I used to read that the low pressure in tornadoes was what caused the windows to explode from a house. Then, I read that that’s a misconception, that windows break in tornadoes because they’re simply blown in by the wind and what it carries. Now, I read a similar story (link below) about Bernoulli’s principle being the cause - in hurricanes. The theory is that the speed of the wind across the windows lowers the pressure and causes the internal pressure of the house to blow the windows out. Sounds a lot like the discredited tornado theory, including the advice to open windows when a storm’s coming. Any meteorologists or fluid mechanics around to take this one on? Can someone confirm this or call bs on it?
About 10 years ago I worked as an engineer at a firm that builds software used to asses the risk from large natural disasters. I was on the hurricane modeling team (I have my BS and MS in structural engineering).
At that time, as I recall, the “pressure differential” idea was considered incorrect. If a tornado doesn’t cause it a hurricane certainly won’t (the pressure in a hurricane is higher than in a tornado and the windspeeds are correspondingly lower, so any “Bernoulli” effects will be less).
Hurricanes and tornadoes do wind damage by sheer force (for example, peeling the roof off of a structure) or by debris - wind gusts will be 100-250+mph (depending on the type of storm) and anything that isn’t securely held down will get blown all over the place; a bit of gravel coming off a roof at 100mph will break a window quite nicely, never mind a 2x4 at 200mph. Once the building envelope is punctured (windows broken, roof torn off, etc) wind can blow directly inside and more debris and rain (for hurricanes) enters and that’ll pretty much total a house.
Houses are also leaky structures so any difference in pressure will equalize quickly.
Hurricanes are very large and don’t move very quickly, so any pressure difference is going to be equalized on both sides of the window. In tornadoes, the pressure drop can be rapid, but it isn’t all that extreme. Certainly not enough to cause breakage.
In regards to the Bernouli principle, it is a nice theory, but in practice it doesn’t make much sense. First of all, lots of cars, trains, airplanes go at 150 mph or greater with galss windows not all that much more sturdy than a house window. The real issue is that the ammount of debris hurled around by the wind in a weather event will cause glass to break much sooner than any natural effect.
The only good reason for opening a window in a storm is to prevent the glass from breaking and getting thrown around the inside of your house.
[Ron White] It doesn’t matter that the wind is blowin, it’s what the wind is blowin. If you get hit with a Volvo it doesn’t matter how many sit ups you did that morning[/RW]
In Florida, all windows and glazed doors must be able to withstand both positive and negative pressure. Here in Hillsborough County, where the standard design wind speed is 110-120 mph, the window must withstand about 26 psi inward and 36 psi outward pressure. In addition, if the home is located in a “windborne debris region” or in any area with design wind speed over 120 mph, the glass must be impact-resistant.
The Florida Building Code has tables and diagrams that define the maximum pressures that are expected to occur on walls and roofs. Depending on wind direction, some pressures are inward (positive pressure) and some are outward (negative pressure). These tables are based on wind tunnel testing. Depending on wind direction and roof slope, a windward roof may have positve pressure and the leeward roof may have negative pressure, or they both may have negative pressure. Same with walls. Within 4 feet of the building corners and roof ridges, there will be higher pressures than elsewhere.
Specifically Dade and Broward. Not only the code, but the implementation. They are driving contractors out of the area, out of business with the entire process. It’s crazy, and will end up costing homeowners in the end.
Meaning it’s too stringent? Part of the problem in Andrew was absolutely horrible construction; I’m not sure if the code was up to snuff at the time but the implementation certainly wasn’t. For a while that was the single most expensive natural disaster in US history, homeowners paid by having their homes go bye-bye and insurance companies had some rude awakenings.
the weak spot in any code system. I live in S. Louisiana and understand about hurricanes and such. I am all in favor of tough building codes, as I believe is fisha. The problem is that enforcing the codes is very very tough. Either the local government comes up with a LOT of money, tripling their code enforcement budget probably isn’t enough, or they make the contractor wait days to weeks for an inspector. Remember, the work usually has to stop when the inspection is due. It kills the ability of the contractor to make any money. The historical practice is to skip over the inspections or make them cursory. But now the county might be held liable, so they don’t want that. The solution requires that the local government suddenly becomes efficient, responsive and diligent. It isn’t working :(.
Well said. It’s not just about the money, it’s about leaving the house open to the elements. Not just for a few hours. For weeks.
I’m all for strong codes, and strong code enforcement- it weeds out the bad guys and gives homeowners a better, safer product. But they have now designed the codes without having any idea how to efficiently implement or enforce them.
