"[T]he infill fields of a Gothic cathedral’s ing behave as masonry shells, and by the 1860s masonry s of great beauty were developed by in both Spain and the United States. Gustavino’s work and graphic design methods... influence[d] Antonio Gaudí... whose work has been shaped by an interest in force-derived architectural form. While early engineers such as Guastavino pushed unreinforced masonry’s limits in thin shell construction, it was the introduction of steel reinforcement, initially in concrete, that sparked the Twentieth century’s interest in thin shell construction. Early [twentieth century] shell-builders were often engineers, and were engaged to design economical long-span structures... as aircraft hangars, train sheds and factories."