The Coronado Bridge Zipper

The Coronado Bridge is a five lane bridge with a concrete zipper running down the center lane. While local Coronado residents and visitors take it a bit for granted, it really is quite an engineering feat.

History:

The zipper was installed over a weekend in April 1993 by the designers, Barrier Systems Inc. and performed its first run on April 8, 1993 during a dedication ceremony.

The cost of the materials for the zipper, its installation and the cost of the two transfer machines that zip and unzip the zipper totaled about $8 million dollars.

Statistics:

·The transfer machines each weigh 25 tons and cost $500,000 each.
·They are equipped with 900-pound tires made for mine-excavating machines.
·They are powered by 350-horse Cummins diesel engines.
·They travel at approximately 3mph.
·The zipper is comprised of 1.6 miles of concrete segments.
·Each 3-foot segment cost $250 & weighs 1,400 pounds.
·The concrete segments of the zipper have a total weight of over 3.5 million pounds.
·The 12-foot lane change is completed in 25-45 minutes.

Pulling the Zipper:

The zipper is made up of concrete segments 2 feet wide by 3 feet long each linked to adjoining segments with large steel pins. These concrete segments are “zipped” and “unzipped” by two transfer machines working in tandem.

Five days a week, twice a day, the transfer machines work their magic on the barrier. The front machine moves the segments 6 feet across the middle lane and the second machine moves it another 6 feet to complete the lane change direction.

Inside each transfer machine, the dashboard looks like a simpler version of an aircraft cockpit. There is also a TV monitor that shows the view of the machine behind. The machines are guided automatically by radio wires embedded in the concrete.

Even though there is a green autopilot button, the drivers have to monitor the speed of the machine in fractions of miles per hour. They also have to adjust the amount of pull or drag on 3-foot “capstan wheels” under the machine that keep the barrier segments from stretching on uphills and downhills.

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