The excavation phase of construction entails preparing the site for the structure's foundation. Excavation activities generally include stripping topsoil, digging dirt, and removing rocks. If the soil has adequate backfill qualities, it can be stockpiled on site for backfilling into the foundation. If the dirt is unsuitable for use as backfill, it is removed from the site and dumped elsewhere. There are costs associated with removal of the soil from the site, and therefore it is important to know the swelling characteristics of the excavated soil (Gallo & Campbell, 1934). In this example, the swelling of the excavated soil is assumed to be 10% of the volume of the original soil.
Question 1
The volume of topsoil to be removed and stockpiled is computed as follows:
Enlarging the footprint by 5 feet (2.5 feet on all sides added from the outer edge of the footing), the total area is calculated as follows:
Area = ( 10.5' × 17' ) + ( 14.5' × 12.5' ) + ( 15' × 14.5' ) = 577.25 sf
Volume = Area × depth = 577.25 sf × 9" = 432.94 cf
Assume 1 cf = 0.037 cy. Converting volume in cubic feet (cf) to volume in cubic yards (cy):
Volume = 432.94 cf × 0.037 cy / cf = 16.02 cy
Question 2
The total volume of soil after swell is 110 / 100 × 16.02 cy = 17.63 cy
The number of 7 cy trucks that will be used to cart away the topsoil are ( 17.63 cy ) / ( 7 cy ) ≈ 3 trucks
It is important to note that changes in the humidity of the atmosphere will affect the swelling characteristics of the soil. The swell rate should be re-adjusted appropriately in the case of changes in the humidity of the atmosphere (Zeigler, 1981).
Foundation
Foundations transfer loads from the building to the ground. The mode of transfer of these loads to the ground varies depending on the type of foundation. The foundation of a building varies depending on the type of the building. Common shallow foundations include mat foundations, strip foundations, and pad foundations. Deep foundations include pier foundations, pile foundations and caisson foundations. These foundations may or may not have concrete in them. Pile foundations may be made of steel, timber or concrete (Pratt, 2010).
The foundation in the drawings provided is a strip foundation running along the major structural walls of the house.
The depth of foundations of proposed structures is not known until comprehensive geotechnical investigations are conducted (Foster, 1972). In addition, the depths of foundations on one site may vary. Therefore, during the preliminary takeoffs, a depth is assumed subject to confirmation on site.
Question 3
The total linear feet of footing required are computed from the centerline dimensions of the building. The centerline dimensions are determined by subtracting 12” (half the width of the footing) from the dimensions provided in the plan.
The total linear feet of footing are as computed as follows:
Total Linear Feet = { ( 8' + 12' ) × 2 } + 16'' + ( 10' × 2 ) + 8' = 84'
The total linear feet of footing required is 84.
Question 4
The dimensions of the footing are 24” by 9”. The cross-sectional area of the footing is calculated as follows:
Cross - sectional area=24 in. × 9 in. = 1.5 sf
The cross-sectional area of the footing is 1.5 sf
Question 5
The volume of concrete for the footing is obtained by multiplying the cross-sectional area of the footing by the total linear feet of footing required. The computation is as follows:
Volume of concrete = ( 1.5 sf × 84' ) = 126 cf
Adding 5% waste: Volume of concrete = 126 cf × 1.05 = 132.3 cf
Converting volume to cubic yards: Volume of concrete = 132.3 cf × 0.037 cy / cf ≈ 5 cy
Slab
Slabs transfer loads to beams, columns or the ground, depending on the structural system. Slabs may be ground slabs or suspended slabs. Slabs may be constructed of steel, timber or concrete. Concrete slabs have different types and sizes of reinforcement. Slabs may be reinforced by steel reinforcement or fiber reinforcement. The steel reinforcement may be pre-tensioned or posttensioned to add strength to the slab (Popescu, Phaobunjong, & Ovararin, 2003).
Question 6
The area of the slab is computed up to the edge of the outer wall of the building (Ding, 2009). The area is computed as follows:
Slab area = ( 8'-8" × 12' ) + ( 10'-8" × 16'-8" ) ≈ 282 sf
Volume of concrete = Area × depth = 282 sf × 4" = 94 cf
Adding 8% waste: Volume of concrete = 94 cf × 1.08 = 101.52 cf
Converting volume to cubic yards:
Volume of concrete in cubic yards = 101.52 cf × 0.037 cy / cf ≈ 4 cy
Question 7
The total slab area is 282 sf. Adding 20% for lap and waste, the area is1.2 × 282 sf = 338.4 sf
1 roll of WWF is 750 sf. The number of rolls of WWF that will be required are ( 338.4 sf ) / ( 750 sf ) ≈ 1 ⁄ 2 roll of WWF
The assumption made in the computation is that the WWF rolls can be obtained in fractions of a whole. If the rolls can only be obtained as wholes, 1 WWF role will be required for the slab.
Question 8
The productivity rate for the slab is 0.4 labor hours/cy. The total number of labor hours required for the 4 cy slab is 0.4 ( labor hours ) ⁄ cy × 4 cy = 1.6 labor hours
The productivity rate for the WWF reinforcement is 1 labor hour/roll. The total number of labor hours required for ½ roll is 1 ( labor hour ) ⁄ roll × 1/2 roll = 0.5 labor hours
The total number of labor hours for both slab and WWF reinforcement is 1.6 labor hours + 0.5 labor hours = 2.1 labor hours
The labor rate for both the slab and the WWF is $14.50/hr. The total labor cost for both the WWF and the slab is obtained by multiplying the total labor hours by the labor rate as follows:
Total cost = 2.1 labor hours × $ 14.50 ⁄ hr = $ 30.45
The cost to provide and install the slab and WWF is $30.45.
References
Ding, A. (2009). DEWALT Construction Estimating Complete Handbook. New York, NY: Cengage Learning.
Foster, N. G. (1972). Construction estimates from take-off to bid. London, United Kingdom:
Gallo, F. J., & Campbell, R. I. (1934). Small residential structures: Construction practices and material take off estimates. New York, NY: John Wiley & Sons.
Popescu, C. M., Phaobunjong, K., & Ovararin, N. (2003). Estimating Building Costs. California, CA: CRC Press.
Pratt, D. (2010). Fundamentals of Construction Estimating. New York, NY: Cengage Learning.
Zeigler, C. D. (1981). Walker's Manual for Construction Cost Estimating. Boston, MA: Creative Homeowner Press.
