Structural assessment of St. Charles hyperbolic paraboloid roof

At the time of completion in 1961, the roof of St. Charles Church became the largest unbalanced hyperbolic paraboloid structure in the United States and the only shell structure in Spokane, WA. Situated on an 8-acre site on the north side of the city, St. Charles is a modernist structure designed through partnership of Funk, Molander & Johnson engineers, architect William C. James and in consultation with Professor T.Y. Lin of the Structural Engineering Laboratory at the University of California, Berkeley. This asymmetric structure spans over 33.5 m (110 ft) and utilizes folded edge beams that taper from 1067 mm (42 in) at the base to a 76.2 mm (3 in) thickness at the topmost edge using regular strength reinforcing steel and concrete load carrying components. The novelty of the pre-stressed shell structure serves both architectural and structural design criteria by delivering a large, uninterrupted interior sanctuary space in materially and economically efficient manner. This structural assessment summarizes the roof’s historic design and construction according to the original construction documents, newspaper reports and historic photographs. The FEA is completed using UBC 1955 design loads and ACI 334 Concrete Shell Structures provisions.