If there were any doubts about the dangers of poorly anchored rooftop equipment during a hurricane, building consultant Thomas Lee Smith can point to case after case of flying mechanical units, rolling condensers, and whipping lightning protection systems causing extensive damage to buildings. But one incident during Hurricane Ivan in September 2004 is particularly disturbing.
“Hurricane Ivan sent an HVAC unit that was 30 feet long and weighed 18,000 pounds rolling off a medical office building on the Gulf Coast,” says Smith. Sixteen straps were not enough to anchor the unit against wind speeds estimated at just 85-95 mph. The massive unit crushed several cars below, but luckily no one was hurt. “It was quite scary,” says Smith, AIA, CSI, who is president of TLSmith Consulting Inc., Rockton, Ill., and a member of the Federal Emergency Management Agency Mitigation Assessment Team.
“You don't want to be around when one of those rooftop units start to fly,” says Phil Kabza, AIA, FCSI, director of technical services with SpecGuy, a Charlotte, N.C.-based specification and technical consulting firm to the AEC industry. Kabza says even though there are industry standards and best practices for securing rooftop equipment in high-wind areas, the design and installation specifications often fall through the cracks.
“The mechanical engineer is thinking about the systems, not about wind loads, and the structural engineer is worrying about the major building structure, not about the stuff the mechanical engineer is going to put up on the roof,” says Kabza.
To help Building Teams avoid failure of rooftop equipment during high-wind events, Smith provides some important design tips based on his work with the FEMA Mitigation Assessment Team:
1 Determine wind loads using ASCE 7-05. In 2002, the American Society of Civil Engineers added rooftop equipment design requirements into its ASCE 7 code document, “Minimum Design Loads for Buildings and Other Structures.” Smith says to reference the 2005 edition of ASCE 7, “because the 2002 criteria were somewhat non-conservative.” Calculations should take into account both lateral loads and wind-uplift loads for each piece of equipment. A minimum safety factor of 3 is recommended for critical facilities, and a safety factor of 2 for all other buildings.
2 Avoid exposed ductwork. Even ductwork that is properly anchored to the rooftop can sustain damage from high winds and flying debris. Smith recommends that Building Teams avoid installing exposed ductwork on rooftops in high-wind regions. “If you absolutely need it, then make sure to custom design the system and pay particular attention to loads and resistance,” he says. Beefing up the ductwork may call for heavier-gauge sheet metal and substantial anchoring, such as bolted steel angles that wrap the ducts.
3 Spring isolators should provide uplift resistance. If the design calls for mounting equipment on vibration isolators, make sure to specify spring isolators that accommodate uplift resistance in addition to lateral resistance. Smith says the FEMA team found numerous incidents post-Katrina where equipment blew off because the isolators were not designed for uplift resistance, and the equipment was not heavy enough to stay grounded.
4 Make sure fans and cowlings are well anchored. If not properly anchored, fan systems and cowlings can become windborne. Fans should be anchored to the curb with at least two screws on each side, and cowlings should be attached to the curb with steel cables. Two cables are recommended for areas with less than 120 mph wind speed; four cables for regions with 120+ mph winds. For cowlings less than four feet in diameter, 1/8-inch-diameter stainless steel cables are recommended. Larger cowlings should use 3/16-inch cables.
5 Use straps and screws to anchor condensers. Rooftop-mounted condenser units are often placed on wood sleepers that simply rest on top of the roof system. They have no chance against a high-wind event. The base of the condenser unit should be anchored to the equipment stand, and the stand should be attached to the roof. Wrap the unit with two metal straps and attach the strap ends to the equipment stand with two side-by-side #14 screws or bolts at each strap end.
6 Strap or latch equipment access panels. Even when mechanical equipment is well anchored, access panels and other small ancillary pieces can blow off if not properly fastened. Smith says access panels that use latches (vs. hinged swing doors) and can be completely removed from the equipment are particularly susceptible. He recommends that Building Teams install a hasp or other locking device that is approved by the equipment manufacturer.
7 Construct mechanical penthouses for critical facilities. Additional precautions should be taken to protect rooftop systems for hospitals, schools, and other critical facilities. Smith recommends the construction of a mechanical penthouse to shield vital equipment.
8 Make sure the equipment will not corrode. To avoid corrosion-induced blow-off, all exterior-mounted equipment (including stands, anchors, and fasteners) should be nonferrous, stainless steel, or steel with a minimum G-90 hot-dip galvanized coating.
9 Choose anchors wisely for lightning protection systems. Failure of lightning protection systems during high-wind events is usually the result of anchors that open up (thereby releasing the conductor cable) or completely debond from the roof. The result is large cables, often with connector ends still attached, whipping around, tearing and puncturing the roof membrane. Looped connectors should be specified in lieu of pronged units. The connectors should be mechanically fastened to parapets with #12 screws that have a minimum 1¼-inch embedment. For installation on a built-up or modified bitumen roof, fasten the connectors with asphalt roof cement. Use a liquid sealer for single-ply roofs.
For more, see FEMA 549, “Mitigation Assessment Team Report, Hurricane Katrina in the Gulf Coast,” chapter five and appendix E. www.fema.gov/library/viewRecord.do?id=1857