Study On Seismic Behaviour of Tall Irregular Buildings Under Influence of Non Parallel And Offset Irregularities

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Study on Seismic Behaviour of Tall Irregular Buildings under Influence of
Non Parallel and Offset Irregularities
Lohith Kumar B C 1 Manjunath N K 2 Gangadhar N 3
1,2Asst.Professors Department of Civil Engineering Madawalabu University Bale Robe Ethiopia
3Asst.Professor Department of Civil Engineering RRCE Bangalore India
---------------------------------------------------------------------------------------------------------------------------------------------------
Abstract -The present study aims at understanding the
importance of codal provisions, which are particularly
provided for the analysis of torsionally unbalanced
structures. IS 1893-2002 Part 1 Code gives the information
about number of parameters which influences the
irregularity of the structure. However, in the present study
the worst affected irregularity under the influence of
torsion are studied, In the present study, seismic analysis
has been performed by Equivalent Lateral Force Method
(ELF) ie the codal method, for all zones and for all soil
types irregularities such as Non parallel irregularity and
Offset irregularity for 10, 15, 20 storey buildings
The results in form of torsional moment, fundamental time
period and base shear results are compared for different
irregularities and the analysis is done with Etabs 9.2
software.
STATEMENT OF THE PROBLEM
Seismic induced torsion in asymmetric RC buildings has
been studied for various parameters. Equivalent Lateral
Force Method (ELF) adopted to study the induced torsion
as per IS 1893(Part 1): 2002 codal provisions. ETABS
v9.5.software package is used to carry all the static and
dynamic analysis. Since the present study is devoted for
the investigation of torsional behaviour of asymmetric
structures, in order to capture exact three dimensional
behaviour, all the analysis are performed on complete
three dimensional models of the structures.
OBJECTIVES OF THE STUDY
The present study aims at understanding the importance
of codal provisions, which are particularly provided for the
analysis of torsionally unbalanced structures. IS Code gives
the information about number of parameters which
influences the irregularity of the structure. However, in the
present study the irregularity under the influence of
torsion are studied in detail.
Hence, the following objectives were identified based on
these parameters. The present study focuses on the
discontinuities in a lateral force resistance path, such as
out-of-plane offsets of vertical elements etc.
1. To study the influence of discontinuities in a
lateral force resistance path, such as out-of-plane
offsets of vertical elements etc.
2. To study the influence of Non parallel System ie
the vertical elements resisting the lateral force
which are not parallel about major orthogonal
axes.
REVIEW OF LITERATURE
Anil K. Chopra et.al (2004) [2] study compares the
seismic demands for vertically irregular and ‘‘regular’’
fram9es determined by rigorous nonlinear response
history analysis (RHA), due to an ensemble of 20 ground
motions. Forty-eight irregular frames, all 12-story high
with strong columns and weak beams, were designed with
three types of irregularities stiffness, strength, and
combined stiffness and strength introduced in eight
different locations along the height using two modification
factors. The effects of vertical irregularity on the median
values of story drifts and floor displacements are
documented. Next, the median and dispersion values of the
ratio of story drift demands determined by modal
pushover analysis (MPA) and nonlinear RHA were
computed to measure the bias and dispersion of (MPA)
estimates leading to the following results: (1) the bias in
the MPA procedure does not increase, i.e., its accuracy
does not deteriorate, in spite of irregularity in stiffness,
strength, or stiffness and strength provided the
irregularity is in the middle or upper story, (2) the MPA
procedure is less accurate relative to the regular frame in
estimating the seismic demands of frames with strong or
stiff-and-strong first story; soft, weak, or soft-and-weak
lower half; stiff, strong, or stiff-and-strong lower half, (3)
in spite of the larger bias in estimating drift demands for
some of the stories in particular cases, the MPA procedure

