Hurricane experiences and afterthoughts

We got a direct hit on our house in Palm Beach County, Florida from Hurricane Frances. Hours and hours of high winds gusting well over 100
mph, See my photo gallery at:
http://www.truetex.com/frances
It was a novel and challenging experience, especially as an engineer and inveterate do-it-yourselfer.
I learned quite a bit about the mechanics of wind damage and what works to stand up to it. Even a good mechanical intuition is likely to be incorrect about these things. I thought I'd pass along some of the lessons I learned.
First, the pressure from hurricane force winds is only on the order of a few pounds per square foot. This I learned by perusing _Marks_ handbook during the long night of winds when I ran out of reading material. Heavy debris does not leap up into the air and fly around. Structural problems are due to the wind catching things in a sail-like or a can- opener-like manner, and collecting and concentrating the few lbs/sq-ft force onto critical fastenings.
Before the storm came I was all concerned to get shutters on the windows, and fussing with a shortage of fasteners drained by a panicked populace. But it turns out you don't need plywood fastened with 1/2- inch lag bolts over your windows. A 4x4 foot sheet, say, 16 sq ft, is only going to experience at most about 100 lbs of pull-off force. It is not like there is some horrific suction that wants to tear things off. My aluminum channel strut reinforcements were way overdone [metalworking content!].
I was worried about my 10' x 7' garage doors that were 1970s pre- hurricane-code construction. I bought pieces of 10' SuperStrut to reinforce and anchor them to the concrete floor, but didn't have time for that metalworking project before the winds hit. During the highest winds, these experienced at most a few hundred pounds of force, which certainly made them slowly bow in and out a few inches with the slowly rising/falling gusts, but not fail, rather like a crude windspeed indicator. I actually wedged myself in between the garage door and the back of my Jeep, and bonded with the breath of the beast as she huffed and puffed. It was like a giant hand was outside pushing and relaxing a few times each minute.
While we tried to clear our 0.85 acres of loose stuff before the storm, there were a few things left around. It was odd how light little things didn't get moved by the wind, while healthy, well-rooted 60-foot trees were knocked down. For example, I had a 3-foot assembly of 3/4" PVC pipe leaning against an outside wall, directly facing the worst winds, and it didn't even get tipped over. Small scraps of plywood stayed where they fell from cutting.
Second, the handy guy in the neighborhood with the tools and know-how is very popular in a calamity. Having the materials, skills, and chutzpah to improvise in a crisis is a shining moment.
Third, while many neighbors had all their roof shingles stripped, we lost none except the corners of a few on the peak row. I wasn't the owner when the last re-roof was done 12 years ago, but it appears to me that the heavier shingles made all the difference, especially considering the minimal increment in cost. I believe the peak shingles were broken by the way the slope of the roof must have caught the wind and highly concentrated the force with an airfoil effect.
Fourth, securing a household in the absence of time and retail hardware is like being on a boat in the middle of the ocean. No one is going to help you, there is no place to go for supplies, and the mechanical problems are extreme and puzzling in a way you never have dealt with. Everything depends on your own wits and what you have on hand.
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On Sat, 11 Sep 2004 13:57:57 -0500, Richard J Kinch

I can relate to this phenomenum. My house during Hurricane Fredrick back in late 70's was literallay tore apart. My entire front porch 65 foot long was ripped off and god only knows where it wound up. I never found a lot of my original roof either. I had a pair of steps on each end of the porch leading up 10 feet from groundlevel to a small 4 x 4 foot landing, before the porch. The entire porch was screened in. We were in a hurry to go to Montgomery, Al., so we cold take care of some business come Monday morning and then head back home, and the wife had just washed the floors down and got all the tracked in sand up as we had some company that weekend before Fred hit, and I had already locked the screen doors on the porch. She left an emplty 10 quart red plastic scrub bucket setting on the one landing. We got delayted longer than expected in Montgomery, and had to fight and fuss with law enforcement to allow us back in as Fred was now heading to the area. Up until that point Fred did not know where he wanted to go, or even if he was really going to pay a visit........anyway after the storm and loosing most of the house, only the steps on each end of the porch and the house floors and some walls were left, but that bucket wsa still setting on the landing area right were the wife placed it, and it did not have as much as a drop of water in it for as much rain that fell from the hurricane. Really really strange. Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wife, I had no input whatsoever. Remove "nospam" from email addy.
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My calculations show something rather different. Can you point out where I went wrong?
Drag = drag coefficient * Dynamic Pressure * Area Dynamic pressure - 0.5 * fluid density * (fluid velocity)^2
Coeeficient for a square plate: 1.16 Density of air at sea level: 0.00238 Area of plywood sheet: 32 square feet Let's assume a wind velocity of 100 mph (147 feet per second)
Dynamic pressure: 0.5 * 0.00238 * 147^2 = 25.7 Drag = 1.16 * 25.7 * 32 Drag = 954 pounds
I get almost 1000 pounds force on a 4x8 sheet of plywood, which is over 29 pounds force per square foot.
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[...]
[...]
I'm a physicist by education, but by no means an expert in fluid dynamics. Here's my largely intuitive take on it anyway.
He's working on pressure, you're working on drag. I suspect neither are the whole story, but I can see more problems with using drag than pressure. I assume that drag coefficient is for moving a flat plate *alone* though the medium (or vice-versa) - this is not the situation for a window which is *attached to a building*. In particular the vortices / lower pressure behind a plate moving through a fluid would cause very significant drag.
Drag calculations would only be valid for the entire building. Your dynamic pressure calculations are broadly in agreement with the OP's calculations.
Anybody here done finite element analysis of such a situation? :)
Tim
--
Guns Don’t Kill People, Rappers Do.

