Clash Detection Is Where BIM Pays for Itself
An HVAC duct runs through a structural beam at the same elevation. The structural drawing shows the beam; the mechanical drawing shows the duct. In a traditional 2D drawing workflow, this conflict might not be caught until the contractor tries to install both in the field and discovers the problem - generating an RFI, a potential change order, and schedule delay. In a BIM coordination workflow, the duct and beam are both three-dimensional objects occupying real space, and software can flag the intersection automatically before construction begins. This is the fundamental value proposition of BIM coordination, and it is tested on the ARE PDD exam.
The Federated Model Approach
In a multi-discipline BIM project, each consultant produces their own discipline model (the architect’s architectural model, the structural engineer’s structural model, the MEP engineer’s mechanical model, etc.). These are combined into a federated model - a composite view that overlays all disciplines in the same coordinate space without merging the underlying files. The federated model is used specifically for coordination: finding clashes, reviewing spatial conflicts, and running clash detection. Each discipline retains control of their own model file.
Hard Clashes vs. Soft Clashes
Hard clashes (geometric clashes) occur when two objects physically occupy the same space: a duct runs through a structural member, or a pipe passes through a wall without a sleeve. Hard clashes are straightforward to detect and must be resolved before construction. Soft clashes (clearance clashes) occur when two objects are too close together to be installed properly, even if they do not actually intersect: an MEP system installed within the required maintenance clearance of another system, or equipment located in a space too tight for access. Soft clashes require tolerance rules to be set up in the clash detection software before running the analysis.
The Clash Detection Workflow
Clash detection is typically run in coordination software such as Autodesk Navisworks or Solibri. The process: discipline models are imported; clash rules are set (what objects should not intersect or be within certain clearances of each other); a clash report is generated listing all detected conflicts with location and the conflicting objects; the design team reviews the report in a BIM coordination meeting; trades or disciplines responsible for each clash resolve the conflict by adjusting their models; and the process repeats until the clash count is reduced to an acceptable level for construction.
Key ARE PDD Exam Points
- Federated models overlay multiple discipline models in a shared coordinate space without merging the source files.
- Hard clashes are geometric intersections; soft clashes are clearance violations that prevent installation or access.
- Clash detection software (Navisworks, Solibri) generates clash reports showing location and conflicting elements.
- Clash detection does not replace architect’s review - it generates information for the design team to resolve.
- BIM coordination meetings use the clash report as the working agenda; resolved clashes are tracked across coordination cycles.
Study PDD on AREprep
AREprep’s PDD flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.