Tallbuildings_2013.pdf

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Analysis and Design of RC Tall Building Subjected to Wind and Earthquake
Loads
Conference Paper · January 2013
DOI: 10.3850/978-981-07-8012-8_166
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National Conference on Emerging Technologies in Civil Engineering
(ETCE’13) 12th April 2013
103
Department of Civil Engineering, K.S.Rangasamy College of Technology
ANALYSIS OF TALL BUILDING SUBJECTED TO WIND AND
SESIMIC LOADS
K. Rama Raju*,1
, M.I. Shereef3
, Nagesh R Iyer2
, S. Gopalakrishnan4
1
Sr. Principal Scientist, 2
Director, CSIR-Structural Engineering Research Centre, Chennai.
3
PG Student, 4
Professor, K S Rangasamy College of Technology, Tiruchengode.
*
krraju@serc.res.in
ABSTRACT
Today, tall buildings are a worldwide architectural phenomenon. The behavior of the
structures during seismic and wind loads definitely has a major role, not only from structural
engineering point of view, but also safety of humans living in the structure. It is a major challenge to
study the impact and performance of tall structures under wind and seismic loading. In this paper,
the response of tall buildings under wind and seismic load as per IS codes of practice is studied.
Seismic analysis with response spectrum method and wind load analysis with gust factor method are
used for analysis of a 3B+G+40-storey RCC high rise building as per IS 1893(Part1):2002 and IS
875(Part3):1987codes respectively. The building is modeled in 3D using STAAD.Pro software. Safety
of the structure is checked against allowable limits prescribed for inter-storey drifts, base shear,
accelerations and roof displacements in codes of practice and other literature for earthquake and
wind.
Keywords-Tall buildings, seismic loads, wind loads, response spectrum method, gust factor method.
------
Introduction
Buildings are defined as structures utilized by the people as shelter for living, working
or storage. With rapid growth in population along with the development of industrial and
commercial activities rapid urbanization has taken place which has resulted into continuous
movement of rural people to metro cities. So it is clear that the horizontal space constraint is
reaching an alarming situation for metros. To manage with the situation maximum utilization
of space vertically calls for the construction of multistoried buildings in large numbers.
Today, tall buildings are a worldwide architectural phenomenon. Many tall buildings are built
worldwide, especially in Asian countries, such as China, Korea, Japan, and Malaysia. From a
structural engineer's point of view high rise building or multi-storeyed building is one that, by
virtue of its height, is affected by lateral forces to an extent that they play an important role in
the structural design.
In general, tall multi-storey buildings need to be designed for wind as well as
earthquake loads. Governing criteria for carrying out dynamic analyses for earthquake load is
different from wind load. According to the provisions of Bureau of Indian Standards for
earthquake load, IS 1893(Part 1):2002, height of the structure, seismic zone, vertical and
horizontal irregularities, soft and weak storey necessitates dynamic analysis for earthquake
load. The contribution of the higher mode effects are included in arriving at the distribution
of lateral forces along the height of the building. When wind interacts with a building, both
positive and negative pressures occur simultaneously, the building must have sufficient
strength to resist the applied loads from these pressures to prevent wind induced building
failure. Load exerted on the building envelope are transferred to the structural system, where
in turn they must be transferred through the foundation into the ground, the magnitude of the

104
pressure is a function of the following primary factors: exposed basic wind speed,
topography, building height, internal pressure, and building shape.
The main objective of this study is carry out the analysis of a 3B+G+40 multi stored
residential building against earthquake and wind loads as per Indian standard codes of
practice IS 1893(Part 1):2002 and IS 875(Part 3):1987. The structural model and its loading
conditions are taken from the INSDAG report. The structure was subjected to self weight,
dead load, live load, wind load and seismic loads. Wind speeds, design wind pressure, loads
to the building are calculated using IS 875(Part 3):1987. Seismic loads are calculated using
IS 1893 (Part 1):2002.
Description of Building Model
In the present study, an RCC 3B+G+40 storeyed residential building model is taken
for analysis and design from INSDAG report.
(2007). The building is assumed to be in
Mumbai. The general features of the building model and beam sections used in the building
are given in Table 1. The grades of reinforcing steel and concrete used in the building are
assumed to be of Fe500 and M35 respectively. The material properties used for concrete and
steel are given in the Table 2. The dimensions of the column sections used in the building are
given in Table 3. The plan of the building model indicating the Beam and Column schedule is
shown in Figure 1.
Table 1 General features of the model structure
Length 30.18 m
Width 29.7 m
Height 139.3 m
Height (including basement) 148.9 m
Height of each basement floor (3
nos.)
3.2 m
Height of ground floor 5 m
Height of typical floors 3.5 m
Height of refuge floor*
1.6 m
Slab thickness 0.15 m
Shear wall thickness 0.25 m
Beam schedule
B1 : 0.3x1.2 m
B2 : 0.3x0.7 m
Column schedule
Given in Table
3
*
10th
, 20th and 30th
floors are designed as refuge
floors
Table 2 Material property
Property Concrete Steel
Young's Modulus, E
(kN/mm2
)
21.718 205
Poisson's ratio(µ) 0.2 0.3
Density, 3
) 2549.291 7833.413

