Two recent projects for helicopter hangars on U.S. military bases, one in Colorado and one in Kentucky, offer examples of how out-of-the-box thinking about crane design can overcome some big construction obstacles.
It took three years to design and construct a $750,000, 35-ton overhead bridge crane for the 88,000-sq-ft aircraft maintenance hangar at Fort Carson Army Installation in Colorado Springs.
The project’s design-builder, Hensel Phelps of Greeley, Colo., drove the team’s vision for the 13th CAB, ASB [Combat Aviation Brigade, Aviation Support Battalion] hangar with the ability to service as many as 14 helicopters in the hangar bay.
The project began in January 2012 and wrapped up at the end of 2014. “It was the most challenging design of my career,” says Tony Strobl, CEO of Cincinnati Crane & Hoist, the project’s crane contractor.
The job entailed designing a crane that could traverse the hangar’s entire 115-ft ceiling span smoothly and evenly, while taking into account the allowable variances in the steel support structure and the integrity of the building—all while working around a 160-ft opening on one side of the hangar. The result is a 350-ton, 115-ft span double girder underhung on one end and top running on the other end.
“With this project being a net-zero facility, there wasn’t a lot of extra head room [no wasted volume to heat/cool] in the ceiling,” says Hensel Phelps’s John Naccarato, area superintendent.
“Custom coping of the trolley beams was performed on the 35-ton crane to allow the MEP systems to be installed at the design elevation above it. The ASB Hangar project performed airfield apron paving concurrent with the installation of the cranes, which required large mobile support cranes to enter the hangar bay during installation. Overall, the design build team did a tremendous job,” he adds.
“The load requirement was 3.5 times greater than the typical 10-ton load customary on these types of projects, and the reality of a 35-ton crane was in question for quite some time,” explains design manager Blake Hoskisson, whose firm, Steel LLC of Atlanta, along with Brittingham & Associates of Norcross, Ga., the structural engineer, designed and fabricated structural steel for the crane. Hoskisson is currently the president of SunSteel, Vancouver, Wash., a company bought by Steel LLC.
“The design loads are much higher than we usually encounter on a long span structure, so that proved challenging for the designers. As a result, we had to pay special attention to the preparation of the bottom chord of the truss where the crane underhangs its bottom flange,” Hoskisson says. “We also had to coordinate the truss camber with Cincinnati Crane, as you run the risk of creating too much slope for the crane.”
“Normally, at a 35-ton capacity, you’d be able to support the crane with columns or other structures so that the span of the big opening isn’t so great,” Strobl says. “However, the runway beam the crane is hanging from had to span this whole distance. To have those two elements in combination on a crane of that capacity is something I’ve never seen; we couldn’t find any existing designs that came even close.”
Cincinnati Crane developed an anti-skew system that incorporated photo-eye sensors to ensure the 35-ton crane would travel at the same speed and track true on both sides of the runway even though the wheel-to-surface condition was dramatically different. This addressed the extreme span and the large, allowable deviations of steel sections as well as the possibility of camber changes with wind and snow loading.
“This was a one-of-a-kind solution that required innovative thinking while keeping safety in mind for the soldiers using the crane,” Strobl says. A veteran of the Army National Guard, Strobl employs a number of veterans and reservists who “strive to give the military a piece of cutting edge equipment that’s just right for them.”
Height Restrictions
In early 2013, while the Fort Carson project was well underway, Steel LLC was pursuing a project to construct a five-ton, 165-ft span crane for an unmanned aerial support hangar at Fort Campbell, Ky.
The already-in-progress hangar featured a tight space, both horizontally and vertically, explains Joe Scappaticci, senior project manager for Walsh Construction’s regional office in Detroit, the project’s general contractor. Hoskisson referred Walsh to Cincinnati Crane and Hoist, thinking the firm’s experience with the challenging Fort Carson design would come in handy.
“We were limited to 80 inches due to building height restrictions with the flight line at the top and minimum clearance requirements for the aircraft at the bottom,” Hoskisson says.
With a span that long, the crane is typically very deep—and so is the height of the girder. This extremely low-profile design, combined with support and stability issues at that length, presented another difficult crane design.
Cincinnati Crane and its affiliate, Diamond Construction, also of Cincinnati, worked with Walsh Engineering Services of Idaho Falls, Idaho, along with the Steel LLC and Brittingham teams to fit the crane in a too-short envelope. The result is a much-wider-than-conventional box girder design, which increased the weight of the steel girders. The additional weight was spread out along the runway with an innovative end-truck design that dispersed the weight of the crane over several columns. That kept the foundation reactions within the originally designed loading scheme, Strobl says.
Approximately 175 Lejeune torque bolts were field installed as temporary support for the heavy steel elements during construction. This amounted to approximately one bolt every 10 in. on a crane more than half the length of a football field, Strobl says.
“The Lejeune bolts did not produce the connection stiffness necessary to keep the steel support structure within allowable deflection tolerances, so we had to come back with mobile lifting equipment and lift our girder above the positive camber point and then weld the steel plates together to allow the camber to relax to its dead load status,” Strobl adds. “After the weld was applied, we were within design tolerance and good to go for final live-load and dynamic-load testing.”
Design and construction of the $250,000 Fort Campbell crane was completed in October 2014.
