Michigan Highways: Since 1997.

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M-22 & M-109 junction route signage in Glen Arbor, Michigan

The Zilwaukee Bridge: From the Beginning

The following pages reproduce a 1987 report issued by the Michigan Department of Transportation in response to public concerns about the safety of the Zilwaukee Bridge, then nearing completion. The text here is reproduced word-for-word from the original report and no alterations—grammatical or otherwise—have been made.


MDOT conducts literally thousands of tests and has devised numerous methods and devices to confirm the strength and structural integrity of the bridge, and its components, and to assure that it is being built according to design specifications. In addition to engineers and technicians at the bridge site, nearly three dozen specialists in MDOT's Materials and Technology Laboratory in Lansing have been assigned to various aspects of the project. Following are some examples.


Before erection of the bridge segments began, MDOT conducted load testing in one of the first segments to be fabricated. Inspectors had observed small cracks over and above the webs, or vertical portions, of the segments at the point where they joined the outside wing, or cantilever, of the horizontal deck.

The cracks were formed during the curing cycle of the concrete when the segments were still in forms. Engineers concluded that the cause was a combination of shrinkage and thermal expansion and contraction, and were to be expected on concrete pours of that magnitude.

It was decided to test the strength of the segments. Prior to the load testing, the transverse post-tensioning of the steel cables in all segments was increased. That had the desired effect of preventing the opening and closing of the cracks under live loads.

The load test consisted of placing 39 tons of concrete slabs on the outside of each of the two wings of the segment, simulating maximum loads that can be expected when the bridge is opened. Of the 52 locations where crack widths were monitored, seven showed a very small crack opening of .002 (two one-thousandths) inches or less when the maximum load was placed. The other 45 locations showed no crack width increase, even at maximum load. Added protection will be provided by the application of a layer of high-density latex-modified concrete to the bridge deck.


Load-testing a bridge deck segment. Thirty-nine tons of concrete slabs were placed on each of the segment 'wings.'


Next: Section 10: Conclusion & Appendix
Back to: Section 8: Bridge Safety
Back to: Table of Contents