At 26,372 feet (8,038.2 m) in length, the Mackinac Bridge is a suspension bridge spanning the Straits of Mackinac, where it connects the upper and lower peninsulas of the U.S. state of Michigan. According to the Mackinac Bridge Authority, it is North America’s third-longest suspension bridge. But in all the years since opening in 1957, the bridge’s main towers had never been completely cleaned and coated, owing to access difficulties and harsh regional conditions.
As difficult as the work appeared, approximately 60 years of use meant that maintenance could no longer be put off. Yet at the same time, officials with the bridge authority couldn’t afford extended closures to traffic. Fortunately for them, nearby coatings contractor Seaway Painting, LLC — based in Livonia, Mich. — had a plan and proved ready for the challenge.
“We developed a patented leg and strut containment system to provide a safe environment for our employees, and for traffic crossing the straits of Mackinaw,” said Steve Vlahakis, owner of Seaway Painting. “The Mackinac Bridge is the most challenging bridge we have ever done, and we have worked on bridges all over the country.”
Typically, Seaway Painting has up to 100 employees, and they needed approximately 20 at a time for work on the two 552-foot-high (168.3 m) towers. Personal protective equipment (PPE) for crew members consisted of RPB Nova 2000 blast hoods, 3M full-face respirators, and full-body harnesses from Miller Fall Protection.
In all, more than 350,000 square feet (32,516.1 m2) of structural steel needed to be coated for each tower, which meant that this wasn’t a job that could be completed quickly. Yet, before they could even begin with preparing the surfaces and coating, Seaway needed a plan for access and containment. In doing so, they worked with other contractors on the development of a movable scaffold system, designed by Ruby+Associates of Bingham Farms, Mich., and fabricated by Moran Iron Works of Onaway, Mich. These custom-designed platforms allowed crews to enclose and move up and down the towers above the roadway deck. The platforms encircled the tower legs and allowed workers to adjust it to accommodate tower tapering near the top, while another platform allowed access to the struts joining the tower legs.
All working areas on the platforms were fully contained with parachute-fabric wrapping. Because the bridge’s original paint was lead-based, Seaway had to contain all paint as it was removed before shipping it to
a landfill facility. Engineers with the bridge authority limited each working area to 20 feet (6.1 m) at a time to avoid putting excess weight on the bridge.
Full containment in the Great Lakes region presented additional challenges. “The containment system had to be designed to withstand 100 mph [160.9 km/hour] winds,” said Vlahakis, who estimates that winds were usually 10-to-15 mph (16.1–24.1 km/hour) faster on the bridge’s road span than they were on land.
Other prominent considerations included roadway traffic and seasonality. “There was a four-to-five month working window [each year] due to weather,” Vlahakis said. “Traffic was another major concern. If there was heavy traffic, our lane closures had to be taken up, and our equipment removed. We had a night shift in order to deal with less traffic and keep on schedule.”
Ultimately, the meticulous planning by Seaway proved essential. In all, the contractor estimated that it was able to cut its working hours by more than half when compared to using a conventional scaffolding system. “Despite facing challenging weather and traffic, our patented leg platform system allowed for our workers to work safely and adhere to MDOT [Michigan Department of Transportation] 100-percent containment restrictions,” Vlahakis said.
Working section by section, crew members navigated the platforms to apply a zinc-epoxy-urethane coating system to all areas of the steel towers.
First on the application agenda, of course, was surface preparation. An 8-ton (7,257.5 kg) Schmidt blast pot by Axxiom Manufacturing, powered by a 1,800 cfm (51.0 m³/min.) air compressor from Atlas Copco, was utilized for abrasive blasting, with GMA garnet abrasives acquired from BlastOne used as the blast media. All areas were blasted to achieve the NACE No. 2/Society for Protective Coatings (SSPC) Surface Preparation (SP)10 standard for a near-white metal blast cleaning, while a 45,000 cfm (1,274.3 m³/min.) dust collector from ARS Recycling Systems helped to keep the area clean.
After each area was prepped, crews advanced to applying the multi-layer coating system. Using 64:1 airless paint spraying units from WIWA, the coating system from Carboline began by applying the Carbozinc 859 zinc-based primer at approximately 4 mils (101.6 microns) dry film thickness (DFT), followed by the Carbomastic 615AL aluminum-filled epoxy at 5–10 mils (127.0–254.0 microns) DFT in any areas where a mastic was required.
Once primed, the final two layers on the towers consisted of an intermediate coat of the Carboguard 893 epoxy at roughly 9 mils (228.6 microns) DFT, followed by an ivory-colored urethane topcoat of the Carbothane 133 VOC of at least 1 mil (25.4 microns) DFT. “The towers are ivory, and the structure itself is foliage green,” Vlahakis said. Thanks to the work of Seaway’s crews, the towers should remain that way for quite some time.
With periodic maintenance, the bridge’s new paint system is expected to last for at least 35 years, according to the bridge authority. The south tower was completed during working windows in 2017 and 2018, while crews progressed to the north tower — using the same procedures and processes — in 2019 and 2020. “It’s a team effort with everybody involved, including the Mackinac Bridge Authority,” Vlahakis said. “They stay on top of things.”
So far, the work of the Seaway crews has been met with rave reviews from the client and the industry at large. In particular, the access platform has been viewed as a major breakthrough, with various awards presented from the Structural Engineers Association of Illinois; SSPC: and the National Council of Structural Engineers Associations.
“We love the Mackinac Bridge, and it’s been an honor to work on that project,” Vlahakis concluded. “It’s an icon for Michigan, truly.”
Editor's note: For further analysis of this unique project, listen to our exclusive podcast with Steve Vlahakis, owner of Seaway Painting.