per lineal foot of bridge. This corresponds to a sustained wind velocity of 120 miles per hour. By incorporating this safety factor, the bridge is expected to withstand stronger wind forces and continue to remain stable, thereby ensuring the safety of the individuals using it.
3.3 Ice
The Straits experience an average maximum ice thickness of 18 inches, although there has been a recorded deep-water thickness of 30 inches. Strong winds cause massive ice floes to accumulate in the Straits, which can pile up to a height of 50 feet or more. To ensure the stability of the foundations, they were designed to withstand both vertical dead and live loads, with a bearing pressure of 15 tons per square foot. This value was considered conservative for the type of rock involved, as it was based on loading tests.
For the superstructure, it was designed to withstand a wind pressure of 50 pounds per square foot. Pier design took into account the very severe assumption of ice that is 4 feet thick, with a crushing strength of 400 pounds per square inch. Even with this conservative assumption, the net bearing pressure increased to 25 tons per square foot, ensuring the stability of the structure.
3.4 Geologic Structure
The Mackinac Straits are located in the northern part of the Michigan Basin, where the rock strata strike in an eastward direction and dip southward towards the center of the basin. While the rate of dip of the beds increases with stratigraphic depth, the average dip was believed to be 50-65 feet per mile.
The dips of the St. Ignace and Bois Blanc formations were calculated to be 52 and 55 feet per mile, respectively, which are greater than the average dips for the Michigan Basin. These regional dips are thought to be the result of greater subsidence at the center of the basin during Silurian and Devonian times, compared to around the margins.
The rock units used in the construction of the bridge included various types of sedimentary rock such as Upper and Lower Silurian, Lower and Middle Devonian shale, limestone, dolomitic limestone, dolomite, chert, and thin evaporites.
4. Foundation Details of the Mackinac Bridge
The engineers initially had doubts about the rock formation underlying the Straits due to its unusual brecciated formation. However, through exhaustive geological studies, laboratory compression tests, and in-situ load tests conducted underwater at the site, it was established that the rock could safely support over 60 tons per square foot, which was four times more than the maximum load expected from various factors such as dead load, live load, wind load, and ice load on the bridge.
The maximum ice pressure ever recorded at the Straits field was approximately 21,000 pounds per lineal foot of pier width, while the greatest ice pressure achievable in laboratory-controlled conditions for theoretically maximum pressure was 23,000 pounds per lineal foot. Despite this, the Mackinac Bridge was designed to withstand five times more than the theoretically assumed ice pressure, with the piers designed for a hypothetical, impossible ice pressure of 115,000 pounds per lineal foot.
This level of safety exceeded what the engineers had initially anticipated, as the open caisson foundation was originally designed for a capacity of only 15 tons per square foot. However, the rock formation underneath the Straits was found to be capable of providing a much higher level of support, allowing for a robust and safe bridge design that could withstand extreme loads and conditions, including ice pressure beyond what had ever been recorded or theoretically assumed.
Foundation of the Mackinac Bridge
The safe foundation pressure for the design of the Mackinac Bridge was achieved by dividing the maximum possible ice pressure by a factor of 4, which resulted in an overall factor of safety of 20 for the piers against ice pressure. This is why the Mackinac Bridge is often referred to as the safest bridge in the United States. In addition to this, the piers of the bridge were further protected against potential ice damage with steel sheet piling, steel caissons, and armor plate.
The massive size and weight of the foundations of the Mackinac Bridge were designed to provide stability against severe wind reactions, ice pressure, and other possible loads or forces. Each pier had a diameter of 116 feet and was filled with concrete weighing over 145 kilo-tons. The steel embedded inside the concrete piers of the open caisson foundation added an additional weight of around 30 kilo-tons. This resulted in a total weight of the piers of approximately 175 kilo-tons, making them capable of resisting any kind of load combination.
The two main cables of the Mackinac Bridge provided a tensile force of 30 kilo-tons on the anchorage block. Moreover, the weight of the concrete in the anchorage block was more than 170 kilo-tons, providing a factor of safety of 5.5. This further ensured the stability and safety of the Mackinac Bridge against various loads and forces.
5. Aerodynamics Stability of the Superstructure of the Mackinac Bridge
The Mackinac Bridge, designed using the latest knowledge of suspension bridge aerodynamics, boasts the most stable aerodynamic design to date. The main span of the bridge is a suspension bridge, which is inherently considered the safest type of bridge. Notably, the stiffening trusses are 38 feet deep, making up 1/100th of the span length, a ratio that is 68 percent greater than that of the Golden Gate Bridge. This generous depth ratio ensures ample aerodynamic stability for the Mackinac suspension bridge span.
What sets the Mackinac Bridge apart is not its hefty construction, but rather the scientific approach used in the design process. The bridge’s cross-section was meticulously designed to eliminate the causes of aerodynamic instability. By addressing the vertical and torsional aerodynamic forces that tend to trigger oscillations, the Mackinac Bridge’s design effectively eliminates these factors, resulting in enhanced stability. This achievement was made without the need for excessive spending on additional weight and stiffness in the structure, and instead focused on eliminating the root causes of aerodynamic instability through innovative design techniques.
