To ensure the stability of any structure against wind, it is essential to perform a structural analysis by applying the net intensity of the wind speed. For calculating the net intensity of wind, it is important to understand the factors that influence wind load.
In this article, we provide full details of the factors affecting wind speed and their influence on wind load. Read the article till the end, and if you get any valuable information from this, please share it with your friends. Let’s Start.
Table of Contents
What is Wind Load
On earth, wind flows everywhere, but when an obstruction is encountered in the wind’s path, it exerts pressure on the structure. This pressure is referred to as wind load.
Wind Speed Influencing Factors
Wind load is directly depends on the basic wind speed, and basic wind speed is varies with locations. This brings us to the first wind load influencing factor: Location.
1. Location
Basic wind speed varies with location. For example, a structure located near marine areas are subjected to higher wind speeds, whereas other areas experience lower wind speeds compared to marine regions.
As per the basic wind speed map of India in IS 875 Part-3, locations near the sea experience the maximum wind speed value of 55 m/s, whereas locations in the interior regions of India, far from the sea, have the minimum wind speed value of 33 m/s.
However, the wind speed is also affected by the roughness of ground, and here the next influencing factor come in picture: Terrain Category.
2. Terrain Category
Ground roughness is one of the major influencing factors for the design wind speed at a particular location. In flat terrain, the wind speed remains uniform across the area or increases slightly compared to the normal speed. However, in rough terrain, the wind speed decreases due to the ground’s roughness.
Ground Features like buildings, trees, hill, vegetations, etc. contribute to surface roughness. The friction between wind and such features slows the speed.
Indian Standard IS 875 Part -3 : 2015 divide terrain of ground into the four category that are listed below.
Category – 1:
Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the structure is less than 1.5 m. The equivalent aerodynamic roughness height, (z0,1) for this terrain is 0.002m. Typically this category represents open sea-coasts and flat plains without trees. For Examples, Open Ground, Flat Surface of Sea, river, Lake, etc.
Category – 2:
Open terrain with well scattered obstructions having heights generally between 1.5 m and 10 m. The equivalent aerodynamic roughness height, (z0,2) for this terrain is 0.02 m. This is the criterion for measurement of regional basic wind speeds and represents airfields, open park lands and undeveloped sparsely built-up outskirts of towns and suburbs. Open land adjacent to sea coast may also be classified as Category 2 due to roughness of large sea waves at high winds.
Category – 3:
Terrain with numerous closely spaced obstructions having the size of buildings/structures up to 10 m in height with or without a few isolated tall structures. The equivalent aerodynamic roughness height, (z0,3) for this terrain is 0.2 m. This category represents well wooded areas, and shrubs, towns and industrial areas full or partially developed. It is likely that the, next higher category than this will not exist in most design situations and that selection of a more severe category will be deliberate.
Category – 4:
Terrain with numerous large high closely spaced obstructions. The equivalent aerodynamic roughness height, (z0,4) for this terrain is 2.0 m.
3. Topography
However, ground features like hills and cliffs also affect wind speed. You may have observed an increase in wind speed at the top of a hill. Wind speed increases with an increase of ground slope and decreases with a decrease of ground slope. And, here the third wind speed influencing factor come in role that is Topography.
As per IS 875 – Part 3 : 2015, the basic wind speed in hills, valleys, cliffs, escarpments, or ridges is not the same as the values mentioned in the wind speed map of India, as it varies with the local topography.
The effect of topography is to accelerate wind near the summits of hills or crests of cliffs, escarpments or ridges and decelerate the wind in valleys or near the foot of cliffs, steep escarpments, or ridges.
As per IS 875 Part – 3 : 2015, the effect of topography shall be significant at a site when the upwind slope (θ) is more than about 3°, and below that, the value of k3 may be taken to be equal to 1.0.
The value of k3 is confined in the range of 1.0 to 1.36 for slopes more than 3°. A method of evaluating the value of k3 for values more than 1.0 is given in Annex C of IS 875 Part 3. It may be noted that the value of k3 varies with height above ground level, at a maximum near the ground, and reducing to 1.0 at higher levels.
4. Importance Factor
The wind speed during cyclones or storms is significantly higher compared to the basic wind speed considered for a location. In fact, studies show that wind speeds during storms can reach up to 70 m/s, which is much higher than the highest basic wind speed of 55 m/s in India.
In cyclone-affected regions, it is necessary to ensure the stability of critical structures like hospitals, communication towers, bridges, cyclone shelters, schools, etc., against heavy storms by considering an additional factor that is known as the importance factor.
As per IS 875 Part – 3 : 2015, The effect of cyclonic storms is largely felt in a belt of approximately 60 km width at the coast. In order to ensure better safety of structures in this region (60 km wide on the east coast as well as on the Gujarat Coast), the following values of k4 (as recommended in IS 15498) are stipulated as applicable according to the importance of the structure.