A Swirl of Questions, Calculations and Tests

Deanne writes:
A lot of interesting questions on the topics of mobile home construction and the practicalities of carbon fiber came up for us during the testing for this show. We had our own theories, but it would be interesting to find out what you think.

1. How would you shape a mobile home to give it the most defense against hurricane force winds? And make it architecturally functional/practical?
2. Do you think that the mass production of carbon fiber will ever drive down cost enough to see this as a potential solution for mobile homes, perhaps in 10 years? (The high cost of this application was a big concern for us, but our show is about testing proof of concepts, so we proceeded nonetheless. Many airplanes will be made of carbon fiber soon enough, and manufacturers are increasing its production, so it was an interesting debate.)
3. Carbon fiber is extremely lightweight. It has high tensile strength, but only low to moderate impact resistance. Can you think of a lightweight material that may work even better in our application?
4. Mobile homes are elevated and rest on blocks. If you could put up a temporary cladding around the open space below the home, would you? And what would you use that is removable after the storm?
5. I’ve met two guys who tried to make diamonds out of carbon fiber. I’ve never tried it and they couldn’t get it to work, but could it be done?
6. The Smash Lab team met Burt Rutan, the creator of Spaceship One, at a small, local diner at the Spaceport in Mojave. Would you go into space if you had the chance?

Now, for my unofficial lab notebook pages. Our challenge for this episode was to find out if carbon fiber cloth could protect a mobile home against hurricane force winds. We limited our tests to carbon fiber and only testing the effects of high-speed wind.

Note that many alternative building materials are available and that a hurricane is comprised of not only strong winds, but also immense rain,  flying debris, storm surge and, of course, potentially long duration, which equals structural fatigue.

Material:
Carbon Fiber Cloth and Resin.

Tensile strength of our woven carbon fiber fabric is 150,000 psi. A mild steel, ASTM A36, has a tensile strength of 36,000 psi. Unidirectional cloth is often used in hoop stress applications such as reinforcing pipelines. Bidirectional cloth is the better choice for application in a hurricane because the winds may come from any direction. We used bidirectional cloth in our test, which has an equivalent strength of a 1/16-inch-thick plate of steel.

Hypothesis:
Carbon fiber has incredible potential when applied to conditions requiring tensile reinforcement, so its ability to minimize deflection of panels due to high pressure wind is almost certain. On the other hand, carbon fiber is not very impact resistant, but may prove to have some minor improvement regarding flying debris damage. The focus of our debris failure is solely linked to wind, and how wind may further damage a home once debris initiates damage to the home.

Testing:
Qualitative tests were performed for the audience to describe carbon fiber cloth's tensile strength and impact resistance qualities. (We used old samples that were falling apart, but they still worked.)

Wind tunnel tests were conducted (and omitted from the final show) that studied the flow of air over a 1/12-scale model of our trailer.  Of course, a wind tunnel creates laminar, smooth flow, but a mobile home would be mainly subjected to turbulent winds during a hurricane. Nonetheless, here's a list of the major failure points of a mobile home:

1. Hurricane force winds create positive pressure along the windward face and low pressure on the leeward face. Basically, this creates a huge force along the windward face of the mobile home and can cause deflection or even shear fracture – much like we saw on the shed test.
2. Large flow separation occurs at the corner of the front wall and roof.  This creates a negative pressure region along the roof that creates suction and  can cause the roof to peel back at the corners.
3. The wind along the front face causes a moment/torque, giving it a tendency to tumble as opposed to shift backward.
4. Even though we could barely see them, a research specialist in hurricane structure damage told me that hairpin vortices along the roof corners of the home are one of the most significant causes of damage.

We also explored various ways to reshape/retrofit an existing mobile home to make it more aerodynamic during a hurricane and/or create a downward force toward the ground, but found that the shape change would have to be very drastic to have any effect. There are two main issues:

1. Hurricane winds can come from any direction.
2. Since the fluid dynamics of any wind is that it has a velocity of zero on the ground (standard boundary layer stuff) and the velocity increases as you go up in altitude, a change in roof shape would have little to no effect in pushing the home down. The only potential change we decided on would be to change the entire structure to have more of an ellipsoid profile, to create a shape with a lower drag coefficient, thus minimizing the stress on the corners. (The classic Airstream trailers got their name for a reason ...)

