Video is great, but fastest mile answer falls short

Discussion in 'Design, Development and Standards Discussions' started by Craig Lekutis, Jan 10, 2008.

  1. Craig Lekutis

    Craig Lekutis Friend of the Community

    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.

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