Wind load

Introduction and Calculation of Wind Load

1. WIND LOAD SANDHYA – 1AN15AT025 SPURTHI – 1AN15AT029

2. WIND Wind is a moving mass of air . Buildings and other structures represent obstacles that deflect or impede the wind , covering the kinetic energy of the moving air mass into the potential energy of pressure. Wind loads result from the forces exerted by the kinetic energy of the moving mass of air , which can produce a combination of direct pressure , negative pressure or suction , and drag forces on buildings and other obstactles in its path. Wind forces are typically assumed to be applied normal , or perpendicular , to the affected surfaces of the building. Wind pressure increases as a function of wind velocity. The average mean wind velocity for any particular area , measured over a long period of time , generally increases with height . The rate of increase of the mean velocity is also a function of the ground roughness and the interference offered by surrounding objects that include other buildings , vegetation , and land forms.

3. Load acting on high rise buildings Vertical loads:  DEAD LOADS  LIVE LOADS Lateral loads:  SEISMIC LOAD (EARTHQUAKE LOAD)  WIND LOAD

4. Variation of Wind Velocity with Height Variation of Wind Velocity with Height-Near the earth’s surface, the motion is opposed, and the wind speed reduced , by the surface friction. At the surface , the wind speed reduces to zero and then begins to increase with height , and at some height, known as the gradient height, the motion may be considered to be free of the earth’s frictional influence and will attain its ‘gradient velocity’. Gradient Height 300 m for flat ground& 550 m for very rough terrain

5. Wind Effects on Structures Wind effects on structures can be classified as ‘Static’ and ‘Dynamic’. Static-Static wind effect primarily causes elastic bending and twisting of structure. Dynamic for tall, long span and slender structures a ‘dynamic analysis’ of the structure is essential . Wind gusts cause fluctuating forces on the structure which induce large dynamic motions , including oscillations.

6. Design wind speed – Vz =Vb.K1.K2.K3 Vz =Design wind speed at any height z inm /s; K1= Probability Factor (K1=1.06 For Important Buildings of Life 100 years) K2= Terrain, Height and Structure sizefactor (Varies for Height) K3= Topography Factor)   Design Wind Pressure Pz Pz = 0.6 ×Vz x Vz N/m2 Design wind speed Value of K2 From Table 2 Of IS 875- Part3

7. Wind Load Calculation- F= Ae pd Cf a/b = 13/17=.76 h/b = 20.5/17 = 1.21 Cf = 1.3 using graph F=wind load; Cf =force coefficient; pd =design wind pressure; A= effective frontal area obstructing wind,which is identified for each structure

8. GIVEN DATA No. of stories=10 Height of each storey =3m Height of the structure =3x10=30m Location of the building= Vadodara Design life of the structure =50 Category 3 Topography=plane with upwind slope less than 3 deg Calculate WIND LOAD at different heights (10m,15m, 20m, 30m ) and floor levels using static method with help of the given data.

9. PLAN ELEVATION 15M 30M

10. STEP 1 CALCULATING DESIGN WIND SPEED

11. Design wind speed Design wind speed – Vz = Vb.K1.K2.K3 Vz =Design wind speed at any height z in m/s; Vb = Basic wind speed at any height z in m/s; K1= Probability Factor K2= Terrain, Height and Structure sizefactor (Varies for Height) K3= Topography Factor

12. BASIC WIND SPEED ( Vb ) As per IS 875 part 3 Clause 5.2 Vb VALUE = 44m/s , ACCORDING TO GIVEN DATA

13. K1 ( PROBABILITY FACTOR ) As per IS 875 part 3 Table 1 Clause 5.3.1 K1 VALUE = 1, ACCORDING TO GIVEN DATA

14. DIMENSIONS OF THE BUILDING L=24M B=15M MAX DIMENSION H=30M H=30M K2 VALUE , ACCORDING TO GIVEN DATA HEIGHT K2 VALUE FOR TERRAIN CATEGORY 3 10 0.88 15 0.94 20 0.98 30 1.03 K2 ( TERRAIN,HEIGHT AND STRUCTURE SIZE FACTOR) As per IS 875 part 3 Table 2 Clause 5.3.2.2

