Video is great, but fastest mile answer falls short

Discussion in 'Design, Development and Standards Discussions' started by Simon Weisman, Jan 10, 2008.

  1. Simon Weisman

    Simon Weisman Industry Observer

    Thank you for the great video of the Florida installation. What a wonderful service!

    While reading the "Because you asked" portion, regarding the question, Can you explain the difference between fastest mile and three-second gust?, the answer implies a number of things that are not quite so:
    a) that 1-in-50 year winds are recorded;
    b) that the three-second gust will provide more accurate values; and
    c) that the three-second gust will provide the designer with the possibility of using instantaneous loads.

    None of these are correct. Typically, winds are recorded hourly, although they may be measured continuously. Fastest-mile and 3-second gust are ways of specifying averaging period for the wind measured. The fastest mile wind is averaged over the time required for the passage over the measuring device of one mile of wind; the 3-second gust is averaged over 3 seconds. Using the first method, a 70 mph wind will be averaged over 51 second, while a 90 mph wind will be averaged over 40 seconds. Using the second methods, each wind will have been averaged over 3 second. This consistency of the averaging period is what scientists find alluring.

    The shorter the averaging period, the smaller the variation between the average wind speed observed and the maximum wind gust in the period. This is why wind speeds averaged over longer periods are called mean wind speeds, while those averaged over very short periods are called gust winds.

    In any event, the design wind speeds given under fastest mile and the 3-second gust, are the results of statistical analysis and not directly measured values or recorded values. The 50-year return is simply a way of saying that there will only be a 2% probability of having that wind exceeded in any one year. If we were to talk about a 25 year return wind speed, then there would be a 4% chance of its being exceeded in any one year.

    It is possible to convert from the three second gust to the fastest mile and vice-versa. For the winds used in TIA 222, you can go from the fastest mile value to the 3-second gust by multiplying the fasted mile wind speed by 1.2.

    There is nothing inherently more accurate about the 3-second gust wind. The only real difference is that smaller amplification factors will be used in calculations of the gust wind with the 3-second gust than with the fastest mile. In fact between 222 F and 222 G the gust factor has gone from between 1.00 and 1.25, in F, to 0.85 in G, so there is no significant difference in the resulting calculated pressures.

    All loads we apply in our analyses can be considered instantaneous loads, no matter how they are calculated, because they are applied at the very same time.
  2. Richard Bell

    Richard Bell Friend of the Community

    Mr. Weisman;
    I have read your response and although I don't completely understand all you say, I trust your opinion completely. I would like to know your overall opinion of the new "G" Specification.
    Maybe you can shed some positive light on the purpose of these drastic changes. In my opinion the "G" Specification not only imposed radical changes in how a tower is designed, but the objective of even having the standard has changed. The "F" standard objective was to provide a minimum criteria for specifying and designing steel towers and supporting structures. The "G" standard objective is to provide recognized literature, and while the design committee believes it to be accurate, it should not be relied upon without the examination and verification of a licensed Professional Engineer.
    It appears the objective of the authors of the "G" standard is more self-serving for the engineering community than a minimum standard that is comprehensive to the industry and easily proven mathematically to be compliant.
    The "G" standard has unnecessarily increased concrete volume by as much as 60% and the steel content by 20%. Today we pour over 120% more concrete than we did under the "C" specifications and I have never seen or heard of a "C" Spec. anchor pulling out.
    I reviewed the design of a 1100' TV tower designed under the "G" standard and it was much lighter than the "C" and "F" standards, yet it required 75% more concrete.
    Can you enlighten me on how this can be and how it is an improvement over the old standards?
    Thanks for your time.
    Richard Bell
  3. Simon Weisman

