Quantity-from-quantity goods are estimated based upon the quantity of another item. For example, the number of gallons of paint may be estimated by dividing the area of the surface to be painted by the coverage rate for a gallon of paint. Alternately, roofing nails may be estimated by multiplying the area of the roof in squares (100 square feet) by the average number of nails used per square. Items that may be estimated by the quantity-from-quantity method include mortar, per- lite, fasteners, saw blades, waterproofing and dampproofing, blown insulation, stucco, shims, drywall tape and mud, adhe- sives, grout, tack strip and seaming tape for carpet, paint, and electrical wiring. Quantity-from-quantity estimates are per- formed using one of the following equations:
(4-20)
where
Quantity ⫽ Quantity of Material Needed
QuantityBase⫽ Quantity of the Item that the Estimate is Based Upon
Coverage ⫽ Coverage of the Material Needed
(4-21)
where
Quantity ⫽ Quantity of Material
Needed
QuantityBase ⫽ Quantity of the Item that the Estimate is Based Upon Average Quantity Required⫽ Average Quantity
Needed to Cover One Unit of Base Material The coverage and average quantity required may be obtained from historical data, manufactures’ data, or reference books. Estimating the quantity from quantity is shown in the fol- lowing two examples:
EXAMPLE 4-7
Determine the quantity of paint needed to paint 5,000 square feet of wall. One gallon of paint will cover 300 square feet of wall.
⫽ (QuantityBase) (Average Quantity Required) Quantity
Quantity⫽ QuantityBase Coverage Volume⫽ (100 ft)(100 ft)a 4 in 12 in/ftba 1 yd3 27 ft3b ⫽ 123.5 yd 3
Solution: Using Eq. (4-20), we get the follow quantity:
䊏
EXAMPLE 4-8
Determine the quantity of roofing nails needed for a 14 square (square⫽ 100 square foot) roof. Historically, 340 nails are needed per square.
Solution: Using Eq. (4-21), we get the follow quantity:
䊏
WASTE
Ordering the foregoing quantities may not provide the quan- tities necessary to complete the construction project because some of the materials are lost because of waste. Unavoidable waste is waste that is the result of not being able to use scrap materials (as we saw in Examples 4-4 and 4-5). Avoidable waste is waste that is due to improper use of materials, lost or damaged materials, and the difference between the actual di- mensions and the design dimensions. Where possible, un- avoidable waste should be included in the original quantity takeoff of the materials as it was in Example 4-5. This is neces- sary to accurately track and control avoidable waste. A waste factor is often added to the quantities to account for avoidable waste, as well as unavoidable waste that has not already been included. The waste factor is expressed as a percentage of the calculated quantity. The quantity of material needed, includ- ing the waste, is calculated using the following equation:
(4-22)
Volume, area, or number may be substituted into Eq. (4-22) for the quantity. The inclusion of waste is shown in the following example.
EXAMPLE 4-9
Determine the volume of concrete for the slab in Example 4-6. Include 10% waste in the calculated volume.
Solution: From Example 4-6, the volume of the slab is 123.5 cubic yards. Using Eq. (4-22), we add the waste as follows:
䊏 The waste factor should be determined from historical data. Historical data is obtained by comparing the estimated quantity without waste to the actual quantities used on the project, using the following equation:
(4-23)
Waste Percentage⫽ 100 a QuantityUsed QuantityEstimated⫺ 1b
⫽ (123.5 yd3) (1.10)⫽ 135.8 yd3
Volumewith Waste⫽ (123.5 yd3)a1 ⫹
10 100b ⫽ Quantitywithout Waste a1 ⫹
Waste Percentage
100 b
Quantitywith Waste
Quantity⫽ (14 sq)(340 nails>sq) ⫽ 4,760 nails Quantity⫽ 5,000 sf
2
EXAMPLE 4-10
The slab in Example 4-6 required 138 cubic yards of concrete. Determine the actual waste percentage for the slab.
