Wind analysis of building

Wind Load Analysis
Concept of Wind Load Analysis:
Buildings are subject to horizontal loads due to wind pressure acting on the buildings.
Wind load is calculated as per IS 875(Part III)-1987. The horizontal wind pressures act
on vertical external walls and exposed area of the buildings. Some of the pressure
acting on exposed surfaces of structural walls and columns is directly resisted by
bending of these members. The infill walls act as vertical plate supported at top and
bottom by floor beams, thus transferring the loads at slab level. The parapet wall is at
terrace transfers the wind loads to the surface slab by cantilever action. For simplicity,
the wind loads acting on exposed surfaces of a given storey are idealized to be
supported by upper and lower floors.
Wind load analysis as per IS 875 (Part 3)-1987:
Wind forces acting on a given surface is equal to the wind pressures multiplied by the effected
area.
Design wind speed (Vz):
Design wind speed is given by the equation
Vz= Vb K1 K2 K3 where
Vz =Design wind velocity (m/sec)
Vb= Basic wind speed in m/sec (Based on Appendix -A of various cities in IS 875
–Part 3)
Basic wind speed Vb, depends on the location of the building. For this purpose, the
country is divided in to six zones with specified wind speeds ranging from 33m/s to 55
m/s. Basic wind speed is based on gust velocity averaged over a short time interval of
3 seconds at 10m height from mean ground level in an open terrain and for 50 years
return period. Appendix A (Fig.1) of the code specified for some important cities/
towns is given.

Vb has 6 values 33, 39,44,47,50 &55 m/sec.
K1=Risk Coefficient (Table 1 of IS875-part3)

The risk coefficient k1 takes in to account the degree of reliability required and the
expected life of structure.
1.All general buildings (Design life 50 years)
2.Temporary sheds (Design life 5 years)
3.Less important Buildings (Design life 25 years)
4.Important Buildings (Design life 100 years)
K2=Terrain, height and structure size factor (Table 2 of IS875-part3).
It depends of Terrain category and building class/size of structure.
Four terrain categories are specified by the code defending on the availability of obstruction
to the flow of wind.
Category 1: Refers to no obstructions available to the building (e.g) sea coasts and
flat treeless plains where other structures if any have heights less than 1.5m.
Category 2: Refers to open terrain with scattered obstructions of 1.5m to 10m
height. (e.g) industrial area.
Category 3: Refers to areas of closed spaced buildings of height up to 10m (e.g)
buildings at outskirts of city

Category 4: Refers to area with highly closed buildings of large heights (e.g) dense
city area.
K2 factor also depends on the dimensions of the building under considerations. Based
on dimension of building, the structures are classified as Class A, Class B, Class C
Class A: Maximum of l, b, h <20m.
Class B: Maximum of l, b, h  20m to 50m.
Class C : Maximum of l, b, h > 50m.
K3 = Topography factor (Clause 5.3.3.1 of IS875-part3). It depends on the
topography i.e hill region, cliffs and ridges. If the upward ground slope 0 ≤ 3◦ , value
of K3 shall be taken as 1.0.
For 0> 3◦, the value of K3 lies between 1.0 to 1.36.
Design Wind Pressure (pz)
pz= 0.6 Vz
2 where
pz= Design Wind speed in N/m2 at height z
Vz= Design wind speed in m/s.
Wind load on a building is calculated for the “ Building as a whole.”

Force coefficients are given for the building as a whole in the code for clad or unclad
buildings. In this we considered clad (covered with side cladding or walls) buildings
only.
The wind force acting normal to the building surface is given by
F= Cf Ae pd where
Cf= force coefficient (Figure 4A & 4B of IS 875-part 3-1987).
Ae=effective frontal area.
pd =design wind pressure.

WIND LOADS IN STAAD-PRO
Built in program automatically calculates the wind loads on the structure (from the
pressures given at different heights) and distributes the loads at the lateral joints.
Wind loads are converted in to joint loads and exposure factors due to openings are
incorporated.
Exposure factor (c) is the fraction of the influence area associated with the joint(s) on
which the load acts. Exposure factors depends on the openings provided in the walls of
the buildings.
Joint load F= Cf Ae Pd
The above formula can be rewritten as,
F= (Cf Pd) Ae
F= pi Ae where
pi = wind intensity pressure at that heights.
Hence in STAAD Pro, the wind intensities at different heights are given in the input as
follows:
Intensity pi= p1, p2, p3 … Height h1, h2, h3 … where pi= p1, p2, p3 etc.
The program automatically calculates the wind loads on the structure (from the
pressure given at different heights) and distributes the load at the joints.
Two stages in Wind loads command
1. Define the wind load
2. Apply the defined wind load.
Note: The above “Define” command must be given immediately after the support
command before giving any load command.
1. DEFINE COMMAND:
SUPPORTS
……………
DEFINE WIND LOAD
TYPE 1 (wind intensity in X Direction)
Intensity 1.8 2.0 2.1 Height 10. 15.0 20.0

