jobsite. Let's assume that you estimate 8 cubic yards or 216 cubic feet will be placed in the form per hour. In this case, the vertical rate of placement is 216 cubic feet, divide by 30 square feet of horizontal area, or about 7 feet per hour. See the formula below.

NOTE. For an economical design, try to keep the rate of placement to 5 feet/hour or less.

Pressure from Vertical Rate of Placement

To determine the maximum concrete pressure, the Builder must know the temperature of the concrete and the rate of placement per square foot. When you know these things, you can determine the maximum concrete pressure by using the chart, as shown in figure 3-1.

For example, to find the maximum concrete pressure, first make a reasonable estimate of the temperature of the concrete, let's say 70°F, and it has a rate of placement at 7 feet per hour. Move across the

Figure 3-1. - Maximum concrete pressure graph.

table to 70°F, then move down the table to 7 feet per hour. In this case, the maximum pressure of concrete is 1,050 pounds per square foot (psf).

MAXIMUM SPACING OF WALL FORM STUDS. - Suppose you want to know the maximum spacing when using 3/4-inch sheathing with a concrete pressure of 600 pounds per square foot. (Refer to table 3-1). Move down the chart to 600 pounds per square foot, then go down the chart to 3/4-inch plywood sheathing. You will find that 14-inch spacing per stud is required. This chart refers to the forms with the face grain running across the supports. If the concrete pressure value falls between the two values in the column, round up to the nearest given value.

To determine the uniform load on a stud (ULS), you multiply the maximum concrete pressure by the maximum stud spacing. Then convert the answer to pounds per linear foot by dividing the result by 12. For example, the maximum concrete pressure is 1,050 pounds per square foot and the stud spacing is 14 inches. Multiply the two values together then divide by 12 which equals 1,225 (lbs/lin ft).

MAXIMUM SPACING OF WALL FORM WALES. - When you know the spacing of the studs, the sheathing, and the maximum concrete pressure, the maximum wale spacing is not difficult to determine using the chart shown in table 3-2 and table 3-3. For example, suppose you want to find the maximum wale spacing for 2 by 4 studs, and the concrete pressure is 600 pounds per square foot. Move down the chart until you reach 600 pounds per square foot, then go across to 2 by 4 lumber and you will find that the spacing for the waler and ties are 24 inches.

To determine the uniform load on a wale (ULW), you multiply the maximum concrete pressure (600 pounds per square foot) by the maximum wale spacing (24 inches). Convert the answer to feet by dividing the result by 12, which equals 1,200 (pounds per linear foot).