Last week we looked at concrete solutions for impact-resistant home building in hurricane and tornado areas.
This week, we'll look at wood frame houses and how to build them to withstand as much as they can. A wood frame house will never face down a direct hit from an EF5 tornado, but if secured to the foundation properly, it should be able to hold its own against a near miss or a lesser tornado. A safe room or storm shelter is a very good idea in a wood frame house in Tornado Alley.
As I said last week, I have been emailing with Alex Lukachko of Building Science Corp. (BSC) about rebuilding after Hurricane Katrina and the Greensburg, Kan., tornadoes. He told me that there was no demand for thick-walled concrete houses in this area mostly because of cost:
In Greensburg, we were proposing energy-efficient new construction with a relatively low cost—something like $120,000 per house. But many of the houses in the area were only worth a fraction of that. I seem to remember that the average sale price of a house in the area was something like $75,000 ... and you could buy a lot with an old house on it for as little as $5,000.
I asked him if they specified safe rooms in the replacement houses and he said:
We recommended that safe rooms be added to all of the houses. Some were, but those things are expensive. People are pretty used to heading down to the basement to shelter. There was one more part of the cost equation that was important: After quite a few energy-efficiency seminars for homeowners looking to rebuild, one guy came up to me and said, 'OK, I save 40% on energy costs, but 50% of my electricity bill is a fixed service charge. Can you make that go away?' Well, no.
He went on to point to some work on wood frame walls with spray foam to see how they compared to the impact resistance test for windows conducted by Peter Baker, also of BSC. Lukachko commented:
One surprising result of the 'normal' walls was that when it comes to a flying 2x4, it is probably better to stand in front of the window (which has been designed for impact resistance), then a light frame wall (which has not). There is a pretty useless but fun to watch video here:
Lukachko said that they did a few houses in New Orleans that were wood framed and designed for hurricane-type wind events. The basic approach is to put one piece of metal at the ends of every piece of wood nailed together while framing a house. Metal connectors anchor studs to rims and rafters, rims to foundations, and rafters to ridge beams.
Their plans were based on the Wood Frame Construction Manual: Guide to wood construction in high wind areas 130 mph Exposure B.
Critical Connections for Wood Framing with Hardware
Anchor the sill plate to the slab: Continuous tie-down begins at the foundation. The traditional way to anchor a sill plate is with J-shaped anchor bolts and large washers. Metal straps are also available. Place bolts or straps every 2 feet and as close to the center of the plate as possible.
Tie the walls together: High wind causes high pressure, which stresses a wall or roof. One small hole can pressurize a house and blow it apart from the inside. Windows and doors are potential weak spots. Metal straps tie each stud to the bottom plate and the top plates. At window and door openings, more U-shaped straps tie the header to the window framing.
Bolt down the corners: Sheer strength often is achieved with plywood, but engineered and site-built sheer panels can be added to hold a house plumb, which prevents the house from diagonal collapse. Heavy-duty hardware—called hold-downs—bolts the sheer panels to the foundation.
Connect the bottom of the roof to the walls: Uplift pressure of strong wind under the roof overhang can pull a roof off a house, which is usually the beginning of the end. ‘Hurricane clips’ are available in many styles for many particular uses; the basic concept is a metal strap that holds perpendicular framing members together.
Hold the ridge together: The top of the roof can pull apart under the pressure of very strong winds. Metal straps placed over each rafter pair, or standard wooden collar ties in the upper third of the rafter, will hold the roof together.
Balloon frame the gable ends: Standard platform framing adds a hinge-point to tall walls. For gable walls shorter than 20 feet it is best to keep the studs continuous and balloon frame the wall. Add fire blocking as required by local code.
'Fortified for Safer Living' is another report with everything that insurance companies say will reduce claims ... if first construction cost isn't an issue. Lukachko said that the cost equation was the same in Katrina a few years earlier:
It is really expensive to build a house that meets the prescriptive wind design requirements, let alone something that will have a chance in a tornado. In both Greensburg and NOLA, the reconstruction involved going from no building code or an unevenly applied building code to new codes and standards that demanded much better construction.
And that made progress difficult to pay for.
[Sources: Wood Frame Construction Manual, 130 mph, Exposure B’ (AFPA Wood Council); Simpson Technical Bulletin T-TORNADO12]
Image courtesy Simpson Strong-Tie