Exposure 1.0 YRANGE 15.0 17.0
TYPE 2 (wind intensity in Z Direction)
Intensity 1.8 2.0 2.1 Height 10. 15.0 20.0
2. APPLYING THE ABOVE DEFINED WIND LOAD:
LOAD 1 (DL)
……….
LOAD 2 (LL)
………
LOAD 3 (WL in X Direction)
WIND LOAD X 1.0 TYPE 1
LOAD 4 (WL in -X Direction)
WIND LOAD X -1.0 TYPE 1
LOAD 5 (WL in Z Direction)
WIND LOAD Z 1.0 TYPE 2
LOAD 4 (WL in -X Direction)
WIND LOAD Z -1.0 TYPE 2
WIND ANALYSIS GRAPHICAL METHOD
1.Defining Wind Load
Click Loads & Definitions Click Definitions 
Wind Definitions Add.
Click Type 1 Comments: Wind 1
Intensity(Kn/m2) Height(m)
15.0 2.0
20.0 2.1  AddClose.
Click Exposures Factor 1.0  Add Close.
Highlight Exposure and select node cursor and entire structure   Assign to
selected nodes Assign Yes Close.
2.Applying the Wind Load:

1.Click Load case detail Add
Number : 3, Loading type: Wind
Title : Wind in X +ve dir Add.
2. Click Load case detail Add.
Number :4, Loading type: Wind
Title : Wind in (-)X dir  Add
Number :5 Loading type: wind
Title : Wind in Z +ve dir Add.
Number :6, Loading type: Wind
Title : Wind in (-)Z dir Add Close.
3. Assigning Wind load:
Click Wind in X +ve  Add.
Wind Load  X Direction Factor 1
Define Y range Mini: Max:  Add.
Click Wind in X -ve  Add.
Wind Load  X Direction Factor -1
Click Wind in Z +ve  Add.
Wind Load  Z Direction Factor 1
Click Wind in Z -ve  Add.
Wind Load  X Direction Factor -1
Load combination to be considered in Wind Load :
Load combination for limit state of collapse as per IS 456-2000.
1. 1.5(D+L) 1.5(D+L)
2. 1.5(D+W) 1.5(D+W in X +ve)
1.5(D+W in Z +ve)
3. 1.5(D-W) 1.5(D+ W in X -ve)
1.5(D+ W in Z –ve)

4. 1.2(D+L+W) 1.2(D+L+W in X +ve)
1.2(D+L+W in Z+ve)
5. 1.2(D+L-W) 1.2(D+L-W in X –ve)
1.2(D+L+W in Z –ve)
6. 0.9D+1.5W 0.9D+1.5W in X +ve
0.9D+1.5W in Z +ve)
7. 0.9D-1.5W 0.9D-1.5W in X –ve
0.9D-1.5W in Z-ve
Total load cases =13.
Procedure to calculate Wind Intensity of Pressure at Particular height to use
in Staad Pro
Data:
Size of building : Length(b)=36.0m Width (a)=29.0m
Height of building (h)=27.0m
Vz= Vb K1 K2 K3
Vb= 50m/sec
K1= 1.08 m/sec
K2= For Terrain category 2 and class B for 10.m height=0.98 ; 15m height=1.02
For 20m height=1.05 and 30m height=1.10
K3 =1.0 for plain category.
Vz= 50 x 1.08 x 0.98 x 1.0=52.9
The coefficient Cf is found from Figure 4A and 4B of IS 875-Part 3
a/b= Width of Building/Length of Building=28/36=0.8
h/a=Height of Building/ Width of building= 27/28=0.96
h/b= Height of Building/length of Building=27/36=0.75
Referring Fig. 4A and 4B of IS 875- part 3
For h/a =0.96 Cf=1.24 in X direction in STAAD
For h/b=0.75 Cf =1.215 in Z direction in STAAD
Wind force in a Building = Cf x Pd x area exposed.

The values are tabulated as follows:
Height in
metres
K2
Vz= Vb K1 K2 K3 Pd=0.6 Vz
2
N/m
Pi = Cf x Pd KN/m2
X
direction
Z direction
10 0.98 52.92 1680.32 2.08 2.04
15 1.02 55.08 1820.28 2.26 2.21
20 1.05 56.70 1928.93 2.39 2.34
30 1.10 59.4 2117.02 2.63 2.57
1.Defining Wind Load
Click Loads & Definitions Click definitions 
Click Type 1 Comments: Wind 1 (W in X dir)
2.04 10
2.21 15
2.34 20
2.57 30  Add.
Click Loads & Definitions Click definitions 
Click Type 2 Comments: Wind 2 (W in Z dir)
2.08 10
2.26 15
2.39 20
2.63 30  Add.
Click Exposures Factor 1.0  Add Close.
Highlight Exposure and select node cursor and entire structure   Assign to
selected nodes Assign Yes Close.

Wind analysis of building

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