Two-Way (Waffle) Flat Slab Design

Abstract

A “waffle” flat slab is a two-way joist system. Under the general provisions of the 1995 Code, Chapter 13, the two-way joist portion may be combined with a solid column head or with solid wide beam sections on the column centerlines for uniform depth construction. These are the usual applications, favored for economy of formwork and the unobstructed level ceiling with a regular pattern of void spaces utilizable for lighting or ventilating fixtures. The arrangement of two-way joists with a solid head results in a slab stiffness distribution similar to that for a solid slab with a drop panel. The use of solid “beam” sections on the column centerlines also provides a solid full depth for shear transfer to the columns and permits concentration of moment reinforcement at the column. Other common variations are cantilevered edges or use of spandrel beams deeper than the uniform depth elsewhere.