The shed tests were fun. Nick and I made weak structures to see where the wind damage occurs first. The front wall of the shed cracked in half, shearing at a nail.  If there is one thing we have reaffirmed on this show it’s that fasteners are the most common failure point … over and over again. We’ve seen it on planes, buildings, cars and everything else. (So, choose your fasteners wisely!)

Calculations:
Kinetic energy (N*m) = 1/2 * mass * v^2

For a gas, the mass is spread out over volume, so we replace it with a density yielding:

Stagnation Pressure applied to the exterior of a structure (N/m^2) = 1/2 * density * v^2

At a height of 10 meters (33 feet), the standard atmosphere (defined in 1976) says that the density is 1.22 kg/m^3.

Let’s assume a wind speed of 150 mph (67.056 m/s). The quick and easy version, which rolls all of the above assumptions into a coefficient-style formula, uses this equation:

Stress = 0.00256 * v^2, where v is in mph and p is in lbs/ft^2

In English units, this equates to .4 psi. It doesn’t sound like much; however, a person has a cross-sectional area of 6 ft^2, which means s/he gets blasted with 346 pounds of force in a category 4 storm. It’s not easy to stand in that -- and Kevin proved it!

I won’t do more math, but knowing that carbon fiber has a tensile strength around 150,000 psi, Nick and I were obviously confident in our design.

Application:
The home that was purchased for us to use was literally rotting. So, in order to give the carbon fiber a realistic substrate, we added a thin plywood cladding on the outside of the mobile home. Carbon fiber strapping was then applied around every corner at 3-foot intervals, and carbon fiber sheets were applied to the entire façade of the home with at least a 6-inch overlap between panels.

To prevent the mobile home from rolling we came prepared with anchor straps. Unfortunately, we assumed we would be on a concrete foundation, but Kevin and Chuck’s secret locale was a spaceport that allowed us to anchor to sand, and only sand. We improvised a set of welded spikes, since that was our only resort due to time constraints.

Final Test:
Airplane thrust ranging from category 1 to category 5 winds and debris from an amazing 2x4 air cannon.

Results:
As suspected, carbon fiber was successful at structurally reinforcing the mobile home, but it still suffered from impact resistance exerted by flying debris.

We also learned the hard way that anchors are critically important in saving a mobile home from hurricane damage. If you don’t anchor the mobile home down, it may even become debris itself that destroys other nearby structures. While Nick and I  had to improvise on the spot since we were anchoring the structure to sand, not a concrete slab foundation, homeowners should not improvise if their mobile home is at risk. Buy the proper anchors, check them annually for corrosion, and no matter where you live, listen to your local emergency manager’s advice and orders to evacuate during serious storms.

Hurricane Facts:
One of the best ways to find out more about how hurricanes work is to collect meteorological data from them such as wind speed, temperature, pressure and precipitation – from an airplane! Flying an airplane into a storm  is a very effective way to get observations of the hurricane, since it can directly measure the data needed even when the storm is far away from land. The U.S. Air Force and National Oceanic and Atmospheric Administration (NOAA) "hurricane hunters" fly into several storms every season, collecting data that is used by researchers, forecasters and numerical weather models to help predict hurricane tracks and intensity. I know this because my brother happens to be a meteorologist who flies into the eyes of these storms. He’s flown in Katrina, Rita and many other types of weather phenomena to deposit dropsondes, little GPS-enabled parachutes, which measure useful meteorological data.

Another interesting thing that I’ve learned from my brother is that monetary assessment of hurricane damage has been estimated to be proportional to the wind speed anywhere from the 3rd power up to the 9th power. So, the difference in damage of a category 4 and category 5 storm is HUGE in comparison to the damage between a category 1 and 2.

For tons more info about hurricanes, go to the following Web site: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html

Comments

[Bill Sappington]

Hey guys,

OK let me preface these remarks by saying that I *know* you guys have to put on a show that gets ratings. That being said, please avoid whenever possible substituting theater for science.

