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.

On Sat, 11 Sep 2004 13:57:57 -0500, Richard J Kinch
wrote:

===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
===
=sip

===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.

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.
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I had no input whatsoever.
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Richard J Kinch wrote in
:

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.


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 pressu 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.

Murray Peterson wrote:
Richard J Kinch wrote in
:

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.

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
[...]
Dynamic pressu 0.5 * 0.00238 * 147^2 = 25.7
[...]
Drag = 954 pounds

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.

Tim Auton wrote in :

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.


If you drop the turbulence and edge effects completely, then the drag
coeeficient drops to 1.00 (by definition I believe). That only drops
the pressure down to 822 pounds from 954 pounds.


Drag calculations would only be valid for the entire building. Your
dynamic pressure calculations are broadly in agreement with the OP's
calculations.

I don't consider 25 pounds per square foot to be much in agreement with
"on the order of a few pounds per square foot". That's why I was asking
about the discrepancy.

I did some more searching, and found this document (Florida building
code for garage doors):

http://www.dasma.com/PDF/Publication...al/TDS155o.pdf

It shows the design load for a 100mph wind to be 15 through 28 pounds per
square foot, depending on the exposure. The figures for the higher wind speeds
get scary.

"Richard J Kinch" wrote in message
. ..
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.

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.


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.

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

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.

"Richard J Kinch" wrote in message
. ..
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.

Murray Peterson wrote:
Tim Auton wrote in :
[snip]

Drag calculations would only be valid for the entire building. Your
dynamic pressure calculations are broadly in agreement with the OP's
calculations.

I don't consider 25 pounds per square foot to be much in agreement with
"on the order of a few pounds per square foot". That's why I was asking
about the discrepancy.

Ahh, you didn't put any units on the calculations so I guessed the
calculations were for the whole panel - ie 25/32 per square foot. I
should have noticed the lack of a 32 in the calculations.


Tim
--
Guns Don’t Kill People, Rappers Do.

Richard J Kinch wrote in message ...
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:


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

Rich writes:

Rich, I'm happy you didn't sustain any major damage, but I don't think
Frances was the storm to learn from. ... Specialists later determined
that the winds exceeded 200mph in Country Walk.

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.

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.

"Richard J Kinch" wrote in message
. ..
Rich writes:

Rich, I'm happy you didn't sustain any major damage, but I don't think
Frances was the storm to learn from. ... Specialists later determined
that the winds exceeded 200mph in Country Walk.

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.