Preserving the Single Largest Transportation Asset in Michigan

Opened in 1957, the 26,372-foot-long Mackinac Bridge — “The Mighty Mac” — is the fifth longest suspension bridge in the world. It is the longest suspension bridge in the western hemisphere, with the third-longest suspended main span. Its towers rise over 550 feet above the Straits of Mackinac as it connects the upper and lower peninsulas of Michigan.

Four million vehicles cross the bridge annually. Approaching the bridge from the south on I-75, motorists can first glimpse Michigan’s crown jewel from miles away, as they prepare to cross its nearly 5-mile length spanning the straits.

Since 2021, HDR has supported the Mackinac Bridge Authority as general engineering consultant, supplementing the authority’s engineering staff with experts who perform inspections and contribute as-needed engineering design and construction support services to preserve and maintain this landmark structure for generations to come.

Preventive Care for The Mighty Mac

A regular program of inspections is a fundamental component of preventive care for The Mighty Mac. Annual inspections provide the MBA with a critical evaluation of existing conditions to report the structure’s overall condition. Inspections include a thorough evaluation of the bridge superstructure and substructure components, including the towers and anchorages, as well as unique elements of a suspension bridge such as the cables and suspender ropes.

In summer 2021, HDR’s eight-person bridge inspection team spent three weeks on the Mackinac Bridge. The team inspected connections and members of the bridge including the various superstructure framing configurations in the suspended spans, deck truss approach spans, and approach viaducts. The resulting inspection report identified bridge components in need of repair and prioritized recommendations for specific repairs and general maintenance.

A notable feature of the bridge is its open grid deck on the middle two lanes of the suspended spans roadway, which was designed to improve the aerodynamics of the structure. An open grid deck grating is more exposed to the elements than a solid deck and therefore requires a detailed condition survey to identify segments for repair or replacement. Much of the grid decking remains from the original construction, making it more than six decades old. Based on age and deterioration, the MBA has been replacing panel sections systematically. HDR’s team performed a detailed inspection of the grating that identified panel sections meeting replacement criteria.

Safety While Traffic Kept Moving

Summer is peak travel season for Michigan tourism and a very busy time for the Mackinac Bridge. Over 600,000 vehicles cross the bridge each summer month, so it was important for the inspection to occur without disrupting travelers.

The inspection workplan considered traffic patterns, other work on the bridge, and availability of staff and equipment. Inspection schedules adapted as needed to accommodate weather. HDR coordinated with another contractor, who was painting the south tower at the time, to keep motorists moving while our engineers worked safely.

Even with the traffic delays caused by needed lane closures for this inspection, our team was greeted with friendly comments of “Great job out here” and “Thank you for your work” from passers-by.

Inspection crews focused on safe access practices using standard guidelines for fall protection which require personnel to be 100% tied off when working at height. MBA staff escorted inspectors to access the structure safely, especially when inspecting the suspended spans and using the travelers, work platforms and catwalks. The MBA operated inspection equipment including a safety boat while crews worked on the spans over the water. The state’s “Reach-All” UB-62 was brought in for a portion of the inspection for special access locations.

Preserving Michigan’s Landmark

The Mackinac Bridge is Michigan’s single largest transportation asset. Annual inspections and assessments are key components of the work by the MBA to carry out its mission to provide safe, pleasurable and expedient passage over the Straits of Mackinac for economic benefit and improved quality of life.

HDR will continue serving the MBA through 2024, performing annual inspections and as-needed engineering services to help preserve this iconic suspension bridge.