5/24/2018 3 Second Gust and Hourly Mean Wind Speeds GustAndMeanWind

3 second gust and hourly mean wind speeds

The figure on the left is taken from the old code, CP3: Chapter V Part 2 1970 and that on the right

is from the new code, BS 6399-2: 1997. In CP 3 the basic wind speed is defined to be .the 3-

second gust speed, at 10m above ground in an open situation, that is likely to be exceeded on

average only once in 50 years. In BS 6399-2 the basic wind speed is defined to be the hourly

wind speed with and annual risk Qof being exceeded of 0.02, irrespective of wind direction, at aheight of 10m over completely flat terrain at sea level that would occur if the roughness of the

terrain was uniform everywhere (including urban areas, inland lakes and the sea) and equivalent to

typical open country in the United Kingdom.

The two sets of basic wind speeds differ by a factor of about 2. The 3-second gust makes more

sense for building design, but the hourly wind (the hourly mean) is more representative of the wind

in a particular geographical location. Wind pressures are proportional to 12"v

2, but after applying

various factors to the basic wind speeds, the two codes give very similar pressures.

Clearly confusion can arise if the factors meant to be used with the 3-second gust are applied to

the hourly mean or vice versa. The factors involved are roughly 4 (2 squared). As a guide, wind

pressures are usually of the order of 1kN/m2.

Confusion is especially likely when codes from different countries (or national and European

codes) are used together, taking wind speeds from one and factors from the other.

Chris Williams

5/24/2018 3 Second Gust and Hourly Mean Wind Speeds GustAndMeanWind