105
Figure 1 Plan of the model
Analysis
The analysis of reinforced concrete structure has been done considering the entire
structure as a three dimension model framed structure using STAAD package. Beam and
columns are considering as beam elements. The slabs are considered as plate elements. There
are 2710 number of joints and 7585 elements in the STAAD analysis model of the structure.
The main objective of modeling the whole structure as 3D model is to take into account the
behavior of each and every component in space structure environment. The slab is modeled
as an element to carry the live load as distributed pressure load. Lift well and staircase walls
are modeled as shear wall to resist the lateral loads like wind and earthquake load. Plate
elements are used for shear walls. From the analysis, the natural frequencies and time periods
in first ten modes are found to be 0.17, 0.244, 0.544, 0.622, 0.623, 0.686, 0.706, 0.73, 1.081,
1.24Hz and 5.9, 4.1, 1.84, 1.61, 1.6, 1.46, 1.42, 1.4, 0.93, 0.81s respectively. The RCC
Table 3 Column schedule
Column Basement to 5th
floor
6th
to 10th
floor
11th
floor to
roof
C1 0.5x1.25 0.3x1.25 0.3x1
C2 0.5x1.25 0.3x1.25 0.3x1
C3 0.3x1.5 0.3x1.5 0.3x1.5
C4 0.3x1.5 0.3x1.5 0.3x1.5
C5 0.5x1.5 0.5x1.5 0.5x1.5
All column sections are in ‘m’

106
members need to be designed as per IS 456: 2000 using Limit State Method. Member forces
need to be calculated with load combinations for Limit State Method given in IS 456: 2000
and the members need to be designed for the most critical member forces among them.
Modeling of Loads
The basic loads considered in this study are dead load, live loads, earthquake loads
and wind loads. The values of Dead loads (DL) are calculated from the unit weights as
specified in IS 875 (Part 1): 1987. The live load (LL) intensities for the various areas of
residential buildings are obtained from IS 875 (Part 2): 1987. The summery of dead load and
live loads considered for the building is given in Table 4. In load combinations involving
Imposed Loads (LL), IS 1893 (Part I):2002 recommends for loads upto and including 3
kN/m2
, 25% of the imposed load to be considered for seismic weight calculations. However
to be conservative, in the present study, 100% imposed loads are considered in load
combinations. The earthquake loads are assigned in X and Y directions as EQLx and EQLy
respectively as per IS 1893(Part 1): 2002.
Analysis of Building for Earthquake Loads
As per IS 1893(Part 1): 2002, dynamic analysis with time history or response
spectrum method need to be performed to obtain the design seismic force, and its distribution
to different levels along the height of the building and to the various lateral resisting
elements, for the regular buildings those greater than 40 m in height in Zones IV and V, and
those greater than 90 m height in zone II and III. The building taken for study has 148.9m in
height and it is situated in seismic zone III. Since it is more than 90m height and situated in
Zone III, the dynamic analysis is required to be carried out. As Per Clause no 7.8.2 of IS
1893(Part 1): 2002, dynamic analysis may be performed either by time history method or by
the response spectrum method. Here, response spectrum method is used for carrying out
dynamic analysis.
Table 4 Summary of Dead Load and Live Loads
Load description Value
Superimposed load on each floor
Finish load
Live load
Wall load with hollow bricks
- 240 mm thick external wall
- 125 mm thick internal wall KN/m
1.25 kN/m2
2.0 kN/m2
10.0 kN/m
3.6 kN/m
Intensity of superimposed load over refuge
floor
Finish load
Live load
1.25 kN/m2
4.0 kN/m2
Additional service load over roof top
Water proofing load
Live load
Service load
3.0 kN/m2
1.5 kN/m2
4.0 kN/m2
Overhead reservoir load
(Column load at the column of shear wall)
500 kN

