Schedule Compression and Time-Cost Trade-Off - Construction Project Scheduling and Control - page 236

Cycle 7
Path

Duration (days)

ABEH 24 24 23 22 21 20 19 18

ACEH 25 24 23 22 20 19 19 18

ACFH 23 22 22 21 20 19 19 18

ACFG 24 23 23 22 21 20 19 18

ADG 3 3 23 22 21 20 19 18

Now the entire project is completely crashed, with a duration of 18 days.
The results may be summarized as follows:

• The normal duration is 25 days, at a total cost of $7,950.
• The least-cost is $7,780, at a duration of 22 days.
• The crash duration is 18 days, with a crash cost of $8,100.

Notes

1. Accelerating can become complicated even in a small network. For
larger projects, we use computers, as explained in the next section.
2. Activity D is not completely crashed, which demonstrates an important
point: to accelerate a project, we do not need to compress all activities.
If we did, we would be wasting money with no benefit to the schedule.
3. We assume linearity within each activity (e.g., it costs the same amount
of money to compress activity C from 8 days to 7, from 7 days to 6,
or from 6 days to 5). However, we see that the change in total cost for
the project is not linear.
4. The project manager may choose a project duration between 22 days
and 18 days to meet the contract-required finish date (22 days for least
cost and 18 days for least duration).

ACCELERATING PROJECTS USING COMPUTERS
Real-life projects usually contain hundreds or thousands of activities. There may be
tens or even hundreds of paths, each with 50, 60, or more than 100 activities. Per-
forming calculations such as those done in example 8.1 would be too cumbersome.
The practical approach is to use trial and error. However, nine guidelines follow:

1. Always make a backup copy of the schedule before you make any changes.
2. Focus on the critical path. A good idea is to sort activities by total float (first
criterion) and start date (second criterion), in ascending order. You may also