Some homeowners who lost large structures in the October wildfires have engaged local engineers seeking fresh ideas with fire defense incorporated into design.
For one of those engineers, Kevin Zucco, executive principal of ZFA Structural Engineers, based in Santa Rosa, it’s personal. His father Greg, founder of ZFA, lost his own Santa Rosa home on Foothill Dr. The company has offices in Santa Rosa, Napa, Sacramento, San Francisco and San Carlos.
A home or commercial building can resist fire. “It would have non-flammable exterior cladding,” Zucco said, “stucco, plaster.” The structure itself, including roof framing, would be concrete, masonry blocks or steel.
Better window glazing is essential to survive heat and wind. Roof design has to change so windblown embers don’t create a fire pit on top of the structure that ignites anything flammable.
John Cook, senior principal at Rohnert Park-based MKM & Associates Structural Engineers, spent 30 years designing office buildings, schools, industrial complexes, parking structures and homes. Hundreds of homes and businesses designed by MKM burned. “We did the majority of Coffey Park,” he said, where more than 1,000 homes burned, “and a lot of Fountaingrove.”
Fire-protective exterior sprinklers
Cook has started discussions of fire-protective watering systems with homeowners whose places burned. “That would be a relatively inexpensive additional safety measure,” he said. “Putting sprinkler heads in is not that expensive. You need an emergency generator” in case of power failure.
Each home or commercial building could have its own koi pond or pool. If automated sensors detect heat from oncoming flames, perhaps 250 degrees, a pump draws water out of the pond and sprays it over the building. Water running down the sides of the structure would collect in concrete troughs that carry water back. A pump could have automated backup — generator or batteries in case of power failure.
Such a structure, with a watering system similar to those used for landscaping, might cost an additional $20,000 but would obviate fire insurance and nearly guarantee the home’s survivability. Interior sprinklers already help quench indoor fires, but current design does not protect a building’s exterior.
“I know of individuals who saved their houses by spraying them down” with garden hoses, Cook said. “A few people around the perimeter made sure embers didn’t stick on the house or get under the eaves. This is very feasible,” he said.
“A lot of newer houses had interior sprinkler systems,” Cook said. “It didn’t do any good” in the intense heat generated in this firestorm. “Most house fires are from the inside out. There could be a cost-effective way to install sprinklers on the roof as part of construction. Make them look like weather vanes, ornate,” he said.
“People are thinking about” ways to use on-site stored water to fight a fire, Zucco said, such as a swimming pool or pond. “They have systems where you can drop a nozzle into your pool and shoot it out” with a gas-powered pump, he said.
“I’ve never heard of one that’s automatically piped into your house. It’s pretty interesting.”
Ugly vs. fire resistance
As for designing the building itself to be fire-resistant, “the issue is not, can it be done,” Cook said, but cost. “What are the architectural ramifications (ugly). Walls could be completely fireproofed. The building would still burn down because windows blew out. Unless you enclose windows, you’re not going to” stop fire.
“You could design shutters” that would protect windows and doors, “impenetrable to any fire, including this fire,” he said. “What would that look like? What would it cost? Your $1 million structure now costs $1.5 million. You have a $500,000 additional cost for the chance that another event like this might happen.”
In advising property owners how to rebuild, Cook assesses the probability of another similar fire happening in the next 100 years — cost-benefit analysis. “My hunch is that it wouldn’t be cost-effective to retrofit all buildings or incorporate fire-resistant construction” in rebuilding, he said.
Cook visited many burned sites. “Aluminum melts at 1,200 (degrees Fahrenheit),” he said. “Steel melts at 2,400 and will start deforming if under load at less than that (about 2,000°.)” He observed warped steel beams.
“It got very hot,” Zucco said.
Engineers look at fire-ruined building sites for clues to the fire’s temperature. “We saw puddles of aluminum,” he said.
He examined custom homes that burned. “Structural steel, bent down and sagging over,” he said. “Steel turns red and loses its strength. It deforms, a nice S-shape, doesn’t do any good.”