Solution: From Example 4-6, the estimated volume of the slab was 123.5 cubic yards. The waste percentage is calculated using Eq. (4-21) as follows:
䊏
Waste Percentage⫽ 100 a 138 yd
3
123.5 yd3⫺ 1b ⫽ 11.74%
Data on the quantities used should be available from the company’s accounting system.iWhen performing the quan- tity takeoff, the estimator should keep an accurate estimate of the quantities before adding avoidable waste. This quan- tity is to be used to measure the actual quantity of waste. On one apartment project the author reviewed, he found that the number of studs needed to complete identical apartment units varied by almost 10% because of differences in the use of materials.
CONCLUSION
The quantity takeoff is best performed by the estimator building the project in his or her mind, keeping track of the materials, equipment, and labor tasks needed to complete the project. The estimate should be broken down into work packages that contain related materials and tasks, such as materials that will be ordered together. The planned use of materials, labor, and equipment must be communicated with the field personnel.
The quantity takeoff for materials can be divided into counted items (both repetitive and nonrepetitive), linear components, sheet and roll goods, volumetric goods, and quantity-from-quantity goods. Similar methods are used to determine the quantities in each of these categories.
PROBLEMS
1. What is the best way of performing the quantity takeoff? 2. Why should the estimator communicate the planned
material use and construction methods to the field personnel?
3. Determine the number of studs needed for a 75-foot-
long wall where the studs are spaced at 1 foot on center.
4. Determine the number of studs needed for a 120-foot-
long wall where the studs are spaced at 16 inches on center.
5. Determine the number of 20-foot-long pieces of pipe
needed to complete 150 feet of pipe. The pipe is con- nected with a butt joint.
6. Determine the number of 20-foot-long pieces of rebar
needed to complete a 240-foot-long footing. The pieces are spliced with a lap of 18 inches and form a single bar running the length of the footing.
7. An 8-foot-high by 50-foot-long wall is constructed of
15 5>8-inch-long by 7 5>8-inch-high blocks. The mortar joints between the blocks are 3>8 inch thick. Using the area method, determine the number of blocks needed to construct the wall.
8. An 8-foot-high by 50-foot-long wall is faced with 7 1>2-
inch-long by 2 1>2-inch-high bricks. The mortar joints between the bricks are 1>2-inch thick. Using the area method, determine the number of bricks needed to face the wall.
9. Use the row and column method to solve Problem 7. 10. Use the row and column method to solve Problem 8. 11. Determine the volume of a 200-foot by 75-foot concrete
slab. The slab is 6 inches thick. Express your answer in cubic yards and include 10% waste.
12. Determine the volume of a 20-foot by 15-foot by 4-
foot-high concrete footing. Express your answer in cubic yards and include 5% waste.
13. Determine the quantity of paint needed to paint 12,620
square feet of wall. One gallon of paint will cover 250 square feet of wall.
14. Determine the quantity of joint compound needed to
finish 3,000 square feet of gypsum-board wall. One gallon of joint compound will cover 200 square feet of wall.
15. Determine the quantity of roofing nails needed for a
27.65 square (square⫽ 100 square foot) roof. Historically, 375 nails are needed per square.
16. Determine the quantity of shims needed to install 22
doors. Historically, 0.75 bundles of shims are needed per door.
17. The slab in Problem 11 required 300 cubic yards of
concrete. Determine the actual waste percentage for the slab.
18. The footing in Problem 12 required 47 cubic yards of
concrete. Determine the actual waste percentage for the slab.
REFERENCE
1. For more information on construction accounting systems and tracking quantities, see Steven J. Peterson, Construction
Accounting and Financial Management, Prentice Hall, Upper
66
In this chapter you will learn how to apply the principles in Chapter 4 to concrete, forms, and reinforcing for continuous and spread footings, walls, square and round columns, beams, slab on grade, raised slab, and stairs. This chapter includes sample worksheets that may be used in the quantity takeoff. You are encouraged to set these worksheets up in Excel and learn how to use them. This chapter also includes example takeoffs from the residential garage drawings given in Appendix F.