    Simon Weisman Industry Observer

    Mr. Bell:
    The authoritative response should come from the TIA, themselves, so you may wish to write to them about this.
    For the sake of this discussion, I will attempt an unofficial response.
    The term "recognized literature" was, I think ill advised, but intended to say that the authors wished that TIA 222 G should serve as the material the building officials and courts should recognize as being authoritative on the subject of towers. This was, I believe, meant to assist designers in having their designs accepted.
    TIA 222-G is still the minimum standard for design of towers in the USA. This does not mean that it is less than some other standards, only that the designer should, as a minimum, satisfy this standard. In this respect there is no difference between the intent of TIA 222 F and TIA 222 G.
    Nor can it be properly claimed that it serves the engineers' narrow commercial interests to design heavier structures. While this may be true of tower suppliers who charge by the weight of steel and yard of concrete, it is not true for the engineer, because it should not matter to his work whether the tower is lighter or heavier. The work could be the same.
    The change from TIA 222 F to TIA 222 G was required for a number of reasons.
    To be accepted by the Building Codes, and, therefore, become law, the standard had to use the latest approach in design that was also used for other structures in these Building Codes. This is why TIA 222 G has seismic design provisions and references the 3 second gust wind instead of the fastest mile wind as was the case with TIA 222 F.
    Further, there are US manufacturers who wish to export to the rest of the world. To have their designs accepted, the standard must be seen as being at least as reliable as that of the other competing suppliers, like those from England, Germany, France, Italy, and Denmark, for example.
    While the way the wind is specified and the analysis and design procedures have been changed, they were changed in such a way that in most cases the resulting structure will not be much different than it was under TIA 222 F. For example, the design wind increased by about 20% when we went from the fastest mile to the 3-sec gust, but at the same time the gust factor has decreased by that amount, giving a change in loading of nearly 0%.
    Some design criteria did indeed get more stringent, but that was because certain things had been previously ignored and that had to be corrected.
    Still, as you pointed out, the new standard can yield lighter designs for tall towers. This, when it happens, is because TIA 222 G recognizes that the maximum wind does not cover the entire structure at the same time but acts in bursts called gusts.
    Why, then, do you get heavier concrete designs? From the numbers you quote, I get the impression that the designer forgot to allow for the fact that the results of the analysis in G, in this case the loads onto the foundations, are the effects of factored loads (LRFD) not the foundation loads due to unfactored loads. So, if the designer of the concrete uses the old approach where he factors up the loads applied to the foundation by 60% or 70%, then it can be expected that his foundation will be that much heavier.
    Clause 9.4.1 of TIA 222 G stipulates that the designer should compute the ultimate strength of the foundation and make sure that the foundation design loads calculated using TIA 222 G are less than or equal to the factored value of the ultimate strength calculated, where the factor is anywhere from 0.4 to 0.75, depending on failure conditions outlined in that clause.
    It sounds as if you need to have a second opinion on your foundation designs to make sure they are in keeping with the standard.
  4. Richard Bell

    Richard Bell Friend of the Community

    Mr. Weisman;
    Thank you for your response. I think that your suggestion that I get a second opinion on design is tacitly consistent with the intent of this specification.

    I agree that we charge our customers by the pound for steel and by the yard for concrete, so therefore, it makes me no difference so long as all of the suppliers are using the same design procedures to arrive at their conclusions.

    If all designers are using the same design approach, then the burden is put on the individual designer to develop a more efficient design. A good example of efficient design was the trend to abandon the old 60 degree angle leg microwave towers that were so widely accepted in the 60's and 70's.

    They were replaced by the more efficient solid round, welded section designs that were more aerodynamic than the flats. And the welded section greatly reduced the amount of field construction labor which was very costly. This is a healthy competition that drives commerce and improves quality while lowering prices to the end users.

    The problem that I see with "G" is that it does not provide a minimum criteria for tower design, but rather complicates the design procedure to the point where designers are in disagreement.

    What I see on a regular basis is a gross variation in the designs by the different engineers while supporting the identical loads in the same locations. The only logical reason for this is simply interpretation, or maybe, misinterpretation. And I have yet to see any of these very light "G" designs challenged by opposing engineering firms.

    I don't know of any cases to date where the issues of compliance with the "G" specifications have been litigated. Quite possibly, there may never be such a case because this would be a litigation nightmare.

    On the other hand, I could imagine a stream of expert witnesses giving conflicting testimony before a totally confused jury where justice would never be served.

    I have discussed the "G" Spec. subject with a number of engineers and frankly, none of them are supportive of this specification. But in my layman's opinion, why, when none of the properly designed "C", anchors have ever pulled out, do we need to double the volume of concrete to accomplish the same result?
  5. Simon Weisman

    Simon Weisman Industry Observer

    Mr Bell,

    In your response there is no indication that you have checked my guess that your calculations of foundations, in the case of TIA 222 G calculations, double up on the safety factor.

    This should be the first check.

    For the long term, however, I suggest that instead of not being "supportive" you and these engineers raise some funds, which, if you are correct, will come back to the contributors in future savings, and get some proper research done. This research will then inform the TIA TR14.7 committee when they go to reexamine these provisions.

    There is an ASCE Telecommunications Facilities Committee dedicated to looking into these issues, but they need funding from the industry.

    Everybody is willing to give an opinion, but nobody will support research.

    Your claim that: "The problem that I see with "G" is that it does not provide a minimum criteria for tower design, but rather complicates the design procedure to the point where designers are in disagreement." confuses a couple of points:

    1. Any standard automatically, by its very definition, provides minimum criteria for tower design. EIA 222 A, B, C, D, E, and F, all provided minimum criteria for design of towers. It is just that these criteria have changed from one release to the next.This is reflected in the changes to the standard.

    2. The confusion is not because there is a different G standard for every engineer, but that every engineer could see it differently. This does not mean that they are all correct, or even that any of them are correct. It says more about the engineers than about the standard.

    3. I believe that you are confusing minimum criteria for design of towers with criteria for design of towers of minimum weight or cost. The G standard is equally applicable to efficient structures and inefficient structures. The difference from one tower to the next is in the choices made by the respective designers.

    As to the relative efficiencies of towers, that was easier to asses when they were built for a single broadcast antenna and the wind load on the tower was the most significant load. For the wireless towers and broadcast towers carrying more and more equipment, the loads due to appurtenances are becoming more significant that those due to the tower, and these do not depend on the shape of the tower or the shape of the tower members.

    You may change some of your opinions when the new construction is old enough to start showing signs of fatigue in welded connections of leg splices in particular.

Share This Page