One of the US’s biggest tandem lifts came from running late. Site problems and manufacturing hang-ups had left the contractor Landow & Company two months behind on a project working on the a new aircraft hangar at the Dulles Jet Centre at Dulles airport, near Washington, DC.
If we could somehow engineer lifting the entire roof section in one piece, it would get the project on track,” says Dale Riddle, regional manager for the crane subcontractor, W. O. Grubb. The EagleSpan steel-truss roof was originally intended to be lifted a section at a time.
“We looked at our experience, combined from steel erectors and crane rental people, and what cranes were available, and what could go wrong, and what we could do to prevent it,” he says. “We decided that it was very doable. It was very involved and critical, but it could be done.
“We were originally approached with a weight of 730,000 lbs (331t). We looked at the site to see where we could position the cranes, and looked at the structure with the help of the manufacturer to see where we would hook on and the working radiuses.
“Naturally, one obvious thing was how to keep these cranes coordinated so that the load was lifted level and plumb, and we don’t overload any particular crane. Before we had relied lots on LMIs for multiple crane blind picks, so we thought that we would use LMIs to help us.
“Basically we took what we usually do for multiple-crane lifts and multiplied it – more people, more communication, more meetings and test lifts,” Riddle says. That included monitoring the weather – the lifts eventually took place in late autumn 2005 – and increasing the crane and rigging safety factor from 85% to 75% – standard practice for multi-crane lifts, he says.
It took about two weeks for sales manager Sam Fattori and Grubb’s critical lift team to plan the job. The company initially specced eight cranes, ranging from 365 US tons to 220 US tons capacity, to lift the roof in two parts. It mobilised two safety supervisors, Riddle, Fattori, an operations manager, crane operators and three apprentices to the site. Steel erector Pinnacle rigged slings supplied by I&I Sling. The airport authority required that the crane booms be kept under 90ft (27m) high so not to interfere with aircraft.
“Each of the crane operators had a radio. We designated one signal man. He flagged the cranes via radio and was in sight of the cranes. He was also in radio communication with two spotters, one on the short side and one on the long side to ensure the roof went up plumb.”
First, they started with a test lift. And the job provided two surprises. “What we discovered as we started to hook onto structure to pick it was that the pick points previously engineered would carry the load, but they would not carry the load without the structure flexing,” he says. Worried about the load flexing, they added another two cranes – a hundred tonner and an eighty tonner, not to bear the load, but to make the structure more rigid where it was sagging.
“Once we made the test lift, we monitored and documented the weight and radius of each crane, and communicated it to all crane operators, to continue to maintain a constant weight and radius,” he says. By watching their LMIs, the operators would make sure that their crane was not overloaded – because if one crane fell, they all would. Although the team had calculated on paper how the load would hang on each of the cranes, they did not know exactly until the load was in the air.
And this was the second big surprise: the total weight of the roof section was only 430,000 lbs (195t), nearly half of what they were expecting. Riddle explains: “All the manufacturers had to go on was the weight of steel per foot. Then they had to add in bolts, nuts, shim plates and purlins to tie it in. As it got to figuring the thing out, I don’t think they wanted to be too close to the chart,” he says, so the manufacturer’s engineers added in extra weight for safety.
Although the job had over-specced craneage, finding smaller-capacity replacements would have caused more delays, Riddle says, so the customer stuck with the configuration.
The roof was assembled on the concrete pad of the hangar in two pieces. Cranes were positioned along each of the long sides, at the front and rear of the hangar. Columns supporting the roof had already been erected at the rear of the building about every 30-40ft. The front was to be left mainly open for access doors, apart from a central column. Two steel trusses attached to the column would help support the roof spans.
The cranes raised the roof up from the deck and held it at its 60ft final height while workers bolted it into place at the rear and the side. Then another 30 US ton capacity rough terrain crane lifted the front column into place. This was tricky, because there was no space to lift it from above – the roof was above it. The steel erectors slung the several-tonne column so it hung in at an angle, bolted in the bottom and then pulled it into line with chain tensioners. Once the column was up, two middle and two end cranes were cut loose and lifted the trusses up into place from cribbing where they had been assembled.
Despite the meticulous planning, the job was not quite that straightforward – once the roof section was up, the erectors realised that there was a manufacturing problem with the way the centre column and front trusses tied in together. On-site modifications dragged on for hours. “We were hooked on to it probably a total of 24 hours until it was completely tied in and the cranes were taken down,” Riddle says. The company needed to call in relief operators to take over the cranes as they worked into the night. The second roof lift took about 18 hours.
Although his team had looked for a window of good weather, Riddle had even considered what would happen if it started to snow, or they faced other weather emergencies. They had already had to delay the test lift by two days because of worries about snow loading. “The plan was that we would bring it down as quickly and as carefully as we raised it up. When it was partially tied-in, then we would have to make a judgement call on how quickly to tie it in or remove it from the columns and bring it down to the deck.”
