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.

 

[hokie66]

I don't think the problem is in interpretation. Either the design was faulty, or CompuWeather is wrong.

 

[jimstructures]

How was it damaged? Did the sheet metal fail? Did the foundation fail? Did the foundation overturn? What happened?

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

 

[jgailla]

cityplan,
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.

 

[btrueblood]

Wind speed at a weather station might not match the wind speed from a microburst, or wind shear event (or a tornado, for that matter) even a few hundred yards away.

 

[hokie66]

jgailla and btrueblood are both correct. The problem could have been construction rather than design, and after the fact assessment of wind speed is fraught with difficulty.

 

[Ron]

We commonly see structural failures at less than the "design wind loads". These usually occur because of construction anomalies or defects.

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.

 

[JStephen]

There can be some variation in how the wind speeds are figured, but it shouldn't make THAT much difference.

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.

 

[paddingtongreen]

I don't know if this is relevant, but worth a look if there is a possibility of funneling.

http://www.mechanics.citg.tudelft.nl/~pierre/CT5143_handout_15.pdf

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[weab]

First of all, I'm amazed at how accurate CompuWeather thinks that they can be in such a localized area for an instant in time.

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.

 

[TXStructural]

All wind is local, and you CANNOT say for certain what wind pressure was experienced by a structure at any moment in thunderstorms. Hurricanes are a bit more predictable on the macroscale but not necessarily at the microscale.

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.

 

[msquared48]

What it the topography like at the two sites - the weather station and the site of the silo failure? Both dead flat?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[cityplan]

Thanks for all inputs. they did give a lot of useful information. I know some consulting engineering company is also hired to do field investigation, no result come out yet though.

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.

 

[Lion06]

This must have been one of those 842 torsional wind cases that are new and wasn't considered during the original design!!

 

[JedClampett]

Another aspect to consider is the motivation of the organization paying for the analysis. My guess is the insurance company wants to blame the original design, so they can go after them for damages. So they hire someone who will give an answer that is to their advantage. Now they have "proof" that the wind didn't exceed the code design requirement, so it puts the original design and construction into question. And questions are good to them.

 

[structuresguy]

Cityplan: i think you have your assumptions/conclusions backwards regarding applicability of wind speeds. The way they work is that the longer the duration of the wind is averaged over, the lower the calculated wind speed.

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.

 

[JStephen]

I suggested reading the fine print in the report. I know on the internet weather sites, when looking for temperature, you can enter ANY location and it will give you a number. But, as far as I know, measurements are only made at stations in main cities. So presumably, the numbers in between are interpolated from results at those stations. Using this kind of reasoning, you could entirely miss a tornado.

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?

 

[TXStructural]

The ASCE 7-10 winds are the same once the load factors are considered, so you can ignore that difference as long as you are consistent using one code - you cannot mix V from one edition and load combinations from another.

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.

 

[ToadJones]

I'm with SEIT here...those torsional cases really pi$$ me off $-)
But, you would think a round silo would be really good in torsion huh SEIT?

 

[TXStructural]

As if more load cases and a sharper pencil will offset the inaccuracy of the oh so precise math. We have become obsessed with precision, rather than working to the degree of accuracy & precision sufficient for design.

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.