The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 82

we need to go to Door Installation and manually move it. With logic, all we have to
do is recalculate the schedule.
It is also possible to combine logic and constraints. Assume that an activity may
be completed as early as 08 June 2015, but the project manager wants to delay it until
15 July 2015 for a reason that may have nothing to do with logic, such as cash-flow
requirements. The scheduler may add a constraint (without taking out the logic) that
will not allow it to start before 15 July 2015, assuming this date is LF.

The author believes that the overwhelming reason for using constraints in lieu
of logical relationships is a lack of understanding of the concept of the CPM. Some
contractors, especially those who work on small projects, used to make bar chart
“schedules” by using a spreadsheet program or even by drawing a bar chart manu-
ally without any CPM calculations. In such schedules, the contractor would choose
the start and finish dates for every activity rather than calculating them. Such a bar
chart is subjective and not based on CPM. Furthermore, it may not have much value
in the court of the law. Consequently, using such schedules is strongly discouraged.
Nevertheless, constraints may be extremely useful. The owner may have, in addi-
tion to the project completion date, other deadlines, such as Substantial Completion,
which means reaching the stage when the facility can be used for the main purpose
for which it was intended. Even though the deadline for Substantial Completion may
be in the contract, the contractor must show it as a milestone in the schedule. The
owner bears the responsibility of noting the absence of such milestones in the schedule
before accepting it.

≤

Tip Box 4.13

If you have too many constraints (more than you can count on your fingers), review
them. You may have used constraints in lieu of logic.

The “Hub” Concept

Airlines use the term hub to describe a central airport where the airline has the most
flights: arriving as well as departing. This concept applies to activities or milestones in
logic networks with la arge number of predecessors and/or successors. An example of
this is Building Dry-in, Substantial Completion, Walk Through, or Punch List. Each
of these activities/milestones has many predecessors. An activity such as Materials
Delivery, when it means the arrival of a delivery truck carrying materials for several
activities, could be a predecessor to many activities. There is a high likelihood for hub
activities to be critical. In fact, special attention must be given to hub activities, as a
delay in one of them can be a cause of serious delays in the schedule.

The Definition of Critical Path, Revisited

We defined critical path earlier as “the longest continuous path in a network from start
to finish. It represents the summation of the durations of activities and lags along that