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Old 10-26-2010, 10:02 PM   #1
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90 MPH wind? - Structural engineer needed

Hi there -- not sure where to post this, but hoping someone can help out. For our business, the city permit department needs us to hire a structural engineer to prove that our airstream (1965 tradewind) can withstand 90MPH wind. Clearly, airstreams were built to withstand travel with winds, but does ANYONE know ANYWHERE out there that proves that our "bubbles" can withstand 90MPH wind. I called airstream and they claim there has never been a test for this. Any and all help would be greatly appreciated. Thank you!
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Old 10-26-2010, 10:05 PM   #2
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I know a really good one but that kind of analysis is time consuming and therefore expensive.

So too is purchasing time in a wind tunnel big enough to hold a traylah, sideways.
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Old 10-26-2010, 10:11 PM   #3
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I've towed over 90mph. Does that count?

More to the point, An Airstream will probably not "pass" a 90mph test. Most of the building my firm designs would not. Especially the roofs.

Do understand what this type of "test" means. Is this a sustained wind force test?

I would imagine that an airstream hit on the side with a 90mph wind is going to minimally lift, if not turn over.

Then again, after considering the shape a little more, maybe it would do nothing?

I've got a friend that works in the wind tunnels at NASA. perhaps I'll drop him an email...
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Old 10-26-2010, 10:18 PM   #4
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make an appointment for the inspector to ride in the trailer because a test is much better than theory ;-)

there is a vendor here that may have driven one at that speed. maybe he can write you a letter.

have you checked with any other vendors to see what they did for this requirement?
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Old 10-26-2010, 10:25 PM   #5
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business

Quote:
Originally Posted by CupcakeGirl2 View Post
Hi there -- not sure where to post this, but hoping someone can help out. For our business, the city permit department needs us to hire a structural engineer to prove that our airstream (1965 tradewind) can withstand 90MPH wind. Clearly, airstreams were built to withstand travel with winds, but does ANYONE know ANYWHERE out there that proves that our "bubbles" can withstand 90MPH wind. I called airstream and they claim there has never been a test for this. Any and all help would be greatly appreciated. Thank you!
If you are the same member who is doing a food business in your stream this sounds like the city is being anal and trying to find an excuse to not give you a permit.Maybe because of flack from brick and mortar establishments(as is happening in NYC currently).
I would probably check what the city of Austin is doing with their "food streams".Also the wagons that go to the construction sites certainly will not withstand 90 plus winds.Might be cheaper to consult an attorney who has dealings with the city than to hire a structural engineer.Exemptions are given all the time and an attorney will probably know how to get one.Good Luck.
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Old 10-26-2010, 10:39 PM   #6
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Quote:
Originally Posted by CupcakeGirl2 View Post
Hi there -- not sure where to post this, but hoping someone can help out. For our business, the city permit department needs us to hire a structural engineer to prove that our airstream (1965 tradewind) can withstand 90MPH wind. Clearly, airstreams were built to withstand travel with winds, but does ANYONE know ANYWHERE out there that proves that our "bubbles" can withstand 90MPH wind. I called airstream and they claim there has never been a test for this. Any and all help would be greatly appreciated. Thank you!
In the early 70's, when I worked for the insurance division of Airstream, we did a number of different tests involving the Airstream trailers.

One was having a camera mounted on the top of the drivers seat, so that it could record the speedometer reading and a airspeed indicator, that was properly mounted and vented, as well as see on coming traffic.

The test was to see what happened to wind speeds when meeting on coming trucks , busses and cars. This filming took place on single lane highways, to observe the maximum impacts from the wind from an on coming vehicle.

This test clearly showed a wave of air currents, that rapidly diminished after the two vehicles passed each other.

The speed of the oncoming vehicles, of course had to be estimated since there was no easy way to find out an exact speed.

When our vehicle was going 60 mph and met, head on, a large vehicle going the same estimated speed, the airspeed indicator would suddenly jump to 120 mph, quickly return to almost zero mph, and just as quickly went to near the 120 mph , then to about 10 mph. These oscillations continued to swing up and down, but each time they would reduced the difference in the 2 speeds, until the airspeed indictor showed the same speed or nearly so, as the speedometer in the car. That typically took just a very few seconds.

Granted, these were very crude tests, but the results answered the questions. Anyone can duplicate those tests if they wish.

We also wanted to know, "how fast" can an Airstream trailer be towed. The trailer under this test, was assigned to me by Airstream, and was a 1973 31 foot, fully loaded. The total weight of the car and trailer was 14,500 pounds. The car was a 1973 Buick Electra, with all the goodies on it to assure adequate control.

