One-Way Concrete Joist Construction - Page 2

TABLE 1 Standard Dimensions of Forms for One-way Joist Construction¹

NOTES
1 . All dimensions are in inches, except the module designations.
2. Width is the horizontal clear distance, between two consecutive joists, measured at the bottom of the joists.
3. Depth is the vertical distance, measured between two consecutive joists, from the underside of the concrete slab to the bottom of the joists.
4. Special filler forms may be available only in limited quantities. Availability should be investigated before specifying these forms.
5. Tapered endforms are available for the one-way 3'-0" module. These forms are 30 inches wide at one end and 25 inches wide at the other end, and they are 36 inches long. Standard depths of these forms are 8, 10, 12, 14, 16, and 20 inches.
6. Tapered endforms are available for the one-way 4'-0" module. These forms are 40 inches wide at one end and 34 inches wide at the other end, and they are 36 inches long. Standard depths of these forms are 12, 14, 16, 18, 20, 22, and 24 inches. These forms are generally available only on the West Coast.

    Sectional steel pan forms can adjust to varying site conditions without extensive detailing and fabricating of special shapes. The Architect/ Engineer is allowed great freedom in varying joist widths for accommodating concentrated loads by slightly adjusting the center-to-center spacing of the ribs. Clearing blockouts, drops and other interferences is accomplished by workers simply starting and stopping pan runs as required. Steel pan forms are a proper forming system to consider when evaluating design choices because they provide inherently stiff floor systems for the volume of concrete and reinforcing steel, and the forms are economical to obtain and erect when concrete esthetics are not a concern.
    Project specifications are often vague with reference to laps and single one-piece voids. The Architect/Engineer's expectations are generally different from those of the Contractor. The Contractor should be very sensitive to the Class of finish for which the pan forms are intended. Lap pans are generally inappropriate for exposed work. The Architect/Engineer's attention should be focused on the end product results during pre-construction meetings as to the finish that these forms are and are not capable of producing. For instance, when pans are lapped, both the joist width and slab thickness vary slightly. ACI 117 tolerances for joists and slabs are +3/8, -1/4 inch in width and thickness (Section 4.4.1). The Contractor needs to ensure that the erection of the formwork is performed with a reasonable degree of accuracy. Finally, the Architect/Engineer may want to recognize the challenges with this type of forming and specify a joist width one inch larger than required by design. While it is usually better to cast an onsite mockup section, it may be more practical and prudent to have the Architect/Engineer and Owner participate in a site visit to a structure of similar construction and application to measure both esthetics and performance.

FABRICATION AND ERECTION
    The typical lap pan is a 16-gauge or a 14-gauge piece of sheet metal, 3 feet long, bent into one of three traditional shapes (see Figure 2) with varying flange widths dependent on style and Supplier. Both ends are open. A chalkline on the deck or soffit form should be used to align the pans. End caps are placed first and work proceeds toward the center of the member from both ends, overlapping the pans until proper closure is achieved. Flanged pans are nailed into position. After the pans are tightly in place, they should be oiled before other trades proceed with their work. See Figure 3.
    The normal procedure for setting pans is to set the end caps first, nailed to the deck form on the line where the coffer begins. A long section of pan is first placed over the end cap. Then, through pre-punched matching holes in the top flange of the end cap and the top surface of the pan section, nails are dropped in to form a bond between the form sections. It is not uncommon to see small machine screws or center pin rivets used. However, form stripping procedures need to be considered with these types of fasteners. This connecting procedure also assists in preventing the end cap from collapsing inward under the pressure of concrete placing. The pan section is then nailed in place and a free standing steel or wood diaphragm (internal brace) is inserted into the form (suggested spacing is 18 inches on center under normal concrete placing conditions and should include the lap point between pans) and nailed in place. The next pan section is then installed, reasonably lapping (1 to 5 inches) the previous section and the previous procedure is repeated until the coffer is completely formed. It should be noted that all pans may require diaphragms to resist lateral pressures.