Hanger Spacing and Seismic Bracing in a Renovated Building
A contractor is installing sprinkler piping in a renovated 5-story building in Boston. The building is in a seismic design category that triggers seismic bracing requirements. The contractor plans to use 1-inch Schedule 40 branch lines spaced 12 feet apart with hangers every 12 feet, and to brace the main only at the ends of each run. The building has a new mechanical mezzanine with a 20-foot ceiling, and the contractor wants to use 1-inch pipe for the 20-foot drops to the mezzanine heads. The AHJ has asked for the hanger and bracing submittal before rough-in inspection.
1. Are the proposed hanger spacings acceptable, and what standard governs?
Read the worked answer
Hanger spacing must comply with NFPA 13's table for the pipe size and material. For 1-inch Schedule 40, the maximum spacing is generally 12 feet, so 12-foot spacing is at the limit and may be acceptable if the hanger is listed for the load and the pipe is not subject to additional loads. However, the contractor must verify the specific table and the hanger listing, and must account for concentrated loads such as valves and risers. The submittal should show hangers at the required spacing with the correct rod sizes and attachments.
2. Is bracing only at the ends of each main adequate for seismic resistance?
Read the worked answer
Generally no. Seismic bracing requires lateral and longitudinal bracing at specified intervals along the main, not just at the ends. The contractor must follow the standard's bracing spacing rules, which depend on pipe size and the seismic category, and must provide both lateral and longitudinal bracing at the required locations. End-only bracing leaves long unsupported runs that can swing and damage the system during an earthquake. The submittal must show brace locations, sizes, and attachments.
3. What concerns arise from using 1-inch pipe for 20-foot drops to the mezzanine heads?
Read the worked answer
Long drops of 1-inch pipe can be subject to sway and to loads from the heads and any fittings, and they may need additional support or restraint. The contractor should verify that the drop is supported per the standard, that the pipe size is adequate for the flow to the heads, and that the drop does not create an unsupported cantilever. If the drop is a riser nipple, the standard's rules for riser nipples and drops apply. The submittal should show support for the drops and confirm the hydraulic adequacy of 1-inch pipe for the number of heads served.