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50 & 20 Years Ago

Strictly for the birds

Taken from Building with Steel, 1966

Architects: Daniel, Mann, Johnson and Maidenhall. Structural Engineer: Donald J Neubater.

An aviary in the United States has been created using structural steel members.

The Great Flight Cage at the National Zoo, Washington D.C. is 130 ft. diameter and 90 ft. high and dominated by six parabolic arches tilting outwards at 30°. It is built on the side of a hill and the need to keep the top of the arches on a level plane posed something of a problem for the designers. The arches are welded box sections tapering from 12 in. by 24 in. at the bottom to 12 in. by 16/ in. at top.

The central mast is a tapering Y-section fabrication tapering from its heaviest point about 18 ft. above ground. From a conical steel anchor ring near the top of the mast 72 half inch diameter wire rope cables radiate to the rims of the arches from where cables of similar dimensions extend downwards to anchorages around the periphery of the cage. In all, two miles of rope are used. Draped over the structure and clipped to the cables is a vinyl-coated steel mesh.

Although of such slender proportions the structure is extremely strong and stable, and capable of withstanding the worst weather conditions, particularly heavy wind loadings and ice build-up. The mesh, or instance, has been tested to a wind loading of 100 mph. The strength of the structure is due largely to the employment of 50,000 psi minimum yield corrosion resistant steel for the mast and arches, coupled with the design of the sections.

Corrosion has been eliminated by the corrosion resistant properties of the steel used in the main members, and the plastic coating on the steel mesh.

The size of the cage provides plenty of space for the birds to stretch their wings and the internal landscape has been planned to simulate natural conditions. The reduce the discomforts of wet and cold weather there are shelters and electrically heated perches are provided. Visitors can walk through and see the birds at close quarters.

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