Ductwork clash detection

Why Ductwork Drives So Many Clashes

Ductwork is the largest physical service in most ceilings, a 24x12 supply trunk takes more plenum than most pipes and conduits combined. Ductwork also branches frequently, has insulation that increases outside dimensions beyond the duct's nominal size, and serves equipment with tight elevation requirements. Most above-ceiling clashes involve ductwork.

The Most Common Ductwork Clashes

• Supply or return ductwork crossing structural beams without coordinated openings
• Insulated duct dimensions exceeding available plenum
• Ductwork crossing fire sprinkler branch lines at the same elevation
• Return air plenum interrupted by hung piping or conduit
• VAV box installed above a ceiling that lacks the required service access
• Diffuser locations conflicting with sprinkler heads or light fixtures
• Duct hangers conflicting with cable tray or other suspended utilities
• Equipment dimensions on shop drawings larger than design drawings

Modeling Ductwork for Clash Detection

Effective clash detection starts with accurate modeling. Specific items to verify in the model:

• Ducts modeled at outside dimensions including insulation thickness
• Hangers and supports modeled, not just ducts
• VAV boxes, fire/smoke dampers, and other in-line components included
• Service clearance volumes modeled for VAVs and similar equipment
• Diffuser neck height and trim dimensions modeled to ceiling face
• Risers modeled with the actual transitions, not just rectangular boxes
• Equipment listed with actual manufacturer dimensions, not generic library blocks

Clash Detection Workflow

• Run clash detection at DD with ductwork vs. structure to verify available plenum
• Run at 60% CD with ductwork vs. all MEP and structure
• Run at 95% CD with ductwork vs. everything including ceiling fixtures
• Categorize clashes by severity: hard clash, soft clash (clearance), or acceptable
• Assign each clash to a discipline owner for resolution
• Re-run after resolution to verify the fix didn't create a new clash

Field Verification Before Sheet Metal

Even with BIM, field conditions don't always match the model. Verify the critical items on-site before sheet metal is fabricated:

• Existing structure dimensions and elevations
• Ceiling height vs. designed clear height
• Beam web depths and stiffener locations
• Existing piping and conduit not in the model (typical on renovations)
• Equipment locations vs. designed locations

Resolving Hard Clashes

• Reroute the smaller / more flexible service (typically conduit before duct)
• Raise the ceiling or modify the soffit in localized areas
• Drop a beam or add a coordinated opening (requires structural review)
• Resize ductwork (slower air velocity may allow smaller duct)
• Relocate equipment to a less congested area

Resolving Soft Clashes (Clearance Issues)

• Confirm code or manufacturer minimum clearances
• Verify maintenance access is preserved
• Document any clearance variance approval from the AHJ or designer