C
oncrete work consists of three tasks: installation and removal of the forms, installation of the rein- forcement (rebar and wire mesh), and placement and finishing of the concrete. The installation of the forms and rebar may occur in many different orders. For footings, the forms are placed and then the rebar is placed inside the forms; for columns, the rebar is placed and the forms are placed around the rebar; and for walls, one side of the forms is placed, the rebar is placed, and then the forms are finished. After the rebar is placed and the forms are completed, the concrete is placed and then finished. After the concrete has cured, the forms are removed. With careful planning and scheduling of the concrete work, the forms can be reused many times on the same job.FORMS
Forms or formwork may be bid by itemizing all of the com- ponents of the forms, or they may be bid by the square foot, the lineal foot, or set of forms. When bidding formwork by itemizing all of the components of the forms, the estimator must be familiar with the forming system to be used and build the forms in his or her mind, counting the items as he or she goes.
Estimators will often bid the formwork based on the square footage, the lineal footage, or the set of forms, rather
than accounting for each of the items needed. To do this the estimator must have historical data on the equipment, mate- rials, and labor needed to complete a square foot, lineal foot, or set of forms.
Beams, girders, walls, and tall footings (footings greater than 12 inches high) are often bid based on the square footage of forms needed to form the concrete member. When figuring the area, the estimator must make sure to in- clude forms on all sides and the bottom (if needed) of the concrete member. For example, on a foundation wall the es- timator will need to include both sides of the wall and the ends of the wall. The area of the forms should be based on the area of the forms, not the area of the wall. Wall forms typically come in 2-, 4-, and 8-foot heights. When forming an 18-inch-high footing or wall, a 2-foot-high form would be used. In this case the forming height would be 2 feet rather than the wall height of 18 inches.
Short footings (footings 12 inches high or less) can be formed by using 2⫻ 10s and 2 ⫻ 12s; therefore, they are often bid based on the lineal footage of forms. Just as in the case of the walls, the estimator needs to be sure to include forms on both sides and ends of the footings.
Columns are often bid based on the square footage (square and rectangular columns only), lineal footage of the column, or set of forms. When prefabricated forms or Sonotube are used, they may be bid based on the set of forms required.
Raised slabs are often bid based on the square footage of the slab, often ignoring the small amount of forms needed around the perimeter. When forming a raised slab, the bot- tom forms of the slab need to be supported until the slab has cured sufficiently to support its own weight.
When estimating forms, the estimator must determine not only the quantity of forms to be installed, but also the quantity of form material that must be purchased for the project, as it affects the cost of the forms. The quantity of forms that needs to be purchased for a project is based on how many times the forms can be reused, whether the forms
CONCRETE
can be used on other projects, and the life of the forms. If the concrete for a project can be poured in four separate pours with sufficient time between the pours to allow the concrete to cure, the project will take one-fourth of the forms it would need if the concrete on the project was poured in a single pour. If the forms are available from another project or can be used on a subsequent project, the purchase of the forms may be spread out over many projects, reducing the cost of forms that need to be included in the estimate for the project. However, if a project requires a custom form that will not be reused, the entire purchase of the forms must be included in the estimate. The final factor is how long the forms will last before they wear out or are consumed as they are cut to meet specific dimensions.
REINFORCING
Concrete reinforcing consists of rebar, welded wire fabric, chairs to position the rebar, and so forth. Rebar is designated by bar size, which represents the nominal diameter of the bar in eighths of an inch. For example, a #7 bar has a nominal di- ameter of 7>8 inch. Rebar is taken off by the lineal foot or by the number of pieces. Small quantities of rebar are often purchased by the piece or by the lineal foot, whereas large quantities of rebar are often purchased by the pound or ton. The weight per foot for common rebar sizes is shown in Table 5-1.
Continuous rebar must be lapped. The lapping is speci- fied by the structural engineer or other design professional. In the event it is not specified by the design professional, the contractor has to meet the lapping requirement in the build- ing code of the jurisdiction where the building is being con- structed. Lapping is specified by a minimum length or a number of bar diameters or both. For example, the drawings may require that the rebar be lapped 24 bar diameters or 1 foot, whichever is greater. For a #3 bar, 24 bar diameters is 9 inches (24⫻ 3>8 in). The bar would have to be lapped the greater of 9 inches or 1 foot; therefore, the bar would have to be lapped 1 foot. For a #6 bar, 24 bar diameters is 18 inches (24⫻ 6>8 in), and the bar would have to be lapped the greater of 18 inches or 1 foot.