I must take umbrage with shooting a pristine 2x4 at the building being tested. It is a statistical improbability that a 2x4 will be pristine AND fly arrow like AND strike the target perfectly perpendicular in all respects.

It is far more likely, statistically speaking, that any debris striking a dwelling of any type will strike at an oblique angle, have a large sail area ( thus taking flight ) and will be tumbling in flight when striking.

The baseball gag, was just that, nothing more then a gag, it had no scientific or theoretical basis in the context of this experiment, in my opinion.

I will watch at least one more episode and give you guys another chance. I hope you put up some solid science.

[spambutcher]

when I saw you guys were wrapping the -entire trailer- in carbon fiber (after adding a layer of plywood) - there wasn't much of a question if it would survive or not.

however, I suspect the carbon fiber used would've cost several-10's of thousands of dollars (if it wasn't provided by the manufacturer).

people don't live in trailers because they're rich.

why not test something less exotic and more affordable like fiberglass? (for these purposes - I suspect it would've provided almost all the benefit at small fraction of the cost)

In other revelations - cars would be safer and more fuel efficient if they were all made of titanium...

[bob]

that trailer was not a mobile home. its an office trailer. straps are fastened on the frame not the roof.everything you do has been done on mythbusters.

[Richard Bentoski]

I believe a lot of the trailers strength was gained by the plywood that was SCREWED to the trailers studs. The main reason mobile homes (and your test shed) fail in high winds is because of the use of nails and staples. Your test shed had nothing to prevent the front wall from caving in at the bottom, like a floor. If you built a bridge that was 16'x8'x3.5" thick how much weight do you think it would hold?
I lived in central Florida when hurricane Charlie came through. I built a shed/workshop 16'x24' on blocks, wood floor, similar to the way a mobile home sits. The walls were complete, the roof was sheeted, but the gable ends were not closed in at the time of the hurricane. After hurricane Charlie there was absolutly no damage to the shed, why? I do not use nails for any wooden construction projects only screws. I believe the MAIN reason your trailer survived was the plywood and screws not the thousands of dollars in carbon fiber.

[John]

I must second the post above who mentioned the cost of carbon fiber, I checked a common bidirectional-weave carbon fiber cloth costs anywhere from 50 to 70 dollars a yard!! Furthermore, your weakened structures used for your initial tests were misleading to the viewer, anyone who has any carpentry experience could easily tell something funny was going on. You really need to tighten this show up, the rip-offs from mythbusters is not helping your show, they do it better, seem more meticulous and respect the intelligence of their viewers a bit more.

[Mitch]

Let me start by saying I’ve seen two episodes and they will be my last. I'm not a very technical guy but, either is this show. The fact is these people have "degrees" in specific areas and I have yet to see them use there knowledge. Anybody with four friends, a backyard, and alot of money could do this "stuff".

[Erick]

I wanted to comment on the Carbon Fiber cost, but someone beat me to it. But as Bill noted, Carbon Fiber is EXPENSIVE! and NO Mobile home manufacturer, nor buyer would ever invest the money in a CF wrapped home on wheels. No offense, but this show is weak. Sounds like the producers saw how people enjoyed the destructive episodes of Mythbusters, and said, "lets make a cheesy version where all we do is break stuff. Better luck next time. I would be surprised if this lasts a season. E>

[greenlaserboy]

This show is a joke. Like another post said the shed had not floor. You seemed to want to make the shed like a house. Houses are bolted to a concrete footer. Who can afford to cover their house in carbon fiber? What I want to see is details on the air cannon, how about a parts list. That I would watch, I find that more interesting that a carbon fiber covered trailer.

[Rob]

Just to expand upon the shed experiment. Not only was the shed just resting on the ground without being anchored to a foundation, it was very clear that the shed was constructed in just an "A" Frame roof without any ceiling joists, which is not, I repeat not built in accordance with building codes. Also, to make it realistic, use nails, not screws or a staple gun and use proper sheathing both on the roof and walls. This is where the strenght lies and I dont even have an engineering degree.

