GBJ9-87 changed the national roughness grade from TJ9-74 to A, B and C. However, with the vigorous development of China's construction, the height and density of urban housing are increasing day by day, so the roughness of the central area of big cities has also been improved to varying degrees. Considering the standards of most developed countries such as the United States, Britain and Japan, as well as the international standard ISO4354 and the European unified standard IN 65438+,
In order to adapt to the current development form, this revision will also change the three categories into four categories, in which the parameters of Class A and Class B remain unchanged.
Class A refers to offshore sea surface, island, coast, lakeshore and desert, and its roughness index is 0.12;
Class B refers to open fields, villages, jungles, hills, small and medium-sized towns and suburbs of big cities with sparse houses, and its roughness index is 0.16;
Class C refers to the urban area with dense buildings, and its roughness index is changed from 0. 12 to 0.22;
Class D refers to the metropolitan area with dense buildings and a large number of high-rise buildings, and its roughness index is 0.3. Roughness index is mainly based on the wind speed observation data of towers and balloons in recent years through statistical analysis. When calculating the change of wind speed in different places, the wind speed at each height will be calculated according to the local roughness grade.