107
Response Spectrum Method
The response spectrum method is also known as the “modal analysis procedure” and
is performed in accordance with the requirements of Clause 7.8.4, IS 1893(Part 1): 2002. The
method is based on superposition of modes. Hence, free vibration modes are computed using
k) termed as the modal
response, is obtained for each mode (say kth
mode). The number of modes considered is based
on a quantity termed as the mass participation factor for each mode. Sufficient number of
modes (r) to capture at least 90 percent of the total participating mass of the building (in each
of the horizontal directions), should be considered in the analysis. In the present study, 12
modes are considered. The modal responses from all the considered modes are then combined
together using either the square root of the sum of the squares (SRSS) method or the complete
quadratic combination (CQC) method. The peak response quantities (for example, member
forces, displacements, storey forces, storey shears, and base reactions) are combined as per
CQC method. The details of data used for the seismic analysis are; zone factor, z=0.16,
importance factor, I=1.5, response reduction factor, R=3 and type of soil is soft.
In response spectrum method, the design base shear (VB) shall be compared with a
base shear ( B) calculated using a fundamental period Ta, where Ta, is as per clause no 7.6 of
IS 1893(Part 1): 2002. Where VB is less than B, all the response quantities (for example
member forces, displacements, storey forces, storey shears and base reactions) need to be
multiplied by a factor / VB.
The base shear is found by response spectrum method (VB) and seismic coefficient
method ( B) using STAAD.Pro and they are 4768.24 kN and 7043.327 kN respectively. The
base shear found by response spectrum method (VB) is less than the base shear seismic
coefficient method ( B). Hence, all the response quantities (member forces, displacements,
storey forces, storey shears and base reactions) need to be multiplied by a factor 1.477 ( B /
VB =7043.327/4768.24).
Storey Drift for seismic loads
Storey drift is the displacement of one level relative to the other level above or below.
Drift is a common phenomenon for high rise and this may hamper the integrity of the
structure and cause serious loss of life and properties in case of a major earthquake. As per
Clause no. 7.11.1 of IS 1893(Part 1): 2002, the storey drift in any storey due to specified
design lateral force with partial load factor of 1.0, shall not exceed 0.004 x hs, where hs is
storey height (3500 mm). So maximum drift allowed= 0.004 x 3500 = 14 mm. From the
analysis, peak storey drift is found to be 6.11 mm. Since, peak inter-storey drift is less than
the allowable, the design is safe.
Base shear comparison for building located at different earthquake zones and site soil
conditions
The building is assumed to be located at different seismic zones and site soil
conditions and corresponding base shear is found by seismic coefficient method and response
spectrum method using STAAD.Pro and the results are given in Table 5. The base shear
calculated with response spectrum method (VB) is less than the base shear calculated with
seismic coefficient method ( B). So, as per as per clause no 7.6 of IS 1893(Part 1): 2002, all
the response quantities (member forces, displacements, storey forces, storey shears and base
reactions) need to be multiplied by a factor / VB.

