For all the time it took to set up the demolition of the Jamestown Bridge in Rhode Island, people watching from the shore had to make sure they weren’t looking somewhere else when it happened.
“You could blink and miss it,” said John McNulty, general superintendent of Cashman Equipment of Boston, the general contractor of the project.
It took workers four months to prepare the bridge and it will take another six weeks to remove the parts of the through truss that fell into the water far below. But in between then it took just a matter of seconds for the 66-year-old bridge to go splashing into Narragansett Bay between Jamestown on Conanicut Island and North Kingstown on the mainland.
The middle part of the 7,000-ft. (2,121 m) bridge was successfully demolished precisely at 11 a.m. on Tuesday, April 18. A second controlled blast occurred on May 18. And approximately a dozen smaller controlled demolitions are planned sometime between July and October.
McNulty explained that several blasts are needed for two reasons: there will be a smaller mess to clean after each one; and this way engineers can work with the structure of the bridge to remove it properly.
“It depends on the age of the bridge,” McNulty said. “You can utilize the design of the bridge to help you.”
Only the through truss (the middle section), measuring 1,100 ft. (333.3 m), came down in the April blast. The two bookend pieces will come down next and the freestanding piers that held up the through truss will be detonated later.
The bridge has become somewhat of an eyesore in recent years. Built in 1940, it was replaced during the ’90s by a much more modern span just 100 ft. away. Even though it hadn’t been used for more than a dozen years, it stood as a reminder of how technology and design have changed.
Many local residents remember the bridge when it was used, but not all the memories were fond.
It rose 135 ft. (40.9 m) over the water with much of the drop visible through the open grating that made up the deck. It was only 26 ft. (7.9 m) wide, just enough for two lanes of traffic, but not enough for a breakdown lane. Whenever strong winds blew, it would shake the entire structure. Those who dared to cross it often held their breaths until they got to the other side. Others simply refused to use it at all.
Wind was about the only problem the weather had to offer during the preparations this spring. “The winds never stop in Narragansett Bay,” McNulty said.
It also was difficult to work over such a large body of water. Narragansett Bay is approximately 70 ft. deep in the channel where they were working. “Water is a challenge in itself,” he said.
Workers needed three consecutive days of good weather to get all the charges attached to the proper parts of the bridge.
“We don’t want to work one day and have four bad days,” McNulty said. “We’re trying to project things and be Mr. Weatherman.” If the process goes much longer than three days, problems may occur, he explained.
“The worst case scenario is that we have to go back. The longer they [the charges] sit up there, they will chafe and come undone,” he said.
That didn’t happen in this case. The three days ahead of the blast were nearly ideal for installing the charges. Demtech Inc. of Dubois, WY, was in charge of the actual explosion. The company has traveled around the world demolishing bridges, buildings and towers from the Virgin Islands and Jamaica to Korea and Kazakhstan in the former U.S.S.R.
Perhaps the company’s most unusual assignment has been to destroy 300 different Minuteman II nuclear missile silos. They also have demolished the largest lock and dam on the Mississippi River (Alton, IL) and the longest over deck truss bridge (Clays Ferry, KY).
In Rhode Island, workers installed 350 linear shape charges, weighing a total of 75 lbs. (33.75 kg). Thin pieces of copper pipe were wrapped around the major connecting points. The pipe then got hot enough to burn through the steel causing them to separate and fall away. While some people have called it an “implosion,” McNulty explained that that is not the correct term.
“It looks like someone took a torch and cut the beam. It is like a very hot torch,” he said. “It is different than an implosion because the bridge is falling straight down.”
The blast actually happened in a flash. One moment the bridge was there, the next moment it was gone, but not without a boom like a crash of thunder and a flash like a momentary fireworks display.
“You push the button, that’s the easy part,” McNulty said.
After most of the bridge had splashed into the water below, workers discovered that a couple pieces of steel had tangled and did not fall as had been planned. They were soon removed without incident.
“You can only plan so much. Some things are completely out of your control,” McNulty said.
The first part of the $20-million job actually took four months to complete with all of the concrete and steel grids removed beforehand. Joseph B. Fay Company of Pittsburgh, PA, was in charge of that. After the blast, it will take another six weeks to clean up all the remnants of the through truss from the water. The recovered steel will then be recycled.
Cashman Equipment will haul the large chunks of concrete to several nearby islands where they will be used as manmade reefs. There they will create habitat for fish and other underwater wildlife, according to McNulty.
Pulling the debris from the water is an immense job in itself. In order to do so, a 4100 Series Manitowoc Ringer 300-ton (272 t) crane is being used to lift all of the 2 million lbs. (900,000 kg) onto a barge.
“There is a lot of engineering going into taking the bridge down. We make it look simple, but there is a lot of planning needed,” he said. CEG