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. |
SAFE TODAY & TOMORROW
It's only natural that people are concerned about the safety of the Zilwaukee Bridge. After all, it has received lots of publicity following the construction accident of 1982 that stopped work on the bridge for several years.
It's not suprising that people don't understand how safe the bridge actually is today because it is a highly technical subject. Few people know about the stresses and strains of building materials. And few people realize the extent of of built-in safety factors used in bridge design.
For those of you who may have concerns about using the bridge when it is completed, here are a few facts that may help you better understand why we are confident that it will serve the motoring public safely for many decades. Additional details appear on the following pages.
1. The 1982 construction accident was the result of human errors, not basic bridge design or building materials.
Too much weight was allowed to accumulate on one side of a pier while a concrete segment was being added to the structure. Until a span is completed between two piers and the segments become one strong unit that will carry traffic, balancing the weight of new segments as they area added is crucial. Imagine a teeter-totter with equal weight on each side. A little weight can be added alternately on each side while keeping it in balance. And that's the same idea that we're using to add segments to the bridge. But if you add too much weight to one side, you've got an overload...just as we did at the bridge.
2. Following its repair, the damaged pier and its footing are now the strongest on the bridge.
A very innovative and thorough engineering design for the repair was developed by state engineers working with some of the nation's top bridge engineering consultants. New concrete and steel columns six-feet in diameter were constructed about 90 feet deep to bedrock. A new concrete footing was built to completely surround the existing footing. The new footing is about three times bigger than the old one.
3. As a result of our added concern for bridge safety following the accident, outside engineering consultants are now assigned full time to the bridge and provide daily advice too both MDOT and the contractor building the bridge,
MDOT hired the engineering consulting firm that assisted with bridge repairs. MDOT's new construction contract required that the prime contractor also hire a consulting firm experienced in design and construction of segmental concrete bridges.
4. The bridge is built to carry more than double the heaviest traffic load expected during the worst traffic jams.
Imagine bumper-to-bumper traffic in all four bridge lanes with six feet between vehicles. Six of 10 vehicles are cars, and four (40 percent) are fully loaded 40-ton tucks. (Our traffic studies show that trucks actually represent only 10 percent of traffic on I-75 at the Zilwaukee Bridge.) Total weight of the vehicles you've imagined is less than half of the bridge's ultimate design traffic load. In addition to the traffic load, the bridge also is built to carry is own weight plus an additional one-third of its weight.
5. Right now, the bridge is getting its most severe safety test almost every day. Structurally, the bridge is weakest during construction when spans are only partially completed, yet it is now carrying loads far greater than it will ever carry when opened to traffic.
The huge steel girder equipment used to attach new segments to the bridge weighs 1,700 tons, and the tractor/trailer that carries segments to it weighs about 262 tons-a total of nearly 4 million pounds. To create the same weight in the same space on the bridge would require lining up 49 fully loaded 18-wheeled trucks and trailers bumper-to-bumper across four lanes.
Next: Section 2: Background
Back to: Letter from James P. Pitz, Director
Michigan Department of Transportation
Back to: Table of Contents