108
Table 5 Base shear (kN) comparison
Seismic coefficient method Response spectrum method
Zone
Soil Type Soil Type
Hard Medium Soft Hard Medium Soft
II 2628.61 3572.21 4414.717 1820.45 2449.99 2980.15
III 4212.52 5729.022 7043.327 2912.71 3919.98 4768.24
IV 6335.62 8593.533 10548.141 4369.07 5879.97 7152.36
V 9503.436 12907.151 15839.061 6553.61 8819.95 10728.54
The Lateral Wind Force (Fz) As Per IS 875 (Part 3):1987
According to the provisions of Bureau of Indian Standards for wind loads, IS 875
(Part 3):1987 dynamic analysis for wind load is suggested for closed buildings with height to
minimum lateral dimension ratio of more than 5 or fundamental frequency of the building
less than 1 Hz. It is suggested to check for wind induced oscillations and a magnification
factor called gust response factor needs to be included in the dynamic effects of the wind.
Since the building considered for study has height to least lateral dimension ratio is
5.2 and it is more than 5 and, the natural frequency calculated using the formula given IS 875
(Part 3):1987, (here, H is height of the building and d is the maximum base
dimension of building in metres in a direction parallel to applied wind force) is 0.412 Hz and
by dynamic analysis of building using STAAD.Pro the natural frequency is 0.17 Hz are less
than 1 Hz, the dynamic analysis of the building for wind loads need to be carried out. For
considering dynamic effects in the present study, guest factor method given IS 875 (Part
3):1987 is used.
The design wind speed, Vz at any height z, Vz = Vb k1 k2 k3 (1)
Where, Vb is basic wind speed in m/s, k1 is probability factor (risk coefficient), k2 is
terrain roughness and height factor and k3 is topography factor as per Clause 5.3.3.
The lateral force along wind load on a structure on a strip area (Ae) at any height, z,
Fz = Ae Pz Cf G (2)
where, Cf is force coefficient for building, calculated from clause no 6.3.2.1(fig.4), Ae
is effective frontal area considered for the structure at height z, Pz is design pressure at height
z due to hourly mean wind obtained as 0.6 Vz (N/m2
), G is gust factor (peak load / mean load)
as per Clause 8.3. The data considered for the guest factor method are wind speed, Vb=44m/s,
force coefficient, Cf =1.38, K1=1.07, K2 is varying with height as per Terrain Category I,
K3=1, Life of the structure 100 yrs, Gust factor, G=1.787.
For the analysis purpose, the lateral force Fz is considered in kN/m2
and these wind
intensities at various heights are given as input to the STAAD.Pro software as given Table 6.
From the analysis, the base shear and base moment due to wind load are found to be
13648.03 kN and 205982.86 kN-m respectively.
Table 6 Wind Intensity at various heights
Height (m) 5-8.5 12 15.5-19 22.5-29.5 33-48.6 52.1-99.2 102.7-139.3
Intensity (kN/m2
) 1.995 2.205 2.369 2.539 2.836 3.214 3.479

109
Roof displacement due to wind
The maximum value of deflection in serviceability limit condition obtained for wind
loads from the finite element 3-D model of STAAD.Pro is 268.9 mm. According to BS 8110-
Part 2: 1985 the maximum allowable deflection is calculated as hs/500, where hs is the storey
height for single storey building. Therefore, maximum allowable deflection value for
building height of 139.3 m is 278.6 mm. Since roof displacement found from analysis
(268.9mm) is less than allowable (278.6 mm). The design of structure is safe.
Peak inter-story drift due to wind
For the performance of the building envelope to be adequate, the peak inter-story drift
must not exceed 1/300 to 1/400 of the storey height under un-factored loads, although this
criterion may vary depending on type of cladding or glazing and cladding attachment details.
In absolute terms, inter-story drift should not exceed 10 mm unless special details allow non
structural partitions, cladding, or glazing to accommodate larger drift. However, this criterion
must also be qualified, depending on specific building features (Simiu and Miyata, 2006). For
the present problem, the maximum allowable drift is assumed to be 1/400 of storey height.
For a storey height of 3500 mm, the allowable drift is 8.75 mm. For the present problem, the
peak inter-story drift due to wind loads is found to be 7.6 mm which is less than the
allowable, so the design is safe.
Peak accelerations due to wind
In the case of wind load, serviceability limit states for tall buildings are specified in
literature (Simiu and Scanlan, 1996) in terms of degree of discomfort caused by wind induced
acceleration as given in Table 7.
In this study the a simple expression, equation (3) by Islam et al, 1990 is used for
finding the acceleration levels caused by wind induced on a building in urban environment.
Acceleration at height z (m/s2
), A(z) = 0.0116 B0.26
z ) (3)
Where, U is mean hourly wind speed at the top of the building (m/s), B is plan
dimension of building (m), M is generalized mass of the building (kilogram), K is generalized
stiffness (N/m) = M (2 N) 2
, N is frequency (Hz) and is damping ratio.
Using equation (3), the maximum acceleration at a height of 139.3m is found to be
0.1096 (%g). Therefore the degree of discomfort from the Table 7 is imperceptible.
Table 7. Degree of human discomfort and
the acceleration levels
Degree of discomfort
Acceleration
(%g)
Imperceptible <0.5
Perceptible 0.5 to 1.5
Annoying 1.5 to 5
Very annoying 5 to 15
Intolerable >15