Cables supporting the main tower of the Mackinac Bridge
The high degree of aerodynamic stability in the suspension bridge was achieved through the provision of wide-open spaces between the stiffening trusses and the outer edges of the roadway. The trusses were spaced 68 feet apart, while the roadway was only 48 feet wide, leaving 10 feet wide open spaces on each side for the entire length of the bridge.
To further enhance aerodynamic stability, a wide longitudinal opening was incorporated in the middle of the roadway. The two outer lanes, each 12 feet wide, were made solid, while the two inner lanes and the center mall (24 feet in total width) were constructed with an open-grid design.
In addition to the design features mentioned above, the use of yielding steel ensured aerodynamic stability. Maximum torsional stability was achieved by providing two systems of lateral bracing in the planes of the top and bottom chords, respectively.
Lateral bracing system
Based on the results of wind-tunnel tests, it has been determined that no modifications to the design of the Mackinac Bridge are necessary or desirable. The tests conclusively demonstrated that the bridge design possesses complete and absolute aerodynamic stability against vertical oscillations, regardless of wind velocities or angles of attack. Additionally, the design also exhibited complete and absolute aerodynamic stability against torsional oscillations at all wind velocities and angles of attack. Furthermore, the bridge design demonstrated complete and absolute aerodynamic stability against coupled oscillations, which involve a combination of vertical and torsional movements, under all wind velocities and angles of attack.
FAQs
What is the total length and height of the Mackinac Bridge?
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The Mackinac Bridge is a structure that spans a distance of 8 kilometers. This suspension bridge connects the two peninsulas of Michigan, providing an important transportation link between the Upper and Lower Peninsulas of the state. The bridge was built in the mid-20th century and has since become an iconic landmark of Michigan.
One of the distinguishing features of the Mackinac Bridge is its height. The bridge towers above the water below, standing at a height of 168 meters. This height not only provides a stunning view of the surrounding area, but also allows for large ships to pass underneath without interference from the bridge.
Overall, the Mackinac Bridge is an impressive engineering feat that serves an important purpose for the people of Michigan. Its length and height make it a significant landmark and an essential transportation link for the state.
What is the total weight of the Mackinac Bridge?
The weight of the Mackinac Bridge, a suspension bridge spanning the Straits of Mackinac in Michigan, is estimated to be around 1.5 million tons. The bridge, which opened to traffic in 1957, is a popular landmark in the region and serves as an important transportation route between Michigan’s Upper and Lower Peninsulas. It is one of the longest suspension bridges in the world, with a total length of about five miles. Despite its immense size and weight, the bridge is designed to withstand the harsh weather conditions and strong winds that are common in the area. The weight of the bridge is distributed across its cables, towers, and deck, allowing it to remain stable and secure even in adverse weather conditions.
When did the construction of the Mackinac Bridge begin?
On May 7th, 1954, the construction of the Mackinac Bridge began.
Who was the design engineer of the Mackinac Bridge?
The Mackinac Bridge’s design engineer was D.B. Steinman.
What was the cost of construction of the Mackinac Bridge?
The construction of the Mackinac Bridge incurred a total cost of USD 100 million. The bridge, which is located in the United States, was built to connect the upper and lower peninsulas of the state of Michigan. Its construction involved extensive planning, design, and engineering to ensure that it was structurally sound and could withstand the harsh weather conditions in the region.
The Mackinac Bridge is a suspension bridge that spans the Straits of Mackinac, which is a narrow waterway that connects two of the Great Lakes: Lake Michigan and Lake Huron. It was completed in 1957 after several years of construction, and it has since become a significant landmark in the area.
The cost of constructing the Mackinac Bridge was substantial, but it was considered necessary to improve transportation in the region. Prior to the bridge’s construction, the only way to travel between the upper and lower peninsulas of Michigan was by ferry or a lengthy detour around Lake Michigan. The construction of the bridge reduced travel time and increased economic growth in the area, making it a valuable investment for the state.
What was the need of the Mackinac Bridge?
The purpose of building the Mackinac Bridge was to minimize the travel time between the upper and lower peninsula of Michigan state. Prior to the construction of the bridge, people had to rely on ferry rides to cross the Mackinac Strait, which took more than an hour to complete. However, with the construction of the bridge, the time taken to cross the strait was significantly reduced to a mere 10 minutes, thus providing a quicker and more efficient mode of transportation.
What is the type of foundation used in the Mackinac Bridge construction?
The construction of the piers of the Mackinac Bridge utilized open well foundation, a specialized technique. This type of foundation was specifically chosen for the project due to its suitability for supporting the weight and load of the bridge. Open well foundation was used to establish a solid base for the piers, which are critical structural elements of the bridge. This technique ensured that the piers were securely anchored to the ground and could withstand the environmental and load demands of the bridge’s construction and operation. The decision to employ open well foundation was based on its proven effectiveness in providing stability and durability for large-scale structures like the Mackinac Bridge.