Mark Meshulam is an expert witness and consultant for wind damage to buildings.
NEW: Download Chicago Window Expert’s Wind Pressure Chart and Calculator
Here in the windy city, the pressure of the wind plays a big role in the specification and design of buildings, and especially the part of the envelope we like to lick: the windows and glass.
When designing and testing windows, curtainwalls, panel systems and other vertical facade elements, we typically see three categories of wind pressure utilized:
1. Pressures used for testing air infiltration and water penetration. These can vary from .56 psf to 15 psf depending on product type, building type and application.
2. Pressures used for structural design (this is called “design load” or “design pressure”). These can vary from 20 psf to 70 psf and more.
3. Pressures used for safety factor. These run 50% greater than design load, so they can vary from 30 psf to 105 psf and more.
“Psf” means “pounds per square foot”. It refers to the number of pounds of force applied (in this case by the wind) to each square foot of the building’s exterior, including our beloved windows. Psf is a measure of pressure which is an amount of force per an amount of area.
There are other ways to express this. Wind pressure can also be expressed, for instance, as kilograms per square meter (kg/m^2) for you sick Euro-freaks out there who espouse the so-called “metric” system.
Not only can you convert wind pressure to other units of measure, you can also convert wind pressure to an equivalent wind speed, because it has been calculated that a uniform wind blowing against a building at a given speed will produce a predictable wind pressure. This phenomenon has earned its very own formula, the “Emswiler Formula”, widely used in our industry, thanks to John Edward Emswiler:
If you know the velocity, find the pressure with this formula: P = .002496 V^2
If you know the pressure, find the velocity with this formula: V = SQRT(P/.002496)
In order to convert from wind velocity V to wind pressure P, simply multiply the velocity (in mph) by itself (square it), then multiply the result times .002496. That’s it. The result is in psf.
Let’s try one. According to one study, Chicago’s average wind speed is 10.3 mph. Calc it out and you find that it converts to .26 psf. Surprisingly low – only one quarter pound per square foot.
And so, you might think: What gives? Chicago Window Expert just said structural design pressures involve a range of 20 psf to 70 psf, yet our average wind speed only produces 1/4 psf!
Well, the wind “gives”. Notice that the wind pressure formula involves squaring the wind velocity. Math whizzes out there will immediately recognize that the equivalency between wind pressure and wind speed is not linear. It will not provide a straight line on a graph, and here’s a graph to prove it. Each time the wind speed increases just a little, the wind pressure increases a lot.
Let’s try it again. In a 66 year study (1930-1996) Chicago’s peak wind gust recorded at O’Hare airport was 84 mph. I remember it being a bad hair day. Calc that on your abacus and you find that 84 mph converts to 17.61 psf.
So let’s compare these two conditions:
| Chicago’s average wind | 10.3 mph | .26 psf |
| Chicago’s 66 year peak wind gust | 84 mph | 17.61 psf |
Comparing an average wind with a gusting wind, the wind speed increased by a factor of about 8 (10.3 mph to 84 mph), but the wind pressure increased by a factor of over almost 68 (from .26 to 17.61 psf).
We can see that the range of historical wind behavior is similar to the test pressures commonly used for air infiltration and water penetration, but structural design pressures are very much higher.
There are three main reasons higher structural design loads are used:
Reason 1: Historical official wind data is typically taken at only 33 above the ground, yet our big buildings are much higher than that where the wind roams freely across the skies. The Willis Tower is over 1400 feet tall. What? You haven’t heard of the Willis Tower? You will never believe this. To induce a new tenant, Willis Group Holdings, to lease a few floors of office space, the Sears Tower changed its name! First Frango Mints were outsourced, then Marshall Fields became Macy’s, and now this! At least they didn’t take away the bean!
Reason 2:Our urban canyons create constricted spaces where the wind becomes funneled between buildings, dramatically increasing wind speeds and pressures. This “venturi effect” was put to good use in the carburetors of pre-fuel injection cars, causing the intake air to accelerate and better mix with the gasoline.
Reason 3: As the wind passes a building, it creates a negative pressure on the leeward side that can be even more powerful than the positive pressures on the windward side. We see this effect a lot in our lives. It is the same negative pressure (meaning it really sucks) that lifts airplanes and propels sailboats.
If you are an engineer attempting to design a big building, you will need to arrive at a reasonable determination of the design pressure for that building. In order to do this, you first consult the local building code. This will tell you the minimum design pressure you can use, by law. Then, you will need to go beyond the code and look for compelling reasons to increase the design pressure. There may be special atmospheric reasons, such as adjacency to large body of water (we have a rather large lake to consider) or mountains (we have them, too. Ever heard of Mount Trashmore (elev +67′-2″ above Lake Michigan)?
