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Describe the process of Critical Path Method (CPM) in construction scheduling.

Writer's picture: BHADANIS Quantity Surveying and Construction Management Training Institute for Civil Engineers & Construction Professionals OnlineBHADANIS Quantity Surveying and Construction Management Training Institute for Civil Engineers & Construction Professionals Online

What is CPM?

The Critical Path Method is a step-by-step technique used to identify the longest sequence of dependent tasks in a project, known as the "critical path." This critical path determines the shortest possible duration to complete the project. In other words, if any task on this path gets delayed, the entire project will be delayed by the same amount of time.


Step-by-Step Process of CPM

1. List All Tasks

First, you need to list out every single task required to complete the project. For example, in building a shopping mall, tasks might include “Excavate site,” “Pour foundation,” “Erect steel framework,” “Install electrical systems,” “Finish interior walls,” and so on. Each task is a piece of the puzzle.



2. Determine Task Dependencies

Next, figure out which tasks depend on others. For instance, you can’t start “Erect steel framework” until “Pour foundation” is complete. These dependencies help you understand the order in which tasks need to be completed.

For example:

  • Task A: Excavate site

  • Task B: Pour foundation (depends on Task A)

  • Task C: Erect steel framework (depends on Task B)


3. Estimate Duration for Each Task

Now, you need to estimate how long each task will take. Let’s say:

  • Task A: Excavate site – 5 days

  • Task B: Pour foundation – 7 days

  • Task C: Erect steel framework – 10 days

These durations are crucial because they help you calculate the total time needed to complete the project.


4. Create a Network Diagram

With your tasks and dependencies in hand, you can create a network diagram. This is a visual representation of all the tasks and their dependencies. Each task is represented by a node (a box or circle), and arrows are used to show the sequence in which the tasks must be completed.

In our shopping mall example, the diagram might look something like this:

  • Task A → Task B → Task C



5. Identify the Critical Path

Now comes the key part—identifying the critical path. To do this, you’ll look for the longest path of tasks that determine the project’s duration. In our example:

  • Task A (5 days) + Task B (7 days) + Task C (10 days) = 22 days

If Task A, B, or C gets delayed, the entire project will take longer. This sequence of tasks (A → B → C) is your critical path.

6. Monitor and Manage the Critical Path

Once you know the critical path, you need to monitor these tasks closely throughout the project. If any task on this path is delayed, you’ll need to take immediate action to get back on track, whether that’s reallocating resources, working longer hours, or finding ways to speed up the task.


Example in Practice


Let’s say you’re managing the construction of the shopping mall in Riyadh. The critical path includes excavating the site, pouring the foundation, and erecting the steel framework. If the excavation takes an extra two days because of unexpected soil conditions, your entire project is now two days behind schedule. To compensate, you might decide to bring in additional workers for the foundation phase to make up lost time.


Float or Slack Time

Another concept that CPM introduces is "float" or "slack" time. These are tasks that aren’t on the critical path and have some flexibility in their scheduling. For example, if installing interior walls can be delayed by a few days without affecting the overall project timeline, that task has some float. Knowing which tasks have float allows you to prioritize the critical tasks that can’t afford delays.

Benefits of CPM

CPM is incredibly valuable in construction management because it helps you:

  • Focus on the tasks that are most crucial to the project’s success.

  • Predict potential delays before they happen.

  • Optimize resource allocation by understanding which tasks can be delayed without impacting the project timeline.

  • Keep the project on track by closely monitoring the critical path.


Real-World Scenario

Imagine you’re overseeing the construction of a high-rise building in Jeddah. The critical path might involve key structural tasks like laying the foundation, constructing the core, and installing the steel framework. Meanwhile, tasks like landscaping and painting could be non-critical with some float. By focusing on the critical path, you ensure the building structure is completed on time, even if less crucial tasks like landscaping run slightly behind.

In short, the Critical Path Method is all about prioritizing the most important tasks and making sure they’re completed on time so the overall project stays on schedule. It’s a powerful tool that helps you manage complex construction projects with confidence, ensuring you meet deadlines and deliver successful results.



Critical Path Method (CPM) Template for Construction Scheduling

Task ID

Task Name

Duration (Days)

Dependencies

Start Date

End Date

Critical Path (Yes/No)

Float/Slack (Days)

1

Excavate Site

5

None

[Start Date]

[End Date]

Yes

0

2

Pour Foundation

7

Task 1 (Excavate Site)

[Start Date]

[End Date]

Yes

0

3

Erect Steel Framework

10

Task 2 (Pour Foundation)

[Start Date]

[End Date]

Yes

0

4

Install Electrical Systems

8

Task 3 (Erect Framework)

[Start Date]

[End Date]

No

[Float Days]

5

Finish Interior Walls

6

Task 4 (Install Electrical)

[Start Date]

[End Date]

No

[Float Days]

Additional Sections for CPM Analysis

Critical Path Analysis

Details

Total Project Duration

[Total duration of the critical path tasks]

Critical Path Tasks

[List of tasks identified as critical]

Non-Critical Tasks with Float

[List of tasks that have float/slack time]

Key Risks on Critical Path

[Identify potential risks that could impact tasks on the critical path]

Mitigation Strategies

[Plans to address risks if delays occur on the critical path tasks]

Example Data

Task ID

Task Name

Duration (Days)

Dependencies

Start Date

End Date

Critical Path (Yes/No)

Float/Slack (Days)

1

Excavate Site

5

None

01/09/2024

05/09/2024

Yes

0

2

Pour Foundation

7

Task 1 (Excavate Site)

06/09/2024

12/09/2024

Yes

0

3

Erect Steel Framework

10

Task 2 (Pour Foundation)

13/09/2024

22/09/2024

Yes

0

4

Install Electrical Systems

8

Task 3 (Erect Framework)

23/09/2024

30/09/2024

No

3

5

Finish Interior Walls

6

Task 4 (Install Electrical)

01/10/2024

06/10/2024

No

2

Key Points for CPM Analysis

  • Total Project Duration: 22 Days

  • Critical Path Tasks: Task 1, Task 2, Task 3

  • Non-Critical Tasks with Float: Task 4, Task 5

  • Key Risks on Critical Path: Potential delays in the steel framework delivery

  • Mitigation Strategies: Expedite material procurement or adjust the sequence of non-critical tasks to cover any delays

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