Clash detection finds places where elements of a federated BIM model conflict, so they can be fixed on screen instead of on site.
Types of clashes
- Hard clash: two objects occupy the same space — a duct through a beam.
- Soft (clearance) clash: an object intrudes into a required clearance — maintenance access around an AHU, insulation thickness.
- Workflow (4D) clash: sequencing conflicts — deliveries or crews scheduled into the same zone.
A practical workflow
- Agree model origin, levels and naming so models federate correctly.
- Define a clash matrix: which disciplines are tested against which (e.g. MEP vs Structure first).
- Set tolerances — ignoring overlaps of a few millimetres removes noise.
- Run tests in Navisworks (or a similar tool) and group results by system and location; 2,000 raw clashes may be 60 real issues.
- Assign each group to a responsible party, track status in an issue log or BCF.
- Re-run after each model update until the agreed zones are clash-free.
Priorities
Gravity systems (drainage) first, then large ducts, then pipes, then cable trays — smaller, flexible services move last.
What clash detection doesn’t do
It won’t check code compliance or design intent. Coordination meetings and engineering judgement are still essential.