Any building code requires you to consider basic wind speed, ground snow load, exposure, occupancy importance factor, seismic zone coefficient, shape factors, snow drift, snow sliding, wind drag, wind gust, wind suction, wind pressure, dead loads, live loads, construction loads and all the possible load combinations. When in doubt, the most conservative combinations should be considered in analysis and design. Most of the time, this will drive the cost upward.
A wind-tunnel test is warranted when the structure is large, tall, complex in shape, a new system or a public safety concern. But usually the key reason is the project wants to cut the cost. Normally the wind-tunnel test will lower wind/snow loads to be used for analysis and design. When the structure is small, the saving doesn’t pay for the cost of the test.
Unfortunately wind-tunnel test is not an exact science; it takes a lot of experience. There are not that many reputable outfits in the world. Otherwise the cost will not be so high.
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