Field-Verified Offset Transition at a Sprinkler Main
Shop drawings show a 24 in. by 12 in. rectangular supply main at elevation 9 ft 6 in. connecting to an 18 in. by 10 in. branch at elevation 8 ft 0 in. over a horizontal run of 18 in. The field measurement shows the branch centerline is offset 8 in. to the left, and a 10 in. sprinkler main crosses the run with its bottom at elevation 8 ft 10 in. The layout mechanic has a laser, a 4 ft folding rule, and a 3 ft framing square, but no CAD workstation in the field. The shop can fabricate 20-gauge galvanized duct with a Pittsburgh lock and can roll a maximum 48 in. wide pattern. The foreman asks whether the fitting can be a straight taper, how to lay out the cheek, and what must be checked before the shop cuts metal.
1. Check the elevations and determine whether the run can clear the sprinkler and still make the transition.
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The branch is 10 in. high with its centerline at 8 ft 0 in., so its top is at 8 ft 5 in. The sprinkler bottom is at 8 ft 10 in., leaving 5 in. of clearance if the duct top stays flat at the branch end. The main is 12 in. high with its centerline at 9 ft 6 in., so its top is at 10 ft 0 in. and its bottom is at 9 ft 0 in. The transition therefore drops 18 in. in centerline elevation over an 18 in. horizontal run, which is a 45-degree average slope, and it also shifts 8 in. laterally. A concentric straight taper would move both cheeks inward and would not account for the 8 in. offset. An eccentric transition with one cheek nearly flat and the other sloped can work, but the high side must be checked against the sprinkler and the low side against headroom. The 5 in. clearance is minimal for a fitting with seam allowances and insulation, so the field layout should confirm actual elevations before fabrication.
2. Describe how to lay out the cheek pattern using triangulation, including the true-length checks you would make.
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Divide the cheek into two triangles by drawing a diagonal from the main-end corner to the opposite branch-end corner. In plan, measure the horizontal distance from the main face to the branch face, 18 in., and the lateral offset, 8 in. In elevation, measure the vertical rise between corresponding corners, 18 in. For each triangle, compute the true length of the diagonal as the square root of the sum of the squares of the plan distance and the elevation difference. For example, the plan diagonal of the 8 in. offset and 18 in. run is about 19.7 in.; combined with the 18 in. vertical rise, the true diagonal is about 26.7 in. Use those true lengths to strike the pattern with a trammel or divider, and check that the two diagonals of the developed cheek are equal before cutting. Add Pittsburgh lock allowance on the longitudinal seam and 3/8 in. on the transverse edges, and mark the offset direction clearly so the shop does not build a mirror image.
3. What field checks should be completed before the shop cuts and breaks the fitting?
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The mechanic should confirm the actual top and bottom elevations of both ducts, not just centerlines; verify the sprinkler main size, location, and insulation; and check for concealed beams, conduit, or fireproofing that could reduce clearance. The offset should be measured from a common reference line such as a column or wall, not from the duct centerline alone. A template or story pole should be set at the branch location to confirm the 18 in. run and 8 in. offset. The layout should also confirm that the 48 in. pattern width is sufficient after seam allowances and that the 20-gauge metal can make the required bends without oil canning. If the measured clearance to the sprinkler is less than about 1 in. after insulation and seam allowance, the fitting should be redesigned or the sprinkler relocated by others. These checks prevent a fabricated fitting from being scrapped or field-modified.