Anchoring Equipment and Ducts in Seismic Regions

Why this matters

In an earthquake the building moves and everything sitting on it tries to stay where it was. A furnace on a platform, a condenser on a pad, a rooftop unit on a curb, a run of duct on hangers: each one feels a sideways push proportional to its weight and to how hard the floor under it shakes, and the floor shakes harder the higher it is. Restraint is what keeps that push from sliding, toppling or dropping the equipment, and flexible connections are what let the equipment and the building move a little differently without tearing the gas line, the refrigerant line or the duct apart. Both get installed once, by a crew that was thinking about it, and then get quietly undone by service work done by crews who were not. This card is about recognizing the restraint that is there and not being the visit that removed it.

What restraint is for

Three separate jobs, and a given detail may do one or all of them.

Keep it in place. Anchors, clips and bolts stop sliding. A condenser sitting loose on a pad can walk off it; a unit bolted to its curb cannot.

Keep it upright. Tall, narrow or top-heavy equipment overturns before it slides. Overturning is resisted by anchors at the base working in tension on one side, which is why a tall unit's anchors are sized for a pull, not just a shear.

Keep the connections whole. Even anchored equipment moves relative to whatever its pipes and ducts are fastened to, because the building flexes. A flexible connection between them absorbs that difference. Without it, the connection is the weak link and it fails first, which for a gas line means a leak beside a flame and for a refrigerant line means a release.

A fourth job applies to anything overhead: keep it from falling on someone. Suspended equipment, unit heaters and duct runs above an occupied space are a falling-object hazard in strong shaking, which is why commercial codes treat them seriously.

Where the requirement comes from

Houses under the IRC. IRC M1307.2 (2021 edition, as adopted) requires appliances designed to be fixed in position to be fastened or anchored in an approved manner, and it carries the specific water heater strapping rule for Seismic Design Categories D0 to D2. Residential duct systems are not generally required by the IRC to carry seismic bracing. That makes the house side mostly a matter of the manufacturer's installation instructions, the water heater rule, and good practice.

Commercial and multifamily under the IBC. The IBC, as adopted, sends nonstructural components to ASCE 7, Chapter 13, which sets seismic design forces for mechanical and electrical equipment, ducts and piping and requires anchorage and bracing designed for them in the higher Seismic Design Categories. In practice that design is done by an engineer, or comes with the equipment or curb as a manufacturer's seismic certification, and it shows up in the field as specific anchors, restrained isolators and brace locations.

The exemption that depends on flexibility. ASCE 7 exempts some small equipment from those design requirements. In the 2016 edition, Section 13.1.4 exempts, in Categories D to F, components with an importance factor of 1.0 that are mounted at 4 ft or less above a floor and weigh 400 lb or less, or that weigh 20 lb or less, provided flexible connections are installed between the component and its ductwork, piping and conduit. The exemption is conditional. A light unit that was legitimately unbraced because it had flexible connections stops qualifying for the exemption the day someone hard-pipes it. The numbers and conditions have been revised across editions, so check the edition your jurisdiction adopted.

The service acts that undo it

This is the part every trade needs. Each of these is an ordinary step on an ordinary call, and each one removes a piece of resistance the house or building was given on purpose.

Pulling the anchors to move a unit and not putting them back. A condenser or package unit gets unbolted to slide it for coil access or to replace it, and the new or reset unit goes back on the pad with no clips or bolts. On a house this is the most common one. Reinstall the anchors or the clips the pad was drilled for, or new ones the manufacturer lists, before the unit is reconnected.

Replacing a flexible connector with hard pipe. Gas: a listed flexible appliance connector is replaced with black iron because the tech had pipe on the truck. Water: a flexible connector on a water heater or boiler is replaced with sweated copper. Both convert a joint designed to move into one designed to break.

Replacing the duct's fabric collar with sheet metal. The flexible canvas or fabric connection between a furnace or air handler and its plenum is there for vibration and for differential movement. A replacement unit that does not line up gets a rigid transition instead. The equipment and the duct are now one stiff assembly.

Swapping restrained isolators for plain ones. On commercial rooftop and suspended equipment, seismically restrained spring isolators carry snubbers or housings that limit travel. A replacement fan or compressor goes back on open springs from stock. In service it runs quieter; in a quake it can bounce off its supports.

