Transcript Chapter 2 Arithmetic and Principles of Measurement

Chapter 2
Arithmetic and
Principles of Measurement
How is Work
Measured in a Takeoff?
• Work is measured in five different kinds of units:
– By counting the number of pieces; for example, the
number of doors
– By measuring the length of an item such as a pipe
– By measuring an area of a material; for instance the
area of floor sheathing
– By measuring the volume of a material such as
concrete
– By measuring the weight of materials such as
structural steel beams
Calculating Number of Items
• Enumerated items are often counted from the drawings, but
sometimes you need to calculate the number. Examples:
– How many bolts spaced at 24" are required on a 13'-0" wall (the
end bolts are 2" from the end of the wall)?
– Answer: 13'-0" - 4" = 12'-8" (12.67')
12.67' / 2.0'= 6.3 (round up to 7)
+ 1 for the end bolt = 8
– How many studs spaced 16" are required for a 21'-0" wall?
– Answer: 21.0'/ 2.0' = 10.5 (round up to 11)
+ 1 for the end stud = 12
Length Calculations
• Usually the length of an item is obtained directly from the
dimensions on the drawings.
• Scaling lengths is not recommended but may be necessary when
dimensions are lacking.
• Many buildings have perimeter walls. The volume of concrete in
a perimeter wall is obtained by multiplying the centerline length of
the wall, the width of the wall, and the height of the wall.
• The centerline length may also be used to calculate a number of
other items: forms for walls, trenching, concrete, and forms for
wall footings.
Perimeter Calculations
• The perimeter length of the basement wall
shown of Figure 2.3 is calculated in this way:
2 x 42'-0" =
84'-0"
2 x 26'-0" =
52'-0"
= 136'-0"
Less 4 x 8"
=
<2'-8">
133'-4"
Calculating Areas
• Most areas in estimating will be rectangles, but
triangles, circles, and other shapes sometimes have
to be calculated.
• The area of the building (to outside of walls) shown
on Figure 2.12 is calculated in this way:
42.0' x 38.5' = 1617
+ π x 21.0'2 / 2 = 692
2309 sf
Calculating Volumes
• To calculate the volume of concrete in a continuous footing to the
perimeter of a building use this formula:
– Volume = centerline length x width x depth
– Example: If there is a footing size 2'-0" x 1'-6" under the wall
shown on Figure 2.4, what is the amount of concrete in this
footing?
less
2 x 25.0 =
2 x 16.0 =
4 x 0.5
volume: 80.0 x 2.0 x 1.5
50.0
32.0
=
<2.0>
=
80.0
= 240 cu. ft.
=
9 CY
The Quantity Takeoff
• A quantity takeoff is the process of measuring the work of the
project.
• The document that is produced in this process is also called a
takeoff.
• A takeoff is a series of quantified items that correspond to the
tasks required to construct the project.
– A takeoff item comprises a description and a set of dimensions
that define the quantity of the item.
• Items are measured according to standard rules of measurement
so that all estimators measure the same kind of items in the
same way.
– For example, the standard units for measuring concrete footings
are cubic yards.
What is Measured in a Takeoff?
• A takeoff for an estimate is more than a materials
takeoff because additional information is required for
pricing.
– For example: “160 square feet of ½″ G1S ply” is fine
for a material takeoff, but is to be applied to walls or
floors? This information is needed for pricing.
– Also, some items do not involve materials such as:
“hand troweling.”
Rules of Measurement
• Dimensions are entered onto the takeoff in this order: length,
width, and depth (or height).
• Dimensions are written in feet to two decimal places, thus, 5'-10"
would be written into a dimension column as 5.83.
• Deductions listed with the dimensions are written in red or
enclosed in brackets and noted as deductions.
• Extensions (the result of multiplying dimensions together) are
calculated to the nearest whole number whether it is linear feet,
square feet, or cubic feet.
• Totals in the extensions column are rounded off to the nearest
whole number.
• See text for a full list of measurement rules.
Takeoff Order and Strategy
• The order of the takeoff generally follows the
sequence of the work on the project.
• Strategy for larger projects:
– Divide the project into manageable parts
– Takeoff one part at a time
– Within each of these parts, measuring the work as a
sequence of assemblies
– Where an assembly is a collection of related items
Wall Assembly Example
• Items included in a wall assembly:
–
–
–
–
–
The concrete in the wall
The forms to the sides of the wall
The forms to openings and blockouts in the wall
The rubbed finish on the exposed concrete of the wall
The reinforcing steel in the wall
Computer Estimating
• Use of computers can increase the efficiency of the process.
– The computer can perform all the arithmetic to determine total
net quantities from the dimensions input.
– Items are automatically sorted and recapped into the required
order.
– Items can be automatically priced from the database.
• Keeping backup notes on paper is recommended.
– It provides a record of how the estimate was prepared.
– It enables the takeoff to be checked to ensure that all the work
has been measured as required.