Scheduling as Part of the Project Management Effort - Construction Project Scheduling and Control - page 277

quantity by the daily production rate. For instance, in the preceding example,
if we had to place 166 CY of concrete, the duration would be 166∕90 ≈ 2days.
The PM might choose to use multiple crews or to adjust the productivity as
a result of favorable or unfavorable conditions. Regardless, one source of pro-
ductivity will feed both the estimate and the schedule.
Materials costs do not affect the productivity level, the durations, or the schedule.
One important note regarding materials is how estimators apply the waste factor. A
waste factor must be added to the estimate of the materials; however, good estimators
differentiate between “takeoff quantity,” which does not include waste allowance, and
“order quantity,” which does include such allowance. The problem with adding the
waste allowance to the takeoff quantity is that it will apply to labor and equipment,
although it shouldn't. This erroneously affects both labor productivity and cost. For
example, suppose that you are taking off the number of studs for framing a building
and you have calculated a total of 536 studs (2 by 4, 9' 4'' long 9 ) are needed, that is,
3,335 board-feet. You want to order 600 studs (2 by 4, 10' 0'' long), that is, 4,000
board-feet. The total cost should be:
Total cost = takeoff quantity ∗[unit price for labor and equipment
+ unit price for materials ∗(1 + waste factor %)]
The scheduler must be extremely careful about using estimate-generated infor-
mation, for three reasons. First, activities in such a schedule are defined by the
estimator, not the scheduler. Second, the durations in such schedules are based
on the productivity level built into the estimate items. In reality, the productivity
level may need to be adjusted because of many factors, such as project size and
complexity; crew size; skill level; weather conditions; site conditions; equipment size,
type, and condition; overtime or the lack thereof; and management competency
and experience. Formulas and adjustment tables and curves have been developed
to allow productivity-level adjustments for individual factors such as weather and
overtime (Oglesby, Parker, and Howell, 1989). 10 Some authors have even suggested
using one formula that incorporates these factors for the productivity adjust-
ment (Neil, 1982). Third, in some activities, two or more crews may be working
simultaneously (a common occurrence in large projects), which will reduce the
duration accordingly.
After the scheduler has adjusted the productivity rate, the logic (relationships and
lags) must be built. The importance of such a step cannot be overemphasized.
9 For rough carpentry such as framing, the waste percentage is usually higher than for other items because
waste allowance not only covers broken, defective, and missing pieces but also waste resulting from cutting.
For example, in the studs case above, the true length may be 9' 4'' (9 feet + 4 inches) but studs are sold only
in increment of 2 feet, so the estimator has to order studs 10 feet long.
10 See also the discussion on overtime in Chapter 8.