Concrete Reinforcing Steel Institute

Concrete joist construction consists of a monolithic combination of regularly spaced ribs and a top slab. It may be constructed with permanent or removable fillers between ribs (joists) arranged to span in one direction or in two orthogonal directions (Section 8.11.1). Two-way joist construction which spans in two orthogonal directions, commonly referred to as waffle slab construction, must conform to the requirements of Chapter 13 of the 1995 Code for slab systems reinforced in flexure in more than one direction. Chapter 7 of this book gives a detailed explanation of these requirements. One-way joist construction with joists parallel to one another must conform to the requirements for analysis and design of the 1995 Code which will be discussed in this chapter.

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.