By: Allen Zeyher Managing Editor
Jumping off a bridge is a difficult way to commit suicide when the bridge is less than 20 ft above the water and the water is only 8 ft deep. It seems unlikely that Suicide Bridge in Secretary, Md., got its name because it was a favorite spot for jumpers. It seems more believable that it got its name because long ago it was only one lane wide and so long that if two cars met in the middle, they would have to crash into each other or drive off the edge.
Suicide Bridge is now two lanes (28 ft) wide. It stretches 608 ft long across Cabin Creek in 32 spans, a timber bridge supported by timber piles. It was replaced recently, primarily because the timber piles were deteriorating.
“The existing bridge was timber, and we replaced it with timber. It just looks good there,” Robert Tenanty, P.E., administrator, Department of Public Works at the Dorchester County Highway Department, told Roads & Bridges.
The design of the new bridge is basically the same as the design of the previous bridge, built in 1968 to replace an even older bridge. The timber piles of the 1968 bridge were gradually failing, and traffic on the bridge was restricted to vehicles lighter than 3 tons.
“The timber piles were being eaten away by worms and various other bugs,” said Tenanty. “Basically, the timber piles that were supporting the bridge were failing.”
Part of the problem was a few mild winters in the area. “Typically a cold snap will kill off the insects,” Tenanty said. “Unfortunately, we had a couple of warm winters.”
As an added measure of protection for the piles, the timbers for the new bridge are pressure treated with copper naphthenate as well as being creosoted like the old bridge.
“We specified a preservative treatment that the timber industry uses for most of their marine applications,” Mike Blair, P.E., a senior associate at Johnson, Mirmiran & Thompson (JMT), Sparks, Md., and project engineer for the Suicide Bridge replacement, told Roads & Bridges. “In this case we went with a dual treatment, which is a creosote and also copper naphthenate. For marine applications, especially in saltwater conditions, it gives you a real extra protection.”
Suicide Bridge crosses Cabin Creek where it enters the Chesapeake Bay, so the piles are at the border where fresh water mixes with salt water.
From old to new
The bridge location, where Cabin Creek meets the Chesapeake Bay, is a flat, low-lying coastal landscape with areas of forest.
“Anytime you get near the bay in this area in Maryland, it’s always a little tricky with the environment,” commented Blair. “They prohibit in-stream work between Feb. 15 and June 15 for certain species” of fish and marine plant life.
The contractor for the replacement decided to use a method suggested by the engineers, a method that used the old bridge as a platform for constructing the new bridge on the same alignment. JMT designed the new pile bents to be driven every 19 ft halfway between the old pile piers, which also were 19 ft apart. Each pile bent consists of four driven piles with a pier cap supporting the timber superstructure.
At the position of the new pile bent, the contractor, E.A. & J.O. Crandell Inc., West River, Md., cut a hole in the old deck and drove the new piles. A small crane held the pile while a drop-hammer drove it into the muddy ground. All the construction equipment needed for the job stayed on the bridge, off of the land and out of the water.
“By doing that they were able to lessen the impacts on the creek and environment,” said Blair. “We had a staging area for a barge that was on the side of the bridge. They didn’t have to use any of that area. They were able to leave the land the way it is. That was one of the challenges: how to do it without disturbing too much of the marine environment.”
When all the piles were driven, Crandell placed new pile caps, tore out what was left of the old deck and put in the new deck in 19-ft sections.
The piles had to be driven about 70 ft deep to get enough load capacity to support the bridge and its traffic. The coastal soil in the area is relatively soft, and there’s no bedrock in reach.
“The depth of [the piling] was really a function of the geotechnical investigation,” said Blair. The first 20 ft below the mud line is really just muck and mud. Because we had to go so deep, we had to go with a fairly big-diameter pile [14 in.].”
The new Suicide Bridge opened to traffic in August of this year capable of handling all vehicles. The replacement work started in December 2004, according to Tenanty. Blair said the bridge saw about 563 vehicles per day in 2002 and is projected to see about 720 vehicles per day in 2020.
The new Suicide Bridge will be inspected every two years to detect any deterioration of the structure. The dual-treated timbers should have a lifetime of at least 30 years, said Blair, and could last as long as 40 years.
