A box beam configuration may be used at openings in a floor or wall framing assembly. The American Iron and Steel Institute’s AISI S100 contains design provisions for built-up flexural members consisting of two C-sections oriented back-to-back to form an I-shaped section, i.e. Section D1.1, but does not contain design guidance for a box-section formed by orienting two C-sections lip-to-lip. For built-up members to act as a single composite unit, the members must be connected with sufficient fasteners at the maximum spacing as calculated below. This article illustrates the extrapolation of S100 Section D1.1 provisions to a box beam configuration.
The AISI S100 provisions are based on stabilizing the shear flow in the flanges. The same shear flow exists in a box-shaped section; therefore, the S100 provisions could apply to form a box-shaped section with the two C-sections oriented lip-to-lip.