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Saturday, October 31, 2009

Weather Considerations not Usually Considered in Log Home Design and Construction








Weather Considerations not Usually Considered in Log Home Design and Construction

Reblogged from LHOTI
By Kim Elmore, PhD.
Kim Elmore received his B.S., M.S., and PhD. in meteorology from the University of Oklahoma. He has done extensive research at the National Center for Atmospheric Research (NCAR) in Boulder, CO, and worked at NOAA's National Severe Storms Laboratory. See end of article for a better glimpse of our resident PhD.

Classic Tornado

As a research meteorologist who specializes in severe weather, I thought long and hard about severe weather as we designed our log home. A colleague once put the problems severe weather pose as follows: it’s not the odds, it’s the size of the bet! Most information about how weather affects homes details the problems of making the home weather-tight, insulation properties, heat loads, etc. These items mainly refer to HVAC considerations. But weather is much more than seasonal averages, or even seasonal extremes, such as the 90th percentile of low or high temperatures. My concern here is with severe weather: the stuff that gets your NOAA weather radio beeping, or your local TV meteorologist showing off their radar. Here I’ll discuss mainly wind survival. Note that I’m not a structural engineer: make sure you work with one who understands your concerns.

On 3 May 1999, a complex of tornadic storms traversed Oklahoma and Kansas, killing 49 people and leveling entire neighborhoods. In the Oklahoma City metropolitan area alone, thousands of homes were destroyed. Damage ranged from F3 (roofs and some walls torn off well-constructed houses; trains overturned; most trees in forests uprooted; heavy cars lifted off the ground and thrown) to F5 (strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters; trees debarked; incredible phenomena). FEMA commissioned a Building Performance Assessment Report to examine the nature of the damage. Their report is contained online at http://www.fema.gov/library/viewRecord.do?id=1423. At 200 pages, the report is thorough, covering both residential and commercial structures. In this brief write-up, I’ll cover only the highlights of what was learned in making that report and how it might affect your home design.

No log homes were mentioned in the report and I know of none that were in the damage path. However, the historic Tri-Cities tornado of March 18, 1925 did pass over many log homes. Subsequent research showed that these homes typically remained intact, losing only their roofs. All other homes were typically destroyed. Some of these log homes remain intact to this day. Local residents attributed log home survival to their spiked construction and heavy log walls.

What about safe rooms? A safe room is a structure that is built to be independent of the rest of the home. It is your last refuge in the event of a tornado. In most cases, a log wall at least 8” thick is as strong as the wall of a safe room, but the problem of missiles remains: they probably won’t penetrate the walls with any remaining energy, but they will come through the windows and, while not posing a structural issue, they do pose a survival issue. Safe rooms protect the occupants from wind-borne missiles. Without a safe room or storm cellar, you must find someplace in your home where you will be protected well. Yet, your home must also protect you from severe weather short of a strong tornado. This is where proper engineering and careful thought come into play.

In typical frame construction, the walls and roof form an integrated structure. In severe wind storms, the roof is usually separated from the walls, at which point the walls fail and the entire structure collapses. Log homes have self-supporting walls and so will probably remain intact should the roof be separated from the walls. However, there are things that every log home owner should do if they live anyplace where high winds might occur in severe weather (this includes wind storms such as Chinooks and boras). While your home is very unlikely to suffer a tornadic onslaught, it is likely to endure winds of up to 100 mph anywhere that thunderstorms occur. Along the Gulf and southeast coasts, hurricane winds are also a significant threat.

First and foremost: maintain a continuous load path from the roof joists/trusses/rafters all the way to the ground. And make sure that it terminates in something (like a stem wall) that will not be pulled out of the ground. You can accomplish this with hurricane clips, straps, or any number of other methods.

Simply keep in mind that the forces involved will tend to lift the roof, so you must use a method that will not fail under tension.

Nails and screws must not be prone to pull out: these attachment methods are strongest in shear and weakest in tension. One obvious exception to this are the long screws used to build up a log wall in a milled log home: by design, these are quite strong under tensile loads.

Regardless of building codes, the first course of all external log walls should be attached to the stem wall with J-type bolts set in the concrete, though there may be alternative fasteners such as continuous through-bolts. The next course should be attached to the bottom course with screws intended for log applications, and so on. Roof trusses, joists, or rafters must then be attached to the last course of logs such that they will not easily be pulled free. When more than one floor is involved, how the load path is maintained depends on the construction method. In our milled log home, the second floor is attached with large screws and specialized clips to the top course of logs, and all subsequent external framing is attached to this floor using hurricane clips.

Another important consideration is outbuildings: make sure your outbuildings are as wind-resistant as possible. Many structures fail under otherwise survivable winds because of wind-born missiles. Imagine how much damage a few 2x4 studs could do if hurled at 100-150 mph at your house. And in all cases, it’s best to have your home engineered with continuous load paths and wind survivability in mind.

Assuming you have succeeded in building your house with a continuous load path, the next consideration is siting. The most common roof failure mode in and around the Oklahoma City area was overpressure due to garage door collapse. In these cases, the garage door collapsed inward due to strong winds. This created a large opening that was pressurized by the wind. This pressurizes not only the garage but any attached structures and so lifts the roof off of the walls. Large garage doors, such as those used for single-door two-car garages, are particularly vulnerable. In addition to the door itself failing, the rails on which it runs are also subject to failure: these must be reinforced with wind survivability in mind. In our case, we have a detached two-car garage (a failed garage door will result in damage to only the roof of the garage) with two single doors that face north. Single doors have twice the fastening to the structure and are far less likely to buckle.

In most of North America, strong straight-line (non-tornadic) winds will typically have a south-westerly to westerly component. This is because the strongest thunderstorms tend to be associated with south west to west winds aloft and thunderstorms have the ability to bring these winds to the surface. Strong surface winds can occur from other directions (the Santa Ana winds of California are easterly and microburst winds can be from any direction) but the very strongest winds are routinely south westerly and westerly. Hurricanes are the exception: the strongest winds are always in the onshore flow. Along the Gulf coast, these winds will be southerly, but on the East coast they will be easterly.
So, avoid exposing any particularly vulnerable structural components to these winds. Vulnerable components include not only garage doors but also large windows.

A north-facing garage means that any ice accumulation will be slow to melt, because of the low sun angle in winter. A more winter-dominant climate would have to deal with ice and snow buildup on the north side of the garage. Yet, in the southern plains we are nearly assured of seeing 100 mph winds over a 20 yrs period. A slick driveway is far less inconvenient that a destroyed home.

Our home was built in an exposed area, well clear of any trees. However, our previous home (milled yellow pine logs fastened with spikes) was nestled in the trees, on the northeast side of a northwest-southeast ridge. We seldom experienced strong winds there. Trees will indeed lessen the damage wind can do to your home, as long as you are well within the trees. They may fall on your home, but they will also significantly lessen the strength of even tornadic winds and so protect you and your home to some extent.

Keep these ideas in mind as you plan and build your home. If in doubt, contact your local National Weather Service Forecast Office and learn about historical wind damage in your area.

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