The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 52

In most construction projects, durations are calculated in workdays ,which
usually, but not always, follow a 5-day workweek. In some cases, the project, or
only certain crews, follow a 6- or 7-day workweek. In some other countries, a 6-day
workweek may be the norm. In others, such as those in the Middle East, a workweek
starts on Sunday not Monday. The scheduler must be aware of such calendars.
The scheduler must also account for nonworkdays , such as holidays, rain
(weather) days, shutdowns, and the like. The contract usually specifies official
holidays and the allowed number of rain days . Rain days represent the number of
days (per month) during which work on the project is expected to be suspended
because of weather delays. This prediction is usually calculated on the basis of
past meteorological data for the location. It does not include unusual weather and
disasters. (Delays due to unusual weather or disasters are discussed in Chapter 13.)
The distribution of this time allowance is discussed in Chapter 10.
Tip Box 4.4
The scheduler should never unilaterally assign durations to activities without consul-
tation with and getting an approval from the project manager and the technical teams'
leaders.
Although the workday is the most often used calendar unit in construction
projects, other units of time, such as hour, week, or month, are not unthinkable.
Industrial and process (oil/gas) projects may have to be scheduled by the hour,
especially with multiple-shift schedules. The scheduler, along with the management
team leaders, must decide on the unit of time before preparing the schedule because
changing the unit of time after the project starts is extremely difficult and confusing
and can lead to unnoticeable errors. When choosing a time unit for duration, you
can roll up the timeline in reporting, but you cannot roll it down. For example,
if the time unit is “week,” you can report in weeks, months, quarters, or years, but
you cannot report in days.
Tip Box 4.5
If the activity has a planned nonwork (pause) period, it may be better to split it so that
the resulting activities don't have any nonwork “gaps.”
3. Determine logical relationships
Again, this step is a technical matter. The scheduler should obtain this information
from the project managers and technical team leaders. Logical relationships should
not be confused with constraints.
a. A logical relationship exists between two activities when the start of one activ-
ity (or finish, as is explained in Chapter 5) depends physically on the finish (or