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Bob Crager
Bob Crager of Lewis Street Glass is a 26 year veteran in the glass business. Lewis Street Glass is a leading Wichita Glass company, serving the entire Wichita/Sedgwick County area since 1919. They do anything and everything having to do with glass, both residential and commercial. They also do Auto glass. They are located at 743 South Market, facing Kellogg on the South, and you can reach them by phone at (316) 263-8259. You can email Bob Crager at bcrager@lewisstreetglass.com
Glass
2012-08-01 13:52:21
Tornado season is here again
Q- I’ve heard that when a tornado is approaching my location, that I should open all my windows, and that will prevent the ‘low barometric pressure’ inside my home exceeding the pressure outside my home, thereby causing it to “explode”…I was wondering…how good is that glass in my windows? Is it…that good?
A- There are some folks in the SE part of the Wichita area who have recently seen first hand the effects of these tornados. And even though most homes in the Wichita area have never seen one, they’re awesome in their destruction, and can take a while to recover from. Back to your question about how good your glass is. I don’t know of any window glass, in the real world, that anyone could actually afford to buy for windows that could stand up to a tornado. (grin) If you hear anyone telling you your windows and/or window glass will withstand winds of a tornadic velocity…don’t walk away from them…run hard and get away from them! That bad advice could be contagious! If tornadoes blow away wood, brick, stone, steel, and concrete, and they do, glass doesn’t stand a chance. Seriously, over the last 2 decades or so, there has been a lot learned through studying the extent of building damage left behind by tornadoes. There are still lots of problems in deciding whether the damage was caused by tornadoes, or straight line winds, in the absence of a video of the actual storm. This is partly due to the fact that some objects were moved along straight line paths inside a tornado, and the same kind of objects followed curved trajectories inside straight line windstorms. Many variables can cause this, not the least of which are the characteristics of the objects themselves, but also the variations of the surfaces over which they are moving. That’s why the investigators who go out to assess the damages are sometimes unable, from the evidence they see, to determine the exact kind or type of wind field they’re looking at. What I’ve never seen in the wake of a tornado though, is the home blown away, and the windows still standing there unbroken. (g) Have you? The Fujita Scale is used by those who study this stuff, to rate the intensity of a tornado, by examining the damage caused by the tornado after it has passed over a man-made structure. Assigning f scale numbers to structures based on the degree of damage is a subjective visual procedure. However, when trying to derive the intensity of the winds, it is important to consider how well the buildings are constructed and to recognize weak links or flaws within such structures. Large variables in the strength of wood-framed buildings will yield an f scale number with no greater confidence than plus or minus one f scale. The popular beliefs some folks still have about opening windows as the tornado heads for your house just don’t stand up to the facts. There’s also a bit of advice for folks in hurricane pathways. They’re sometimes told to board up their windows. In a very light storm, that might keep them from being blown out or penetrated by flying objects, but in the big winds, that roof is probably coming off and the rest of the home or other buildings are leaving too. Some of the lessons learned in analyzing tornado damage and hurricane damage is somewhat confusing, but the end results are still that when Mother Nature unleashes her mighty powers, we’re pretty much “holding on” till she blows by. Just make sure that when the warnings are given, you and those you care about are in a well constructed storm shelter or a room in the basement of your home that allows the highest level of safety you can get. Pre-plan for this emergency like you do for other emergencies, and stay away from the windows…glass will break and become flying shards of lethal destruction! Back to your original question about preventing your house from “exploding” from the changes in pressures…Even though it was once thought that the low pressure within tornadoes caused buildings to explode, it was based upon the erroneous assumption that a building somehow remains structurally intact after passing the radius of maximum winds on the periphery, or edge, of the tornado. In addition, that theory assumes that the building could have remained sealed so that the barometric pressure inside the building could become significantly greater than outside. That’s not likely! However, studies of tornado damage have, for some time now, indicated that building damage initiates from wind pressure ‘breaching the building’, not from low barometric pressures at all. The winds typically enter the building through broken windows or doors. Evidence of mud, insulation, glass shards, and wood missiles inside buildings that remain partially intact have indicated that the winds had entered the interior of the buildings. Openings on the windward side of a building actually increase the internal wind pressures, resulting in additional uplift pressures on the roof itself. If you’ve watched videos of tornadoes as they strike homes or other buildings, you can actually see the roof of the home or buildings being raised up into the funnel and blown away in the early stages of the strike. Then the rest of the home or buildings normally follow rapidly. Thus persons are no longer advised to open their windows in advance of a tornado, by anyone who knows what they’re talking about. You don’t want to “listen to a tornado approach your home”! All of that flying debris will likely break the windows anyway, thus everyone in the path of one of these violent storms should use every second of advance warning time to seek appropriate shelter rather, than running around opening their windows. The fact that a tree, house, or object is twisted during a tornado does not always indicate that the varying direction of the wind caused the damage. Although the primary wind flow in a tornado at the ground is rotational, the rotating wind field extends over a diameter much larger than the dimension of most objects. The width of an average house is much smaller than the diameter of an average tornado. Thus at any given instant, a building in the tornado path would receive winds that are approximately unidirectional. Tornado damage studies have indicated that twisted buildings are usually the result of variations in the strength of foundation anchorage, and not the rotating winds. You may have heard the experts say for you to get into the hall bathroom if you’re in a house with no basement. That’s because the bathroom plumbing usually provides the greatest anchorage of a house to the foundation, and the house will pivot around this point. That sounds kind of scary to me. How about you? Studies have concluded that a twisted house was more likely the result of different resistances in foundation anchorages, rather than the spiraling winds themselves. The same kind of damage has been known to occur even in straight-line winds from violent and severe thunderstorms. In the Northern Hemisphere where we live, the greatest wind velocities typically occur on the right sides of cyclonically rotating tornadoes, as the effects of translation are added to the rotation. Computer simulations have shown that fast translating, weak tornadoes can leave straight-line damage paths, the same kind of straight-line damage trajectories that were seen in the debris fields left behind in the Mesquite, Texas tornado. So, use the time wisely when a warning is issued, get underground if you can, and after the storm has passed, you will then, because you were blessed to survive it all, be able to call a good, full service glass dealer to come out and help you with the new windows you’ll need when you rebuild your home after the storm. (g) Some information for this answer was researched on the world-wide web. More next month…
 
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