15. IF DEG IS LESS THAN 3 THEN K3=1 IF DEG IS GREATER THAN 3 THEN K3= 1 TO 1.36 K3=1, ACCORDING TO GIVEN DATA K3 ( TOPOGRAPHY FACTOR ) UPWIND SLOPE DEGREE As per IS 875 part 3 Clause 5.3.3.1

16. Design wind speed – Vz = Vb.K1.K2.K3 Vb VALUE = 44m/s , ACCORDING TO GIVEN DATA K1 VALUE = 1, ACCORDING TO GIVEN DATA K2 VALUE , ACCORDING TO GIVEN DATA K3=1, ACCORDING TO GIVEN DATA HEIGHT K2 VALUE FOR TERRAIN CATEGORY 3 10 0.88 15 0.94 20 0.98 30 1.03 HEIGHT VZ VALUE ( mls ) 10 38.72 15 41.36 20 43.12 30 45.32 Vz = 44 X 1 X K2 X 1

17. STEP 2 CALCULATING DESIGN WIND PRESSURE

18. Pz = 0.6 ×Vz x Vz N/m2 CALCULATING DESIGN WIND PRESSURE HEIGHT VZ VALUE (m/s) 10 38.72 15 41.36 20 43.12 30 45.32 HEIGHT Vz VALUE (m/s) Pz VALUE (N/m2) 10 38.72 899.54 15 41.36 1026.38 20 43.12 1115.60 30 45.32 1232.34

19. STEP 3 CALCULATING DESIGN WIND LOAD

20. 1.CALCULATING DESIGN WIND LOAD Wind Load Calculation- F= Ae x Pd x Cf F=wind load Ae= effective frontal area obstructing wind, which is identified for each structure Cf =force coefficient pd =design wind pressure

21. FIND H/B RATIO AND CHECK IF IT IS GREATER OR LESSER THEN 1 ( HELPS TO DECIDE WHICH GRAPH TO FIND Cf) HEIGHT (H)= 30 LENGTH (B) = 24 BREADTH (A) = 15 A / B = 0.62 H/B = 30 / 24 = 1.25 1.25 > 1 USE GRAPH 6A FINDING Cf VALUE USING GRAPH As per IS 875 part 3 Clause 5.2 IF H/b VALUE IS GREATER THAN OR EQUAL TO 1 THEN USE GRAPH 6A IF H/b VALUE IS LESSER THAN 1 THEN USE GRAPH 6B

22. A / B = 0.62 H/B = 1.25 Cf=1.2

23. 2.CALCULATING VALUE OF FRONTAL AREA (Ae) Ae= effective frontal area obstructing wind, which is identified for each structure Here, we are calculating load per unit area Therefore, frontal area= 1 x 6 (shortest span between columns ) Ae = 1 x 6 =6 m2

24. PLAN ELEVATION 15M 30M

25. 2.CALCULATING VALUE OF DESIGN LOAD AT DIFFERENT HEIGHT HEIGHT Pz DESIGN PRESSURE F = Ae X Cf X Pz ( in N/m) F = WIND LOAD ( in KN/m ) 0-10 899.54 6584.6 6.58 10-15 1026.38 7513.10 7.513 15-20 1115.60 8166.19 8.166 20-30 1232.34 9020.72 9.020

26. 30m 20m 10m 0m 15m 6.58 Km/m 7.513 Km/m 8.166 Km/m 9.02 Km/m

27. 10 9 8 7 6 5 4 3 2 1 19.74 19.74 19.74 19.74 21.39 22.53 23.7 24.75 25.5 26.28 27.06 STOREY NUMBER LOADING LEVEL HEIGHT OF EACH FLOOR DESIGN FORCE KN/m FORCE AT EACH STOREY (KN) 10 27-30 3 9.02 27.06 9 24-27 3 8.76 26.28 8 21-24 3 8.50 25.5 7 18-21 3 8.25 24.75 6 15-18 3 7.90 23.7 5 12-15 3 7.51 22.53 4 9-12 3 7.13 21.39 3 6-9 3 6.58 19.74 2 3-6 3 6.58 19.74 1 0-3 3 6.58 19.74

28. 10 9 8 7 6 5 4 3 2 1 19.74 19.74 19.74 19.74 21.39 22.53 23.7 24.75 25.5 26.28 27.06 30m 20m 10m 0m 15m 6.58 Km/m 7.513 Km/m 8.166 Km/m 9.02 Km/m

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