The Corridor Duct That Never Fit the Corridor Beam
Six-story medical office, design-bid-build, GMP contract with a 45-day BIM coordination obligation stating that the contractor shall coordinate and resolve all spatial conflicts among all trades. The drawings route a 24 in. by 30 in. insulated supply duct down the main corridor ceiling. Conditions: 12 ft floor-to-floor, 8 in. slab, W18 beams with 2 in. of spray fireproofing, 9 ft ceiling, and 4 in. of hanger clearance required. Your detailer flags the conflict in week 16. The sheet metal submittal was approved in week 9, the duct was fabricated in week 12, and 60% of it is sitting in a staging trailer. The mechanical engineer's RFI response reads: coordinate with the architect. The architect's response reads: the ceiling height is not negotiable.
1. Establish whether this is a coordination failure or a design failure, and identify your own exposure.
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Do the arithmetic before assigning blame.
Space available at each beam crossing: 12 ft floor-to-floor = 144 in.; minus 8 in. slab = 136 in. to the deck; minus 18 in. beam and 2 in. of fireproofing = 116 in. at the beam soffit; minus a 108 in. (9 ft) ceiling = 8 in. of clear plenum.
Space the duct needs: 30 in. duct plus 2 in. of liner = 32 in., plus 4 in. of hanger clearance = 36 in.
Deficit: 28 inches at every beam. No arrangement of hangers, offsets, or supports creates 28 inches. This is not a clash coordination can resolve; the corridor as designed on the mechanical drawings never fit the corridor as designed on the structural drawings.
Who owns it: coordination is not design. Clash detection, identification, and reporting are your obligation. Reducing the duct, cutting a beam web, or lowering the ceiling changes a sealed design and requires the engineer of record - the sheet metal contractor cannot decide to cut a W18 web and you cannot authorize it.
Your exposure: the clash existed the day the structural and mechanical models were federated. You found it in week 16, after submittal approval in week 9 and fabrication in week 12. The owner's argument will be that timely coordination would have stopped the fabrication, and on that narrow point they have a case.
2. Present the options to the design team and state your cost and time entitlement position.
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Put three real options in front of the design team, each with a cost and a schedule number:
- Beam web penetrations at each crossing. Cheapest in duct dollars - the fabricated duct largely works with short transitional sections. Requires the structural engineer's sealed approval, shear and moment checks at each location, stiffeners where needed, and re-fireproofing of the penetrated zone. Get it in writing with a repair detail.
- Reduce the duct at beam crossings, for example to 24 in. by 18 in. with transitions. The sheet metal can be reworked rather than scrapped. Requires the mechanical engineer to confirm added static pressure and fan capacity - a design change, not a contractor convenience.
- Lower the corridor ceiling. The architect has refused, but that is the owner's decision about the owner's building. It should be refused in writing by the owner, with the cost of the alternatives on the table.
Entitlement: you are entitled to the cost of the design change plus time, because the documents as issued cannot be built. The fabricated duct is recoverable to the extent it conformed to the approved submittal and to the extent you gave notice when you learned of the conflict. Expect the owner to negotiate the fabrication cost off your change order and expect to concede part of it - but make that concession a term of a written settlement, not an admission in an email.
3. The corridor is on the critical path. How do you manage the schedule and the decision?
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Corridor work drives the ceilings, the sprinkler drops, and the corridor finishes. Model three outcomes before you send the RFI answer back:
- Penetration approval in 10 days: you keep the fabricator's queue and lose about two weeks.
- Duct redesign: add submittal, approval, and fabrication - typically 6 to 8 weeks.
- Ceiling change: fastest in the corridor, but it ripples into door heights, corridor lighting, and the architect's ceiling coordination.
Give the owner a written decision date. Every week the decision sits is a week of extended general conditions that the owner will later claim they did not cause. A dated RFI with a stated time and cost impact is the cheapest insurance available on this job.
In parallel, protect the material: confirm with the fabricator what can be reworked versus scrapped and in what sequence, so that when the answer comes you already know your recovery path. Do not let the duct sit in a trailer while the answer is negotiated and then bill for re-fabrication nobody approved.