[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We run clash detection on a federated BIM model for an office building. The first report shows several thousand clashes between ducts, pipes, cable trays, sprinklers and structure. The coordination meetings get stuck on who moves.

• Is there an accepted priority order between systems?
• How should clash tests and tolerances be set up so the report is useful?
• What checks should be done besides hard clashes?

[MEP Center Engineering]

Give priority to what is hardest to move: structure and gravity drainage first, then large ducts, cable trays and busbars, sprinkler mains, large pressurised pipes, and small pipes and conduits last. Agree this order and the clearance rules in the BIM execution plan before the first clash run.

A practical priority order

1. Structure and architecture fixed elements (beams, columns, ceilings).
2. Gravity drainage and rainwater pipes: their falls cannot be changed without changing their level everywhere upstream.
3. Large ductwork and smoke extract ducts.
4. Cable trays, busbar trunking and ladders: limited bending radius, and spacing from wet services.
5. Sprinkler mains and branch lines (sprinkler heads have deflector distance rules from the ceiling).
6. Large chilled water, heating and domestic water mains, including insulation.
7. Small pipes, conduits and flexible connections.

Worked example: drainage fall budget in a ceiling void

A DN100 drain (110 mm outside diameter) runs 18 m at 1 % fall in a 600 mm void between slab soffit and ceiling.

• Fall over the run: 18 x 0.01 = 0.18 m = 180 mm.
• Start: top of pipe 50 mm below the slab for supports, so pipe bottom at 50 + 110 = 160 mm below soffit.
• End: pipe bottom at 160 + 180 = 340 mm below soffit, leaving 600 – 340 = 260 mm to the ceiling.
• A 400 mm deep duct cannot cross under the drain near its downstream end; it must cross near the upstream end (440 mm available) or be re-routed.

Doing this kind of check before modelling saves many clash cycles; it shows that drains, not ducts, decide the void zones.

Setting up useful clash tests

• Run tests discipline against discipline (e.g. ductwork vs structure, drainage vs ductwork), not “everything against everything”.
• Use a small tolerance to ignore touching elements and modelling noise, and soft-clash (clearance) tests for maintenance and insulation zones.
• Model insulation thickness, or test with an equivalent clearance.
• Group clashes by element or location so one rerouted duct closes dozens of clashes.
• Assign each group an owner and a due date, and track status between runs.

Beyond hard clashes

• Access and maintenance zones: filter withdrawal, valve handles, damper access panels, ceiling access hatches.
• Spacing from cable trays to wet services as the specification requires.
• Supports and hangers: room for threaded rods, unistrut and seismic bracing if required.
• Installation sequence: can the large duct actually be lifted into position after the pipes?
• Penetrations: sleeves and fire stopping coordinated with structure before casting.

Common mistakes

• Models at too low a level of development for the clash run, e.g. ducts without flanges or pipes without insulation.
• Resolving clashes by moving sprinkler branches below ceilings or drains to lower falls without approval.
• Ignoring the ceiling height when “solving” clashes vertically.

Related: MEP coordination clash checklist, BIM LOD for MEP models explained, Drainage falls and cleanouts.

Check the edition of the code that applies to your project.

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