Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.79
One-way joist construction was developed to reduce dead load. For long spans,
the utility of solid-slab construction is offset by the increase in dead load of the
slab. One-way concrete-joist construction provides adequate depth with less dead
load than for solid slabs, and results in smaller concrete and reinforcement quantities
per square foot of floor area.
Uniform-depth floor and roof construction can be obtained by casting the joists
integral with wide, supporting band beams of the same total depth as the joists.
This design eliminates the need for interior beam forms.
9.54 STANDARD SIZES OF JOISTS
One-way concrete-joist construction that exceeds the dimensional limitations of the
ACI 318 Building Code must be designed as slabs and beams. These dimensional
limitations are:
Maximum clear spacing between ribs—30 in
Maximum rib depth—3.5 times rib width
Minimum rib width—4 in
Minimum top-slab thickness with removable forms—2 in but not less than onetwelfth
the clear spacing of ribs
Minimum top-slab thickness with permanent forms—1 1 ⁄2 in but not less than
one-twelfth the clear spacing of ribs
Removable form fillers can be standard steel pans or hardboard, corrugated cardboard,
fiberboard, or glass-reinforced plastic. Standard removable steel pans that
conform to ‘‘Types and Sizes of Forms for One-Way Concrete-Joist Construction,’’
(ANSI/CRSI A48.1-1986), American National Standards Institute, include 20- and
30-in widths and depths of 8, 10, 12, 14, 16, and 20 in. Standard steel square-end
pans are available in 36-in lengths. Widths of 10, 15, and 20 in and tapered end
fillers are available as special items. For forms 20 and 30 in wide, tapered end
forms slope to 16 and 25 in, respectively, in a distance of 3 ft.
9.55 DESIGN OF ONE-WAY
CONCRETE-JOIST CONSTRUCTION
One-way concrete joists must have adequate structural strength, and crack control
and deflection must be satisfactory under service loads. Approximate methods of
frame analysis can be used with uniform loads and spans that conform to requirements
of the ACI 318 Building Code (see Art. 9.41). Table 9.15 lists minimum
depths of joists to limit deflection, unless deflection computations justify shallower
construction (Table 9.14). Load tables in the Concrete Reinforcing Steel Institute’s
‘‘CRSI Design Handbook’’ indicate when deflections under service live loads exceed
specified limits.
Economy can be obtained by designing joists and slabs so that the same-size
forms can be used throughout a project. It will usually be advantageous to use
square-end forms for interior spans and tapered ends for end spans, when required
with a uniform depth.