Critical Path Method (CPM)

Many planning systems use the critical path method (CPM) in their approach, such as Primavera Project Planner® [1] and Microsoft Project® [2]. These traditional systems are based on a purely hierarchical top-down approach where every project is broken down into smaller activities. As traditionally being practiced, general contractors would prepare project schedules to cover the entire project duration and in turn, the superintendents may create look-ahead schedules that reveal more detail on activities to be executed in the near future. These look-ahead schedules are normally valid for up to 3 or 4 weeks out. Specialty contractors may also prepare their own schedules to meet the project’s needs.

There is a difference, however, between the schedules prepared by the specialty contractor and the main contractor. This is because each specialty contractor’s detailed design, fabrication, procurement and construction process normally takes the form of a job shop, which view differs from the project view adopted by general contractors in their CPM schedule [3]. CPM focuses on the project itself, which has a definite start date and finish date that is used to compute activity floats and level resources. By contrast, a job shop scheduling focuses on the continuous flow of work or jobs, where jobs have due dates but their execution may be interwoven so that there is no clear start or finish. Therefore, shop capacity is critical in maintaining a steady flow of work for all resources. Floats become a secondary issue provided that the shop has adequate capacity to perform all scheduled work. This is an important schedule constraint to be considered.

Existing CPM-based project management tools used in construction do not provide appropriate support to field crews, such that they do not help shield the crew from uncertainty and provide no insight into resource availability. This is because traditional CPM-based tools release to construction what should be done, instead of what can be done. In other words, the tools cannot give advice on which actions to take, especially when the actual project schedule differs from the planned project schedule. Resource leveling using CPM determines the total project duration or completion date by referring to the resource usage at completion, a quantity which is usually assumed at the initiation of the schedule; resource usage to date, which is the actual resource usage; and the resource quantity to complete, which is simply the resource usage at completion minus resource usage to date [1]. Therefore, the total project duration or completion date is still based on the initial assumption and do not reflect the actual condition of the project. It will be a great advantage if there is a tool that provides a method that can systematically evaluate project performance and resource sufficiency during project pre-planning and construction.

[1] Primavera Project Planner. (2004). Version 3.1. PA: Primavera Systems Inc.
[2] Microsoft® Project. (2002). Copyright© Microsoft Corporation. Redmond. WA.
[3] Tommelein, I. D. and Ballard, G. (1997). Coordinating Specialists. University of California. Construction Engineering and Management Program. Civil and Environment Engineering Department. Technical Report 97-8. Berkeley. CA.