A question that has been perplexing me for a while.

[SunshineR12]

Hello all, as a meteorologist I have recently been trying to put together an advanced report on why the standard model of homes are very dangerous during a period of advance snowfall, winds, downburst, or tornado.

My theory is somewhat simple, but at the same time, very complicated.

Given the normal structure of a home being similar to a cube with a triangular top, I imagine the sides would experience a great amount of force being blasted by even a weak tornado, which can reach wind speeds of around 60-150MPH.

The low end wind speeds can be quite normal in a severe thunderstorm, 60MPH-75MPH winds in a thunderstorm are actually quite common in many areas.

Downbursts are particularly dangerous due to the fact that these come with little to no warning, downbursts frequently reach wind speeds of 100mph or greater.

Here's a diagram of what happens during a downburst.

As you can see, the area that would experience the most severe damage would be the initial impact of the downward wind, I'd imagine the pressure put onto a roof would be quite considerable due to the shape, I'm not sure how to explain it, but there is little to no room for the wind to roll off the roof so to say.

The flat surface also comes to play in the dangers of snow, large snowfall amounts can put tremendous weight onto a roof, and ultimately, lead to the collapse.

During a tornado, the period that causes the most damage would be the wind extending from the center of the funnel, which is where the standard cube shaped homes are flawed.

I'd imagine that the winds place a severe amount of pressure on the outside walls, which could lead to the collapse of the walls, or the winds filtering into the house through a weak-spot such as a window or previous crack, and basically exploding the house like an over-inflated balloon.

Here is a chart showing the pressure drop within a strong tornado.

Anyways, onto my question, I've tried using the formula provided in a previous thread, but it's a bit confusing, so I was wondering, given a model house of 50 feet long, and 20 feet high, (forgive me if I'm totally off on the size of average homes, I'm just guessing on search results from Google), what would the PSI be on the outside walls given a wind gust of 120MPH?

 

[PhanthomJay]

Anyways, onto my question, I've tried using the formula provided in a previous thread, but it's a bit confusing, so I was wondering, given a model house of 50 feet long, and 20 feet high, (forgive me if I'm totally off on the size of average homes, I'm just guessing on search results from Google), what would the PSI be on the outside walls given a wind gust of 120MPH?

Thanks for the meteorology lesson! When I used to work in nuclear power plant design, we used as I recall a tornado wind of 300 mph over a 70 foot length, and tapering down to lesser value on either side of that length! I guess this would be an F5 tornado? I'm not sure for what forces homes are designed...I think for several feet of average density snow on roofs, and I'm not sure about winds. There are also pressure drops and uplift to consider. But as for your question, the formula I use for wind forces is psf = 0.00256V² times a shape factor, where V is in mph, and the shape factor is 1.6 for a flat surface. Thus a 120 mph wind has a pressure against a flat surface of (.00256))(120)(120)(1.6) = nearly 60 psf, or about 60,000 pounds total on a 20 X 50 foot wall. This formula comes from the drag force equation for average density air. There are also gustfactors and height factors to consider.

 

[SunshineR12]

Thanks for the meteorology lesson! When I used to work in nuclear power plant design, we used as I recall a tornado wind of 300 mph over a 70 foot length, and tapering down to lesser value on either side of that length! I guess this would be an F5 tornado? I'm not sure for what forces homes are designed...I think for several feet of average density snow on roofs, and I'm not sure about winds. There are also pressure drops and uplift to consider. But as for your question, the formula I use for wind forces is psf = 0.00256V² times a shape factor, where V is in mph, and the shape factor is 1.6 for a flat surface. Thus a 120 mph wind has a pressure against a flat surface of (.00256))(120)(120)(1.6) = nearly 60 psf, or about 60,000 pounds total on a 20 X 50 foot wall. This formula comes from the drag force equation for average density air. There are also gustfactors and height factors to consider.

Previously, you would be correct, but they recently adapted a new system for rating tornadoes, now, not only does it base the rating on wind speeds, but also damage to structures.

A tornado could have wind speeds of 300mph, but be over an open-field, and they'd be forced to rate it the highest an EF-1 due to the lack of damage.

Thanks for the answer, here's a video you might be interested in.

https://www.youtube.com/watch?v=v5KI63L22UU

It shows a bank being hit by an EF-5 tornado.

 

[pallidin]

Dang. Glad no-one was in the bank at the time.
Nice vid!

 

[PhanthomJay]

Thanks for the clip! Would you mind looking at these 2 threads and put in your 2 cents worth? Not that any of the answers are wrong, but I'd be very much intersted in hearing from a meteorologist on these topics. Thanks, and welcome to Physics Forums!

https://www.physicsforums.com/showthread.php?t=13090

https://www.physicsforums.com/showthread.php?t=451292

 

[harrylin]

In addition to Sunshine, I recently saw a documentary (I regret, don't remember by whom but surely you can find out) that explained the error of only considering direct damage by winds. The documentary discussed the investigation why a town had been completely raised from the map, to the surprise of experts. It turned out that in towns the debris from damaged houses act as projectiles that can significantly increase the destructive power, especially if the tornado is slow (for it then collects increasingly more projectiles). You should certainly discuss that effect in your report.