Leaving a hanger or brace off after duct or pipe work. A seismic brace on a commercial duct or pipe run is a diagonal strut or cable, and it is often in the way. It comes off for access and does not go back. A run that was braced now hangs on vertical rods alone.

Replacing a unit with a heavier or taller one on the old anchorage. The anchors were sized for the old unit's weight and center of gravity. A heavier unit, or one raised on a curb adapter, can exceed them by a large margin without anything looking different. The worked example below carries one through.

Setting new equipment where restraint is impossible. A furnace dropped into an attic on a loose platform, a mini-split condenser on a wall bracket fastened into siding rather than framing, a water heater moved to a spot with no studs to strap into.

None of these look like damage. The equipment works afterward, often better, and the restraint shows up only in what the next earthquake does.

Worked example: a rooftop unit replacement on a small office

A shop is replacing a rooftop package unit on a one-story office in a jurisdiction that rates the site in Seismic Design Category D. Weights and heights below are illustrative stand-ins for the two units' submittal data.

• Old unit: 900 lb, center of gravity about 24 in above the curb top, bolted to a seismic curb whose anchorage was designed for it.
• New unit: 1,150 lb. Its footprint does not match the curb, so it sits on a curb adapter that raises it 14 in. Its center of gravity is about 24 in above its own base, so 38 in above the old curb top (24 plus 14).

What the anchors now see. For the same roof acceleration, the sideways force scales with weight: 1,150 over 900 is about 1.28 times the old force. The overturning effect at the old curb scales with weight times the height of the center of gravity: 1,150 x 38 = 43,700 against 900 x 24 = 21,600, about 2.0 times the old demand. The adapter's 14 in did more to the overturning demand than the extra 250 lb did.

What that means on the roof. Nothing will look wrong. The unit sits square, the adapter is fastened, the bolts are tight. But the curb's anchorage was designed for half the overturning demand it now carries, and the adapter itself is a new link in the load path with its own attachment that nobody designed for seismic loads at all.

What the shop actually does. The crew on the roof works under the shop's fall protection plan, since replacement work is construction and 29 CFR 1926.501 sets the trigger at 6 ft, and stays clear of the curb opening once the old unit is lifted off. It does not compute the anchorage itself. It asks the unit or adapter supplier for a seismic certification of the adapter-and-unit combination for this site's category, or sends the submittals to an engineer, before the unit is set. It also reconnects the gas with a listed flexible connector rather than the hard pipe the old unit had, and checks the isolator type on the new unit's supply fan.

What changes the answer. A replacement of equal or lower weight on a matching curb with no adapter changes nothing about the anchorage; reuse it and verify the bolts. On a house, a heavier heat pump condenser on a ground pad at grade is a much smaller problem, because ground-level shaking is lower and the unit is short, but the anchors still go back on. And in a lower Seismic Design Category the commercial design requirements may not apply at all; the manufacturer's installation instructions still do.

How to verify you left it as resistant as you found it

• Photograph the restraint before you take anything apart: anchors, clips, isolators, braces, connector types. The photo is what tells you what to put back.
• Count fasteners in and out. If four anchors came off, four go back on, and they go into the holes or the structure they came from.
• Every connection you touched is the same kind it was, or better. Flexible stays flexible; a rigid joint you found on a gas or water connection to equipment in a seismic area is a recommendation to upgrade.
• Any change in weight, height or footprint got a question asked about the anchorage before the new unit was set, and the answer is in the job file.
• The ticket names the restraint state on arrival and departure, so the next visit has a baseline.

References

• International Residential Code, M1307.2 (2021 edition), as adopted by your jurisdiction
• ASCE 7, Chapter 13, Seismic Design Requirements for Nonstructural Components, including Section 13.1.4 exemptions (2016 edition), as adopted through the IBC
• Equipment and curb manufacturers' installation instructions and seismic certifications for the specific unit and site
• See related: How Service Work Quietly Removes a House's Hazard Resistance; How to Strap a Water Heater for Earthquakes; Seismic Gas Shutoff and Excess Flow Valves