Duct Versus Exposed Riveted Beam in a Converted Warehouse
A 1902 warehouse is being converted to creative office, and the design intent is to leave the structure exposed and painted. Existing riveted built-up steel floor beams have bottom flanges 8 ft 10 in. above finished floor, with deck at about 10 ft 4 in. A 20 in. by 14 in. supply duct with 1 in. external wrap must run east–west across the 6,000 sq ft open office, and every route crosses at least three beams. The mechanical engineer's current layout drops the duct below the beams, putting the bottom of duct and hangers at 7 ft 2 in. along the main circulation spine. The tenant's workplace standard, written into the lease exhibit, requires 8 ft 6 in. clear beneath the lowest obstruction on primary circulation, and the exposed structure is the feature the tenant signed for. Linear pendants are set at 9 ft 0 in., the sprinkler main crosses the same beams, and no penetrations appear on the structural drawings.
1. Frame the coordination problem: what is at stake, who owns each decision, and in what order should it be resolved?
Read the worked answer
This is a coordination failure, not a design preference.
What is at stake
- 16 in. of duct and wrap cannot pass under an 8 ft 10 in. beam and still leave 8 ft 6 in. clear. The current layout puts the obstruction at 7 ft 2 in. - 16 in. below the leased standard, on the main circulation spine, in the one part of the building that is supposed to be the selling point.
- Three beams are crossed, so whatever solves one crossing must solve all three consistently.
- Sprinkler main and lighting sit in the same zone and will be re-coordinated once the duct moves.
Who owns what
- Structural engineer of record: any penetration of a riveted built-up beam. Riveted members were not designed for field cutting; the engineer must determine whether an opening is permissible, where, and what reinforcement is required.
- Mechanical engineer: duct sizing and routing, friction calculations, and whether the same air volume can move in a shallower section.
- Sprinkler designer: head spacing and branch line relocation after the duct moves.
- Interior designer: holds the design intent and the lease standard, frames the options, and documents the client's decision.
Order of resolution: structure first (it has the longest lead time and the biggest safety consequence), then duct routing and sizing, then sprinkler and lighting, then ceiling/obstruction geometry, then client sign-off. Do not let the mechanical layout become the default because it arrived first.
2. Evaluate the realistic options for crossing the beams and rank them.
Read the worked answer
Four options, ranked by risk and by damage to the design intent.
- Resize and reroute the duct (lowest risk). Increasing width while reducing depth - for example a wider, shallower section with the same free area - can bring the assembly under the beam soffit. This requires the mechanical engineer's friction and velocity calculations, confirmation that the aspect ratio is acceptable, and enough width between beams. It is a paper change with no structural consequence.
- Cross at one location only, with a designed soffit (moderate). Run the duct parallel to the beams and cross once, at a point where a furred soffit can be made a deliberate architectural element rather than an accidental box - for instance over a demising wall or a casework run. The client's standard must be formally relaxed at that one location, in writing.
- Engineered beam openings (highest risk). A riveted built-up member crossing three bays: the structural engineer of record must analyze the member, locate permissible openings away from high-shear regions, and design reinforcement plates and welding. On a riveted section this can end in 'no' or in field welding with fireproofing and finish consequences. It requires the engineer's sealed detail, not a contractor's field decision.
- Drop below the beam and accept 7 ft 2 in. (reject). Code-minimum headroom is not the test; the lease standard is. Accepting this silently sets up a tenant claim and destroys the design premise.
Start with option 1 and hold 3 in reserve only if the mechanical engineer proves it is the only way to serve the space. Engage the structural engineer early enough that a 'no' on option 3 does not cost two weeks.
3. The client says the exposed structure is non-negotiable and asks you to lower the pendant height and move on. How do you respond, and what do you document?
Read the worked answer
Treat it as a decision to be made with full information, not a request to be executed. Lowering pendants does not solve the problem; the pendants are not the obstruction, the duct is.
Response:
- Restate the conflict in one line: the duct as laid out gives 7 ft 2 in. on the spine against an 8 ft 6 in. lease standard.
- Present the three viable routes with cost, schedule and visual consequence for each, including the cost of the mechanical engineer's recalculation and the structural engineer's review. Clients decide well when the options are priced.
- Note that a soffit is not automatically a defeat: a single designed crossing, painted out with the structure, often reads better than a duct snaking under three beams.
- Confirm that if the client elects to accept a lower clearance, that acceptance is a lease-exhibit deviation the tenant must sign, not a designer's concession.
Document:
- A coordination RFI to the structural engineer of record asking specifically whether openings are permitted in the riveted built-up beams, with the answer sealed.
- A revised, fully coordinated reflected ceiling and duct routing drawing signed by all three disciplines.
- A client decision memo distributing the three options, the associated costs, and the chosen direction, copied to the tenant's representative.
- Schedule impact - state in writing whether the review period affects the structural or mechanical permit package so a delay claim cannot be assigned to the design team later.