The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 81

Discussion of Example 4.7
We will discuss the details of project compression in Chapter 8, but we note that in
cases such as example 4.7, when the project needs to be accelerated, not all activities
need to be shortened. This often overlooked point may result in a lot of wasted money
in construction projects. Shortening activities B, D, E, and so forth does not help.
At the same time, we observe that activity F, which is not on the critical path, needs to
be accelerated by 2 days. However, this may not be exactly true. If we accelerate any
activity on the critical path, other than G, by at least 2 days, activity F will not need
to be accelerated. Following is a numerical illustration of this point:
1. Thecriticalpath(A,C,G,I,N)is35dayslong.
2. The path (A, C, F, I, N) is 32 days long.
3. We need to shorten any path that exceeds 30 days.
4. Shortening a path is achieved by shortening the duration of an activity or a
combination of activities by the desired amount of days—5 in this case.
5. Since the two paths share all activities except F and G, any reduction in the
duration of any shared activity (A, C, I, or N) will result in a reduction in both
paths.
This subject is discussed further in Chapter 8.
Tip Box 4.12
Don't insert the contract/owner's “imposed finish date” in the schedule until you have
calculated your own finish date (based on your durations and logic).
LOGIC AND CONSTRAINTS
A trend among many schedulers and project managers is to “fix” the dates of activ-
ities rather than allowing the software to calculate them using the CPM concepts.
For example, the project manager might ask the scheduler to make Door Installation
start on 28 APR 2015. The scheduler assigns a constraint to this activity so that it
will start on that particular date. This practice strips the schedule of its most impor-
tant advantage: logic. The difference is simple: logic is dynamic; constraints are not.
This author—when put in the shoes of the scheduler—would ask the project manager,
“Why do you want to start that activity on 28 Apr 2015?” The project manager would
give an answer such as, “That's when the drywall will be finished” or “That's when
doors will be delivered.” The scheduler can then tie Door Installation to Finish Dry-
wall and/or Doors Delivery. To appreciate the advantage of logic over constraints,
assume that the predecessor, Finish Drywall, is delayed. With the absence of logic,