After constructing the 36-storey 1918 Eighth Avenue office tower in downtown Seattle, Washington, commercial general contractor Sellen Construction needed to take down a Liebherr 355 HC-L. Because of the tower crane’s close proximity to the building, it couldn’t lower itself down. The company hired a Liebherr 200 DR 5/10 to dismantle the crane and finish work on the roof.
While deciding to use the derrick, Sellen also looked into erecting a large mobile crane to take down the 355 HC-L. The cost and logistics were both unfavourable considering it would require closing a street for a few weeks in front of the federal courthouse, explains Scott Amick, senior project engineer for Sellen. That would also involve rerouting buses and taking down trolley wires. “It was next to impossible,” he says.
Planning for nearly a year and a half before the summer 2009 dismantle, Amick corresponded with Daryl Hicks, one of the project engineers at Morrow Equipment Company in Salem, Oregon, from which Sellen rented the derrick. Morrow had purchased the Liebherr 200 DR 5/10 with the 1918 project and several others in the Seattle area in mind, and this was its first job.
To prep the building for the derrick, Sellen created a temporary work deck. Then to raise the derrick high enough to complete the dismantle job, a rigid steel structure was built. Once installed, the derrick’s feet were bolted to the steel structure and that was connected to steel embeds cast on top of the building’s concrete core, explains Amick. All of these support pieces would be torn down and removed through the material hoist at the end of the project.
The tower crane’s last operational day was 31 July, and the dismantle would start the next day. In early July, Sellen installed the base of the derrick. “We just brought enough of it up there to make sure we could connect it to the structure and worked out any bugs,” says Amick.
Meanwhile, the rooftop was becoming more and more crowded with pipes, duct work and mechanical equipment being stashed to later be installed. “There’d be no way of moving them once the cranes were removed. So everything had to be up on the building before the tower crane came down,” he explains.
When it was time to assemble the rest of the derrick, the tower crane lifted it up to the roof. “We were able to erect it fairly quickly, in large components,” say Amick. “On the way back down it’s a whole different story when there is no tower crane to help you.”
While testing the derrick and doing final preparations, the tower crane had to finish as much of the exterior as possible, except anything affected by the cranes, which the derrick would do later.
The first section removed from the tower crane was 20ft of mast, leaving just the climbing frame to support the rest of the crane. Without this section, the crane can lower itself as normal by 20ft, says Amick. “At that elevation we can work on it and lower it on to temporary supports.”
Temporary beams with plastic on top were installed on the temporary work deck. Using cable pulls to control and unhinge the boom from the turntable, it was lowered on to these supports and dragged toward the north before being broken down into two pieces. “We moved it so far north that when they took half the jib off the remainder is stable on two supports,” Amick explains.
From there the boom was lowered to street level using two taglines on each end that ran to the street to stabilize the load. The boom was the longest and heaviest component lifted, says Hicks. “The turntable was fairly heavy too, but that was more compact, so it wasn’t as challenging.”
The 355 HC-L tower crane is designed to be dismantled into components capable of being taken down in an average jobsite material hoist, he explains. A 15ft wide aluminium gantry with a 12t capacity and a manual trolley was rented for the job, allowing the lifts to pick the components in larger pieces.
“So we were able to leave the hoist rope on and take the whole counterjib piece, the luffing gear frame, with the hoist drum attached to it,” says Hicks. “We did remove the motors just in order to get it into the material hoist.”
While dismantling the crane, the heaviest load was about 4,200lb without the rope, which added around 1,000lb more. There were 30 sections from the main tower crane, each travelling about 500ft to street level. The derrick was usable for roughly two weeks, one to dismantle the crane and one to finish other work on the roof.
With the stiff legs, the 200 DR 5/10 is limited to a 180° rotation, depending on the angle between the stiff legs. Once the tower crane was gone from the 1918 building, the stiff legs were removed from the derrick, allowing it to rotate a full 360°. “We didn’t have nearly as much capacity at that point, but it’s part of our planning,” says Amick. “What can we pick at certain capacities, with a certain rotation?”
Safety aside, one of the biggest concerns on the job was coordination with all the trades, Amick says, and making sure everything was done in the right sequence. “It was a tough job, but a very interesting one to plan.”