CRSI |
ENGINEERING
DATA REPORT NUMBER 43 |
ONE-WAY CONCRETE JOIST CONSTRUCTION: STEEL LAP PAN FORMING SYSTEM |
 A
SERVICE OF THE CONCRETE REINFORCING STEEL INSTITUTE
933 N. Plum Grove Rd., Schaumburg, Illinois 60173-4758 |
INTRODUCTION
This type of construction has frequently utilized
ready-made steel pan forms of standard sizes. Depths and spacings for one-way concrete
joist construction were standardized in 1932 by the U.S. Department of Commerce on the
basis of an industry study by a committee of Architects/ Engineers, Contractors and Steel
Form Suppliers. That early standard has since been replaced by ANSI/CRSI A48.1-1986
"Forms for One-Way Joist Construction;' which establishes standard dimensions for
one-way joist forms. Standard form widths are 20, 30, 40*, 53* and 66* inches,
corresponding to structural modules ranging from 2 to 6 feet. Standard depths are 8, 10,
12, 14, 16, 18, 20, 22* and 24* inches. Not all depths are manufactured in each form
width. Filler forms and tapered endforms are usually available locally to fit varying
floor layouts and sizes. This type of construction is well established with a long record
of successful use.
Joist construction was developed to reduce dead weight and
reinforcement. As desired spans increase, the efficiency of solid slab construction is
rapidly offset by the increase in the dead load. Joist construction enables the Architect/
Engineer to provide the depth required for adequate stiffness and efficient utilization of
the reinforcement without excessively high dead load/live load ratios. Standard size
reusable forms make it possible to eliminate unnecessary dead weight with overall economy.
Longer spans or relatively heavy loads can be accommodated by using tapered end forms
which permit widening of the ribs in areas of high shear.
DESCRIPTION
One-way concrete joist construction provides a
monolithic combination of regularly-spaced joists (ribs) and a thin slab of concrete cast
in place to form an integral unit with the supporting beams, columns and walls. In one-way
concrete joist construction, the joists are arranged in one direction between parallel
supports. Joist rib widths vary from 4 to 6 inches. Standard endforms consist of square
endforms. A tapered endform for a 2-foot module tapers from 20 to 16 inches wide in a
distance of 36 inches. A tapered endform for a 3-foot module tapers from 30 to 25 inches
(in some systems 26 inches) wide in a distance of 36 inches. See Figure 1.
 |
 |
| PLAN OF STANDARD TAPERED
END WHERE REQUIRED |
PLAN OF STANDARD SQUARE
END JOISTS |
Figure 1 Tapered Endforms
Wide-module joist systems (also referred to as
skip-joist" systems) are defined as joist systems with a clear spacing between the
ribs of more than 30 inches. Since this module, for application of the ACI Building Code,
exceeds the rib spacing limit for standard joist construction (Section 8.11.3),
wide-module joists become repetitive "T" beams and are subject to design
requirements for such members.
Joist widths for wide-module joist systems vary from 6 to 8 inches.
Standard forms for void spaces between ribs are 40, 53 or 66 inches wide and 12, 14, 16,
18, 20, 22 or 24 inches deep. Standard endforms consist of square endforms. Use of square
end joist forms simplifies forming. Tapered endforms are generally not available for
wide-module systems. See Table 1.
FORMWORK SELECTION CONSIDERATIONS
Maximum overall economy in concrete joist construction, as
in any cast-in-place reinforced concrete design, is achieved by considering the relatively
high cost of formwork and construction time versus material costs. Almost invariably
overall economy is achieved by the maximum reuse of the same forms throughout the project,
not only throughout each floor but also the same layout and size of forms for all levels
of the structure.
The use of the lap-type steel one-way pan system is probably one of the
most efficient methods of reinforced concrete construction ever devised in terms of spans
and applied loads versus volume of concrete and weight of reinforcing steel. A steel lap
pan system has one major drawback: typically it can produce no better than a Class 'C'
finish.
* For wide-module joists only
1 inch = 25.4 millimeters
© Copyright 1998 by the Concrete Reinforcing Steel Institute
