90 MPH wind? - Structural engineer needed

[CupcakeGirl2]

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!

[Jammer]

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.

[DanB]

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...

[richinny]

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?

[bwaysteve]

Originally Posted by CupcakeGirl2

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.

[Inland RV Center, In]

Originally Posted by CupcakeGirl2

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

[Goin camping]

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?

[azflycaster]

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.

[barts]

Originally Posted by CupcakeGirl2

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)

[dkottum]

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.

[Goin camping]

We need a scale model and a wind tunnel.

[Canlib]

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!

[mimiandrews]

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.

[pmclemore]

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

[CupcakeGirl2]

Thank you all so much for your thoughts and suggestions. One thing I didn't mention is that the city of Dallas will not give us a permit so we are having to treat it like a building. This means we are going to be anchoring it with hurricane anchors on a nine inch cement slab. Any additional thoughts?

[barts]

Originally Posted by CupcakeGirl2

Thank you all so much for your thoughts and suggestions. One thing I didn't mention is that the city of Dallas will not give us a permit so we are having to treat it like a building. This means we are going to be anchoring it with hurricane anchors on a nine inch cement slab. Any additional thoughts?

Safe as houses.... .

Well, now you're doubly sure it won't blow away or tip over.

Seriously, you're going to need that PE stamp on the permit; it's the city's CYA approach. They can easily size the anchors; in fact they prob. have prescriptive (following their directions and you're automatically ok) instructions for those units. It's the fear that the trailer will disassemble due to wind load that's more difficult to deal with...

- Bart

[kevin242]

If you can't find any 90 mph wind research, show them a picture of the Airstream that stood up well to the atomic bomb blast test in the 1950's!

[CupcakeGirl2]

Just wanted to thank you all for posting all of these responses. This is such a great community and i feel very welcomed in a place where i know very little! Ok -- so im throwing another doozie out there. We spoke with a structural engineer today. He asked the following questions:
- Height off ground?
- Locations of axles
- Does the sub-carriage (frame) have any locations or methods where a tie-down can be fastened? That is, how will we secure it to the ground or foundation. Where are they?

Does anyone out there know where i can find this info?

Thank you in advance for any and all thoughts. I'm overwhelmed by the community here.

[Inland RV Center, In]

Originally Posted by CupcakeGirl2

Just wanted to thank you all for posting all of these responses. This is such a great community and i feel very welcomed in a place where i know very little! Ok -- so im throwing another doozie out there. We spoke with a structural engineer today. He asked the following questions:
- Height off ground?
- Locations of axles
- Does the sub-carriage (frame) have any locations or methods where a tie-down can be fastened? That is, how will we secure it to the ground or foundation. Where are they?

Does anyone out there know where i can find this info?

Thank you in advance for any and all thoughts. I'm overwhelmed by the community here.

There are a number of ways that an Airstream trailer can be secure to the ground with steel cables or chains.

Each side of the A-frame is a very good place to fasten to, along with both sides of the frame at the rear of the trailer.

You can also use a cable or chain around the outside of each axle just next to the axle mounting plate, and before the torsion arms.

For a tandem axle trailer, that would provide secure 8 places to fasten tie downs to.

Do not, attach anything to the axle tube or over it, between the side to side frame
area.

Andy

[slowmover]

Cupcake girl you should look up the story on the mobile popcorn cart once used at Old City Park . . practically rebuilt it to 1500-lbs to meet "interpretation". This is not a new story in Big D. There was also an Airstream or Silver Streak used for years in Deep Ellum as a food concessionaire (early 90's). Reasonable concerns about fire are valid. Being protected from flying debris is something they cannot provide.

As to aerodynamics: low ground clearance (vital) and a radius that is 10% of the length on other edges is found to be valid for FE purposes. Winds pile on flat walls that aren't relieved by radiused edges. The upper portions -- and cross-section -- of an AS is excellent in this regard.

Volvo Trucks Magazine

Exterior shape and all metal framing are conducive to surviving 90 mph winds from any direction what with the shape provided. Wish we could say the same for power poles, directional signals, etc.

An F5 will eventually hit D/FW. But while being east of downtown Dallas is best (and slightly north), the concerns they have are valid. But grandfathered structures make it moot.

Sounds like time for a talk with your Council member.

.