Wind speed vs. wind speed? are they talking different things? 1

[cityplan]

A very interesting and strang case here:

a steel silo was damaged in a storm, according to the manufacturer, the silo was designed to resist 120mph wind and a 3rd party engineer checked the design calculation based on ASCE7-05 and comfirmed it. However, the insurance company hired CompuWeather to do investigation and their conclusion is that the peak wind gust at the silo site at the time when silo was damaged by the wind is less than 70 mph.

My question is: do these 2 wind speeds mean the same thing? i.e. when calculating the wind pressure using formulas in ASCE7-05, q=0.00256kzktI V*V, does V shown in the formula the same meaning of V as recorded by CompuWeather?

Can any body solve this puzzle for me?

Your input will be appreciated very much.

Thanks.

 

[BAretired]

If the height to diameter ratio exceeds 5, vortex shedding is a distinct possibility. This is a dynamic effect where vortices are shed alternately, giving rise to a fluctuating force acting at right angles to the wind direction.

BA

 

[Lion06]

Toad, that was actually my very first thought when writing the reply (that a circular section is the best possible section available for torsion since it sees no warping stresses due to torsion), I just thought that would detract from the point I wanted to make.

 

[racookpe1978]

Where did the structural failure occur? (How far above ground level did the first joint/piece of steel/buckling occur?)

Where was the wind measurement taken? (How far "up" above ground level could their radar sense/was the aenometer mast set/wind estimate made?)

Literally, wind speed at AGL is zero. Just 2 ft above ground level it is still near zero, then rises quadratically: this is why wind turbines are 200 - 300 feet up. So, a "wind speed" is as valid only as its height and degree of turbulence are specified. As shown above, theoretical wind load on the walls will increase as the height above ground increases proportionally to the wind speed squared.

But total load increases with weight of the structure itself (with structural load decreasing slightly as you get higher), and with tension loads (upwind side) and compression loads (downwind side) varying as the instantaneous loads from gusts and vortex shedding varying second-by-second. You do know absolutely that, regardless of what the instanteous local winds might have been, the metal itself failed. So start there, and use the metal to determine what actual wind speed (from whatever reason) actually was present.

Measure the failed metal (torn-out screws and rivets or welds at the first failed joint if upwind side), torn metal itself, buckled metal (if downwind side was the first failure). then go back and find out of that construction was correct - including actual as-built/as-purchased metal thickness and fastener selection. If the metal and fasteners were correct (no extreme rusting, no missing fasteners) back-calculate to find out what stresses were needed to shear the fastener, pull-out the fastener, or pull the metal apart.