Selection Of The Terrain Category And Its Impact On The Structure

In the determination of loads, the correct determination of the terrain category is an important factor for calculating the wind load affected by the structures.Terrain category refers to the effect of the wind path (open areas, forests, cities, etc.) on the wind speed.Different types of terrain can exhibit different resistances and this affects the wind speed and therefore the load.

Eurocode, terrain categories are classified as follows:

Terrain Categories (EN 1991-1-4 Eurocode 1):

Category 0: The lakeside or coastal area is the region where there are no obstacles. It is the category with the highest wind speed.

Category I: Open areas, areas close to the coast or areas where there are small obstacles in front of the wind. Wind speeds are also quite high in these areas.

Category II: Rural areas, open land with trees or few buildings. Wind speeds are moderate.

Category III: Industrial areas or land with many closely spaced obstacles, such as suburban residential areas.

Category IV: Urban centres or densely built-up areas (10 m to 30 m high) and terrain with closely spaced obstacles.

Wind loads are calculated taking into account the wind speed, which varies with height. Different coefficients are used for different terrain categories to calculate this speed.

Variation with height: The average wind speed vm(z) at a height of z metres above ground level in a terrain depends on the terrain's roughness, orographic features and the base wind speed etc. and should be calculated using the equation.

Terrain roughness: The roughness coefficient cr(z) takes into account the variability of the average wind speed depending on the conditions specified at the location of the structure.

Terrain orography: Where orography (hills, slopes, etc.) increases the wind speed by more than 5 %, their effects must be taken into account using the orography coefficient cO.

Neighbouring structures and obstructions of large and considerable height: If a building is located near a structure that is at least twice as high as the average height of the neighbouring structures, wind speeds acting on the building from some directions may increase, depending on the characteristics of the structure. Such situations should be taken into consideration.

For example, when a structure with a speed of 25 m/s and a ground height of 3 m is selected as land category 1, qp(ze) = 0.816 kN/m2

In the same conditions, when terrain category 4 is selected, qp(ze) = 0.459 kN/m2 is calculated.

In cases where the land category is not selected correctly, the wind load that your system will be exposed to may change and pose a risk to the structure.