Wind speed vs. wind speed? are they talking different things?
Wind speed vs. wind speed? are they talking different things?
(OP)
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.
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.






RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
I worked for a Harvestore Dealer and we had to attempt a repair to the metal tube of a Harvestore silo where the the tube was nearly empty and cooled fast enough that the water vapor in the air condensed and the tube crushed inward like a pop can. We were able to inflate it with a very heavy duty high volume compressor so most of the dent came out of it. I bet the silo leaked badly after that.
There are many events that could damge the silo that may not be related directly to the wind. There are also many ways to measure wind speed and calculate pressure. Without knowing more about the event it is hard to answer the question without a lot of conjecture.
Jim H
RE: Wind speed vs. wind speed? are they talking different things?
You say a 3rd party engineer checked the calculations.
Did anyone check the structure itself to see if it was built correctly? hokie66, who normally has very good answers, did not mention this other possibility.
In this case, of course, if the designer is the manufacturer, there is no difference between a design flaw and a construction flaw. The same entity is at fault.
RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
To answer your questions...the wind criteria in the code (ASCE 7-05) are based on a 3 second gust. The wind provided by the weather service was probably an instantaneous gust...less than 3 seconds, which means that the structure probably did not see its design wind load. Based on that, I would start looking at construction anomalies or manufacturing anomalies.
BTB makes a good point...the wind at the site could very well have been different than that recorded by the weather service...we've seen that many times. So that leads you back to a possible design issue, but primarily for the purpose of consideration...there's a big difference between a 70 mph gust and a 120 mph design gust. That level of difference is not likely to have occurred unless you had a spawned tornado involved, but nevertheless, you have to consider it in your evaluation.
RE: Wind speed vs. wind speed? are they talking different things?
I think the current standard is a 3-second gust. If I remember right, the older standard was a fastest-mile speed. Neither one is the same as the instantaneous speed at a particular point.
I'd also be curious how the silo was damaged.
Generally, the wind provisions for metal-plate tanks seem to be fairly reasonable, and you don't see too many problems with wind damage in a completed structure. If the silo was corrugated, there could be some flawed logic in how the stiffness is evaluated or how buckling in the shell is evaluated or something like that.
If ther are other silos at the site, you could get interaction between them that affects wind loading, that is likely not figured into the design.
I would be more inclined to think the wind estimate is off than anything, under the circumstances described. It might be helpful to read the fine print in that report.
RE: Wind speed vs. wind speed? are they talking different things?
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Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Wind speed vs. wind speed? are they talking different things?
I am curious, though, about the actual pressure that the building is designed for. Remember that the structure is not designed for the pressure from a 120mph wind. After accounting for things like importance factor, height, and exposure, your structure is not designed for an actual 120mph wind. These factors actual determine that your structure will be designed for some fraction of 120mph.
If you take your average wind pressure in psf, divide by 0.00256 and take square root, you find out the actual wind speed that the structure is theorhetically designed for. I would still doubt that this is near 70mph, but depending on your wind factors it may be a lot closer.
With this said, everyone else is also right. Construction quality, accuracy of CompuWeather, etc. could all contribute to this discrepancy.
RE: Wind speed vs. wind speed? are they talking different things?
What is anticipated in the code is the winds on the scale which will damage the structure. MWFRS for a typical commercial structure (concrete or structural steel) requires design for lower values than cladding precisely because the local wind can be substantially higher.
The silo may be constructed such that the cladding is the structure and little or no other MWFRS to speak of. A very brief, very strong local wind could buckle the sheet metal, leading to overall failure. If there is a relatively lightweight frame attached or inside, the local buckling of the skin can buckle frame members.
RE: Wind speed vs. wind speed? are they talking different things?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Wind speed vs. wind speed? are they talking different things?
I agree with what weab and TXStructural said. I still think the "V" in ASCE7 formulas can not simply put an equal sign with the measured field "V". especially, now we know that in the coming ASCE7-10 code, the previously 90mph wind zone will become 105mph or even more higher wind zone. that means the V in the formulas of ASCE7 is a symbolic value which guide your wind design. So a field measured 70mph wind might already caused same wind pressure as V=120mph calculated wind pressure with ASCE7 formulas. Plus with agricultural buildings (Category 1, I=0.77), the calculated wind pressure will be much smaller than other category buildings, but a field wind will cause a same pressure no matter what category the building is.
Therefore, my point is that one can not say a building correctly designed (meet ASCE7 code) for 120mph wind zone should be able to sustain a field 120 mph wind. It might only be able to sustain a 70mph actual field wind. Is this viewpoint correct or not? I would like to hear you guys comments.
Thanks.
RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
So in the older codes they used fastest mile. This averaged all the gusts out over the time it took for a column of air to travel one mile. This resulted in an average wind speed of say 90 mph, which equates to the wind speed averaged out over 40 seconds. Now we use 3 second gust, which only averages out the wind speed over 3 seconds. This results in a higher wind speed of 110 mph for the same location. Using the appropriate equations from each era, the calculated pressures for both cases would be approximately the same.
So now if CompuWeather came up with 70 mph as the maximum gust speed recorded, this could actually have only lasted for a fraction of a second. So once averaged out to the same time frame as either of the above examples, the average wind speed would be much lower. In order to be a valid number, they really should provide you with a 3 second average wind speed. This would at least allow you tom compare it with the current code speeds directly.
I do agree with what other have said above, regarding very localized weather effects. Unless there was a weather station sitting on top of the silo, it is difficult to extrapolate after the fact the "exact" wind speed at some remote location. Since you haven't told us how it was damaged, it is very difficult for us to suggest why it was damaged.
RE: Wind speed vs. wind speed? are they talking different things?
On the Compuweather site, they talk about Doppler radar records and whatnot, and I'm not familiar enough with those to know if they actually would show localized spots of higher wind speed. If they don't directly show wind speeds, but only show rain or hail, then the records could be considerably off from what actually happened.
With it being a silo, you have a lower importance factor, as noted above, and also are likely to be farther away from the weather stations. Flat barren fields around the site would increase the wind loading. Colder weather and lower elevations would increase the wind force for a given speed.
With a report like that, I would kind of expect a lot of weasel wording in it, saying winds "probably" didn't exceed a certain amount or that higher winds "were not recorded" or that it is "in the opinion of the writer" or something of the sort. Consider that with a soils report, they can drill two holes 50' apart but can't tell you what's in between them. Is forensic meteorology really that much more exact?
RE: Wind speed vs. wind speed? are they talking different things?
Forensic met is fairly accurate, but consider that standard radar measures rain intensity, and Doppler measures the movement of rain in the air column as a demonstration of wind, not winds speed directly.
The problem comes in that a very local wind event is difficult or impossible to watch on radar even when you know where to point the radar. If you are having to search the files for something which was probably not well captured, you have to take averages and snapshots and figure out what was probable.
Once it is established that the structure was properly designed and built (facts which can be determined from evidence, inspection, and calculation) the variable is the wind, which cannot be determined with the same certainty.
RE: Wind speed vs. wind speed? are they talking different things?
But, you would think a round silo would be really good in torsion huh SEIT?
RE: Wind speed vs. wind speed? are they talking different things?
More engineering, less computation. Stop worrying about taking out that last ounce of concrete and steel while forgetting that an economical, robust design (not excessive) is a good thing.
RE: Wind speed vs. wind speed? are they talking different things?
BA
RE: Wind speed vs. wind speed? are they talking different things?
RE: Wind speed vs. wind speed? are they talking different things?
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.