Substituted Purlins Five Days Before Flock Placement
You are the engineer of record for a 60,000-hen layer barn and attached manure-drying building. Five days before flock placement you find the contractor installed 8-in. 14-gauge Z-purlins where your sealed drawings specify 12-gauge, at the correct 5-ft spacing on a 10-ft span. The shop drawing was never submitted for review. The contractor forwards a supplier email saying "14 ga is what everyone uses." Your sealed design shows the specified 12-gauge purlin at 92% of allowable bending capacity under the full 25-psf roof snow. The owner says the pullets arrive in five days and asks you to sign the final certification.
1. Evaluate the substitution quantitatively and state whether the installed purlins are acceptable.
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Thickness change. 12 ga = 0.105 in., 14 ga = 0.075 in. That is a 29% loss of steel thickness - about 29% less section modulus and moment of inertia for the gross section.
Demand versus capacity. The specified member is at 92% of allowable under full snow. Gross bending capacity scales roughly linearly with thickness, so the substituted member's capacity is ~71% of the specified member's:
Demand/capacity = 0.92 ÷ 0.71 = ≈1.30
The installed purlin is roughly 30% over allowable in bending before anything else is considered.
And bending is the most forgiving check. Cold-formed purlins are frequently governed by web crippling and local buckling at the reaction, which degrade faster than linearly with thickness - closer to t² for some modes, so the real overstress is likely worse than 30%. Deflection rises with 1/I to roughly 1.4× the design value, which matters for the ceiling, insulation and ponding.
Verdict. Not acceptable. The 92% starting point means the original design had already used nearly all of its margin; there was no room for a 29% section reduction. This is not a judgment call about "close enough."
2. What are your professional obligations here, given the schedule pressure and the supplier's email?
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Do not sign the certification. Sealing or certifying a building you know to be under-designed puts the public (and 60,000 birds, and the staff) at risk and violates the fundamental obligation to hold safety paramount over schedule and client preference. You may only seal work you prepared or directly supervised - and you cannot certify a condition you have calculated to be 30% over allowable.
Same-day written notice to both owner and contractor - a nonconformance report that:
- Identifies the drawing, the specified member, the installed member, and the location.
- States the calculated demand/capacity and the basis (25-psf snow, 5-ft spacing, 10-ft span).
- States plainly that the building is not to be occupied until the deficiency is corrected.
- Copies the building official if a certificate of occupancy or concealment is in play.
On the supplier email. A vendor's opinion is not a design; it is not sealed, not project-specific, and not accompanied by calculations for this span, this snow load, or these connections. It carries no weight.
On the submittal failure. Record that no shop drawing was submitted for review, and put a hold on fabrication review for the remainder of the project so the same gap does not recur elsewhere on the site.
If the owner directs you to certify anyway, decline in writing, document the direction, and be prepared to state your position to the licensing board. That is the whole point of the seal.
3. The pullets arrive in five days regardless. Define a defensible path forward.
Read the worked answer
Immediate (day 0–1):
- Written nonconformance and no-occupancy notice, as above.
- Temporary shoring under the affected purlin lines - posts and headers at mid-span, or direct shoring of the trusses - so the roof has a load path that does not depend on the deficient members. This buys time and protects anyone working underneath during the repair. It is not a permanent fix and must be labeled as such.
Repair options, in order of preference:
- Replace with the specified 12-gauge members, bay by bay, working under shoring and keeping the roof temporarily supported.
- Sister/reinforce with a nested member or a mid-span strut, but only if a sealed design by a licensed engineer demonstrates capacity for bending, shear, web crippling, deflection and the connections - including the added dead load of the reinforcement itself.
The contractor may obtain that sealed design from another qualified engineer; you may also agree to prepare it under a change order if you can do it properly and on time. What you cannot do is accept the substitution without a design.
Client communication: give the owner the schedule and cost consequences in writing, and make clear that partial or full occupancy before the repair is a decision they can only make on informed risk - and that you will not certify it. If only one end of the building can be brought into compliance, propose restricting birds to that section rather than the whole flock.
After the repair: reinspection, photographic record, as-built markup, and a closure letter to the file. The seal goes on the corrected building, not on a promise.