[JeffLa]

Good LORD this show is bad!! I have seen 2 episodes of 'Smash Lab' now and I want my 2 hours back! This is like a weak Mythbusters knockoff... Same basic idea but without:

1. A compelling premise for each episode
2. Interesting hosts
3. Uhhh... *science*, guys?! Explosions are great, but explosions alone do NOT make a great show!

This lemon Jumped The Shark within the first 20-minutes of the first episode.

[Firefighter17]

I just watched about fifteen minutes of Gail Force Testing, and I completly agree with the above comments. I would not have trusted that "shed" to withstand the almost constant daily winds in Nebraska, let alone the force of any storm. I realize that I'm repeating statements made in previous comments, but you would think that a person with any type of engineering degree would be able to see that the shed would not hold up to much of anything. I'm a big fan of Mythbusters and while I do not always agree with the way they experiment, never have I felt obligated to make my input known. As for the show as a whole I have seen one other episode and similar mistakes were made in the construction and designs. I'm not giving up hope for the series quite yet, but from I've seen, there is not much to work with.

[singlespeed]

ok, maybe you guys should really point out that you are just testing with the shed. For pete's sake, when you build a building that is supposed to be a "stand in" for a trailer or a house then please build it to "code" or the applicable standards of the industry. I am a carpenter and I can't even build a porch without thinking of building codes.

Please do a little more research and put more thought in to your episodes. I think the State of the Union address will have more entertainment value, maybe not as much fact though.

[StructuralRealitycheck]

OK, let's preface this... I think Mythbusters is an Excellent program. I am also a licensed structural engineer who has used CFRP for strengthing failing concrete structures. It is amazing stuff and expensive!

Your "control shed" wasn't built anywhere near building code standards. Maybe if you added some anchor bolts, 1/2" sheathing with nails at 6" c/c at the perimeters and 12" c/c at intermediate studs, a floor, hurricane clips to anchor the joists, etc. you would have provided a more realistic "control". These are all code req'd elements of light frame construction. Why didn't you just go buy a shed from your local hardware store and put it out there, it probably would have performed the same.

Did you know the design wind speed across most of the US is 90mph? I am pretty sure your shed would not have stood a chance even at much lower wind speeds.

Please make sure your experiments actually have a purpose and don't insult the intelligence of the people who watch this show. Try to solve a problem with a practical solution as I agree with all the statments about those who own trailers cannot afford the CFRP.

Thank you for limiting my worthless viewing time to less than one of your episodes.

[Bill]

If they wanted to just make an indestructo trailer home, you could have done it better and cheaper if you had used 1/8 sheet metal. Flat sheet metal would deflect flying debris better than carbon fiber.
This show needs help!!!!!

[Travis]

When I watched this episode I was very disappointed. The straps to hold the trailer down was stupid. A trailer i held down by the frame which has several concret blocks holding to the ground. You could have atleast anchered the trailer by the frame. If you had done that the trailer would have either ripped the anchors out of the ground or most likely the walls would have ripped from the floor and went flying like a big box after using all the carbon fiber and plywood. The plywood that was added strengthened the walls to much. And I also agree that carbon fiber is way to expensive.

[Waning]

I heard a comment by two members of the cast that thrust comes out of the back of jet engines and then was surprised to see a third cast member indicate on the blog that thrust was directed at the trailer. Thrust does not come out of the back of jet engines. Instead, when the jet pushes air out, thrust is produced in the opposite direction - away from your trailer. There's no thrust coming out of the jets and there was no thrust directed at the trailer. Am I mistaken? If not, it makes me wonder where the science part of this show is. It I am mistaken, then I guess I learned something.

[An Architect]

As a registered architect I design real buildings occupied by real people, not silly little test buildings made to blow over on TV.

First just let me say, there is a reason the Hurricane Strap was invented.

I was quite disappointed in this show, it seemed more silly than scientific. Perhaps the problem was they needed someone who had a clue about how buildings are really built. They even admitted they had no clue, seriously the idea of throwing some tie down straps over the roof in the hopes of holding the trailer in place, yikes…

As for the wrapping a home in carbon fiber idea, wow at least I got a good laugh. But anyhow… Last time I checked there was several companies already producing realworld affordable products to protect buildings from flying debris, and yes there is already a company using carbon fiber reinforced plastic sandwiched between aluminum.

