The Blimp Barns
moffett field historical society2006_1_20In an industrial park two miles south of the waterfront town of Tillamook, Oregon, stands an enormous wooden structure—more than 1,000 feet long and nearly 300 feet wide—that dwarfs all the other buildings in the area. Today it houses an aircraft museum, but it was built for another purpose: It is an airdock dating from World War II. Commonly known as a “blimp hangar,” it is one of two that were built at what was then Naval Air Station Tillamook. During the war it housed blimps, the nonrigid airships that patrolled the coastal waters of the Pacific. A total of 17 of these structures, the largest clear-span wooden buildings ever built, were erected at naval bases along the Pacific, Atlantic, and Gulf Coasts in 1942 and 1943.
The job of the blimps was to escort merchant convoys through coastal shipping lanes where enemy submarines might be lying in wait. In June 1940 Congress authorized the construction of naval bases for these blimps, including power plants, helium storage and handling facilities, barracks for personnel, runways, mooring pads, and hangars. The first new hangars designed to house blimps were built at South Weymouth, Massachusetts, and at Weeksville (Elizabeth City), North Carolina. Because their construction began before America’s entry into the war, when metal was plentiful, they had steel frames.
On December 7, 1941, when Japan attacked Pearl Harbor and America went to war, the Navy had only 10 blimps in operation, all based at Lakehurst, New Jersey. Congress quickly authorized a total of 200. Surveys had determined that there should be 10 bases—3 on the Pacific Coast, 2 on the Gulf Coast, and 5 on the Atlantic Coast—with a total of 17 new hangars. Three would be built at Richmond (Miami), Florida, and 2 each at Santa Ana, California; Sunnyvale (Moffett Field), California; Glynco, Georgia; Tillamook; and Lakehurst. At South Weymouth and Weeksville, 2 new hangars in each location would supplement those already in place. Single hangars would be built at Hitchcock, Texas, and Houma, Louisiana.
don best/tillamook air museum2006_1_21Now that steel was needed more urgently for other purposes, the new blimp hangars would have to be built primarily of wood. The use of timber introduced new and unprecedented structural challenges, and some engineers thought wood construction for such huge, odd-shaped buildings was impractical, but there was no choice. The prototype hangar design was drawn up under the direction of Arsham Amirikian, principal engineer of the Navy’s Bureau of Yards and Docks. Since every airdock built before the outbreak of war had been steel-framed, there were no guidelines or data available to help design wooden blimp hangars.
A major concern was the vast inside clearance needed to accommodate up to nine blimps in a three-by-three grid, along with adequate room and facilities for their servicing and maintenance. An open space at least 220 feet wide by 120 feet high, and stretching the 1,000-foot inside length of the building, would be required to house the airships, which were typically 252 feet long and 65 feet wide. No design precedent existed for creating that amount of clear inside space in a wooden building. Engineers considered several framing possibilities before deciding on an inverted catenary arch truss.
While each hangar would be built using the same plans, there would be some variation in the dimensions of the buildings. A typical hangar was based on a row of 51 trusses, or arches, standing vertically and spaced 20 feet apart, center to center. The bases of the trusses would be bolted to 24-foot-high concrete abutments. The clear span between the facings of the abutments was 2341/2 feet, while the total inside width at ground level (when not directly beneath one of the trusses) was 267 feet. Catwalks ran the length of the building on each side, 137 feet above the floor. The roof was sheathed in 1-inch tongue-and-groove planking. On the outside, a typical hangar was 1,086 feet long by 297 feet wide, for a total footprint of 7.4 acres; its maximum height was 183 feet.
tillamook county pioneer museum2006_1_23Doorways were installed on both ends of the hangar. The doorframe consisted of two reinforced-concrete towers, 174 feet tall, connected at the top by a 304-foot timber box-girder with a 21-foot 91/2-inch cross section. The girder had a 220-foot 9-inch clear middle span and 29-foot extended sections protruding from the doorframe towers on each side; these housed the machinery that opened and closed the doors.
The doors were of the multiple-leaf type, having six leaves (wooden with a vertical steel truss), three on each side of the opening, that slid out of the way in sequence. Each leaf was 271/2 feet wide, 120 feet high, and 3 to 5 feet thick, weighing 26 to 39 tons. When retracted, they nested within 271/2foot enclosed sections on each side. The leaves were mounted on two-wheel trucks that rode on steel rails and were stabilized at the top by rollers on steel guide rails attached to the girder.