Beyond these considerations, the big kahuna of design pressure determination is: how to design for the wild and wacky winds that blow through the jumble of architecture known as downtown Chicago?
We’ve got everything: tight spots where wind roars out of the West, squeezes between buildings and takes off like the Super Chief, sudden open spaces where winds expand and form eddy currents and whirlpools in nooks and corners and mini-twisters in open plazas, and tops of buildings where unfettered Northeasterlies thunder from Canada on the racetrack known as Lake Michigan only to plow into their first resistance since the Arctic Circle. Even our sentences run on. Try boiling all of that down into a formula!
We simply don’t have the computer modeling tools that would enable us to know what will happen on every surface of every building when the wind hits downtown.
When designing big buildings, it is actually easier to build a scale model of the downtown area, load each building model with pressure sensors, and put the whole shebang on a turntable inside a wind tunnel. Turn on the fan and start collecting data.
For one or two hundred thousand dollars, you can commission your very own Boundary Layer Wind Tunnel Test and get all the data you will ever need to adequately design your building. Be sure to bring a hat.
Wind-related definitions from NOAA (National Oceanic & Atmospheric Administration)
Wind
The horizontal motion of the air past a given point. Winds begin with differences in air pressures. Pressure that’s higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated.
Wind Speed
The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall).
Wind Gust
Rapid fluctuations in the wind speed with a variation of 10 knots or more between peaks and lulls. The speed of the gust will be the maximum instantaneous wind speed.
Wind Direction
The true direction from which the wind is blowing at a given location (i.e., wind blowing from the north to the south is a north wind). It is normally measured in tens of degrees from 10 degrees clockwise through 360 degrees. North is 360 degrees. A wind direction of 0 degrees is only used when wind is calm.
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Thanks for writing!
Hi Mark –
I came across your site and found it very interesting. Thank you for posting all of that info. I am an official engineering-school-dropout. Bear with me a second and see if my thinking is sound – – I have a customer that wants a 6’ x 10’ x 8’H prefabricated “boothâ€. My supplier typically builds them to meet a 120 MPH wind load. My customer informs me that they want “blast resistant†windows. My supplier asks “how many PSF/PSI†do they need? Customer responds 3000 PSF, which I know is not correct. Anyhow – – we will get a better answer from them, but if I use your “Emswiler Formulaâ€,would it be fair to say that the booth is built to withstand approx 36 PSF? My point being that it doesn’t make much sense to put 100 PSF windows in a building that would get blown away, right? I am thinking they need some sort of minimal safety glass windows that closer match the booth OR beef up the booth to withstand a similar blast.
Thanks for humoring me!
Hi Jack,
What is the booth being used for?
Mark
Mark – thanks for getting back to me. I don’t know much about it as it is
for military use. They are installing some sort of electronic equipment in
the booth and I suppose they test some explosives/ordinance nearby. Since I
sent my original message, my supplier sent me a spec sheet for a window from
a company called Armortex that has a 42 PSI rating. I was looking at
another web site about the impact of a nuclear blast and it says that just
about everything would be leveled in a range of 5 to 12 PSI – – -which makes
42 PSI hard to believe?
Hi Jack,
Let’s see…
42 psi is 6048 psf. That’s a lot of pressure to suddenly apply to one
square foot of glass, let alone to the structure that is supporting
many square feet.
Not to mention the structure that the new structure is attached to.
It’s not impossible, but would be extremely expensive to do it.
See if you can get some test reports from Armortex.
I ordered a binder from their site but don’t know when it will come.
Is my logic of using a building with a 120 MPH wind load equaling 36 PSF
valid? Thanks for your input. I’ll keep you in my rolodex for future opportunities.
Hi Jack,
Yes, 36psf equals 120 mph.
I have attached a handy tool for your use.
It’s your very own Wind Pressure Equivalent calculator.
Download it here:
https://chicagowindowexpert.com/PressureEquivalents.xls
Remember that 1 psi equals 144 psf
Let me know how I can help.
One thing I can think of where I might be able to help you:: review shop drawings when job is in production.
Hello.
I was just surfing through your site.
Interesting and well vulgarised.
Too bad you feel the need to bash at kilograms. ( Your article on design pressures).
By the way the system is not referred to as the metric system anymore.
It has been named the International System for quite a while. SI.
You do realize that only 5% of the world population still uses the imperial system?
Namely you arrogant self centered United states-ians.
(I do not use the term Americans out of respect for other, more evolved, nations on the continent.)
But money talks.
And one day soon, you will have to recognize that having the rest of the world constantly “translating†for you
is costing you more than it would cost you to switch over once and for all!
Keep up the good work!
Hi robert, (I am using your capitalization out of respect for your individuality),
Thanks for one of the more entertaining messages I have received lately. It’s good to have a bit of conflict to stave off the boredom.
With regard to your assertion that “the (metric) system is not referred to as the metric system anymore” I beg to differ.
