Mackinac Bridge Crews Replace Original Grating
By Karen Gould

Steel hooks secure Mackinac Bridge workers Joe Shampine (from left) of Cheboygan, and Cory Peterson and Conrad Becker, both of St. Ignace. On October 18 the men grind away old welding scars on bridge beams in preparation for a new deck grating section to be welded into place.

Anchored by multiple safety cables, members of the Mackinac Bridge maintenance crew straddled steel beams more than 175 feet above the frigid waters of the Straits to replace an original road grate Wednesday, October 18. The new grate will remain in place until the entire bridge deck is replaced in approximately 12 years.

The grates comprise the two inside lanes of the suspended portion of the bridge and serve to reduce the weight of the bridge deck. The Mackinac Bridge opened to traffic November 1, 1957, almost 50 years ago, and, while still safe, the grates are nearing the end of their life, said Bob Sweeney, executive secretary of the Mackinac Bridge Authority.

If the weather cooperates, 11 grates will be replaced this fall, but more likely, the job will be completed in the spring, he said. Wednesday's grate replacement was the fifth.

At right: With rain threatening and winds increasing from 22 miles per hour to 31 miles per hour in 15 minutes, the Mackinac Bridge maintenance crew remain focused on preparation work for a new deck grate in the southbound lane just north of the north tower. Bridge traffic was reduced to one lane in both directions until the new grate was installed. Working on the installation are (from left) Crane operator Roger Schwartz of Cheboygan, Jeff Fogelsonger of St. Ignace, Conrad Becker of St. Ignace, and (front) Cory Peterson of St. Ignace.

"This is done with our in-house workforce," said Mr. Sweeney. "We'll have replaced 40 sections in eight years. We developed expertise in-house and by doing it in-house, we've saved a substantial amount of money, as opposed to contracting the work out."

Depending on weather, replacing the grate is a two-day job.

Bridge traffic is reduced to one lane in both directions until the new grate is welded into place. A

crew cuts the old weld with an acetylene torch. A crane is used to lift the old grate from its bed on the deck, then the old weld is ground smooth. The new grate is set in place by the crane, and welders step in to fuse the grate to the bridge.

In the early years, routine maintenance was all that was needed to keep the bridge in top condition and delay corrosion, but after 50 years, annual inspections categorize the condition of grates and estimate their remaining life. Categories include generally good condition, moderate condition, and in need of replacement.

When the bridge was built, the grates were painted to protect the steel. The new replacement grates are made of galvanized steel, which resists corrosion and requires no painting. These new silver-colored replacement grates among the older green grates can be spotted by motorists crossing the bridge.

The original 752 grates that make up the two-lane span were initially given a life expectancy of 40 years, said Mr. Sweeney.

"Because the Mackinac Bridge Authority has such an extensive routine and preventive maintenance plan, we are able to extend the service life of this element of our deck system beyond industry standards," he said. "This is an example of the preservation type of work we do on the bridge to extend the life of the deck. We know that for every dollar we spend in restoration work, we save roughly $7 in future repairs and replacement costs."

The grating replacement costs approximately $7,500 per section, with $6,000 paying for each grate and the rest for labor and crane rental, said Mr. Sweeney. Grates will continue to be replaced over the next 12 years as inspections identify deteriorating sections. Mr. Sweeney noted that the grates are replaced well before the load-carrying capacity of the decking is compromised.

The Mackinac Bridge Authority estimates that the bridge deck replacement project, now scheduled for 2018, will cost about $190 million, or almost twice the $99.8 million to construct the entire bridge in the late 1950s.

The new deck may replace the open grate system with a new orthotropic decking system, Mr. Sweeney said. An orthotropic deck was installed on the Golden Gate Bridge in the 1980s, when it was approximately 50 years old. The Golden Gate Bridge, like the Mackinac Bridge, is a suspension bridge.

Orthotropic decking consists of steel plates supported by hollow steel ribs underneath. A driving surface is added to the top. Orthotropic decking, while one solid surface, is lightweight and, like the current Mackinac Bridge deck, which has two open-grated lanes and two asphalt lanes, is designed to withstand wind speeds up to 662 miles per hour, said Mr. Sweeney.

The highest wind speed recorded on the Mackinac Bridge was 124 miles per hour on May 9, 2003.

Other suspension bridges that use orthotropic decking are New York bridges Tri-Borough, which connects Manhattan and Queens, and Throgs Neck Bridge, which connects Queens and the Bronx, and the Alfred Zampa Bridge in Northern California.