Again, the camera was mounted on the top of the drivers seat. With relatively calm wind conditions, and a highway was chosen that had little to no traffic on it, but readily accessible, and was level.

The top speed recorded, was 115 mph, as shown on the speedometer and the airspeed indicator.

The film was given to the big boss of Caravanner Insurance. He viewed it, and quickly got on my case, for going so fast. My only answer was that research always has some kind of risks.

The roads are open and free, if someone wishes to duplicate those tests.

I was much younger then, as well as somewhat foolish, but never the less, the results were in and indisputable. The above tests were done in the early part of 1973.

This perhaps does not answer the question asked, but it is my personal experience, with a film record to support it.

Andy
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Old 10-26-2010, 10:43 PM   #7
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Hook it up to a Porsche Cayanne and take the inspector for a ride.

Isn't there a video of an Airstream being towed at high speed?
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Old 10-26-2010, 10:47 PM   #8
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This summer at the International Rally in Gillette, WY we had a storm with wind speeds measured at 85 MPH. No trailers fell over, but we did loose a garbage can.
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Old 10-26-2010, 11:52 PM   #9
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Quote:
Originally Posted by CupcakeGirl2 View Post
Hi there -- not sure where to post this, but hoping someone can help out. For our business, the city permit department needs us to hire a structural engineer to prove that our airstream (1965 tradewind) can withstand 90MPH wind. Clearly, airstreams were built to withstand travel with winds, but does ANYONE know ANYWHERE out there that proves that our "bubbles" can withstand 90MPH wind. I called airstream and they claim there has never been a test for this. Any and all help would be greatly appreciated. Thank you!
There's two failure modes here - one, the trailer comes apart. Not going to happen; Airstreams are small compared to buildings, and well fastened. More on this later Second, the trailer may become airborne, tip over, etc. That's easy to calculate: PSF = .00256 (mph)**2, so 90 mph is a bit less than 21 lbs/square foot. Assuming that we can model the trailer as a wall 8 ft high and 20 feet long, overall wind load is 8*20*21 = 3360 lbs. This is a conservative model, since 1) the trailer is curved, and the edges don't capture as much force as the middle. This tradewind prob. weighs 4200 lbs; assuming a coefficient of friction of .9 you're ok. If wind speeds go over 100 mph from the side, she will blow away sideways, though; that's what hurricane anchors are for. We should also check to see if it will tip over. Assuming the force above acts 5 feet off the ground, we have a tipping moment of 16800 ft-lbs. If the trailer weighs 4200 lbs and is about tip, all the weight is on the downwind wheels - 4' from center, yielding 16800 ft-lbs. So... the trailer is about to tip over in 90 mph winds if it weighs 4200 lbs. Add some weight (usually not a problem w/ remodels - most trailers go too heavy...) and you'll be just fine. I'd try for 5000 lbs and you'll have a nice margin of safety.

Back to figuring out whether or not trailer structure can take side force of 90 mph. Easiest test is to tip trailer sideways onto large air bag .... This is a very complicated problem to solve analytically, and any answer would likely be way off due to age, vagaries of internal construction, etc. I'd prob. use photo evidence from trailer tip-over accidents that trailer integrity was maintained despite all of the weight landing on the structure sideways.

- Bart (who is not a structural engineer)
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Old 10-27-2010, 07:37 AM   #10
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For what its worth, our trailer sat sideways to 60+ mph winds and rain yesterday on its single axle and stabilizers, solid as a rock and dry as a bone inside.
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Old 10-27-2010, 08:55 AM   #11
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We need a scale model and a wind tunnel.
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Old 10-27-2010, 09:28 AM   #12
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Bway and Barts have nailed your problem. My years of dealing with building department bureaucrats tells me you have a long expensive battle ahead of you to do what is really a simple thing. Save yourself a lot of headaches and rent a storefront for work and go camping in the Airstream as often as possible!
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Old 10-27-2010, 10:14 AM   #13
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Surely the city does not require this sort of analysis for mobile food vendors. If they did, all the construction workers would starve. Never saw a "roach coach" that would take a 90 mph wind, and I see plenty of those around the Dallas/Ft.Worth area.
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Old 10-27-2010, 10:23 AM   #14
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I'm kind of with Mimi; I wonder if they are looking at your Airstream the way they would look at an office trailer as a place of business. They may be wanting to see "hurricane straps" or some such foolishness.

I think some further conversations with the 'crats might help to understand exactly which section of code they are working to - once you know that, then you can evaluate how to satisfy them.

Pat
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