Article Date: 07/26/2013

When Should Constraints Be Used in Scheduling?

By Steve Rizer


To what extent, if at all, should constraints be used when developing or reviewing a schedule for a construction project? “We always tell people, if you are either building a schedule or reviewing [it], … as much as possible, don’t use any constraints because it can throw off your float values in terms of how you may manage the job,” John Crane, a director with Trauner Consulting Services Inc., said during a recent WPL Publishing webinar, the target audience of which consisted of contractors, public and private owners, subcontractors, construction managers, owners’ representatives, architects, and other design professionals providing project-oversight services. “It can dictate your float values throughout and also your longest path.”


But what about instructing engineers and schedulers to put a constraint on a completion milestone based on the contract completion date? Such a constraint can make it easier to keep track of whether the project is behind or ahead of schedule and determine what the float is on secondary or tertiary paths. A webinar attendee made this point during the “Q&A” period of “Two Comprehensive Approaches for Measuring Delay.” 


In response, Crane said, “I have worked with the Virginia Department of Transportation over the years, and one of the things that they do is they actually put a constraint on the schedule -- not [on] one of the activities, like finished activities or anything -- but they will put a ‘must-finished-by’ constraint on the whole schedule itself.… If you have an early completion, let’s just say, on the baseline schedule, if you do have an early completion schedule, that [then may] show 30 days of float to the longest path of the project, not zero days. [This is] exactly what you’re talking about” in the question.


“Now, I will say -- and I will tell everybody: approach [use of the constraint] with caution,” Crane advised. “If you do this, it is great in managing the job; however, [it is important to] understand your change clauses and [who] can use or own the float throughout the job,” Crane said. “This is because … you have now shown, technically, float in the schedule. Can the owner use that to make changes without any damages to you? So, make sure that whatever you do …, it coincides with the other terminology or the other clauses within the contract and, before you start, that everybody understands the consequences or the use of that float. And that’s where sometimes folks get into a heated argument.”


During the webinar, Crane made the following additional points:

  • Critical Path Method (CPM) schedules must be updated periodically and accurately to be a useful tool in delay analysis.
  • One of the weaknesses of contemporaneous analysis is that it requires a complete and accurate current schedule. Other weaknesses are that it requires accurate as-built information and it cannot be performed prospectively.
  • A weakness in the Time Impact Analysis is that it requires development of a “fragnet,” a partial schedule encompassing new work activities added by a change, and modification of the current schedule. Other weaknesses are that the fragnet must be negotiated and the delay is hypothetical.
  • The rules of schedule analysis are as follows: use contemporaneous schedules when available and reliable; recognize that the schedule is a dynamic planning tool and that the critical path changes; evaluate delays based on the status of the project at the time the delays occur; and only delays on the critical path can delay a project.

Crane’s presentation included a discussion of key scheduling terms, CPM scheduling, critical path schedules, delay analysis techniques, recommended practice for forensic schedule analysis, concurrent delays and float, among other things.


To purchase a recording of the 90-minute webinar, visit



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