Gauge and Reinforcement Decision for a 48 in. by 24 in. Supply Duct
A shop is fabricating a 48 in. by 24 in. galvanized supply duct for a 20 ft straight run. The system is designed for 10,000 cfm at 2 in. wg positive static. The shop has 24- and 26-gauge galvanized sheet, a Pittsburgh lock roll former, and a transverse joint machine that can produce snaplock joints at 4 ft or 5 ft spacing. A journeyman proposes 26-gauge metal, 5 ft transverse joint spacing, and one 1 in. by 1 in. angle stiffener at midspan to save material. The duct passes through a 1-hour rated corridor wall, and the project specification requires supports at 10 ft maximum spacing. The shop foreman wants a written check before issuing the cut sheet.
1. Select the duct gauge and transverse joint spacing, and explain the basis for the choice.
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At 2 in. wg positive pressure and a 48 in. duct width, 26-gauge metal with 5 ft joint spacing is too light for the shop's standard detail and would require much closer joints or heavier reinforcement. The safer shop decision is 24-gauge metal with transverse joints at 5 ft, subject to verifying the SMACNA 2 in. wg pressure-class table for the exact panel width and joint type. The 48 in. dimension controls because the flat panel span is large; the 24 in. dimension is less critical. The cut sheet should state the gauge, joint type, seam type, and reinforcement schedule, not just the duct size. If the shop insists on 26 gauge, the joint spacing must be reduced and the reinforcement designed, which usually costs more labor than the material saved.
2. Evaluate the journeyman's proposal of 26 gauge with one midspan angle stiffener.
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One midspan angle does not offset the lower stiffness of 26-gauge metal over a 20 ft run. The duct has five 5 ft joint intervals; each joint is a discontinuity that can deflect, and the midspan stiffener only reinforces one location. The proposal should be rejected unless a qualified person runs the reinforcement calculation for 2 in. wg and specifies stiffener size, spacing, and attachment at every joint. In practice, 24 gauge with 5 ft joints and the standard SMACNA reinforcement detail is simpler and more reliable. The foreman should also check that the Pittsburgh lock and snaplock joints are compatible with the selected gauge; mixing light gauge with heavy joint hardware can distort the duct.
3. What is required at the 1-hour rated wall penetration and for support of the 20 ft run?
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The rated wall penetration requires a listed fire damper sized for the duct, installed in a sleeve or opening protected to the wall rating, with access for inspection and reset. The duct should not be supported from the fire damper or its sleeve. Supports should be at 10 ft maximum spacing or closer per the specification and SMACNA, with the first support near the damper and additional support on each side so the damper is not loaded. The contract documents should be checked for a breakaway joint or flexible connection at the rated wall. The cut sheet should include the damper location, access panel, and support points before fabrication, because adding them after the duct is built usually means field cutting and a compromised rating.