1. Wind Load Analysis on Buildings and Structures
Course Title-Finite Element Analysis
Course No- CE G619
Instructor In Charge- Dr. PN Rao
Birla Institute of Technology and Science, Pilani
Hyderabad Campus
Civil Engineering Department
Prepared by :
Jhaveri Ronak Kirtikumar-2018H1430036H
2. Wind Load Analysis as per American Code 2002
Upcoming Project Work
References
CONTENTS
Wind Load Analysis as per IS 875:1987 (Part 3)
Comparison of Wind Load Analysis with IS 875:2009
Introduction and Background
3. OBJECTIVES
• To Study the Basics of Wind Loads and it’s Variation with
Height.
• To Gain Design Knowledge on various structural elements like
Beam,Column,Slab,Foundation etc.
• To find the wind pressure by considering individual structural
components and structure as a whole by IS Codes and compare
it with American Codes
• To Analyze and Design various Structures such as Multistorey
Buildings, Bridges, Tunnels, Chimneys etc.
• Analyze and Design Wind Load by ETABS.
4. Variation of Wind Velocity with Height
• Variation of Wind Velocity with
Height Near the earth’s surface
where the motion is Opposed
and the wind speed is 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 is considered
as 300 m for flat ground& 550 m
for very rough terrain
5.
6. Wind Load Analysis of Buildings
and Other Structures as per IS
875:1987 Part 3 and IS 875:2009
7. Obtain Base Wind Speed at Location
Calculation of Design Wind Speed
Calculation of Wind Pressure
Calculation of Design Wind Pressure
Wind Forces on Individual Structures
Calculate Wind Forces on Structures and Plot it.
Steps to Calculate Wind Force Acting on
Structure
8. Calculation of Design Wind Speed
• Design wind speed is Mathematically expressed as
Vz = Vb . K1 . K2 . K3
• Design Wind Speed depends on -
1) Risk Level
2) Terrain Roughness, Height and Size of Structure
3) Local Topography
Here,
Vz = Design Wind Speed at any Height in m/s
Vb = Basic wind speed (Appendix A-Clause5.2)
k1 = Probability Factor (Risk Coefficient) (clause 5.3.1)
k2 = Terrain Height & Structure Size Factor (clause 5.3.2)
k3 = Topography factor (clause 5.3.3)
9. Basic Wind Speed (Vb)
• Basic wind speed Vb depends
on the location of the building.
• For this purpose, The country
is divided into various 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.
10. Risk Coefficient (K1)
• The Risk Coefficient K1 takes into
account the degree of reliability
required and the expected life of
structure. The Design Life for
various Buildings are as follows:
1. All general buildings (50 years)
2. Temporary sheds (5 years)
3. Less important Buildings ( 25 yrs)
4. Important Buildings ( 100 years)
11. Terrain Height & Structure Size Factor (K2)
• It depends upon Terrain Category and Building Class/Size
of Structure. Four Terrain Categories are specified by the
code depending 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 are having Height 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 Closely 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.
12. Terrain Height & Structure Size Factor (K2)
•K2 factor also depends on the
Dimensions of the Building
under considerations.
•Based on Dimension of
Building, the Structures are
classified as:
1) Class A: Maximum of l, b,
h<20m.
2) Class B: Maximum of l, b, h in
between 20m to 50m.
3) Class C : Maximum of l, b, h >
50m.
13. Topography Factor (K3)
• It Depends on the Topography i.e Hill Region, Cliffs and
Ridges.
• If the Upward Ground Slope θ≤3◦, Value of K3 shall be taken
as 1.0
• For θ>3◦ , The Value of K3 lies between 1.0 to 1.36. It can be
determined by-
K3 =1 + Cs
• Shape factor varies considerably with proportion of structure
& horizontal angle of incidence of the wind and it can be
determined by using Appendix C as given in IS 875:1987 –
part3
14. Calculation of Design Wind Pressure
• Wind Pressure due to Design Velocity can be determined by the following
formula as given in IS 875:1987 part3
Pz=0.6 (𝑉z)2
Here,
𝑃z = Wind Pressure in N/m2 at Height z.
𝑉z = Design Wind Velocity in m/s at Height z.
• As Per IS 875:2009 - Part 3 ,Few New Factors like Wind Directionality Factor, Area
Averaging Factor, Combination Factor are introduced to make Design Economical,
Hence Design Pressure is different from wind pressure which is given by following
formula :-
Pd = Kd . Ka . Kc . Pz
Here ,
Pd = Design Wind Pressure in N/m2 at Height z.
Pz = Wind Pressure in N/m2 at Height z.
Kd = Wind Directionality Factor.
Ka = Area Averaging Factor.
Kc = Combination Factor.
15. Wind Force on Individual Member
• The Wind Load on individual structural elements such as
roofs and walls, individual cladding units and their fittings,
it is essential to take account of the pressure difference
between opposite faces of such elements or units.
• For calculation of wind force on individual cladding units
following formula can be used:-
F=(Cpe−Cpi) A Pd
Here,
Pd = design wind pressure in N/m2 at height z.
Cpe = external pressure coefficient.
Cpi = internal pressure coefficient.
A = Surface area of structural element
• Internal and External pressure coefficients for different
units having different shapes, structures and slopes are
described in IS 875 with figures.
16. Wind Forces on Structures
• The value of Force Coefficients
apply to a building or structure as a
whole, and when multiplied by the
effective frontal area A of the
building or structure and by design
wind pressure, Pd gives the Total
Wind Load on that particular
building or structure.