The doorframe was supported independent of the hangar to avoid exerting pressure on the main building frame. Power came from two 71/2to 10-horsepower two-speed motors that could open or close the door in two minutes. One location, in swampy Houma, Louisiana, could not use the leaf-style door; the spongy ground couldn’t support its weight. A special curved doorway made mostly of wood was designed to reduce the weight of the door and distribute it over a larger area. These doors projected out horizontally on each end of the hangar in a half-circle with a radius of 120 feet. Engineers called it a “semi-dome” door, but it became better known as an “orange peel” door. It, too, was designed to stand independent of the main building frame. The unique door required less steel than the regular door design but had the disadvantage of taking 10 minutes to open or close.
moffett field historical society2006_1_24The wood specifications called for structural-grade Oregon Douglas fir, although there were exceptions for some redwood in the California hangars and some southern yellow pine for those built in the Southeastern states. Each hangar required 2,500,000 board feet of lumber and as much as 350 tons of steel for such items as bolts and shear plates, pre-stressed concrete, and door rails. The use of wood saved more than 4,000 tons of steel for each structure. All timber parts were treated with fire-retardant salts. This proved effective: At Santa Ana, after high winds knocked down some members during construction, the scrapped wooden pieces were stacked for burning but resisted the flames so stubbornly that in the end they had to be buried.
Trusses were fabricated at the construction sites on huge scaffolds erected on flatcars that ran on steel rails down the center of the hangar floor. Engineers built scale models to help work out the construction process. Each truss was made up of four sections. The usual approach was to hoist the two lower sections, called “haunches,” into place with a traveling derrick and secure them to the concrete abutments with anchor bolts. The two top sections, which formed the crown, were fitted together at floor level, then hoisted into position and joined to the haunch sections with steel bolts and plates. The upright trusses were linked to each other along the length of the hangar with cross-braces. Despite the structural difficulties inherent in wood construction, the prototype design was completed in early fall 1942, and within a year all 17 blimp hangars were in operation. The cost of each structure averaged $2.5 million. To build one today would cost about $28 million.
stephen d. chalker2006_1_24aWhen the war ended, in September 1945, demobilization moved swiftly. Blimps were deflated and removed from their bases and personnel were reassigned or discharged. A few blimps were sold as war surplus, some were scrapped, and others went into long-term storage. The colossal hangars all remained intact, though not always for long.
Fire destroyed 5 of the original 17 hangars—3 at Miami, 1 at Tillamook, and 1 at Weeksville. The most spectacular blaze occurred at the Miami base on September 15, 1945, less than two weeks after the Japanese surrendered. It began with a powerful hurricane headed toward the south Florida coast. In addition to the 25 blimps still in the hangars (14 inflated and 11 deflated and crated), a total of 391 military planes and 100 trucks and automobiles were moved into the hangars for protection from the storm. The hurricane made a direct hit on the base.
The hangars withstood the 126-mph winds until a fire, whose cause was never determined, broke out in one of them and spread rapidly. Violent winds carried sheets of flame to the other two hangars, resulting in the nation’s largest fire that year. When the storm ended, nothing of the hangars was left standing except the concrete doorframe towers and abutments. Today the Gold Coast Railroad Museum occupies the space where the three hangars stood. At its entrance is a concrete tower that was once part of the doorway frame of one of the hangars.
naval air engineering station, lakehurst2006_1_25Seven of the original hangars still stand today, having undergone extensive structural repair and maintenance. The two at Santa Ana were transferred to the Marine Corps in 1951, and for a time the station served as a helicopter facility. The base closed in 1999 and the hangars have been registered as National Historic Landmarks, but the high costs of restoration and annual maintenance are a concern, and a search is on for ways to use the buildings productively.
The future of the two at Moffett Field is uncertain. The Navy closed out its operations there in 1994 and transferred control of the base to NASA’s Ames Research Center. One hangar houses unused aircraft; the other is used by the California Air National Guard. The two at Lakehurst are maintained as part of the Navy’s Air Engineering Center and are in constant use for a variety of functions including training, storage, and testing. Hangar B at Tillamook has been converted to a museum that houses an impressive collection of restored World War II airplanes.
With the postwar development of helicopters, the Navy terminated its blimp operations, which had been housed at Lakehurst, in 1961. The surviving hangars stand as an example of what resourceful engineering, under the impelling force of urgent necessity, can accomplish.
