Comprehensive guide to BIM clash detection covering hard clashes, soft/clearance clashes, and workflow clashes. Includes the discipline coordination matrix, priority classification, resolution workflow, and common clash rules used in Indian construction projects.
Clash Categories
Three fundamental types of clashes detected in federated BIM models
| Clash Type | Disciplines | Tolerance | Priority |
|---|
| Hard Clash | Any two physical elements occupying the same space | 0 mm (physical intersection) | Critical |
| Soft Clash (Clearance Clash) | Element violating required clearance or maintenance access zone around another element | 25–300 mm depending on element type and code requirements | High to Medium |
| Workflow Clash (4D/Time Clash) | Scheduling conflict — two trades or activities requiring the same space at the same time | N/A (time-based) | Medium |
| Duplicate Element Clash | Same element modeled twice — from linked models or copy errors | 0 mm (exact or near-exact overlap) | Low |
| Design Intent Clash | Element violates design standards or code requirements (not physical intersection) | Varies by code requirement | High |
Priority Classification
Classify each clash by severity to prioritise resolution — typical project classification
| Clash Type | Disciplines | Tolerance | Priority |
|---|
| Priority 1 — Critical | Structural vs MEP (major services) | 0 mm | Critical — resolve within 48 hours |
| Priority 2 — Major | Architecture vs Structure, MEP vs MEP (different services) | 0–50 mm | High — resolve within 1 week |
| Priority 3 — Moderate | Clearance violations, maintenance access issues | 25–150 mm | Medium — resolve before construction of that zone |
| Priority 4 — Minor | Cosmetic or non-structural items | 50–300 mm | Low — resolve before finishing works |
| Priority 5 — Acceptable / Approved | Known conditions accepted by design team | N/A | None — approved exception |
Clash Resolution Workflow
Step-by-step process from detection to sign-off
| Clash Type | Disciplines | Tolerance | Priority |
|---|
| Step 1 — Federated Model Setup | All disciplines | N/A | N/A |
| Step 2 — Define Clash Rules | BIM Coordinator | Set per rule | N/A |
| Step 3 — Run Clash Detection | BIM Coordinator | Per rule | N/A |
| Step 4 — Filter False Positives | BIM Coordinator | N/A | N/A |
| Step 5 — Classify and Assign | BIM Coordinator + Discipline Leads | N/A | Assign P1–P5 |
| Step 6 — Coordination Meeting | All discipline leads | N/A | Focus on P1 and P2 |
| Step 7 — Model Update and Re-check | Assigned discipline | N/A | N/A |
| Step 8 — Sign-off and Close | BIM Coordinator + Project Manager | N/A | N/A |
Common Clash Rules
Standard discipline-vs-discipline clash tests used in building projects
| Clash Type | Disciplines | Tolerance | Priority |
|---|
| Hard Clash | Structural vs HVAC Ducts | 0 mm | Critical |
| Hard Clash | Structural vs Plumbing Pipes | 0 mm | Critical |
| Hard Clash | Structural vs Electrical Conduits/Trays | 0 mm | High |
| Hard Clash | HVAC Ducts vs Plumbing Pipes | 0 mm | High |
| Hard Clash | HVAC Ducts vs Electrical Cable Trays | 0 mm | High |
| Soft Clash | HVAC Equipment vs Maintenance Access | 600–900 mm clearance | Medium |
| Soft Clash | Architecture (False Ceiling) vs MEP Services | 50–100 mm below lowest service | Medium |
| Hard Clash | Architecture (Walls/Doors) vs Structure (Columns/Beams) | 0 mm | High |
| Soft Clash | Fire Protection (Sprinkler) vs All Services | Varies — sprinkler deflector clearance per NBC | High |
Notes
• False positive management is critical — poorly configured clash rules generate thousands of irrelevant results. Invest time in refining selection sets and exclusion rules before the first coordination meeting.
• Clash batching: group related clashes by zone/floor/system rather than reviewing individually. A single routing change may resolve 20+ related clashes at once.
• Sign-off process: every resolved clash must be verified in the updated federated model and signed off by the responsible discipline lead. Verbal agreements are not sufficient under ISO 19650.
• Establish a service zone hierarchy at project kickoff: typical order from ceiling down is (1) HVAC ducts, (2) cable trays, (3) plumbing pipes, (4) sprinkler pipes, (5) electrical conduits.
• Run clash detection at minimum fortnightly intervals during design development; weekly during construction documentation phase.
• For Indian projects, clash detection reports should be part of the BIM Execution Plan (BEP) deliverables per IS 19650 adoption guidelines.
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