Firebreaks and defensible space can help protect structures. But the Tubbs fire “jumped Highway 101,” Zucco said. “I can’t imagine a better manmade firebreak.”
Defensible space, recommended as a fire-slowing tool, doesn’t always work. Kmart on Cleveland Avenue in Santa Rosa had huge parking lots around it. “It was an older building,” wood frame with stucco, Zucco said, noting that likely embers collected on the roof and ignited. His firm had previously been hired to design repairs to reckon with leaks.
Zucco, with 24 years practicing as a structural engineer, is an expert in use of pre-cast concrete wall panels, rammed-earth walls, structural insulated panels and fire-station design.
If flames break or melt a window on the outside of a commercial building, “then it’s in,” Zucco said. Venting can also suck in embers.
“It was windy and hot. Attic spaces are cooler. As air around it heats up,” the attic acts as a vacuum for burning embers. “People described embers going up, doing a 180 (turn) and getting sucked into roof vents,” Zucco said. “Then you have an attic full of embers.” A roof draws air in at its low points and vents it at the peak.
Embers sit in the attic for a couple of minutes then the whole thing would go “Whoom — and go on fire,” Zucco said. “It got superheated.”
In wood-frame construction, wooden studs burn along with the paper that faces fiberglass insulation. “That’s highly flammable,” Zucco said.
Sonoma County codes for wildland-urban-interface homes provide that “roof and attic vents shall resist the intrusion of flame and embers into the attic area or shall be protected by corrosion-resistant, noncombustible wire mesh” not exceeding eighth-inch spacing.
“That is supposed to change venting details specifically for this” type of fire condition, he said. “Some newer homes have that. It helped. But the magnitude of this event was much higher.”
Rooftop fire pit
Commercial buildings are vulnerable at rooftop if spot fires start there. Many business buildings have flat roofs made of tar or asphaltic material that ignites at about 500 degrees and auto-ignites at 900 degrees, according to material-safety-data sheets on asphalt.
Thermoplastic polyolefin (TPO) roofing membranes have become common, Zucco said, and offer improved energy efficiency. TPO has a flash point of 650 degrees F. “It’s fire-resistant,” he said. “But at those temperatures, that would ultimately ignite. Then you’re into wood framing.”
Heavy timbers can be oversized to retain strength longer in fires. “It takes an hour to burn an inch” around the outside of a timber, Zucco said. A beam that needs to be 4-by-6 for strength can be upsized to 6-by-8. After an hour in a fire, “you still have a 4-by-6 inside.”
If fire gets into a building and no firefighters are on site, often it will become totally involved. Zucco points to fit-and-finish elements of the interior of a business or home, such as carpeting, drapes, trim, cabinetry. “It’s all flammable,” he said.
Zucco worked on the American AgCredit (Aviation Blvd.) and Luther Burbank Savings (Third St.) buildings in Santa Rosa, both large. “They’re more fire-resistant,” Zucco said. “American AgCredit is steel studs with stucco or plaster and glazing. Structural steel on the outside is zinc with a sunscreen. It’s not going to ignite.” Fire would have to “melt something to get inside” to flammable material, he said.
Structural glass is stronger than usual windows and can be used around railings, but still melts at temperatures near 1,000 degrees, Zucco said. Glass can be made to resist temperatures in the range of 3,000 degrees, as is used in glass doors of wood stoves. Double-pane or triple-pane glass helps resist fire, he said. Fire-resistant glass in significant quantity on a commercial building would be prohibitively expensive.
Ceramic material is being studied for use in place of flammable siding, Cook said. “Different oxides can create relatively thin sheets,” he said, about a quarter inch. “There have been quite a few ingenious ideas lately,” such as a home that looked like logs, but was actually cast concrete. “You couldn’t tell” from even a short distance, Cook said. “They look like wood.”
Interior walls made of concrete masonry units — blocks — help slow fire’s progression once it gets into a building, Zucco said. Construction with steel studs, as in the American AgCredit building, also discourages fire, especially with chip-board on the inside and stucco on the outside.
Fireproof panic room, buried
Since the fires, “we have had a couple of custom-home clients come to us wanting to know about a basement as a refuge” from fire and other disasters, he said. “We have been doing a lot of fire research.”
