FORM DESIGN

Forms for concrete construction must support the plastic concrete until it has hardened. Stiffness is an important feature in forms. Failure to provide form stiffness may cause unfortunate results. Forms must be designed for all the weight to which they are likely to be subjected. This includes the dead load of the forms, the plastic concrete in the forms, the weight of the workmen, the weight of equipment and materials, and the impact due to vibration. These factors vary with each project, but none should be ignored. The ease of erection and removal is also an important factor in the economical design of forms. Platform and ramp structures independent of formwork are sometimes preferred to avoid displacement of forms due to loading and impact shock from workmen and equipment.

When concrete is placed in forms, it is in a plastic state and exerts hydrostatic pressure on the forms. The basis of form design, therefore, is the maximum pressure developed by concrete during placing. The maximum pressure developed depends on the placing rate and the temperature. The rate at which concrete is placed affects the pressure because it determines how much hydrostatic head builds up in the form. The hydrostatic head continues to increase until the concrete takes its initial set, usually in about 90 minutes. At low temperatures, however, the initial set takes place much more slowly. This makes it necessary to consider the temperature at the time of placing. By knowing these two factors and the type of form material to be used, you can calculate a tentative design.

FORM CONSTRUCTION

Strictly speaking, it is only those parts of the form work that directly mold the concrete that are correctly referred to as the "forms." The rest of the formwork consists of various bracing and tying members. In the following discussion on forms, illustrations are provided to help you understand the names of all the formwork members. You should study these illustrations carefully so that you will understand the material in the next section.

Foundation Forms

The portion of a structure that extends above the ground level is called the superstructure. The portion below the ground level is called the substructure. The parts of the substructure that distribute building loads to the ground are called foundations. Footings are installed at the base of foundations to spread the loads over a larger ground area. This prevents the structure from sinking into the ground. It's important to remember that the footings of any foundation system should always be placed below the frost line. Forms for large footings, such as bearing wall footings, column footings, and pier footings, are called foundation forms. Footings, or foundations, are relatively low in height since their primary function is to distribute building loads. Because the concrete in a footing is shallow, pressure on the form is relatively low. Therefore, a form design based on high strength and rigidity considerations is generally not necessary.

SIMPLE FOUNDATION.- Whenever possible, excavate the earth and use it as a mold for concrete footings. You should thoroughly moisten the earth before placing the concrete. If this is not possible, you must construct a form. Because most footings are rectangular or square, you can build and erect the four sides of the form in panels.

Make the first pair of opposing panels (figure 7-1 (a)) to exact footing width. Then, nail vertical cleats to the exterior sides of the sheathing. Use at least

Figure 7-1.-Typical foundation form for a large footing.

1-by-2-inch lumber for the cleats, and space them 2 1/2 inches from each end of the exterior sides of the panels (a), and on 2-foot centers between the ends. Next, nail two cleats to the ends of the interior sides of the second pair of panels (figure 7-1 (b)). The space between these panels should equal the footing length plus twice the sheathing thickness. Then, nail cleats on the exterior sides of the panels (b) spaced on 2-foot centers.

Erect the panels into either a rectangle or square, and hold them in place with form nails. Make sure that all reinforcing bars are in place. Now, drill small holes on each side of the center cleat on each panel. These holes should be less than 1/2 inch in diameter to prevent paste leakage. Pass No. 8 or No. 9 black annealed iron wire through these holes and wrap it around the center cleats of the opposing panels to hold them together (see figure 7-1). Mark the top of the footing on the interior side of the panels with grade nails.

For forms 4 feet square or larger, drive stakes against the sheathing, as shown in figure 7-1. Both the stakes and the 1 by 6 tie braces nailed across the top of the form keep it from spreading apart. If a footing is less than 1-foot deep and 2-feet square, you can construct the form from 1-inch sheathing without cleats. Simply make the side panels higher than the footing depth, and mark the top of the footing on the interior sides of the panels with grade nails. Cut and

Figure 7-2.-Typical small footing form.

nail the lumber for the sides of the form, as shown in figure 7-2.

FOUNDATION AND PIER FORMS COMBINED.- You can often place a footing and a small pier at the same time. A pier is a vertical member that supports the concentrated loads of an arch or bridge superstructure. It can be either rectangular or round. You build a pier form as shown in figure 7-3. The footing form should look like the one in figure 7-1. You must provide support for the pier form while not interfering with concrete placement in the footing form. You can do this by first nailing 2-by-4s or 4-by-4s across the footing form, as shown in figure 7-3. These serve as both supports and tie braces. Then, nail the pier form to these support pieces.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing