influence of wind loads on tall buildings

1

Influence of Wind Loads on Tall

Buildings

Study Project Report

By

Mukul Yambal (2013A2PS581P)

Study Oriented Project

Under Dr. G. Muthukumar

BITS Pilani

May 2016

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Contents

ACKNOWLEDGEMENT ................................................................................................................................................. 3

INTRODUCTION .......................................................................................................................................................... 4

INDIAN STANDARD CODE -IS 875: PART3 ................................................................................................................... 9

Basic Wind Speed: ............................................................................................................................................. 10

Design Wind Speed: .......................................................................................................................................... 10

Probability factor (risk coefficient) (k1) ....................................................................................................... 11

Terrain, Height and Structure Size factor (K2) ............................................................................................. 11

Topography factor (K3) ................................................................................................................................ 12

Importance Factor for Cyclonic Region (k4) ................................................................................................. 13

Wind Pressure ................................................................................................................................................... 13

Design Wind Pressure ....................................................................................................................................... 14

Wind Forces on individual members ................................................................................................................ 14

Wind Forces on structure.................................................................................................................................. 15

Dynamic Effects ................................................................................................................................................. 15

Gust Factor ........................................................................................................................................................ 17

Cross wind Response ........................................................................................................................................ 19

Difference between IS 875:1987 and 2009 (draft) .............................................................................................. 20

ASCE-7: Minimum Design Loads for Buildings and Other Structures (American code) (2002) ......................... 21

Some Basic Concepts ........................................................................................................................................ 21

1. Basic Wind Speed (V) ........................................................................................................................... 21

2. Exposure Category ................................................................................................................................ 22

3. Topographic Factor (KZI) ....................................................................................................................... 23

4. Gust Factor (G or Gf) ............................................................................................................................. 24

Method 1 - Simplified Procedure ...................................................................................................................... 25

Method 2 - Analytical Procedure ...................................................................................................................... 27

Method 3 - Wind Tunnel Procedure ................................................................................................................. 28

Example: ............................................................................................................................................................ 29

1. Structural Data ..................................................................................................................................... 30

2. Properties ............................................................................................................................................. 31

3. Load Combination factors .................................................................................................................... 32

4. Auto Load Calculations Using ETABS 2015 .......................................................................................... 32

5. Analysis ................................................................................................................................................. 41

6. Displacements due to Wind Loads....................................................................................................... 43

Summary ........................................................................................................................................................... 45

Conclusion ......................................................................................................................................................... 47

References ........................................................................................................................................................ 48

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ACKNOWLEDGEMENT

I am indebted to my Instructor Dr. G. Muthukumar, Lecturer Civil Engineering

Group, BITS Pilani who introduced me to this work and provided me an opportunity to work

under his supervision. His constantly supported and encouraged me throughout this work. The

discussions with him were so healthy and fruitful that gave me a chance to increase my domain

of knowledge.

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INTRODUCTION

This Study Project covers some essential study regarding wind loads and related

different codal provisions related to wind and load considered in designing of buildings due to

wind. Study of Indian Standard code (IS 875:1987:- Part 3) as well as (IS 875:2009 part 3-draft)

is explained in this report. American Code for wind loads (ASCE-7:2002 version) and its

comparison with Indian codes is also included in this report.

Some properties and facts related to tall buildings, wind and variation of wind with

respect to its height are introduced below which will give necessity of wind loads on tall

buildings.

TALL BUILDINGS

There is no absolute definition of what constitutes a “tall building” according to

Council of Tall Buildings and Urban Habitat (CTBUH).Height is relative concept, i.e. a building

which should be considered as tall building in country like India may not considered as tall in

case of tall buildings countries like US(Chicago). Measurement of height of Tall Building can

be considered by three methods according to CTBUH. :-

1. Height to Architectural Top

Height is measured from the level of the lowest, significant, open-

air, pedestrian entrance to the architectural top of the building, including spires,

but not including antennae, signage, flagpoles or other functional-technical

equipment.

This measurement is the most widely utilized and is employed to define the Council

on Tall Buildings and Urban Habitat (CTBUH) rankings of the “World’s Tallest

Buildings.”

Some of rankings of world’s tallest buildings are listed below:-

i) Burj Khalifa Dubai (2010) 828m /2,717ft

ii) Shanghai Tower, Shanghai (2015) 632m 2,073ft

iii) Makkah Royal Clock Tower Hotel, Mecca (2012) 601m 1,972ft

v) One World Trade Center, New York City (2014) 541m 1,776ft

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2. Height to Tip


air, pedestrian entrance to the highest point of the building, irrespective of material

or function of the highest element (i.e., including antennae, flagpoles, signage, and

other functional technical equipment).

3. Highest Occupied Floor


air, pedestrian entrance to the finished floor level of the highest occupied floor

within the building.

Note: - A building having height more than 15 m is considered as tall building according to

National Building Code 2005 of India is Called Tall building. (High Rise Building).

Building having height more than 18 m (60ft.) is considered as tall building according to

American code ASCE-7: 2002 version. Building having mean roof height more than least

horizontal dimension is also considered as tall building according to this American code.

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WIND

Wind is air in motion relative to the surface of the earth. Primary

reasons of wind is rotation of earth as well as due to difference in terrestrial radiation.

Over 50 years study of wind and its speed is carried out in different places to find mean

wind speed in different places so that it can be used in further calculations of wind loads

which is listed in IS 875:Part3 -1997 as well as in 2009 draft version. This study is carried

out using anemometers or anemographs which are installed in different meteorological

stations at height of 10 m- 30 m. According to this study India is divided into six different

categories depending upon basic wind speeds in each zone. Average basic speeds as per

IS875 is given in table 1.

Some parts of Jammu and Kashmir as well as Mizoram and Tripura. 55 m/s

Whole eastern Coastal area, Costal part in Gujarat as well as Assam. 50 m/s

Some part of Tamil Nadu and almost Half North India which includes part of Gujarat ,whole Rajasthan ,Punjab ,part of Madhya Pradesh, Utter Pradesh ,Bihar ,Part of Jharkhand, West Bengal and Sikkim.

47 m/s

Western coastal area in Maharashtra as well as some inside land near eastern costal area which is part of Andhra Pradesh ,Maharashtra and Orissa and some eastern part of India , that is , Nagaland , Manipur and Mizoram.

44 m/s

Most of the Middle and South India. 39 m/s

Some part of Karnataka, Kerala, Andhra-Pradesh, Telangana and Tamil Nadu which is mostly land between eastern and western costal area.

33 m/s

Table1-According to IS875:1997 - Part3

In American code, the basic wind speed, V, used in the

determination of design wind loads on buildings and other structures is considered from

a counter curve given in ASCE-7: 2002 version except Special wind regions Estimation of

wind speed from regional data. Data for Special Wind Region is listed below in Table2. Location V (mph) (m/s)

Hawaii 105 47

Puerto Rico 145 65

Guam 170 76

Virgin Islands 145 65

American Samoa 125 56

Table2-According to ASCE-7:2002: Special Wind Regions

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WIND AND ITS VARIATION WITH HEIGHT

Wind speeds are generally very low near earth surface due to earth friction.

It increases with height from zero to some value and then it remains almost constant. At

some height wind can be considered as free from

earth’s atmosphere which leads to almost constant

velocity after certain height. This height is known as

Fetch Height or Gradient Height.

Wind is mostly in horizontal direction in

high speeds as compared to its vertical component.

Vertical component of Wind may be neglected in

mostly design cases for wind loads as it is very small,

hence generally horizontal winds are considered.

Gradient Height

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As per CTBUH, Tall buildings ae also classifies as supertall and mega tall

buildings. Supertall building is a building having height over 300 m (984 feet) while Mega

building is one which has height over 600 m (1968 feet).There were total 91 supertall

buildings and 2 Mega buildings in world till June 2015.Now mostly high populated cities

are coming up with many tall structures. For seismic loads, earthquake loads are usually

known to people but wind loads also plays important role near coastal areas as discussed

above. There are many areas damaged due to cyclones. Wind speed increases with

height from ground surface and hence its impact which eventually leads to Wind as

major factor in consideration for case of tall buildings. Increasing number of tall

structures and unawareness of people regarding wind loads makes this study very

important.

This report will describe about IS 875:1987 –Part 3 which is mainly code used

for designing wind loads in India. Calculation of design wind speeds, design wind

pressure followed by design wind pressure , some Oscillation related phenomenon like

Galloping , flutter ,Ovaling ,Vortex Shedding , etc. according to IS 875:1987 – part 3 as

well as few changes made in IS 875:2009 –Part-3 (draft form) which includes

Interference effect, Dynamic Response Factor ,Cross wind response ,etc.

This report will also describe about American code for Wind Loads, ASCE-

7:2002 version, Wind loading part. Three different methods, i.e. Simplified Procedure,

Analysis Procedure and Wind Tunnel Testing will be described in the report along with

its limitations.

This report will be completed by comparing deformation due to wind loads

which will be calculated by software E-Tabs-2014. A Concrete frame and Steel frame

will be modelled and deformation due to wind will be calculated using IS 875:1987 as

well as ASCE -7:2002 version.

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INDIAN STANDARD CODE -IS 875: PART3

IS 875 was first published in 1957 for the guidance of civil engineers,

designers and architects associated with the planning and design of buildings which was

included the provisions for the basic design loads (dead loads, live loads, wind loads and seismic

loads) to be assumed in the design of the buildings. In its first revision in 1964, the wind pressure

provisions were modified on the basis of studies of wind phenomenon and its effect on

structures, undertaken by the special committee in consultation with the Indian Meteorological

Department. Similarly in 1987 code many changes carried out which leads to better design of

building for wind loads and continuous studied related wind made another version of this code

in 2009 which is still in its draft form.

Design aspect related to wind loads are mentioned in this Indian

Standard Codes. This code is designed by examining few international codes listed below:-

BSCP 3:1973 Code of basic data for design of buildings: Chapter 5 –Loading: Part 2: Wind

loads

AS 117, Part 2 -1983 SAA Loading code Part2 –Wind forces

NZS 4203-1976 Code of Practice for general structural design loading for buildings

ANSI A58.1-1972 American Standard Building code requirements for minimum design

loads in buildings and other structures

Wind resistant design regulations, a world list. Association for Science Documents

Information, Tokyo.

Some points in IS 875 part 3 are listed below:-

i. Basic Wind speed

ii. Design Wind speed

iii. Wind Pressure

iv. Design Wind Pressure

v. Wind Forces on individual members

vi. Wind Forces on structure

vii. Dynamic Effects

viii. Gust Factor

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Basic Wind Speed: India is divided in 6 zones according to Basic wind velocity which is

applicable for height about 10 m from mean ground level which is explained in

introduction of wind.

In Appendix A, IS 875: Part3 -1987 (as well as 2009), Basic wind speed

(Vb) for some important cities is listed by studying wind speeds over 50 years.

Cyclonic storms generally extend up to 60 km inland after striking sea

coast. The influence of wind speed off the coast up to the distance of about 200km

may be taken as 1.15 times the value of nearest coast in case of absence of definite

data. This is also known as off shore velocity.

Design Wind Speed: Design Wind Speed can be determined by estimating Basic Wind

Speed as well as some other factors such as topographical factor, risk factor, terrain

and height factor and Importance Factor for Cyclonic Region. Relation between all

this factors is given by following formulae: -

Vz=K1K2K3Vb - (IS 875:1987 – Part3)

Vz=K1K2K3K4Vb - (IS 875:2009 – Part3)

Where

Vz = design wind speed at any height z in m/s

Vb = basic wind speed in m/s

K1 = Probability factor (risk coefficient)

K2 = terrain, height and structure size factor

K3 = topography factor

K4 = Importance Factor for Cyclonic Region

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Probability factor (risk coefficient) (k1)

This factor depends on estimated life structure required for a

building. Few of K1 Values are given in IS 875 part -3 and it can be generalized

using following formula:-

K1=XN,PN

X50,063=A−B[ln{−

1

Nln(1−PN)}]

A+4B

Where,

N = mean probable design life of the structure in years

PN = risk level in N consecutive years (probability that the design wind

speed is exceeded at least once in N successive years), nominal value = 0.63

XN,P = extreme wind speed for given value of N and PN

X50,0.63 = extreme wind speed for N = 50 years and PN

= 0.63

A and B are coefficients depends on different basic wind speed zones.

Terrain, Height and Structure Size factor (K2)

As per its name K2 factor depends on Terrain, Height of

different structures, Size factor that is Class of Buildings. Values of K2 is gives in IS

875 according to above factors. As per IS 875, Terrain is divided into four different

categories and buildings are divided into three types as per its size which is

described below:-

a. Types of Terrain:

Classified in Four different terrains as follows:-

a) CATEGORY 1:- Exposed Open terrain with few or no obstructions.

b) CATEGORY 2:- Open terrain with well scattered obstructions.

c) CATEGORY 3:- Terrain with numerous closely spaced obstructions.

d)CATEGORY 4:- Terrain with numerous large high closely spaced

obstructions

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b. Classes of Buildings:

Buildings can be classified in three different classes depending upon size

factor:-

a) Class A:-Structure and/or its component having its maximum dimension

less than 20 m.

b) Class B: - Structure and/or its component having its maximum dimension

between 20 m and 50 m.

c) Class C: - Structure and/or its component having its maximum dimension

greater than 50 m

The velocity profile for a given terrain category does not develop to

full height immediately with the commencement of that terrain category but

develop gradually to height ( h, ) which increases with the fetch or upwind distance

(x). The relation between the developed height (h,) and the fetch (x) for wind-flow

over each of the four terrain categories is given in IS 875. For structures of heights

greater than the developed height (h,) the velocity profile may be determined in

accordance with the less or least rough terrain, or another procedure is given in

Appendix B of IS 875:1987-part3

Topography factor (K3)

This is factor for taking local topographic feature like cliffs, hills

valleys, etc. into consideration while calculating design wind speed. This factor is

effective when upwind slop (θ) is greater than 3o. Value of K3 is i) 1 when θ< 3o ii)

1.36 when θ> 17o. And for 3 o < θ < 17 o It can be given by:-

K3 =1 + Cs

Where C and s can be determined by using

Appendix C given in IS 875:1987 –part3

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Importance Factor for Cyclonic Region (k4)

Cyclonic wind speeds may be higher than basic wind speed. To

consider increase in this velocity during cyclones K4 factor is introduced in IS

875:2009 Part 3. This factor will be mostly useful in cyclonic regions like eastern

coastal areas of India.

The following values of k4 are stipulated, as applicable

according to the importance of the structure:

Structures of post–cyclone importance 1.30

Industrial structures 1.15

All other structures 1.00

This factor is useful for 60 km inland from eastern sea coast and Gujarat sea coast.

Wind Pressure

Wind Pressure due to design velocity can be determined by following

formula given in IS 875:1987 –part3 Pz= 0.6 (𝑉z)2

Where

𝑃z = wind pressure in N/m2 at height z.

𝑉z = design wind velocity in m/s at height z.

The coefficient 0’6 (in SI units) in the above formula depends on a number

of factors and mainly on the atmospheric pressure and air temperature. The value

chosen corresponds to the average appropriate Indian atmospheric conditions.

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Design Wind Pressure Design Wind pressure according to IS875:1987 –part3 is same as

wind pressure explained above. In IS 875:2009 –part3 few new factors like wind

directionality factor, area averaging factor, Combination factor are introduced to make

design economical, hence design pressure is different than wind pressure which is given

by following formula:-

Pd =Kd.Ka.Kc .Pz

Where

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.

Wind Forces on individual members The wind load on individual structural elements such as roofs and

walls, and 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

Where

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.

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Wind Forces on structure 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.

F = (Cf) A pd

Where

Pd = design wind pressure in N/m2 at height z. Cf = force coefficient of building. A = Surface area of structural element.

Force Coefficient:

Force coefficient can be classified into 3 types as follows:-

i) Frictional force drag coefficient:-In certain buildings of special shape, a force due to frictional drag shall be taken into account in addition to those loads specified.

ii) Force Coefficients for Clad Buildings:-values of Cf for different shaped clad buildings are mentioned in IS 875

iii) Force Coefficients for unclad Buildings:- values of Cf for different shaped unclad buildings are mentioned in IS 875.

Dynamic Effects For tall, long span and slender structures, wind Fluctuations

causes fluctuating forces on structure which induces large dynamic motions

including oscillations. Wind loads can induce both along wind Oscillations as well

As crosswind Oscillations (vortex shedding). Along wind Oscillations are

calculated more readily. Estimation of Vortex shedding has been introduced in IS

875: 1987 but is still not developed enough.

Flexible slender structures and structural elements have to

investigate for oscillating effects cause due to wind loadings. Criteria for

examining structure or building need not necessary if buildings and closed

structures having ratio of height to minimum lateral dimension, more than 5

OR natural frequency of buildings and closed structures in first mode is less

than 1Hz.

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i) Fundamental time period (T):

Fundamental time period (T) can be determined by experimental

methods of similar structures. In absence of such data T can be calculated

as follows:-

For moment resisting frames without bracings:-

T=0.1n

n=number of storeys including basement storeys

For all others

T=.09𝐻

𝑑

o H=Height of building in metres.

o D=maximum base dimension of building in metres in direction

parallel to applied load.

ii) Wind induced Motions (T):

a) Galloping: - Galloping is the self-induced cross-wind oscillations of

flexible structures due to aerodynamic forces that are in-phase with

the motion of the structure. It is characterised by the progressively

increasing amplitude of transverse vibrations with increased wind

speed. Galloping is generally not an issue for buildings.

b) Flutter: - Flutter is unstable oscillatory motion of a structure due to

coupling between aerodynamic force and elastic deformation of the

structure. Flutter can set in at wind speeds much less than those

required for exciting the individual modes of motion. Long span

suspension bridge decks or any member of a structure with large values

of d/t ( where d is the depth of a structure or structural member parallel

to wind stream and t is the least lateral dimension of a member ) are

prone to low speed flutter. Wind tunnel testing is required to.

Determine critical flutter speeds and the likely structural response.

Other types of flutter are single degree of freedom stall flutter, torsional

flutter, etc.

Example of flutter: - The collapse of the original Tacoma Narrows Bridge,

a suspension bridge in the U.S. state of Washington.

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c) Ovalling: - This walled structures with open

ends at one or both ends such as oil storage

tanks, and natural draught cooling towers in

which the ratio of the diameter of minimum

lateral dimension to the wall thickness is of

the order of 100 or more, are prone to

ovalling oscillations. These oscillations are characterized by periodic

radial deformation of the hollow structure.

iii) Motion due to Vortex Shedding:

For slender structures, ŋ –frequency of Vortex Shedding can be given by

ŋ=SVd

b

S=Strouhal number.

Vd=Design Wind velocity.

b=breadth of structure or structural member in horizontal plane normal to wind direction.

Circular sections

S=0.2 for Vz ≤ 7

S=0.25 for Vz > 7

Rectangular Structures

S=0.15

Gust Factor IS 875:1987 allows Gust method for calculation of wind loads some cases .This

method needs Hourly mean wind speed which is equal to basic wind speed. Wind load

can be calculated from Gust factor method by using following formula:-

𝐹 = (𝐶𝑓) 𝐴𝑒 𝑝𝑑𝐺

Fz = along wind load on the structure at any height z corresponding to strip area Ae

Cf = force coefficient of building

Ae = effective frontal area considered for structure at height z

Pz = design pressure at height z due to hourly mean wind

Ovalling

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G=Gust factor =𝑝𝑒𝑎𝑘 𝑙𝑜𝑎𝑑

𝑚𝑒𝑎𝑛 𝑙𝑜𝑎𝑑

G=1+gtr√𝐵(1 + ∅)2 +𝑆𝐸

𝛽

Where

gf = peak factor defined as the ratio of the expected peak value to the

root mean value of a fluctuating load

Y = roughness factor which is dependent on the size of the structure in

relation to the ground roughness.

B = background factor indicating a measure of slowly varying component

of fluctuating wind load

SE

β=measure of the resonant component of the fluctuating wind load

S = size reduction factor

E = measure of available energy in the wind stream at the natural

frequency of the structure

β = damping coefficient (as a fraction of critical damping) of the structure

ɸ =𝑔𝑡 × 𝑟√𝛽

4

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Cross wind Response

This factor is defined in IS 875: 2009 draft version .The equivalent

cross–wind static force per unit height (We) as a function of z in Newton per

meter height, shall be as follows:

We (z) = 0.6 [Vh] 2 d Cdyn

Where,

d = Lateral dimension of the structure parallel to the wind stream, and

Cdyn = 1.5gR b

𝑑

𝐾𝑚

1+𝑔𝑣𝑙ℎ (𝑧

ℎ)k √(

𝜋𝐶𝑓𝑠

𝛽)

Where,

Km = mode shape correction factor = 0.76 + 0.24 k

Where

k = mode shape power exponent for the fundamental mode of

vibration

= 1.5 for a uniform cantilever

= 0.5 for a slender framed structure (moment resistant)

= 1.0 for building with central core and moment resisting façade

= 2.3 for a tower decreasing in stiffness with height, or with a large

mass at top

Cfs = cross–wind force spectrum coefficient generalized for a linear

mode shape.

GR = peak factor for resonant response (1 hour period) given by

GR = [2 log (3600f0)] (½)

β = ratio of structural damping to critical damping of Structure

Ih = turbulence intensity, obtained by setting z equal to h

gv = peak factor for the upwind velocity fluctuations, which shall be

taken as 3.5

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Difference between IS 875:1987 and 2009

(draft) For calculation of wind speed K4 factor for cyclone is introduced in 2009 version.

Design Wind Pressure is same as wind pressure in case of 1987 code but for 2009 version

three new factors are introduced. Design pressure can be obtained by multiplying this

factors by wind pressure. This three factors are

a)Kd=wind directional factor

a)Ka=Area averaging factor

a)Kc=combination factor

Interference effect is introduced in 2009 code while it was not in 1987. When wind comes

from striking some buildings and original wing may have some phase difference which will

leads to interference effect.

Dynamic Response Factor will tell you wind pressure at certain height of building. In the

1987 version, the dynamic response factor, Cdyn, was applied to the wind loading due to

hourly mean wind speed, as opposed to the 3second gust speed used in the 2009 version.

CROSS-WIND RESPONSE is introduced in 2009 draft for determining equivalent static

wind force and base overturning moment in the cross-wind direction for tall enclosed

buildings and towers of rectangular cross-section

Some parts of the code have been revised from the 1987 version to take into account the

wind incidence angle.

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ASCE-7: Minimum Design Loads for Buildings

and Other Structures (American code) (2002)

This Code is published by American Society for Civil Engineers. As per name

of code this is made for design of all kinds of loads like dead, live, soil, flood, wind, snow,

rain, ice and earthquake loads. In this report mainly we will focus on wind load designing.

The design wind loads for buildings and other structures, including the main

wind force-resisting system and component and cladding elements thereof, shall be

determined using one of the following procedures: 1) Method 1 - Simplified Procedure

2) Method 2 – Analytical Procedure

3) Method 3 – Wind Tunnel Procedure

Some Basic Concepts Before going into further details of procedures, some basic concepts should be

introduced as per ASCE-7:2002 Version.

1. Basic Wind Speed (V)

Basic Wind Velocity can be determined by contour map given in code ASCE-

7:2002 version except for special wind regions and it can also be determined using

Regional Climatic Data.

Special Wind Regions: The Basic Wind Speed shall be increased in case the

wind speeds are higher than given in code by record or experience.

Regional Climatic Data: Reginal Climatic Data should be preferred instead

of code data when approved extreme-value statistical analytical procedure have

been employed in reducing the data and all related factors have been taken into

account.

Hurricane-Prone Regions: This code allows basic wind speed from

simulation techniques instead of given in code data when approved simulation or

extreme value statistics analysis procedures are used. (Not allowed along Gulf and

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Atlantic coasts, the Caribbean or Hawaii). The resulting design speed by this

methods shall not be less than resulting 500 year return period wind speed divided

by √1.5 .

2. Exposure Category

The characteristics of ground roughness and surface irregularities are the

important factors to be considered while designing wind loads. This factors are

included in exposure category which will also take ground variation due to natural

topography and vegetation.

Surface Roughness Category: There are three major categories of surface

roughness as follows

Surface Roughness B: Urban and suburban area, wooded areas or other

terrain with numerous closely spaced obstructions having size of single family

dwelling or larger.

Surface Roughness C: Open terrain with scattered obstructions having

heights generally less than 30ft. (9.1 m). This category also includes flat open

country, grasslands, and all water surfaces in hurricane-prone regions.

Surface Roughness D: Flat, unobstructed and water surfaces outside

hurricane-prone regions. This category also includes smooth mud flats, salt flats,

and unbroken ice.

Exposure Category: There are three different exposure categories defined in

this code along with transition zone explained as follows:

Exposure B: It is mainly for surface category B. Ground surface roughness

conditions prevails in the upwind direction for a distance of at least 2630 ft. (800

m.) or 10 times the height of the building whichever is greater. Upwind distance

can be reduced to 1500 ft. (457 m.), in case of buildings having mean roof height

less than or equal to 30ft. (9.1 m.).

Exposure C: This is for all cases other than where exposure B n exposure

D do not apply.

Exposure D: It is mainly for surface category D. Ground surface roughness

conditions prevails in the upwind direction for a distance of at least 5000 ft. (1524

m.) or 10 times the height of the building whichever is greater. This can extend

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inland fro the shoreline for a distance of 600 ft. (200 m.) or 10 times the height of

the building whichever is greater.

Transition Zone: For a site located in between any two or more exposure

categories, the category resulting largest wind force should be considered.

Intermediate exposure is permitted in case of rational analysis.

3. Topographic Factor (KZI)

Wind Speed-Up effects over Hills, Ridges, and Escarpments

constituting abrupt changes in the general topography, located in any exposure

category, and should be considered while designing wind loads. Conditions to be

considered while designing are as follows:-

1. The hill, ridge, or escarpment is isolated and unobstructed upwind by other

similar topographic features of comparable height for 100 times the height of the

topographic feature (100 H) or 2 miles (3.22 km), whichever is less. This distance

shall be measured horizontally from the point at which the height H of the hill,

ridge, or escarpment is determined.

2. The hill, ridge, or escarpment protrudes above the height of upwind terrain

features within a 2-mile (3.22-km) radius in any quadrant by a factor of two or

more.

3. The structure is located in the upper half of a hill or ridge or near the crest of an

escarpment.

4. H/LH ≤ 0.2.

5. H is greater than or equal to 15 ft. (4.5 m) for Exposures C and D and 60 ft. (18

m) for Exposure B.

Topographic Factor. The wind speed-up effect shall be included in the calculation

of design wind loads by using the factor Kzt which can be calculated as follows:

Kzt= (1+K1K2K3)2

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4. Gust Factor (G or Gf)

In this code Gust Factors are given for rigid structures as wells as for

flexible structures as given below:-

For Rigid Structures:-

G = 0.925 ((l + 1.7gQIQ)/ (1 + 1.7gv lz)

lz = c(33/z)1/6

Where,

Iz = Intensity of turbulence at height z.

Z=equivalent height of structure (0.6h) > zmin .

For Flexible Structures:-

Gf = 0.925 ((l + 1.7Iz (gQ2Q2+gR

2R2)0.5)/ (1 + 1.7gv lz)

25

Method 1 - Simplified Procedure To design any building or structure by this method, it should satisfy few

conditions mentioned as follows:-

i. For the design of main wind force-resisting systems:-

The building is a simple diaphragm building, i.e. an enclosed or partially

enclosed building in which wind loads are transmitted through floor and roof

diaphragms to the vertical main wind force-resisting system.

The building is a low-rise building i.e. enclosed or partially enclosed Buildings

of which mean roof height h jess than or equal to 60 ft. (18 m) or mean roof

height h does not exceed least horizontal dimension.

The building is enclosed and conforms to the wind borne debris provisions.

The building is a regular shaped building or structure, i.e. building with no

geometric irregularity in spatial form.

The building is not classified as a flexible, i.e. Buildings having no natural

frequency less than 1 Hz.

the building does not have response characteristics making it subject to

across-wind loading, vortex shedding, instability due to galloping or flutter;

and does not have a site location for which channelling effects or buffeting in

the wake of upwind obstructions warrant special consideration.

The building structure has no expansion joints or separations.

The building is not subject to the topographic effects i.e., K zt = 1.0.

The building has an approximately symmetrical cross section in each direction with

either a fiat roof, or a gable or hip roof with θ ≤ 45 degrees.

ii. For the design of components and cladding:-

The mean roof height h ≤ 60 ft.

The building is enclosed and conforms to the wind-borne debris provisions

The building is a regular shaped building.

The building does not have response characteristics making it subject to across-wind loading, vortex shedding, instability due to galloping or flutter; and does not have a site location for which channelling effects or buffeting in the wake of upwind obstructions warrant special consideration.

The building is not subject to the topographic effects, i.e., Kzt = 1.0.

The building has either a flat roof, or a gable roof with θ ≤ 45 degrees, or a hip roof with θ ≤ 27 degrees.

26

Design Procedure:-

1. Determine Basic Wind Speed, Importance factor I, exposure category, and

exposure adjustment coefficient λ from code.

2. Main Wind Force-Resisting System:-Simplified design wind pressure (ps) can be

given by following formula:

ps = λ Ips30

Where,

λ = adjustment factor for building height and Exposure

I = importance factor

pS30 = simplified design wind pressure for exposure B, at h = 30

ft, and for I = 1.0.

Note:-ps should not be less than 10lb/ft2 (0.48 kN/m2). This force is applied on

horizontal and vertical projections of building surfaces.

3. Component and Cladding:-Net design wind pressure (pnet) can be given by

following formula:

Pnet = λ Ipnet30

Where,

λ = adjustment factor for building height and Exposure

I = importance factor

pnet30 = simplified design wind pressure for exposure B, at h =

30 ft, and for I = 1.0.

Note:-Positive pnet should not be greater than 10lb/ft2 (0.48 kN/m2) and negative

pnet should not be less than -10lb/ft2 (-0.48 kN/m2). This force is applied normal

to each building surfaces.

27

Method 2 - Analytical Procedure

To design any building or structure by this method, it should satisfy few

conditions mentioned as follows:-

The building or other structure is a regular shaped building or structure.

The building or other structure does not have response characteristics making

it subject to across-wind loading, vortex shedding, instability due to galloping

or flutter; or does not have a site location for which channelling effects or

buffeting in the wake of upwind obstructions warrant special consideration.

Design Procedure:-

1. The basic wind speed V and wind directionality factor K d, importance

factor I, an exposure category or exposure categories and velocity

pressure exposure coefficient KZ or KH, shall be determined for each

wind direction.

2. A topographic factor KZt, a gust effect factor G or Gf, an enclosure

classification shall be determined.

3. Internal pressure coefficient GCpi as well as external pressure

coefficients Cp or GCpf, or GCf shall be determined.

4. Velocity pressure at height z (qz or qh) can be determined using

following formula:-

qz = 0.00256 Kz KZt Kd V2 I (lb/ft2)

[In SI: qz = 0.613 KzKztKd V2 I (N/m2); V in m/s]

Where,

Kd =Wind directionality factor.

Kz =Velocity pressure exposure coefficient.

KZt = Topographical factor.

5. Design wind load p or F shall be determined by considering formulae

given in code as per required case of building.

28

Method 3 - Wind Tunnel Procedure Wind Tunnel Testing is used to determine

wind loads on any buildings and structures. This method is

useful in prediction of wind loads and responses of structures,

structural components, and cladding to verify the wind

conditions and hence to design any building or structure.

Problem with this method is its cost, i.e. it is very expensive.

Example of Structure for which wind

tunnel testing is used is World Trade Centre. In this method

experiments are conducted on small model applying same

Surrounding conditions.

Tests for determining fluctuating mean wind pressure or force by Wind Tunnel Testing

should meet following conditions:-

1. The natural atmospheric boundary layer has been modelled to account for the variation

of wind speed with height.

2. The relevant macro length and micro length scales of the longitudinal component of

atmospheric

Turbulence are modelled to approximately the same scale as that used to model the

building or structure

3. The modelled building or other structure and surrounding structures and topography

are geometrically

Similar to their full-scale counterparts, except that, for low--rise buildings.

4. The projected area of the modelled building or other structure and surroundings is less

than 8% of the test section cross-sectional area unless correction is made for blockage.

5. The longitudinal pressure gradient in the wind-tunnel test section is accounted for

6. Reynolds number effects on pressures and forces are minimized

7. Response characteristics of the wind-tunnel instrumentation are consistent with the

required measurements.

World Trade Centre

29

Example: Reinforced Concrete (RCC) frame with flat roof is designed using ETABS 2015

software having following dimensions:-

Component Length (in m.) Width (in mm.) Depth (in mm.)

Column (Except first floor) 3.2004 600 6000

Column (For first floor) 3.6576 600 600

Beam 6.096 600 6000

Number of Stories =10

Total Length = 24.396 m. (4 Beams in X direction) (Total 5 Gridlines)

Total Breadth = 12.198 m. (2 Beams in Y direction) (Total 3 Gridlines)

Slab Thickness =200 mm.

As shown in figure below:-

Note: - No additional Dead loads and Live Loads are considered as their value is 0 kN/m2.

3D View

30

1. Structural Data

Data related to structure is provided in Table given below:-

a. Story Data:

Elevation and Height of each Floor is given in Table below:-

Table: Elevation and Height

Name Height (mm) Elevation (mm)

Story10 3200.4 32461.2

Story9 3200.4 29260.8

Story8 3200.4 26060.4

Story7 3200.4 22860

Story6 3200.4 19659.6

Story5 3200.4 16459.2

Story4 3200.4 13258.8

Story3 3200.4 10058.4

Story2 3200.4 6858

Story1 3657.6 3657.6

Base 0 0

b. Mass:

Table: Centre of Mass Calculations


Story Diaphragm Mass X

kg Mass Y

kg Mass Moment of Inertia

ton-m² X Mass Center

m Y Mass Center

m

Story10 D1 273531.07 273531.07 20590.7181 12.192 6.096

Story9 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story8 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story7 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story6 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story5 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story4 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story3 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story2 D1 294293.59 294293.59 22648.2132 12.192 6.096

Story1 D1 297259.66 297259.66 22942.141 12.192 6.096

Story Diaphragm Mass X

kg Mass Y

kg XCM

m YCM

m Cumulative X

kg Cumulative Y

Kg XCCM

m YCCM

m

Story10 D1 273531.07 273531.07 12.192 6.096 273531.07 273531.07 12.192 6.096

Story9 D1 294293.59 294293.59 12.192 6.096 567824.65 567824.65 12.192 6.096

Story8 D1 294293.59 294293.59 12.192 6.096 862118.24 862118.24 12.192 6.096

Story7 D1 294293.59 294293.59 12.192 6.096 1156411.83 1156411.83 12.192 6.096

Story6 D1 294293.59 294293.59 12.192 6.096 1450705.42 1450705.42 12.192 6.096

Story5 D1 294293.59 294293.59 12.192 6.096 1744999.01 1744999.01 12.192 6.096

Story4 D1 294293.59 294293.59 12.192 6.096 2039292.6 2039292.6 12.192 6.096

Story3 D1 294293.59 294293.59 12.192 6.096 2333586.19 2333586.19 12.192 6.096

Story2 D1 294293.59 294293.59 12.192 6.096 2627879.78 2627879.78 12.192 6.096

Story1 D1 297259.66 297259.66 12.192 6.096 2925139.45 2925139.45 12.192 6.096

31


Story UX kg

UY kg

UZ kg

Story10 273531.07 273531.07 0

Story9 294293.59 294293.59 0

Story8 294293.59 294293.59 0

Story7 294293.59 294293.59 0

Story6 294293.59 294293.59 0

Story5 294293.59 294293.59 0

Story4 294293.59 294293.59 0

Story3 294293.59 294293.59 0

Story2 294293.59 294293.59 0

Story1 297259.66 297259.66 0

Base 23728.6 23728.6 0

2. Properties

a. Materials:

Table: Material Properties

Name Type E

MPa ν

Unit Weight kN/m³

Design Strengths

4000Psi Concrete 24855.58 0.2 23.5631 Fc=27.58 MPa

A615Gr60 Rebar 199947.98 0.3 76.9729 Fy=413.69 MPa, Fu=620.53

MPa

b. Frame Sections:

Table: Frame Sections

Name Material Shape

Concrete Beam 4000Psi Concrete Rectangular

Concrete Column 4000Psi Concrete Rectangular

c. Shell Sections:

Table: Shell Sections

Name Design Type Element Type Material Total Thickness

mm

Slab1 Slab Shell-Thin 4000Psi 200

d. Reinforcement Sizes:

Table: Reinforcing Bar Sizes

Name Diameter

mm Area mm²

10 10 79

20 20 314

32

3. Load Combination factors

Load combination factors used while analysing are listed below. This factors are taken

for DL+LL+WL combination from IS 456 for Indian case and from ASCE-07:2002 for

American case.

Table: Load Combination Factors from IS456

Name Load Case/Combo Scale Factor Type Auto

DD+LL+WL (Indian) Dead 1.2 Linear Add No

DD+LL+WL (Indian) Live 1.2 No

DD+LL+WL (Indian) Wind Indian 1.2 No

Table: Load Combination Factors from ASCE-07:2002

Name Load Case/Combo Scale Factor Type Auto

DD+LL+WL (American) Dead 1.2 Linear Add No

DD+LL+WL (American) Live 1 No

DD+LL+WL (American) Wind American 1.6 No

4. Auto Load Calculations Using ETABS 2015

a. Indian IS875:1987 Auto Wind Load Calculation:

Assuming frame is located near Orissa, where basic wind speed Vb is 50 ms-1 as per

given in IS875:1987. Other assumed parameters are as follows:-

Table: Factors related to Wind Loads

Structure Class Class B

Terrain Category Category 2

Wind Direction 0;90 degrees

Basic Wind Speed, Vb 50 ms-1

Windward Coefficient, Cp,wind 0.8

Leeward Coefficient, Cp,lee 0.5

Risk Coefficient, k1 1

Topography Factor, k3 1

Table: Calculated Factors

Design Wind Speed, Vz Vz=k1k2k3Vb Vz=50

Design Wind Pressure, pz Pz=0.6Vz2 Pz=1500

33

Story Elevation(m) X-Dir(kN) Y-Dir(kN)

Story10 32.4612 47.4621 0

Story9 29.2608 93.5974 0

Story8 26.0604 90.9865 0

Story7 22.86 88.311 0

Story6 19.6596 85.5586 0

Story5 16.4592 82.4853 0

Story4 13.2588 78.9015 0

Story3 10.0584 75.5657 0

Story2 6.858 74.8726 0

Story1 3.6576 80.2207 0

Base 0 0 0


Story10 32.4612 0 93.7845

Story9 29.2608 0 184.947

Story8 26.0604 0 179.788

Story7 22.86 0 174.5012

Story6 19.6596 0 169.0624

Story5 16.4592 0 162.9897

Story4 13.2588 0 155.9081

Story3 10.0584 0 149.3167

Story2 6.858 0 147.9471

Story1 3.6576 0 158.5148

Base 0 0 0

Case-1 Case-2

34

b. ASCE 7-02 Auto Wind Load Calculations:

Assuming frame is located is in America having same Basic Wind Speed as in Orissa i.e.,

Vb is 50 ms-1 (111.85mph). Other assumed parameters are as follows:-

Table: Factors related to Wind Loads

Structure Class Class B

Terrain Category Category 2

Wind Direction 0 degrees

Basic Wind Speed, Vb 111.85 mph

Windward Coefficient, Cp,wind 0.8

Leeward Coefficient, Cp,lee 0.5

Gradient Height, zg 900

Empirical Exponent, α 9.5

Topographical Factor, Kzt 1

Directionality Factor, Kd 085

Importance Factor, I I = 1

Gust Effect Factor, G G = 0.85

Table: Calculation formulas

Design Wind Pressure, p [ASCE 6.5.12.2.1 Eq. 6-17] 𝐩 = 𝐪𝐆𝐂𝐩,𝐰𝐢𝐧𝐝 + 𝐪𝐡(𝐆𝐂𝐩,𝐥𝐞𝐞)

Velocity Pressure Exposure Coefficient, Kz; For 15 ft.≤Z≤ Zg For Z < 15 ft.

Kz = 2.01 (z/zg)2/ɑ Kz = 2.0 I (l5/zg)2/ ɑ

Velocity Pressure, qz 𝐪𝐳 = 𝟎. 𝟎𝟎𝟐𝟓𝟔𝐊𝐳𝐊𝐳𝐭𝐊𝐝𝐕𝟐𝐈

35


Story10 32.4612 24.447 0

Story9 29.2608 48.1865 0

Story8 26.0604 47.0583 0

Story7 22.86 45.8258 0

Story6 19.6596 44.4622 0

Story5 16.4592 42.928 0

Story4 13.2588 41.1603 0

Story3 10.0584 39.0482 0

Story2 6.858 36.3775 0

Story1 3.6576 36.6011 0

Base 0 0 0


Story10 32.4612 0 56.9014

Story9 29.2608 0 112.4047

Story8 26.0604 0 110.1753

Story7 22.86 0 107.7399

Story6 19.6596 0 105.0456

Story5 16.4592 0 102.0141

Story4 13.2588 0 98.521

Story3 10.0584 0 94.3476

Story2 6.858 0 89.0703

Story1 3.6576 0 90.7399

Base 0 0 0

Case-1

36

Story Elevation X-Dir Y-Dir

m kN kN

Story10 32.4612 18.3353 0

Story9 29.2608 36.1399 0

Story8 26.0604 35.2937 0

Story7 22.86 34.3693 0

Story6 19.6596 33.3467 0

Story5 16.4592 32.196 0

Story4 13.2588 30.8702 0

Story3 10.0584 29.2862 0

Story2 6.858 27.2831 0

Story1 3.6576 27.4508 0

Base 0 0 0


m kN kN

Story10 32.4612 18.3353 0

Story9 29.2608 36.1399 0

Story8 26.0604 35.2937 0

Story7 22.86 34.3693 0

Story6 19.6596 33.3467 0

Story5 16.4592 32.196 0

Story4 13.2588 30.8702 0

Story3 10.0584 29.2862 0

Story2 6.858 27.2831 0

Story1 3.6576 27.4508 0

Base 0 0 0

Case-1

37


m kN kN

Story10 32.4612 0 42.676

Story9 29.2608 0 84.3035

Story8 26.0604 0 82.6315

Story7 22.86 0 80.8049

Story6 19.6596 0 78.7842

Story5 16.4592 0 76.5106

Story4 13.2588 0 73.8908

Story3 10.0584 0 70.7607

Story2 6.858 0 66.8027

Story1 3.6576 0 68.0549

Base 0 0 0


m kN kN

Story10 32.4612 0 42.676

Story9 29.2608 0 84.3035

Story8 26.0604 0 82.6315

Story7 22.86 0 80.8049

Story6 19.6596 0 78.7842

Story5 16.4592 0 76.5106

Story4 13.2588 0 73.8908

Story3 10.0584 0 70.7607

Story2 6.858 0 66.8027

Story1 3.6576 0 68.0549

Base 0 0 0

Case-2

38


m kN kN

Story10 32.4612 18.3353 -36.2302

Story9 29.2608 36.1399 -71.4118

Story8 26.0604 35.2937 -69.7398

Story7 22.86 34.3693 -67.9132

Story6 19.6596 33.3467 -65.8925

Story5 16.4592 32.196 -63.6189

Story4 13.2588 30.8702 -60.9991

Story3 10.0584 29.2862 -57.869

Story2 6.858 27.2831 -53.911

Story1 3.6576 27.4508 -54.2424

Base 0 0 0


m kN kN

Story10 32.4612 21.5974 42.676

Story9 29.2608 42.6641 84.3035

Story8 26.0604 41.8179 82.6315

Story7 22.86 40.8935 80.8049

Story6 19.6596 39.8709 78.7842

Story5 16.4592 38.7202 76.5106

Story4 13.2588 37.3944 73.8908

Story3 10.0584 35.8103 70.7607

Story2 6.858 33.8073 66.8027

Story1 3.6576 34.441 68.0549

Base 0 0 0

Case-2

39


m kN kN

Story10 32.4612 13.7637 -27.1968

Story9 29.2608 27.129 -53.6065

Story8 26.0604 26.4938 -52.3514

Story7 22.86 25.7999 -50.9802

Story6 19.6596 25.0322 -49.4633

Story5 16.4592 24.1685 -47.7566

Story4 13.2588 23.1732 -45.79

Story3 10.0584 21.9841 -43.4403

Story2 6.858 20.4805 -40.4692

Story1 3.6576 20.6064 -40.7179

Base 0 0 0


m kN kN

Story10 32.4612 13.7637 -27.1968

Story9 29.2608 27.129 -53.6065

Story8 26.0604 26.4938 -52.3514

Story7 22.86 25.7999 -50.9802

Story6 19.6596 25.0322 -49.4633

Story5 16.4592 24.1685 -47.7566

Story4 13.2588 23.1732 -45.79

Story3 10.0584 21.9841 -43.4403

Story2 6.858 20.4805 -40.4692

Story1 3.6576 20.6064 -40.7179

Base 0 0 0

Case-3

40


m kN kN

Story10 32.4612 16.2124 32.0355

Story9 29.2608 32.0265 63.2838

Story8 26.0604 31.3913 62.0287

Story7 22.86 30.6974 60.6576

Story6 19.6596 29.9297 59.1407

Story5 16.4592 29.066 57.4339

Story4 13.2588 28.0707 55.4673

Story3 10.0584 26.8816 53.1177

Story2 6.858 25.378 50.1466

Story1 3.6576 25.8537 51.0865

Base 0 0 0


m kN kN

Story10 32.4612 16.2124 32.0355

Story9 29.2608 32.0265 63.2838

Story8 26.0604 31.3913 62.0287

Story7 22.86 30.6974 60.6576

Story6 19.6596 29.9297 59.1407

Story5 16.4592 29.066 57.4339

Story4 13.2588 28.0707 55.4673

Story3 10.0584 26.8816 53.1177

Story2 6.858 25.378 50.1466

Story1 3.6576 25.8537 51.0865

Base 0 0 0

Case-4

41

5. Analysis

a. Base Reaction Calculations:

Table: Base reactions

Load Case/Combo FX(kN) FY(kN) FZ(kN) MX(kN-m) MY(kN-m) MZ(kN-m) X(m) Y(m) Z(m)

Dead 0 0 28918.5181 176287.2864 -352575 0 0 0 0

DD+LL+WL (Indian) Max 0 0 34702.2217 245505.1329 -423089 5837.2475 0 0 0

DD+LL+WL (Indian) Min -957.5537 -1892.1115 34702.2217 211544.7437 -440276 -23068.6228 0 0 0

DD+LL+WL (American) Max 0 962.9247 34702.2217 239404.7455 -423089 14710.6433 0 0 0

DD+LL+WL (American) Min -649.7518 -1547.1356 34702.2217 194045.4059 -434897 -18862.6773 0 0 0

b. Story Forces:

i. Shear Diagrams

.

