Building Design and Construction Handbook - Merritt - Ventech!

9.84 SECTION NINE
centerlines divide the slab into square or nearly square panels, but if desired, rectangular,
triangular, or even irregular panels may be used.
9.59 ANALYSIS AND DESIGN OF FLAT PLATES
The flat plate is the simplest form of two-way slab—simplest for analysis, design,
detailing, bar fabrication and placing, and formwork. A flat plate is defined as a
two-way slab of uniform thickness supported by any combination of columns and
walls, with or without edge beams, and without drop panels, column capitals, and
brackets.
Shear and deflection limit economical flat-plate spans to under about 30 ft for
light loading and about 20 to 25 ft for heavy loading. While use of reinforcingsteel
or structural-steel shear heads for resisting shear at columns will extend these
limits somewhat, their main application is to permit use of smaller columns. A
number of other variations, however, can be used to extend economical load and
span limits (Arts. 9.60 and 9.61).
The ACI 318 Building Code permits two methods of analysis for two-way construction:
direct design, within limitations of span and load, and equivalent frame
(Art. 9.42). Limitations on use of direct design are:
A minimum of three spans continuous in each direction
Rectangular panels with a ratio of longer to shorter span, center-to-center of
supports within a panel, not greater than 2
Successive span ratios, center-to-center of supports in each direction, not to
exceed 2:3
Columns offset from centerlines of successive columns not more than 0.10 span
in either direction
Specified ratio of live load to dead load (unfactored) does not exceed 2
All loads are due to gravity only and uniformly distributed over the entire panel
9.59.1 Design Procedures for Flat Plates
The procedure for either method of design begins with selection of preliminary
dimensions for review, and continues with six basic steps.
Step 1. Select a plate thickness expected to be suitable for the given conditions
of load and span. This thickness, unless deflection computations justify thinner
plates, should not be less than h determined from Table 9.16. With Grade 60 reinforcement,
minimum thickness is, from Table 9.16, for an interior panel
L n
h � � 5 in (9.71)
33
where L n � clear span in the direction moments are being determined. Also, as
indicated in Table 9.16, for discontinuous panels, the minimum h � L n/30 � 5in
if no edge beam is present.