Structural Stainless

Conservatory gets a new lease on life thanks to a corrosion-resistant structure

By Dr. G. Crawford

Nickel Magazine, September 2007 -- The East Conservatory at Pennsylvania’s Longwood Gardens has a classy, long-term lease on life, thanks to a novel roof support structure of stainless steel.

Circular hollow columns of S30403 stainless steel support transverse and longitudinal roof trusses of S30400, all in an attractive, brushed satin finish. An arched roof of glass panels in an aluminum framework completes the renovation, giving the greenhouse a spacious, modern look.

Situated 50 kilometres west of Philadelphia, U.S.A., Longwood sits on more than 10 hectares of gardens, woodlands and meadows, much of which was owned by industrialist Pierre S. du Pont, until his death in 1954. In the early 1970s, the East Conservatory, largest of the greenhouses at Longwood (65 metres long by 30 metres wide) had a self-supporting, barrel-shaped roof of acrylic panels. However, by the 1980s, ultraviolet radiation had embrittled many of the panels, making it necessary to replace the roof.

English architect Sir Peter Shepheard (1914-2002), then at the University of Pennsylvania, was retained to design the replacement, and in 1989 Philadelphia-based Keast & Hood Co. was engaged as the engineering firm. The acrylic panels were replaced by glass, which meant the heavier new roof was not self-supporting. An interior support structure was therefore required. Engineers had to decide what material to use to support the glass. Aluminum was too flexible, and the durability of galvanized steel was too short, given the corrosive atmosphere resulting from fertilizers needed for the plants. Life-cycle analysis proved that stainless steel, in spite of its higher initial cost, was the preferred choice. S30403 was selected for the columns because they are in direct contact with corrosive, fertilizer-bearing soils.

Crescent Iron Works Inc. of Philadelphia supplied and installed the steel for the project. The 35-cm-diameter columns are each about 12 metres long consisting of cold-rolled, welded-seam pipe in two parallel rows. Transverse trusses of S30400 stainless rest on each pair of columns across the width, while longitudinal trusses rest on top of the two rows of columns. There are two expansion joints at the one-third points along the longitudinal trusses to accommodate changes of as much as 4 cm at each joint between summer heat and winter cold. The hollow columns double as downspouts for rainwater from gutters along the length of the roof structure, while vines, now climbing up the columns, add to the aesthetic effect.

The project was not without its problems, most of which stemmed from local inexperience with welding and inspection of stainless steel components. Fred Baumert, structural engineer and a principal of Keast & Hood, acknowledges that “the Nickel Institute was a big help in supplying much-needed assistance with design guides, welding criteria, and material availability.”

The steel used in the Longwood project weighs a total of 130 tonnes. Numerous ageing greenhouses of similar size around the world are a potential market for similar applications of stainless steel.

Dr. G. Crawford is a Mississaugua, Ontario-based consultant to the Nickel Institute.

Photos: Frederick Baumert, Keast & Hood Co.