Shear Forces increases from top to bottom for frame.Folowing table shows shear

forces for different cases which are calculated for story 1 :-

Load Case/Combo Location P VX VY T

Dead Top 28453.12 0 0 0

Dead Bottom 28918.52 0 0 0

DD+LL+WL (Indian) Max Top 34143.75 0 0 5837.248

DD+LL+WL (Indian) Max Bottom 34702.22 0 0 5837.248

DD+LL+WL (Indian) Min Top 34143.75 -957.554 -1892.11 -23068.6

DD+LL+WL (Indian) Min Bottom 34702.22 -957.554 -1892.11 -23068.6

DD+LL+WL (American) Max Top 34143.75 0 962.9247 14710.64

DD+LL+WL (American) Max Bottom 34702.22 0 962.9247 14710.64

DD+LL+WL (American) Min Top 34143.75 -649.752 -1547.14 -18862.7

DD+LL+WL (American) Min Bottom 34702.22 -649.752 -1547.14 -18862.7

Dead Load Case DD+LL+WL Indian case DD+LL+WL American case

42

ii. Moment Diagrams

Moments increases from top to bottom for frame.Folowing table shows Moments for different

cases which are calculated for story 1 :-

Story Load Case/Combo Location MX MY

Story1 Dead Top 173450.2 -346900

Story1 Dead Bottom 176287.3 -352575

Story1 DD+LL+WL (Indian) Max Top 235180.1 -416281

Story1 DD+LL+WL (Indian) Max Bottom 245505.1 -423089

Story1 DD+LL+WL (Indian) Min Top 208140.3 -429965

Story1 DD+LL+WL (Indian) Min Bottom 211544.7 -440276

Story1 DD+LL+WL (American) Max Top 230341.5 -416281

Story1 DD+LL+WL (American) Max Bottom 239404.7 -423089

Story1 DD+LL+WL (American) Min Top 194162.9 -425712

Story1 DD+LL+WL (American) Min Bottom 194045.4 -434897


43

6. Displacements due to Wind Loads

Maximum joint displacements for different cases are shown in following table :-

Load Case/Combo Story no.

UXmax(mm) UYmax(mm) UZmax(mm

) RXmax(rad) RYmax(rad) RZmax(rad)

Dead 10 -2.23E-08 -2.59E-08 -5.43E+00 3.08E-04 2.92E-04 0

DD+LL+WL (Indian) Max 10 1.68E+01 3.79E+01 -6.52E+00 3.70E-04 -2.91E-04 0

DD+LL+WL (Indian) Min 10 -2.77E-08 -3.21E-08 -6.52E+00 -4.87E-03 3.51E-04 0

DD+LL+WL (American) Max 10 3.51E-04 3.32E+01 -6.52E+00 2.90E-03 1.69E-03 8.03E-04

DD+LL+WL (American) Min 10 -5.02E+00 -2.24E+01 -6.52E+00 -4.30E-03 -6.91E-04 -8.03E-04


Deformation Diagrams

-5

0

5

10

15

20

1

15

29

43

57

71

85

99

11

3

12

7

14

1

15

5

16

9

18

3

19

7

21

1

22

5

23

9

25

3

26

7

28

1

29

5

30

9

32

3

33

7

35

1

Dis

pla

cem

ent

in X

dir

ecti

on

(in

mm

)

Joint numbers (from story10 to story1)

Displacement of joints in X Direction

Dead DD+LL+WL (Indian) DD+LL+WL (American)

44

As Shown in above Graphs, Displacement in all direction is maximum for Indian DD+LL+WL

combination, while it is least is case of X & Y direction for dead loads. For American DD+LL+WL

combination, Displacement is least in Z direction as compared to other cases.

For Z direction displacements, Negative displacement means displacement in downward

direction. Some joints are assumed to be fixed and hence their displacement in z direction is zero

In all three cases, Maximum displacement is found to be for top story, i.e. for story 10. It

decreases with height.

-5

0

5

10

15

20

25

30

35

40

1

15

29

43

57

71

85

99

11

3

12

7

14

1

15

5

16

9

18

3

19

7

21

1

22

5

23

9

25

3

26

7

28

1

29

5

30

9

32

3

33

7

35

1

Dis

pla

cem

ent

in Y

dir

ecti

on

(in

mm

)


Displacement of joints in Y Direction


-7

-6

-5

-4

-3

-2

-1

0

1

15

29

43

57

71

85

99

11

3

12

7

14

1

15

5

16

9

18

3

19

7

21

1

22

5

23

9

25

3

26

7

28

1

29

5

30

9

32

3

33

7

35

1

Dis

pla

cem

ent

in Y

dir

ecti

on

(in

mm

)


Displacement of joints in Z Direction


45

Summary Wind Load increases with increase in height and its direction is horizontal hence it is

important to analyse Tall buildings for wind loads.

IS 875:1987 Part 3 is Indian Standard Code for design of wind loads. IS 875:2009 part 3 is

in draft version, having few changes in it as compared to IS875:1987 part3.

ASCE -07 Chapter 6 is meant for design of wind loads. 2002 version of this code is used

in this report while 2005, 2010 versions of this codes are also available.

For IS 875:1987 and 2009 (part 3) versions, some major points for designing wind loads

are as follows:-

o Determining basic wind speed and hence design wind speed depending upon

height of building, terrain, topography, etc. (in case of 2009, cyclone zone also

considered).

o Determining Wind pressure and Design wind pressure for calculated design

velocity.

o Determining force coefficients and hence wind forces on buildings as well as for

individual members.

o This code is yet to be developed for considering Dynamic effects except vortex

shedding.

For ASCE-07:2002 version, some major points for designing loads are as follows:-

There are three different methods for this code

o Simplified method:-This is similar to IS 875 method. It needs to determine Basic

Wind Speed, Importance factor I, exposure category, and exposure adjustment

coefficient λ for calculation of wind loads. This is applicable for limited cases of

buildings and structures.

o Analytical method: - in this method some more factors as compared to simplified

method are taken into account. This can be applicable for most of the buildings

and structure.

o Wind Tunnel method: - This is method in which, actual model of building or

structure for which wind loading is to be designed. Wind conditions are applied

considering scale factors and making same environment as that of actual

building. Response of building and its components are observed and building or

structure is designed.

46

Some Observations from case studied for same conditions using IS875:1987 and ASCE-

7:02

o 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.

o 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.

o Moments for both codes are almost equal.

o Joint displacements are more in case of IS codes that displacements calculated

as per ASCE: 07.

47

Conclusion 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.

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.

Wind Tunnel procedure allowed in case or American code. Indian codes should also contain

wind tunnel procedure in case of designing tall buildings mostly in metro cities where tall

building can be designed considering all factors which will reduce risk for buildings.

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.

48

References

1. Council of Tall Building and Habitat (CTBUH) website.

2. Indian Standard Code:-Code of practice for design loads (other

than earthquake) for buildings and structures- IS875:1987 – Part

3

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. http://www.fluidstructures.com/html/vortex/phenomena.html

http://www.fluidstructures.com/html/vortex/phenomena.html