By Kim Holtgrewe and Gary Johnson, Ceco Building Systems
Where do aircraft go for an oil change? The Des Moines International Airport in Des Moines, Iowa, offers a full service maintenance hangar—a smart solution. The facility provides overnight service for routine maintenance, allowing planes to arrive in the evening and receive aircraft service overnight. While the rest of the world sleeps, the hangar is a flurry of activity. Mechanics work all night to check navigational equipment, change filters, lubricate landing gear, change oil, and perform safety checks to assure safe craft roll-out for scheduled flights in the morning.
The $11.8 million hangar was built by the City of Des Moines and is being leased to Mesaba Airlines, a Northwest Airlines subsidiary. It serves as a regional maintenance facility to support Mesaba's fleet of 50-seat Bombardier CRJ200 and 76-seat CRJ900 aircraft.
The 80,000 square foot building, weighing 1,335,464 pounds, includes 65,000 square feet of open hangar space, large enough to accommodate up to four aircraft side-by-side at one time. The building includes another 15,000 square feet for offices as well as shops for parts and storage.
The hangar is created using 36-inch, 26-gauge ribbed metal MAP panels manufactured by Ceco Building Systems. The clear span structural steel framing provides wide expanses of open space with no need for interior support columns that would impair flexible plane and equipment movement within the facility. “This 246-foot clear span is possible because of the structural steel framing that allows the building weight to be carried by the exterior steel columns in conjunction with highly reinforced concrete footings, which are tied into the floor slab,” says Klint Kruse, project manager with Dean Snyder Construction.
The interior walls are Ceco’s MAP 26 gauge, versatile panels each providing 3 feet of lateral coverage. MAP panels have 1½-inch-deep major ribs, tapering in width from 2 inches to 3 15/16 inches, and spaced 12 inches on center. The entire 55-foot peak height of the walls is lined with 8 inches of insulation and 8-foot-tall liner panels bringing the wall system to an insulation value of R-25. “This is more insulation than is typical in walls,” Kruse says. “But it’s a large, open area to heat; and when you a pull a plane in a 184-foot-wide hangar door, that lets out a lot of heat too. The extra insulation can save energy and make the hangar more efficient.”
The building is capped with a Ceco CXP standing seam metal roof system. Interlocking standing legs at side laps were seamed together into a 360º Pittsburgh double-lock seam using Ceco’s electric seaming machine. The low slope roof, with a 1:12 pitch, has a UL-90 wind uplift rating—an important consideration in an environment where air traffic can create unusually high wind gusts. Eight exhaust fans, weighing 1,200 pounds each and placed in the roof, allow a huge makeup air unit to force fresh air into the hangar, complying with code requirements to offset airplane exhaust fumes. The building is also, equipped with “fall protection” for the maintenance workers.
Aircraft enter through the hangar door, measuring 182 feet by 34 feet. But the public entry to the hangar, on the opposite side of the building, is clearly marked by a metal radius entry canopy. Storefront doors and windows delineate the public space as well. However, the obvious need for high security is built into the design. The airfield/apron side of the building is not accessible by the public. All doors on the public side are wired to the airport terminal and monitored for security at all times. Access is allowed only for employees with security badges and pre-approved visitors.
Due to the nature of the work performed in this building and the value of the equipment it services, fire protection is a paramount consideration. The building includes fire pumps capable of jetting 5,000 gallons of water per minute through 12-inch supply lines to 900 sprinkler heads. “We had to add bracing to the sprinkler lines because of the force emitted by such great pressure,” Kruse adds. “The water can go from the fire pump room to the end of the building in just a few seconds.” The hangar is sprinklered at a rate greater than any other city building. The system relies on two pumps with a third unit provided for backup redundancy. The system also incorporates a fire foam system that unleashes five feet of foam over 65,000 square feet in just three minutes.
Completing such a large hangar in just twelve months was a challenge, particularly when considering the weight and function of the building.
“The magnitude of the structural steel and foundations was amazing,” comments Kruse. The foundation includes 10 inches of concrete with #4 rebar, 12 inches on center, each direction top and bottom. Initially, the design and construction team analyzed their alternatives to decide if it was best to build the foundation slab on grade first or set the steel. If they set the steel after the slab, they would have to drive cranes and forklifts onto the slab to erect the steel. The structural engineer confirmed that the footings were not designed to hold steel without the slab because reinforcement in the slab tied the footings together to carry the weight of the building. So, the team decided to raise the steel before pouring the slab but to put high-strength, half-inch wire cable and tie downs from column to column to secure them. These were left in place until the rebar was connected to the columns and positioned in the concrete, where it would then permanently stabilize and manage the horizontal forces.
Another challenge was how to raise the steel framing with such a massive clear span. This was painstaking work, allowing for completion of just one main frame per day. Each section ranged in height from 25 to 43 feet. “Imagine a five to six-foot-deep mainframe,” Kruse adds. “They are so deep from top to bottom that you could almost lay down in one.” The team decided to use two cranes to erect each mainframe. Essentially they put two main frame pieces together on the ground. Then, one crane would lift half, then the other crane would lift its half, and the two would be bolted together as one piece.
About 75% of the construction was complete when storms surged through Des Moines, bringing tornados, massive gusts, and winds clocked at 70+ to 80 miles per hour. At that point, nearly all of the structural steel was in place, beams standing with no roof. With wind slicing horizontally across the incomplete structure, damage was inevitable. After the storm, construction/demolition crews moved in to salvage the steel, repair the concrete, and to clear the site. The existing foundation and the original footings remained intact with the exception of the pier caps which were demoed out and the concrete repoured. All of the 2-inch-diameter column anchor bolts (36 inches long, approx. 35 pounds each) were cored out and reinstalled with very-high strength epoxy.
Once completed and strength-tested, the replacement of the steel framing was the contractor’s main focus. Temporary bracing was added to circumvent further problems. Cable was strung across every other bay and extra pipe struts were positioned for greater stabilization. Additional days of gusty winds and inclement weather came and went but did not cause further damage. Ultimately the event only impacted the project schedule by about two months because the steel manufacturer and the contractor worked as one team to resolve the residual challenges. “We became a Ceco builder after this project,” says Kruse. “They are easy to work with and accommodating.”
Success In The End
The year-long project resulted in a sleek and solid building that now serves aircraft daily. It is also a showpiece for Ceco who has a variety of aircraft hangar alternatives within its design offerings. And despite its challenges, Dean Snyder Construction is proud to have their first large hangar project successfully completed and functioning to the benefit of the Des Moines community.
About The Des Moines Airport
Des Moines International Airport served over 1.8 million passengers in 2008, amounting to about 336 operations per day. Over a dozen airlines offer service through Des Moines. Users include all major airlines, regional airlines, cargo airlines, general aviation planes, and military aircraft. The airport was created in 1933 as a sideline business to serve planes delivering airmail.
The airport is owned and operated by the City of Des Moines and governed by a seven-member board of Des Moines citizens appointed by their city council.
About the Authors: Kim Holtgrewe, project manager at Ceco Building Systems, can be reached at firstname.lastname@example.org; Gary Johnson, district sales manager at Ceco Building Systems, can be reached at email@example.com.