This Wikipedia article: http://en.wikipedia.org/wiki/Metric_system uses the term “metric” over 70 times, not including footnotes. Not one of those instances includes a variant of “nobody says this any more”.
You go on to somehow correlate the use of non-metric (there, I said it again) units of measure with our being “arrogant self centered United states-ians”. This is curious on a number of levels, not the least of which is grammatical.
First, judging from the tone of your email, I very much doubt that if tomorrow, the US would throw down the inch and embrace the centimeter, your hatred would recede even a little bit. You don’t like the US for some reason, all three hundred million of us. That is certainly your choice, however I would love to hear your reasoning about why other countries in the Americas are “more evolved.” I somehow suspect that the level of your own evolution might not make you the best candidate for making that judgement.
Second, I see from your email address that you are Canadian. According to this article:
http://en.wikipedia.org/wiki/Metrication_in_Canada, “Canadians, today typically use a mix of metric and imperial measurements in their daily lives.” Further, prior to 1970 Canada used Imperial measures almost exclusively. There was much opposition to changing to metric units, and your government has alternately promoted, then backed off of promoting a change to metric units. Obviously, all the Canadians who opposed metrication (were some of these your relatives?), the politicians who backed away from it and the Canadian citizens who use non-metric units must be arrogant and self-centered Canadians.
Speaking of arrogance, you state that the correct name for the metric system is “International System…SI”. If that were true, wouldn’t it be called IS? Actually, SI is an abbreviation of Système international d’unités. It’s French. Notwithstanding the contributions of the French to the development of metric units of measure, once they crossed the threshold and proclaimed the system to be truly international, shouldn’t it have been named in a language that was more truly international? Mandarin would win by sheer numbers. 1,025 million people have Mandarin as their first tongue. Then we would be using åƒåŠ 國際比賽的人 é‡å…· instead.
But if we are too arrogant to go that far, we should name the system in English for its universality, with 1.5 billion who use it as a first, second or foreign language, and not in French which is only spoken by about 1/8 that number.
But speaking the same language, as you and I do, doesn’t always mean sharing the same understandings. Let me try to bridge this gap with help from Dictionary.com:
i·ro·ny
1    [ahy-ruh-nee] noun, plural -nies.
1. the use of words to convey a meaning that is the opposite of its literal meaning: the irony of her reply, “How nice!†when I said I had to work all weekend.
2. Literature .
a. a technique of indicating, as through character or plot development, an intention or attitude opposite to that which is actually or ostensibly stated.
Surprise! I was using a literary device and you didn’t get it. You are hateful AND humorless. But irony is a fun device, so I will continue:
Using units of measure based upon body parts, the foot (feet) and the thumb (inch) works very well because most people have these readily available and don’t have the time to consult with some stuffy French organization in order to cut a piece of wood.
The problem came in when food became more plentiful and thumb sizes varied accordingly. Standardization was necessary. Each neighborhood selected one person to be their standard thumb representative. He ran from place to place adjudicating length disputes.
The pound evolved not from body parts, but from grain. 9,600 wheat grains were the standard for one pound. Aside from the time consuming nature of counting all of those wheat grains each time you wanted a pound of liver, a voracious mouse or flock of ravens could swoop by and throw off the whole standard.
This problem did not stop one of the most arrogant of all rulers, Louis IV from standardizing the thickness of a grain of wheat into a new unit of measure: the millimeter. Ha!
So you won’t become confused again, robert, I made some of that last stuff up.
Whoah!
Hats-off for for your in depth and rigorous psychoanalysis.
I guess I must have touched a sensitive nerve!
Although I don’t think wikipedia qualifies as a reliable source by any means!
I am of an age such that I use both systems with equal ease.
The only reason I still need to use imperial is US hard hardheadedness.
The arrogant accusation is a reasonable response to “eurolovers” as a description of SI users.
Imperial is more user friendly at a certain scale, inch, foot, pounds and miles to a certain degree.
But SI is without contest much better adapted to theorectical, scientific and overall paperwork.
Not to mention unit transfers from weight to force to volumes to sizes etc.
One way to keep the inch pound pint measurements relevant and useful would be to SI them:
Make one inch = exactly 25mm instead of 24.5
One mile exactly 1.5Km instead of 1.6
One pound = exactly one half kilogram instead on 2.2 lbs/Kg
One gallon exactly four liters. (Therefore exactly 8lbs (on earth) )
In other words, go SI and give certain key measurements imperial names.
But I am not holding my breath.
You have some seriously more important issues to deal with.
Cheers to you
I ran across this post while researching urban wind tunnels. Thanks for a thoroughly enjoyable and informative post. It’s great when fun and science/engineering are mixed. Reminds me of that venerable illustrator / explainer of the atmosphere, Eric Sloan.