110
Load Combinations
After the computational model is developed and the loads are assigned, the model
needs to be analyzed for the individual load cases. The internal forces in the members (such
as bending moment, shear force and axial force) for earthquake and wind load cases are
combined as per Table18 of IS 456: 2000, IS 1893(Part 1): 2002, Section 6.3 and IS 875 (Part
5):1987, Section 8.1 are given in Table 8. A total of twenty five load combinations need to
be taken for analysis using software STAAD.Pro. The member forces due to earthquake (ELx
and ELy) found using response spectrum method need to be multiplied by a factor ( / VB )
as described in Section 4.1. Member forces need to be calculated with load combinations
mentioned in Table 8 and the members need to be designed for the most critical member
forces among them.
Summary and Conclusion
In this paper, the response of tall buildings under wind and seismic load as per IS
codes of practice is studied. Seismic analysis with response spectrum method and wind load
analysis with gust factor method are used for analysis of a 3B+G+40-storey RCC high rise
building as per IS 1893(Part1):2002 and IS 875(Part3):1987codes respectively. The building
is modeled as 3D space frame using STAAD.Pro software. Safety of the structure is checked
against allowable limits prescribed for inter-storey drifts, base shear, accelerations and roof
displacements in codes of practices and other literature for earthquake and wind. The member
forces due to earthquake (ELx and ELy) found using response spectrum method need to be
multiplied by a factor ( B / VB ) as described in Section 4.1.
Table 8 Load combination
Seismic Wind
Notation Combination Notation Combination
E1 1.2(DL+LL+ELx) W1 1.2(DL+LL+WLx)
E2 1.2(DL+LL-ELx) W2 1.2(DL+LL-WLx)
E3 1.2(DL+LL+ELy) W3 1.2(DL+LL+WLy)
E4 1.2(DL+LL-ELy) W4 1.2(DL+LL-WLy)
E5 1.5(DL+ELx) W5 1.5(DL+WLx)
E6 1.5(DL-ELx) W6 1.5(DL-WLx)
E7 1.5(DL+ELy) W7 1.5(DL+WLy)
E8 1.5(DL-ELy) W8 1.5(DL-WLy)
E9 0.9DL+1.5ELx W9 0.9DL+1.5WLx
E10 0.9DL-1.5ELx W10 0.9DL-1.5WLx
E11 0.9DL+1.5ELy W11 0.9DL+1.5WLy
E12 0.9DL-1.5ELy W12 0.9DL-1.5WLy
E13 1.5(DL+LL)

111
Member forces need to be calculated with load combinations for Limit State Method given in
IS 456: 2000 and the members need to be designed for the most critical member forces
among them.
Acknowledgment
This paper is being published with the kind permission of Director, CSIR-Structural
Engineering Research Center, Chennai-600113
References
[1] INSDAG PUBLICATION: INS/PUB/104, (2007), “3B+G+40 storeyed residential
building with steel-concrete composite option”, Institute for steel development &
growth (Insdag), December 2007.
[2] BS 8110-2:1985, “Structural use of concrete - Part 2: Code of practice for special
Circumstances”, 30 August 1985.
[3] IS 1893 (Part 1):2002, “Criteria for earthquake resistant design of structures”, Bureau
of Indian standards, New Delhi, 2002.
[4] IS 875 (Part 1):1987, “Code of practice for design loads (other than earthquake) for
buildings and structures”, Part 1, Dead loads, Bureau of Indian standards, New Delhi,
1989.
buildings and structures”, Part 2 Live loads, Bureau of Indian standards, New Delhi,
1989.
buildings and structures”, Bureau of Indian standards, New Delhi, 1989.
[7] IS 456: 2000, “Plain and reinforced concrete - Code of practice”, Bureau of Indian
standards, New Delhi, 2000.
[8] Islam, M. S., Ellingwood, B., Corotis, R. B., "Dynamic Response of Tall Buildings to
Stochastic Wind Load," Journal of Structural Engineering, ASCE, Volume 116, No.
11, November 1990.
[9] Simu, E and Miyata, T. (2006), “Design of buildings and bridges for wind a practical
guide for ASCE-7 standard users and designers of special structures, John Wiley &
Sons.
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