Simplified Design for Two-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 101

Chapter 4
Simplified Design for Two-Way Slabs

4.1 INTRODUCTION

Figure 4-1 shows the various types of two-way reinforced concrete slab systems in use at the present time.

A solid slab supported on beams on all four sides [Fig. 4-1(a)] was the original slab system in reinforced
concrete. With this system, if the ratio of the long to the short side of a slab panel is two or more, load transfer
is predominantly by bending in the short direction and the panel essentially acts as a one-way slab. As the ratio
of the sides of a slab panel approaches unity (square panel), significant load is transferred by bending in both
orthogonal directions, and the panel should be treated as a two-way rather than a one-way slab.

As time progressed and technology evolved, the column-line beams gradually began to disappear. The resulting
slab system, consisting of solid slabs supported directly on columns, is called a flat plate [Fig. 4-1(b)]. The flat
plate is very efficient and economical and is currently the most widely used slab system for multistory
residential and institutional construction, such as motels, hotels, dormitories, apartment buildings, and
hospitals. In comparison to other concrete floor/roof systems, flat plates can be constructed in less time and
with minimum labor costs because the system utilizes the simplest possible formwork and reinforcing steel
layout. The use of flat plate construction also has other significant economic advantages. For instance, because
of the shallow thickness of the floor system, story heights are automatically reduced resulting in smaller overall
heights of exterior walls and utility shafts, shorter floor to ceiling partitions, reductions in plumbing, sprinkler
and duct risers, and a multitude of other items of construction. In cities like Washington, D.C., where the
maximum height of buildings is restricted, the thin flat plate permits the construction of the maximum number
of stories on a given plan area. Flat plates also provide for the most flexibility in the layout of columns,
partitions, small openings, etc. Where job conditions allow direct application of the ceiling finish to the flat
plate soffit, (thus eliminating the need for suspended ceilings), additional cost and construction time savings
are possible as compared to other structural systems.

The principal limitation on the use of flat plate construction is imposed by punching shear around the columns
(Section 4.4). For heavy loads or long spans, the flat plate is often thickened locally around the columns
creating either drop panels or shear caps. When a flat plate is equipped with drop panels or shear caps, it is
called a flat slab [Fig. 4-1(c)]. Also, for reasons of shear capacity around the columns, the column tops are
sometimes flared, creating column capitals. For purposes of design, a column capital is part of the column,
whereas a drop panel is part of the slab.

Waffle slab construction [Fig. 4-1(d)] consists of rows of concrete joists at right angles to each other with solid
heads at the columns (to increase punching shear resistance). The joists are commonly formed by using
standard square dome forms. The domes are omitted around the columns to form the solid heads acting as drop
panels. Waffle slab construction allows a considerable reduction in dead load as compared to conventional flat