Comparing structures to see which withstands wind load best

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Comparing structures to see which withstands wind load best

Postby Haley_Johnson » Tue Feb 12, 2013 4:35 pm

I am building a geodesic dome and other structures to compare which best withstands wind load. I would like to possibly build structures with a gable roof, hip roof, A-frame, tower, or other. I have researched and researched, but I cannot find a website that explains how to make a model of any of the structures except for the geodesic dome. Do you know of any websites that can help?
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Re: Comparing structures to see which withstands wind load b

Postby wendellwiggins » Tue Feb 12, 2013 5:54 pm

Hello Haley_Johnson,

I don't know of any website that explains all the structures. I can offer a few general comments.

Several issues will affect the wind resistance of each structure. The geodesic dome is unique in the respect that it is stable without any internal bracing. The materials you use for building, the way you fasten them together, and any internal bracing are as important in the result you get as is the shape.

I suggest that you decide on the simplest design you can think of for each shape and build each of them out of balsa wood, cardboard, or other simple sheet material with no internal bracing. Make some dimension (perhaps the longest horizontal dimension) the same for each. These considerations make the comparison of strength more meaningful.

Good luck, WW
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Re: Comparing structures to see which withstands wind load b

Postby Craig_Bridge » Thu Feb 14, 2013 2:03 pm

Many years ago, universities with large low speed wind tunnels that were once used for aircraft design either decomissioned them or turned them into equipment to evaluate wind loads on buildings and other structures. Comparing different structure designs in the absense of surrounding buildings and then in with surrounding buildings produces a significant difference in modeled behavior. The wind direction, up and down drafts, and wind speed yield significant differences in behavior. For example, Purdue university built a 9 story building with an open ground level walkway 12 feet above street level in line with a street that went for several miles in the prevailing wind direction. This sidewalk area was needed for pedestrian traffic; however, it also acted like a wind tunnel that magnified the winds going through this opening making it extremely difficult to open the doors or to keep the from flying open. If this building had been evaluated in a wind tunnel before it was built, some significant modifications would have been made to the design and landscaping to reduce the wind tunnel effects.

Coming up with a fair comparison of different structures is a non-trivial task. If you leave out internal bracing as the previous responder suggested, the structure will oscillate at some resonant frequence under some wind conditions and self destruct. Internal cross bracing and spacing will dramatically change the resonant frequencies. Most geodesic dome structures are designed to be self braced so there aren't any large flat surfaces. This complicates their construction, but it eliminates internal bracing. If you try to compare wind performance to simpler structures without their internal bracing, the dome is going to win and I would argue that your evaluation was biased.

The fundamental problem with comparing different structures is to come up with a fair cost function. Architects and structural engineers typically have to estimate the cost of the materials and labor required to build competing designs and then factor in some asthetics when coming up with a "cost" function to evaluate competing designs.
-Craig
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