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General Discussion >> Hurricane Ask/Tell

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Hurricane Wind Loading Factors
      #75918 - Wed Jul 25 2007 05:34 PM

Greetings All,

I've decided to move this question into a new post, in part because it's not really directly related to Daniel's HurriScale, and this seems to have engendered a certain amount of confusion as to what I was asking about.

For a few replies (and links) and some interesting anecdotal testimony to my original question, see the last few replies under the "HurriScale" post, in this forum. ( And for Daniels' very interesting presentation of the HurriScale itself ... a must read! )

So here goes, a fresh post, copied from the previous topic.


What I am *really* looking for, here, is a *number*. It could be either an integer, like 5, or a real number, like 4.7. Let's take the hurricane force wind speeds out of the 'real world' and put them in a 'petri dish'.

Allow me to re-state my question:

First, let's take 5 wind speeds, which stay *exactly* the same for the duration of the 'measurement', say 1 minute, as can be produced in a wind tunnel environment.

Min. Cat. 1 - 74 mph
Min. Cat. 2 - 96 mph
Min. Cat. 3 - 111 mph
Min. Cat. 4 - 131 mph
Min. Cat. 5 - 156 mph

In that wind tunnel, we'll have a 'surface' that's exposed to each of those winds for that 1 minute duration. Let's say, for simplicity, that the surface is a perpendicular 'wall' which exposes itself 'head-on' (flat-wise) to the force of the winds.

Now, for each of those winds, there is a "force exerted" on that surface, which can be *measured* as a function of 'unit area', whether pounds per square inch, per square foot, per square meter, whatever. I believe that the force is measured as (I might be wrong) newtons/m2, or newtons per square meter.

With me so far? Now let's make those measurements for each of the 5 'target' speeds, as listed above, representing each minimal category wind speed.

OK! We've just completed our wind tunnel tests and now have 5 numbers, one for each target wind speed, all expressed as newtons per square meter.

Now if we were to plot those numbers on some x/y axis graph, we should be able to determine (derive) how much stronger the force (wind loading) for each increasing category.

Perhaps the "Scale Factor" doesn't stay the exactly the same between each wind speed category. Perhaps it does.

But in any event, we should be able to say, within the realm of these carefully controlled experiments, just *how much stronger* the force exerted on a perpendicular surface (per square meter) would be for each increasing category.

Per my original post on the subject:


... Something like each increasing Category is 5 times(!) as strong as the previous category, in terms of the 'wind loading' on structures, so that ...

Cat 2 = 5 times the wind loading force as a Cat 1 ...
Cat 3 = 25 times the wind loading force as a Cat 1 ...
Cat 4 = 125 times the wind loading force as a Cat 1 ...
Cat 5 = 625 times the wind loading force as a Cat 1 ...

Which helps to explain, in part, why a MAJOR Hurricane (Cat 3 and above) is *SO MUCH MORE* destructive that a 'sniggling' Cat 1.

Maybe the factor '5 times' is on the high side, but the concept stays the same, whatever the 'factor'.


So, what I am REALLY looking for are those 'scale factor' numbers, which provides an (admittedly) highly idealized 'petri dish' view of the raw data. THAT'S IT !!


Now let me wrap up here. I fully understand that this 'idealized' scenario with its 'rigidly controlled' environment has but limited applicability to the 'real world', with it's many extenuating and complicating factors.

What are these extenuating and complicating factors? (Quotes from prom previous posts are indicated as >>> )

>>> TIME, another parameter to consider if you want to wrap your hands around damage potential ... These storms are very gusty. Time can be a factor.

So (1) how long a storm takes to pass by (fast moving vs. slow moving) any given area and (2) hurricanes are well know for their 'peculiar gustiness' which exacerbates the damage a storm can do.

>>> No we do not want to get into your house vs. my house and how it was constructed...

And, of course, there is (3) the all important issue of building construction, and (4) the shape of the residence (an A-Frame, a rectangular or L-shaped building, or the amazingly hurricane-resilliant "Dome of a Home", etc.)

>>> In addition, the equation does not have an adjustment factor for strengthening or weakening hurricanes...

Whether (5) a storm is strengthening or weakening as it passes by and (6) how the wind veers or backs during storm passage, exposing different parts of the structure to the strongest winds, and (7) the case of direct eyewall passage, where winds come from exactly the opposite direction after the calm eye passes by.

If (8) the same area already experienced a cyclone passage a few weeks previously, already weakening some structures and (9) if the ground is already saturated, causing trees to uproot more easily and (10) if the trees were already damaged from previous storms ...

I could go on and on providing examples or these 'extenuating and complicating factors', but I think you get the idea.

The replies to my post really show how 'well grounded' and pragmatic most peoples' perspective are, really focused on the 'real world issues', and what it means to them, their lives, their homes, their yards (trees), etc., and on how much damage any cat storm can be expected to produce, hence the referrals to various Saffir-Simpson charts and Fujita-Scale articles, etc. That's all very interesting and highly useful information.

But Alas! What I'm looking for are those wind tunnel experimentally-derived numbers, which, when plotted on some kind of graph, results in the much sought after 'scale factor'.

I think Ed was closest (in concept) in his reply that a min. Cat 4 is about 250 times more destructive than a min. Cat. 1. ( See? That's a 'scale factor' number! )

But I'm *NOT* looking at damage assessments, barometric pressures or *anything* to do with the various 'extenuating and complicating factors' as I laid out above.

Just a "simple", though idealized, mathematical construct how the wind loading on an exposed surface exponentially increases for each hurricane category increase. I'm sure the research has been done, just have to find that number.

Sorry for the long post; just thought it was important to clearly lay out my question, and *specifically* what I'm searching for. 'Just' a *number* !

Thanks for your reads and your interest and thanking you in advance if you can provide any insight, links or other info which addresses my inquiry.

Many Mahalos from Hawaii - Norm

Edited by CoconutCandy (Wed Jul 25 2007 05:37 PM)

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Re: Hurricane Wind Loading Factors [Re: CoconutCandy]
      #76433 - Wed Aug 15 2007 03:25 PM

In the building codes, this conversion is common use the formula: P = 0.00256 x V2 , where: P = wind pressure and V = wind speed.

example: 110 (mph) x 110 (mph) x .00256 = 30.97(psf)

This is as close to your request as I can get. There is so many difference factor envolved in this that a firm anwser is just not possible. I have multi-books covering designs and wind loads from direct wind pressure and negative pressure (which is many times more damaging often)

But for your request on a general rule try the above. (remember it is only a guide, and a rough one at that)


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