Such a fire-safe panic room in a custom home would need breathing apparatus to save the lives of its occupants.
A fireproof room in or under a house would need to keep its occupants alive for up to eight hours, Zucco said. “As the thing burns, it falls down and collapses. You end up with a big bed of coals on top of where the house was, really hot.”
“We looked at a site in the Atlas Peak fire (Napa),” he said. “They had a wine cellar, a cave. It totally survived. The elevator shaft burned up. But the cave itself, 10 feet underground, fine. They were not at home.” The site had few nearby structures. “The whole thing sits on a thick slab,” he said, on top of a hill. He’s eager to test the foundation for structural integrity. ZFA already has a contract to rebuild the home. The client is willing to go above code to be fire-resistant, Zucco said.
“Some people want to build a hobbit home,” Cook said, “basically underground, with circular doors.”
He worked with architects who specify rammed-earth construction or sod roofs, both resistant to fire. “One iconic structure in Fountaingrove I did with architect Gary Tucker. We had a big fireplace. The bedroom had a sod roof. It had a lot of glass and rock around the perimeter. It’s gone. The sod roof collapsed. The only thing left was that concrete chimney three or four stories high. Everything else burned.”
ZFA researched building foundations after fire — compromised by heat. A typical residential fire will hit 1,000 degrees, Zucco said. “The cement matrix in concrete starts breaking down at about 600” even though concrete doesn’t burn. “You start to lose strength.”
Footings and foundation for large custom homes cost $200,000 to $300,000, Zucco said, especially if it has drilled piers on a hillside. It takes months to remove an old foundation and replace it.
Steel rebar, used to reinforce concrete walls, conducts readily and draws heat from a fire into the interior of walls, wrecking them. Zucco sketches a concrete footing. “As this all burns and superheats,” he said, “it gets into anchor bolts, rebar, footing. People want to save footings so they don’t have to scrape the whole site, re-grade it all and pour footings back. We have yet to test the footings” in burned properties.
Stem walls, the concrete foundation that sits on footings, usually suffer complete loss in major fire if there’s no dirt on either side, Cook said. “Those stems generally are shot,” he said.
He inspected a burned home site in Coffey Park where stem walls were intact. “At every anchor bolt there’s a vertical crack,” Cook said. “When everything was burning, anchor bolts got hot and split the concrete. In general, concrete below grade is going to be OK,” such as piers used on Fountaingrove hillsides.
Plywood resists earthquakes
Hotel construction often uses concrete on the first floor then changes to wood framing for upper floors, Zucco said. Plywood walls help resist earthquake forces. “But it’s all flammable,” he said. His company designed the renovation for Hyatt hotel in Santa Rosa.
“That’s wood framing, four stories,” he said, with the perimeter fire-resistant using wooden studs treated with fire-retardant chemicals. Studs are pressure-treated to absorb proprietary chemicals. “It has a three-hour rating,” he said; they don’t burn in most fires for three hours. The chemicals release non-flammable gases and water vapor, delaying ignition of wood.
Skyscrapers or high-rise buildings use structural steel for lightness and flexibility in earthquakes. Steel performs well in fires; it must be insulated to keep from deforming.
“Think about mechanical systems of the building,” Zucco said. “Do they have the ability to shut down” in a fire. An operating HVAC system can suck in smoke, damaging ducts and interior walls, requiring weeks to clean.
In earthquake country, overuse of masonry creates vulnerability. “All that weight is bad,” Zucco said. “The forces are much bigger. Fire-resistant construction competes with earthquake construction.”
Zucco’s clients show strong interest in fire-resistant design. “We have been getting inquiries about rebuilding,” he said. More than three-fourths want fire-resistance. “The design industry is just starting to get into it.”
“You can have all the walls be 100 percent fire-resistant,” Cook said. “They would never burn.” But the design has to “insure that windows and doors don’t blow out, which is what happened. They got superheated and shattered.”
James Dunn covers technology, biotech, law, the food industry, and banking and finance. Reach him at: email@example.com or 707-521-4257