4D BIM: Construction’s CPM Rosetta Stone

By: Robert Lamb

Critical Path Method (CPM) scheduling has been the “gold standard” and primary method of managing time on a construction project for more than half a century, but more often than not the schedule is an electronic file stored on a computer and is not used to plan and execute the work in the field. Schedules are presented at meetings and in reports in bar chart format that often fails to successfully communicate the key issues present in the schedule. These barriers are one reason why the CPM schedule is not effectively used as a project management tool on many construction projects today.

4D Building Information Modeling (4D) aims to bridge the CPM communication gap in both proactive and forensic applications. The ability to look into the future with clarity (visualize) affords projects new levels of coordination between trades and greater opportunities for plan optimization and risk management.

CPM was designed to depict the logical relationship between activities and provide a mathematical calculation to support the forecast project completion date—something that could not be done with the more archaic Gantt bar chart. Despite these advances, effective communication and comprehension of the CPM schedule has seldom been realized across trade tiers with traditional CPM reporting methods. This breakdown in communication often results in a disconnect between what the CPM schedule portrays as the plan to construct the project and how the project is actually being constructed.

A 4D is the result of combining a project’s CPM schedule and Building Information Model (BIM), the output being a virtual rehearsal of the construction of a project as sequenced in the “4th dimension” of time. The clarity by which 4D summarizes sequence and scope as outlined in the CPM is described by CPM experts as a seed change on the magnitude of the Gantt bar chart development. Those who have experienced 4D realize its inherent sophistication and value and are beginning to implement 4D requirements into their CPM scheduling specifications. When applied proactively, 4D clearly and efficiently conveys the unique approach to completing a project contained in the complex CPM schedule. 4D can and should be developed to achieve distinct goals such as:

  • Providing an overview of the entire project sequence
  • Conveying crew flow through the project
  • Optimizing concurrent operations
  • Verifying safety requirements as represented in the CPM
  • Analyzing dynamic clash detection between construction equipment/temporary works and the sequence to construct

When applied forensically, 4D summarizes complex analysis results with unrivaled clarity. Impacts can be reviewed visually and presented concisely through snapshots, 3D PDFs, video, or side-by-side comparisons of the construction project. Additionally, most modern projects are designed and/or coordinated in 3D/BIM; as such, the additional level of effort to create a forensic 4D is often minimal after the construction schedule is developed using available scheduling programs. Forensic 4D can be further utilized to highlight schedule flaws and variances between the contemporaneous CPM schedule and as-built sequence. As 4D is specified in greater frequency, the availability of contemporaneously prepared 4D will increase, along with opportunities to examine and use these contractual deliverables to gain and convey valuable insights into the mind’s eye of the project team at that point in time.

The question regarding 4D is no longer whether you should implement it on your project. Rather, the question is how soon you should implement this game-changing project control and analysis platform.