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The Four Types of Constraints used in Determining Task Dependencies

The Four Types of Constraints used in Determining Task Dependencies

As part of project management planning, it is necessary to develop a concise and detailed project network diagram in order to complete an accurate and concise project schedule.  The establishment of task dependencies allows us to determine the methodologies needed to estimate task duration and the dynamic on how the tasks interrelate. By understanding these dependencies, a project manager can contemplate the best approach toward the overall project schedule and identify tasks that have potential to skew the project schedule.  Task dependencies deal with the relationships between given tasks and how they affect each other. The four task dependencies as stated by Wysocki, (2009 pp. 166-167) are Finish-to-start(FS), Start-to-start(SS), Start-to-finish(SF), and Finish-to-finish(FF). The types of dependencies used depends on the constraints that consist between various tasks that have been identified for the project. Understanding the dependencies of each task helps the project management effort to better understand the intricacies and correlation of each task.  There are four types of constraints in determining task dependency as described in our textbook (Wysocki, 2009 pp. 167-171) which are: Technical constraints, Management constraints, Interproject constraints, and Date constraints. During the preconstruction phase of a project, the management team will hold strategic planning sessions to specifically detail out the project network diagram and the overall project schedule.  I have encountered all four types of constraints in project network diagramming and will illustrate how these constraints have affected the sequencing of the tasks on previous projects.

  1. Technical constraints – These constraints deal with the actual construction processes that are tasked to complete an activity or assembly.  It has been my experience,  that technical constraints are the most common type of constraint and in many instances are based on practicality of building methods and standards. For instance, in constructing a footer assembly, excavation and trenching must first be completed(Task 1), then the forms must be put in place(Task2), then the structural rebar must be set (Task 3), then the concrete must be poured(Task4).  In this simple example each task must be completed before moving on to the next task.  This is known as a Finish-to-start(FS) dependency. The completion of each predecessor task determines when the next task can begin. It may be necessary for the project manager to make some decisions that are discretionary. These “judgment calls” (Wysocki, 2009 pp. 167)are decisions or strategies that the project manager may take in modifying the sequencing of tasks to a Start-to-start(SS)dependency. For instance, if the crew forming the footer assembly is ahead of schedule and there is a risk of rain, the project manager might relocate the crew to another task in order to eliminate the possibility of a washed out footer form. The project manager also may use Best-practices constraints(Wysocki, 2009 pp. 168) as he may use his own experiences from similar tasks or projects to adjust the schedule accordingly.  Using the aforementioned illustration, a project manager may determine that the overall project schedule could be accelerated if the footer crew was split and began performing their tasks concurrently at different sides of the building. I have found that it is quite necessary to communicate with field supervisory staff in order to maximize opportunities that may arise in performing tasks concurrently.  There are also Logical constraints (Wysocki, 2009 pp. 168) that a project manager uses based on his own reasoning due to the way he may think the overall project schedule could be optimized. I believe that we must use logic in all our decisions and that sometimes the use of logic, based on our experiences and past practical techniques, can provoke us to establish a certain constraint on a task. As an example,  I have discovered that “Threshold Inspections” on Federally funded projects can cause a lag variable of up to 10 days after the completion of a certain task, therefore, logic tells me that I have to schedule certain tasks concurrently and shift the resources around the project as not to delay productivity on a project. Unique requirements (Wysocki, 2009 pp. 169) are a type of technical constraints that deal with a critical resource or a task that has a unique requirement that affects the successor task. There are unique requirements usually on all projects that can be from materials testing, regulatory inspections, specialized equipment, and other unique resources.
  2. Management constraints – This type of constraint is a management-imposed constraint and is different than the technical constraint since it can be reversed in comparison to the technical restraint that cannot(Wysocki, 2009 pp. 169). Many times, I have changed a 7 day work week to a 5 or 6 day work week because the project may be ahead of schedule and/or there will be a time that certain tasks may be completed a bit too early for the proper sequencing of the project.  Using double or triple daily shifts are also management constraints that are discretionary and usually can be reversed if necessary.
  3. Interproject constraints – This type of constraint is based upon the premise that a deliverable of one project or task is needed to complete another project(Wysocki, 2009 pp. 169-170). It is necessary to identify Interproject constraints during the early stages of project planning in order to account for any potential delays caused by an outside source. I have encountered many Interproject constraints specifically when it comes to custom or  manufactured systems, machinery or products.  For instance, some of our larger development projects have chiller plant facilities  that exceed 30,000 to 40,000 tons of refrigeration; these plants have custom made boilers and associated equipment that is one of a kind and is custom designed and manufactured off site.  It is critical that these various manufacturers and outside sources attend some of the project planning meetings in order to coordinate lead times for design, production, and delivery. This constraint sometimes is used with the Start-to-finish(SF)dependency when an existing boiler system (Task1)is installed then the old boiler system (Task 2) can be discontinued.
  4. Date constraints – Contrary to what the author says regarding Date constraints, this type of constraint is many times necessary and contractually required by the clients that hire our services.  Additionally, Date constraints establishes time milestones or benchmarks on a project that the entire construction team works toward. The three types of date constraints as described by Wysocki (2009 p. 170) are:
    1. No earlier than – Specifies the earliest date that on which a task can be completed.
    2. No later than – Specifies a date by which a task must be completed.
    3. On this date – Specifies the exact date on which a task must be completed.

I have used these types of date constraints on projects and found them to be very useful in maintaining the overall project schedule.  As an example, the “no earlier than” date is often used when installing certain interior finishes to a project. Many times we would not want a certain finish completed due to the possibility that the finish may be marred or ruined is completed prior to some other relevant tasks. In coordinating mechanical and electrical systems, we often use “no later than” date constraint in order to coordinate the various interconnections of these systems.  The “on this date” constraint is often used when there is a utility shutdown or interface with an outside agency that establishes a certain date for a certain task to occur.

As I have mentioned, date constraints are used often on my projects and are generally a requirement from our clientèle.  The US Department of Defense has very strict requirements for completion of projects and often state a definitive completion date and, if not met, they impose liquidated damages that can be considerable.  Many retail and commercial clients use date constraints in order to forecast leases, marketing strategies and financial cash flow projections.

The process of establishing task dependencies is crucial in understanding the dynamic of a project and the development of a functional project management schedule. Understanding the use of constraints is necessary and how the various constraints interrelate to each other and ultimately affect the overall project schedule. A study by Cheng, Choi, Lee and Wu (1999) warns that we must be cautious with potentially being redundant in using constraints, therefore, we need to review our network diagram as a whole and make any necessary adjustments.

Reference List:

  1. Wysocki, R.K. (2009) Effective Project Management: traditional, agile, extreme. 5th ed. Indianapolis: Wiley Publishing. (pp. 167-171)

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