Two weeks ago, I attended a conference on model-based scheduling. In our industry, scheduling is like calculus — all stakeholders nod as though they understand why the derivative goes to zero, but really, only the Schedulers understand why the complicated schedules come together the way they do.
Scheduling is so complex because of the interdependencies involved. Construction projects are high risk projects that are worth millions of dollars with penalties for being late. This reality makes the schedule a critical element and puts additional pressure on its success.
When we connect a model and a schedule, we allow the tasks, logic dependencies, float, and critical path to be easily visualized and therefore more easily understood.
- The tasks are the discrete activities contained within the total scope of the project
- E.g. Pour slab for Area A, erect steel in Bay 1, etc.
- The logic dependencies are the relationships between the activities that define their place and path within the project. These relationships can be defined as Start-Start, Finish-Start, or Finish-Finish and can also include lag time within the dependency. Furthermore, each activity can have predecessors and successors
- E.g. “Hang ductwork in Area A” has a predecessor “Erect steel in Area A” and the relationship is “SS+3”. This means that there is a finish-start relationship where the steel activity needs to be completely finished before the ductwork can start (3 days after)
- The float is the amount of slack time a given activity has without affecting other activities across the project. It is created by logic dependencies and varies by the individual path an activity is contained within
- E.g. “Paint Office 1” has a duration of 2 days, a predecessor of “Tape and Float Drywall in Office 1” which ends no later than 2/9, and a successor of “Install cabinets” that begins no earlier than 2/15. That means “Paint” can start as early as 2/10 or as late as 2/12, giving the activity two days of float
- The critical path is the longest path of activities through the project, which typically has zero float. If the project is ahead, the critical path could have positive float, but if the project is behind, it would have negative float. CPM (critical path methodology) determines the critical path based on a forward and backward pass through each activity duration and relationship to other activities
As model-based schedules continue to grow more advanced, the powerful visualizations contained within help demystify the schedule so that all stakeholders can work together to lower the risk of the project. These advancements are making the schedule more accessible and empowering additional users with visualizations, data, and other outputs. Slowly, we are chipping away at the complexity of the schedule and figuring out additional ways to leverage it for our benefit.
As I reflected on the conference, I was struck by a contrast — on the one hand, our technologies have grown more advanced. With 4D BIM, you see a 3D model run in dated sequence as though you are watching a movie. On the other hand, the core science of scheduling is the same as how it was done in the 60s, and much of the work still gets done over the phone in a trailer at 6 am with some persuasive words. We can continue to improve CPM scheduling dramatically with technology, but it will never go away.
Don Henrich is an accomplished technology veteran in both the MCAD and the AEC industries. As President and CEO of Assemble he brings an distinguished track record of innovation, winning strategies, team building, and the ability to quickly grow revenue and market share. Don and his wife Noel have three children, reside in Marblehead, MA and spend as much time as possible sailing on Massachusetts Bay.
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