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I suspect that many of these roofs were not properly installed. What I have seen on dozens of roofs is that just the bottom "flap" of the shingle is broken off, leaving the bulk of the shingle in place and probably leaving a watertight roof. In other words, the fastener (nail or staple) did not fail. What I am actually seeing on too many of these roofs is dozens of little strips of clear plastic blowing in the wind. I believe that those strips were supposed to have been removed from the bottom of the shingle at installation so they can self-seal. In other words, the roofer had saved a few minutes on the job by doing a half-ass job.

You gotta have that stuff ahead of time. You can't count on the hardware store. That said, Lowes and Home Depot have obviously learned a lot from earlier hurricanes. I was surprised at the speed at which they restocked hurricane items before and after the hurricane.
Vaughn
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Rich, I'm happy you didn't sustain any major damage, but I don't think Frances was the storm to learn from. I went through hurricane Andrew in 1992. I was in the Country Walk development in south Dade, FL (one of the hardest hit areas). This development had about 1,700 homes. After Andrew not one of them was fit to live in. Power and water was out for months. I saw things I thought were impossible, fiberglass/asphalt shingles imbedded inches into trees, a sheet of plywood blown edge first through the outside wall of a house, a row of FPL steel reinforced concrete 3' square at the base electrical poles snapped off at the base, steel reinforced concrete roof beams broken in half inside of my office building. Frances wasn't even a warm-up for this kind of hurricane. Specialists later determined that the winds exceeded 200mph in Country Walk. Ivan can do this kind of damage. It appears that we will be spared this kind of catastrophe this week, but be prepared, there is always next week and next year.
BTW, I lost my house and about everything I owned in Andrew and am now deathly afraid of hurricanes.

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Rich writes:

Not wanting to get into a "can you top this" contest, but remember:
You prepare for the middle forces, a kind of "triage". Below a certain strength, you don't need preparations, above a certain strength, they won't matter. I believe H. Frances was in that middle strength.
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Actually, with the proper preparation and a better roof my house would have survived Andrew. I didn't have shutters. With the steel shutter panels I have now I don't think I would have had any penetration of the exterior (concrete block construction). As it was, I only lost about three windows. The real damage was done when my fiberglass roof shingles peeled off and water came into the ceiling from the joints in the roofing plywood. I only lost about one sheet of plywood from the roof. The insulation got soaked and its weight caused all of the interior ceilings to collapse. That caused all of the damage (totaled the house as far as the insurance company was concerned). Now, I have a cement tile roof which would probably stand up.

won't
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I agree w/ the other poster that Frances wasn't that good of a test, but Charley is the one that really tested the garage door windload engineering. One thing to remember when they are giving the wind speed of a hurricane is that it is at that speed 1 mile up in the air & is considerbly less on the ground.
I live & work in Lee Co & have seen first hand what major hurricanes can do. Charley blew in or blew out hundreds (if not thousands) of garage doors in Lee & Charlotte Co that were installed before the windload codes went into effect. Some of the ones that blew out have yet to be found.
Most of this area is under 130 mph windload (140 on barrier islands & southern Collier Co) so the doors are designed for +/- 30 PSF or higher but they are tested at +/- 45 PSF which is 1-1/2 times the design load. We are not aware of any 130 mph doors failing during Charley although a few got damaged from flying debris.
One of the best examples of the windload working is Pine Island Fire Station #1 which is on a barrier island very near the coast & very close to the path of the eye. A couple of years ago we (www.actiondoor.com) replaced one of four doors w/ a new windloaded door that is required by code. That door had nothing wrong w/ it & still works, but the other three completely left the openings & at least one of them left the building & went into the mangroves. These are 14x14 steel doors & not something that would be considered light weight.
All I can say is that you are lucky that Frances wasn't stronger while you were holding onto it as it bowed in & out. I have also seen where many doors actually started to break in half from the metal fatigue caused by the door bowing in & out & some literally split down the middle. If you would have had a 16' wide door instead of a 10' the bowing of your door would have been much more severe.
When one lives in a hurricane zone they should take the experts advice of "protect for the worst & hope for the best" & in the end there hopefully will be less damage, less injuries, & less loss of live.
Doordoc www.DoorsAndOpeners.com www.ActionDoor.com
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