On small jobs the rebar is often fabricated in the field from straight bar. A common length for rebar is 20 feet; however, lengths up to 60 feet are often available. For large
jobs, the rebar is often shop fabricated and shipped to the jobsite ready for placement.
Many projects are now specifying epoxy-coated rebar. Typically, epoxy-coated rebar is light green and is cut and bent in a fabrication shop. Epoxy-coated rebar that has been field cut and bent must have all cuts and bends re- coated. This will require additional installation time in the field and epoxy paint to coat the rebar. Field cuts and bends should be noted in the estimate so they can be included in the estimate.
Welded wire fabric (WWF), often called wire mesh, is designated by the spacing of the wires and the wire size. For example, WWF 6⫻ 6 10>10 is made up of #10 wires welded at 6 inches on center in both ways. Welded wire fabric is available in flat sheets or rolls 5 feet wide by 150 feet long. Like continuous rebar, welded wire fabric must be lapped as specified by the design profession or building code. A com- mon lapping is the spacing between wires. In the case of WWF 6 ⫻ 6 10>10 it is 6 inches.
CONCRETE
The specifications for concrete vary based on its use. Concrete used for slabs often has a higher strength than con- crete used for footings and foundations. The estimator must read the specification to determine the different types of concrete used on the project. When estimating concrete, the quantities for different types of concrete must be kept sepa- rate. Concrete specifications may be performance based, de- sign based, or both. A performance-based specification sets the requirements for the physical properties of the concrete, such as strength and slump. A design-based specification sets limits on the various components used to compose the con- crete. Concrete is typically composed of cement, large aggre- gate, fine aggregate, and water. Cement powder comes in five types. Type I is normal cement, Type II is moderate-sulfate- resistance cement, Type III is high-early-strength cement, Type IV is low-heat-of-hydration cement, and Type V is high-sulfate-resistance cement. In some concrete mixes, part of the cement is replaced by fly ash, a pozzolan. Admixtures may be added to achieve specific physical properties, such as air entrainment. All of these affect the price of the concrete. The estimator needs to carefully review the specifications to make sure he or she understands what type of concrete is re- quired to complete the job.
Pricing for concrete is best obtained from the concrete suppliers. It is best to provide the supplier with a copy of the specifications for use in pricing the concrete. Concrete companies often charge extra for small or short loads. Care should be taken to note any short loads during the takeoff process.
During hot weather, ice or chilled water is often added to keep the concrete from overheating. This in- creases the cost of the concrete, and the estimator must incorporate these costs into the concrete estimate. During the quantity takeoff the estimator needs to identify the need for ice or chilled water.
TABLE 5-1 Rebar Sizes and Weights (lb/ft)
Bar Size Weight Bar Size Weight
2 0.167 8 2.670 3 0.376 9 3.400 4 0.668 10 4.303 5 1.043 11 5.313 6 1.502 14 7.650 7 2.044 18 13.600
When concrete is placed in cold weather, precautions need to be taken to ensure that the concrete does not freeze. During cold weather the water added to the concrete is usu- ally heated, adding to the cost of the concrete. In addition, the concrete is often protected by covering the concrete with plastic, straw, or concrete blankets and by providing heat. The need for hot water and concrete protection should be included in the concrete takeoff.
The labor required to place the concrete consists of two components: the placement of the concrete and the finishing of the concrete. The placement of the concrete is bid by the cubic yard; the finishing of the concrete is bid by the square foot or square yard of the finished surface. Often the two components are combined into a single cost for a concrete component of a specified size.
A concrete pump is often used to place concrete that can- not be discharged directly from the concrete truck into its final place. Estimators need to be sure they understand what access is available to the site so they can determine the amount of concrete pumping that will be necessary to complete the work. When pumping concrete, care must be taken to ensure that the concrete mix has been designed for pumping. Not all concrete mixes can be pumped with satisfactory results.
Concrete waste varies based on the type of member being constructed. Beams, columns, walls, and other mem-