Shattered homes left in the wake of a Maryland super tornado point out both the strengths and shortcomings of modern building codes. By Matthew Power/Additional reporting by Laura C. Spicer
Imagine that a terrorist triggers a one-ton nuclear suitcase bomb a quarter mile from your latest subdivision. An instant later, a 300-mph blast of wind thunders across the landscape, hurling trash cans, cars, and tree limbs into your buildings at incredible speeds. Would any of your homes--or the people caught inside them--survive?
On April 28, some of the residents of La Plata, Md., discovered the answer to that question, when a monstrous Category F4 (to F5) tornado cut a 24-mile swath through their town, leveling 344 homes and businesses and killing four people. A few days later, a team of Builder editors and photographers hit the streets of La Plata to get a first-hand look at the devastation--and to take measure of how home construction held up against this worst-case scenario.
"We live in that brick house," notes Linda Darosa, pointing to a street littered with downed wires and tree branches. "My husband is blind. When we heard a sound like freight trains coming, we went down in the basement. We could feel the pressure drop as the storm went over, and blew out all of the windows. The house right next to us is completely gone. It was made of wood."
Trail of tears
As we walk the path of the tornado, building weaknesses become immediately apparent. Residents look stunned, even embarrassed, as we look into their homes through walls that have been violently peeled away. A bathrobe still hangs next to an unmade bed. Angry graffiti marks the wall of a teenage boy's room.
Worst hit: older, two-story wooden homes, many built around World War II. They literally went to pieces, as rafters separated from top plates, dormers flew from roof assemblies, and brittle, older asphalt shingles lifted, exposing sheathing underneath. No hurricane clips, no serious anchorage to the foundation--bad news.
Larry Abel, who heads a national architecture and planning firm from downtown La Plata, says the disaster has hammered home the value of better building codes, which include a significant number of details to tie building components together. His own office came through the storm unscathed.
"The interesting thing is that some of the oldest buildings in town survived better than those built 40 or 50 years ago," he says. "What happened was that you had a lot of overconstruction in the oldest buildings, with braces mortared into the foundation, and that sort of thing. Then, when modern construction techniques started back in the 1950s, there was a lack of codes and standards. Over time, the codes became better enforced. More of those buildings are still standing."
In general, single-story homes--many of those sheathed in brick--fared much better than their two-story wood counterparts. Tornadoes can exert enormous pressure on a building. At 300 mph, wind pressure equals 404 pounds per square foot. (For perspective, a car could be parked on 100 pounds per square feet.) The smaller wall area of a single story--and the impact-resistant brick sheathing--protected these buildings to some degree.
Unreinforced brick in the form of chimneys, on the other hand, fared poorly. On most homes, exposed chimneys broke off or took major damage.
"The sheer wall of bricks is what gives them strength," notes Abel. "Most of those chimneys had no vertical reinforcement."
Breaking and entering
Paul Zilio, a structural engineer with Bliss and Nyitray of Miami, specializes in hurricane-resistant design. He notes that tornadoes such as the one in La Plata exceed even the most stringent hurricane codes. "The maximum wind speed we design for is 146 mph," he notes. "But wind pressure is squared as wind speed increases. For normal buildings, you dramatically exceed the design load."
Nonetheless, he says, one key South Florida code provision might have improved building performance in La Plata. These codes include provisions for a heavy projectile impact--not just uplift and wind shear.
Indeed, some of the worst damage to newer, wood-framed homes in La Plata appears to have been related to projectile penetration--most notably in townhouses. On the windward elevation especially, some end units were completely blown out from the inside.
What happened? The builder installed a popular type of rigid foam board sheathing directly over the wood frame, followed by vinyl siding. As vinyl siding came loose or was shattered, an object penetrated the building envelope, making a pathway for high winds. Then things really came unglued.
"What happens when the building is penetrated is that you get a big pressure differential," says Zilio. Result: Walls blow out from the inside.
Structural experts are quick to point out that almost no home is designed to withstand the force of a super tornado (with the possible exception of some concrete monolithic domes).
Tornadoes are "indiscriminate destroyers," says Jay Crandell of the NAHB Research Center in Upper Marlboro, Md. "In a violent tornado, there are only marginal benefits to including enhanced construction requirements in your homes. You should not feel that by doing so, you have now created a safe environment in the face of an F4 tornado."
Good design and close adherence to building codes can, however, improve the odds of a building's survival in any extreme wind situation.
"Not one of the buildings that we designed was destroyed," Abel says. "If you had followed the code explicitly, you would be in pretty good shape. We can accept roof shingle or siding damage, but the structure must stay intact. That's where fastening and clips become very important. Without those clips, your load resistance drops 25 percent or 30 percent, and that can be all it takes."
Laura C. Spicer is a Maryland-based architect experienced in hurricane-related structural failures.
At about the same time BUILDER editors went to La Plata, a team from the NAHB Research Center began a systematic study of the disaster site. Researchers divided the tornado path into 11 zones and held a press conference in La Plata on June 17, which we attended, to discuss their observations. Here are a few highlights from that conference, presented by Jay Crandell of the NAHB Research Center.
* Roofing: Adding a few extra nails to roof coverings made significant difference for homes on the periphery of the vortex. Also, tie-down straps did help hold down roof assemblies.
* Foundations: Homes with or without basements showed no significant difference in performance. Concrete foundations performed somewhat better than block foundations, although the sample size was very small.
* Two-story vs. one-story: Sixty-seven percent of one-story homes suffered minimal damage. Fifty-six percent of two-story homes suffered major structural damage. Single-story homes take the performance prize.
* Bracing methods: The type of wall bracing did not seem to have significant effect on preventing damage. In many cases, the tornado simply lifted the entire roof off the braced wall or lifted the braced wall off the foundation.
Above and Beyond
By applying the following "code enhancements," you can dramatically improve the odds that your homes will survive wind disasters.
1. Roof sheathing attachment: Use minimum 8d nails. Avoid overdriving with nail guns. Space all nails at 6 inches on center, especially at gable end.
2. Roof tie: Connect the trusses/rafters to wall studs using steel straps. The strap must be nailed to both top plates, not just the top one.
3. Foundation anchorage: Properly lay out and install foundation anchors.
4. Wall top plates: Overlap wall top plates and nail them securely at all corners to prevent wall blowouts.
5. Cathedral gable end walls: Use balloon framing and attach the top plate to roof sheathing at a spacing of 4 inches on center.
6. House shape and size: One-story homes with shallow roof pitch fare best. Hip roofs fare better than gable roofs.
7. Wall sheathing: Specify OSB or plywood. Double the nailing of the sheathing of floor deck.
8. Siding: Consider brick, wood, or fiber cement. If you use vinyl, switch to a thicker product line.
Source: NAHB Research Center