Now if it's exotic you want a few companies are producing building panels that make use of materials like Kevlar or composites using Twaron, Technora and Technora-hybrid fibers, Just incase your home needs some serious blast protection.

Hmm... Just think of the possibilities, an episode testing how to protect a home from a rouge RPG attack.

[leonel ledezma]

The tests are not accurate.
Maybe you should start by watching Mithbusters. They can give few examples of how small scale experiments should be conducted. None of the episodes that I have seen have been correct.
It's discovery channel, people expect acurrate results.
Please, focus on the facts, not the ratings.

Carbonfiber, i cant even get a small hood for my car. A house????

[Alex]

I have only seen the episode about hurricane proofing a mobile home. This episode was horrible! Not even close to Mythbusters! Just to name a few problems that I noticed: The "test house" was a complete joke. It could have been pushed over by a lite breeze. The carbon fiber reinforced 2x4: of course, the bare 2x4 is going to break when the last person jumps on the middle of it and not on one of the sides like the "carbon fiber reinforced" 2x4. Carbon fiber is very strong when it placed in a specific structure, not just wrapped around a 2x4 or a mobile home. The straps that go all the way around the mobile home is just ridiculous. And do you not think that all of the 3/4 inch plywood used all the way around the mobile home reinforced it?? I would say that this was were all the strength came from and not from the carbon fiber. This show has a lot to do to catch up with the Mythbusters guys. I will give it another try but I was truly disappointed.

[Clint]

Why didn't they run any tests with just the plywood? I mean we learned about scientific testing contols in elementary school.

[Jean SmilingCoyote]

I have unanswered questions about cost, esp. vs. reinforced concrete options like Monolithic Dome & Insulating Concrete Forms. Use of wrong anchors in desert interfered with results. I did mail (by USPS) inquiry to carbon-fiber fabric manufacturer. Awaiting reply. Presuming cost is a big issue, the only application I can see cf. storms is to wrap a plywood shearwall "belt" around a wood-frame house. I must find out more. Possibly, it might be best used in "This Old House" - type restorations where new walls of ICF's are not part of the deal. TOH doesn't seem to consider that their home area is in the high-risk zone for extreme winds, per FEMA 320.

[Clint the ME]

After looking at everyones education background I realized that one of you is a Mechanical Engineer like myself. I graduated a year ago and remember the design process I went through for my design project. One of the MAJOR aspects of design is cost!! The way I look at it is that you need to design practical solutions to these kind of design projects. Your approch to these problems are very degrading for Engineers.

[Mark]

please,please take this show off the air.

[Paul]

My only question is this :
Is it truly the carbon fiber that protected the trailer, or was it the resin, or was it the combination of the two that actually provided the strength?

[Tim Raisbeck]

Oh my gosh - It sounds like you had to go through the pain of reading Stewart-Bird-Lightfoot's "Transport Phenomena in School (groo-an)

I hope to see Smash Lab develop in to a fun companion to Mythbusters!

As a side note, and the last time I am going to post it:

There is a math problem I am having that isn't directly related to the above program. This is from the perspective of a mathematician/statistical analyst.

As you clearly know, Your Discovery Channel boards have been inundated with complaints about the program. I am confident that many of them are genuine, but there is a conceptual problem I am having.

Many of the most serious complaints are being made by first-time Discovery Channel posters, people who either never had accounts, or if they did, never posted prior to their participation on the Smash Lab board. In addition, Smash Lab has not actually pushed their Discovery Channel boards very hard.

I find it implausible that technically sophisticated viewers, who have been watching seasons of other science and technology related programming on DC, would never have been inspired to join the Discovery Channel boards, or contribute posts about their own favorite shows, yet be so compelled to complain about this one that they would seek out the Discovery Channel fan boards, and gather up the courage to post for the very first time to complain.

This doesn't really make sense to me.

I hope I am being subtle enough here