• It’s expression is given by
F=(Cf ) A Pd
Here,
Pd =Design wind pressure at
height z.
Cf = Force coefficient of building.
A = Surface area of structural
element.
18. Wind Load Analysis of Buildings
and Other Structures as per ASCE-7
American Code (2002)
19. Wind Loads According to ASCE 7
Wind Loads According to ASCE 7
• Wind loads are randomly applied
Dynamic loads.
• It depends on the wind speed, shape,
height and topographic location of the
structure.
• The more the air is streamed, the less
the reaction force exerted by the
structure.
• Wind force highly depends on the shape
of the structure.
20. Wind Loads According to ASCE 7
Wind Speed Curve in Different Region
Sea Side
Terrain Exposure
Open Area Built Up Area Big City
21. Wind Loads According to ASCE 7
Surface Roughness and Exposure
Surface
Roughness and
Exposure
Definitions Examples
B Urban and suburban areas,
wooded areas or other terrain
with numerous closely spaced
obstructions having the size of
single-family dwellings or larger.
C Open terrain with scattered
obstructions having heights generally
less than 30 ft (9.1 m). This category
includes flat open country, grasslands,
and all water surfaces in hurricane
prone regions.
D Flat, unobstructed areas and water
surfaces outside hurricane prone
regions. This category includes
smooth mud flats, salt flats, and
unbroken ice.
22. Wind Loads According to ASCE 7
Basic Wind Speed (V)
• Basic wind speed (V) based
on 3-second gusts, 33 ft (10
m) above ground in a Ground
Roughness Exposure C
(defined in m/s).
• Some regions such as:
Taiwan, coastal China,
coastal USA and Japan have
very high wind speed and
others such as: Indonesia,
India and inland USA have
lower wind speed
23. Wind Loads According to ASCE 7
Wind Directionality Factor (Kd)
• Wind Directionality Factor,
Kd shall be determined from
Table 6-4 of ASCE.
• This factor means to
accommodate the
cross-sectional shape of the
structure
24. Wind Loads According to ASCE 7
Importance Factor, I
• An importance factor (I) for the
building or other structure shall
be determined from Table 6-1
based on building and structure
categories listed in Table 1-1.
• This factor to accommodate the
importance of the structure.
25. Wind Loads According to ASCE 7
Velocity Pressure Coefficient (Kz)
• Velocity Pressure Coefficient (Kz)
depends on the site relative height
to the ground z.
•This means for roof top structure,
z would be the total height of the
component and the building its
installed.
•This factor is to accommodate the
absolute height of the structure
from ground level.
26. Topographic Factor (Kzt), Force
Coefficient (Cf )
• Local abrupt topography affects wind near the ground.
• Wind speed depends on shape of hill, location of building and height above ground.
• The value of Kzt was taken as 1 with assumption of flat region environment.
• This factor is to accommodate the topographic area of the structure location
• Force coefficient, Cf determined based on the shape of the structure is to
accommodate the wind-facing area of the structure.
27. Velocity Pressure (qz)
• From Bernoulli’s equation of flow, the wind pressure can be
calculated as:
(q in psf, V in mph)
• The velocity pressure qz evaluated at height z shall be
calculated by:
28. Wind Loads According to ASCE 7
• Design wind force for each component shall be determined by:
• Gust effect factor, G, could be calculated by
29. Comparison of wind Load Analysis of
Buildings and Other structures by Indian
Standard Codes and American Standard
Codes
30. Conclusion by Comparing with IS and ASCE
Codes for Wind Load Analysis
• ASCE:07 allows to use basic wind data by studying and
analysing wind data which means more accurate local
conditions can be considered and hence structure may be
more economical as well as more strong. Local weather data
study and its analysis should be allowed in Indian Codes too.
• Wind Load for structure is calculated for 2 time taking only 1
case using IS 875: part3 while it is calculated for 12 times
considering 4 different cases in case of ASCE-07:2002.
• Base reactions as well as Shear forces in Y directions are zero
as per IS 875 but it have positive value in case of ASCE-07.
• Moments for both codes are almost equal.
• Joint displacements are more in case of IS codes compared to
that displacements calculated as per ASCE: 07
31. Conclusion by Comparing with IS and ASCE
Codes for Wind Load Analysis
• Gust Factor method in IS 875 which is somewhat similar to
analysis method in ASCE-07:2002 is yet to be developed fully
as compared to ASCE code.
• ASCE-07 consider 4 cases while IS875 consider only two cases
to determine design wind pressure. Indian code should
improve for determining wind load taking more cases, as
more cases mean more precision and hence less risk.
• American code is more effective for designing for wind loads
as it gives less deformation as compared to Indian code. Less
deformation means less chance for failure.
32. REFERENCES
1) Council of Tall Buildings and Habitat Website (CTBUH).
2) Indian Standard Code- Code of Practice for all Design
Loads (Other than Earthquake resisting system) for
Buildings and Structures- IS:875: 1987: Part 3 Wind
Loads.
3) Indian Standard Code Draft Version- IS 875:2009-Part 3 :-
Draft version with commentary.
4) American Code: Minimum Design Loads for Buildings and
Other Structures (ASCE-7:2002 Version).
5) National Programme Technology Enhanced Learning-
(NPTEL) Design of Reinforced Structures Part 1 and Part
2.
