BUTTERFLY SHELLS
Cylindrical shell vaults can be constructed with partial segments of arches, arranged in the form of a Y and called a butterfly roof. This shape is often used for canopies for buildings with skylights and for railroad station platform covering. At the ends of this building, a complete stiffening truss is used to tie the shells together. Horizontal struts are sometimes provided to tie the tops of the shells together at frequent intervals. At supports, the skylight is omitted and may be used to increase the stiffness and strength.
Spans for this type of structure must be quire short in comparison to other barrel shells because the effective depth is, in effect, the minimum depth measured on a slant of one of the individual segments. This depth can be increased by adding a longitudinal stiffening beam at the top of the shell. A transverse stiffening girder is required at each column and may be placed on top of the shell.