identifies stories with largest drift demands and estimates
them to a sufficient degree of accuracy, detecting critical
stories in such frames, and(4) the bias in the MPA
procedure for frames with a soft, weak, or soft-and-weak
first story is about the same as for the regular frame
Babak Rajaee Rad et.al (2007) [3] High-rise concrete
shear walls are often supported near or below grade by
stiff floor diaphragms connected to perimeter foundation
walls. When a large portion of the overturning moment in
the wall is transferred to the foundation walls by force
couples in two or more stiff floor diaphragms, the
maximum bending moment flexural plastic hinge occurs
above the diaphragms and the shear force reverses below
the flexural hinge. Depending on the stiffness of floor
diaphragms, and on the shear rigidity and flexural rigidity
of the high-rise concrete walls, the reverse shear force
below the flexural hinge may be much larger than the base
shear above the flexural hinge. Nonlinear dynamic
analyses indicate the maximum reverse shear force is
proportional to the bending moment capacity of the wall
and inversely proportional to the accompanying base
shear force. An upper-bound estimate of bending moment
capacity of the high-rise wall combined with an assumed
zero base shear force can be used in a simple nonlinear
static analysis to estimate the maximum shear force below
the flexural plastic hinge. A nonlinear shear model can be
used to determine whether diagonal cracking of the wall
and yielding of horizontal wall reinforcement will reduce
the reverse shear force without causing a shear failure.
Increasing the quantity of horizontal reinforcement in the
wall above a certain limit may not prevent a shear failure
and thus a different design solution will need to be found.
An upper-bound estimate of floor diaphragm stiffness
should be used in order to not underestimate the shear
strain demand on high-rise walls.
Devesh P. Soni (2006) [4] study summarizes state-of-the-
art knowledge in the seismic response of vertically
irregular building frames. Criteria defining vertical
irregularity as per the current building codes have been
discussed. A review of studies on the seismic behavior of
vertically irregular structures along with their findings has
been presented. It is observed that building codes provide
criteria to classify the vertically irregular structures and
suggest dynamic analysis to arrive at design lateral forces.
Most of the studies agree on the increase in drift demand
in the tower portion of set-back structures and on the
increase in seismic demand for buildings with
discontinuous distributions in mass, stiffness, and
strength. The largest seismic demand is found for the
combined-stiffness-and-strength irregularity. It can be
concluded that a large number of research studies and
building codes have addressed the issue of effects of
vertical irregularities
Building codes provide criteria to classify the vertically
irregular structures and suggest elastic time history
analysis or elastic response spectrum analysis to obtain
the design lateral force distribution.
Dhiman Basu et.al (2004) [5] made a detailed
investigation on the importance of diaphragm rigidity for
the seismic response of a structure. Even though a rigid
floor diaphragm is a good assumption for seismic analysis
of most buildings, several building configurations may
exhibit significant flexibility in floor diaphragm. In this
paper, the definition of centre of rigidity for rigid floor
diaphragm buildings has been extended to unsymmetrical
buildings with flexible floors. A superposition-based
analysis procedure is proposed to implement code-
specified torsional provisions for buildings with flexible
floor diaphragms, similar to that of rigid floor diaphragms.
The procedure suggested considers amplification of static
eccentricity as well as accidental eccentricity. The
proposed approach is applicable to orthogonal as well as
non-orthogonal unsymmetrical buildings and accounts for
all possible definitions of centre of rigidity.
In this investigation, the building is assumed to have a
single wing only, i.e., buildings with multiple wings (e.g., L,
V, Y, etc. shaped) are not considered. The no-torsion
condition for flexible floor buildings is defined such that
center nodes at either end of the diaphragm are
constrained so that they undergo equal horizontal
displacement. The proposed analysis procedure considers
the final response as the superposition of three cases: the
no-torsion case, amplification of the static eccentricity, and
accidental torsion. The proposed procedure ensures that
the resultant member force is close to that of rigid floor
buildings as the floor diaphragm rigidity increases. It is
seen that treating the diaphragms of such buildings as
rigid for torsional analysis may cause considerable error.
Eduardo Miranda et.al (2004) [6] An approximate
method is presented to estimate the maximum lateral drift
demands in multi story buildings with non uniform lateral
stiffness responding primarily in the fundamental mode
when subjected to earthquake ground motions. The
method is aimed at the estimation of the maximum roof
displacement and of the maximum inter story drift ratio
for a given response spectrum. A simplified model of the
multi story building is used based on an equivalent
continuum structure with non uniform lateral stiffness
distribution consisting of a combination of a flexural
cantilever beam and a shear cantilever beam. The effect of
the type and amount of reduction in lateral stiffness along

the height of the building and of the ratio of overall flexural
and shear deformations on the ratio of the spectral
displacement to the roof displacement and on the ratio of
the maximum inter story drift ratio to the roof drift ratio is
investigated. It is shown that reductions in lateral stiffness
along the height have a negligible effect on the ratio of
spectral displacement to maximum roof displacement and
only a small effect on the ratio of maximum inter story
drift ratio to roof drift ratio.
MODELLING
ETABS is a sophisticated, yet easy to use, special purpose
analysis and design program developed specifically for
building systems. ETABS Version 9 features an intuitive
and powerful graphical interface coupled with unmatched
modelling, analytical, and design procedures, all integrated
using a common database. Although quick and easy for
simple
ANALYSIS USING ETABS
The modelling and analysis of the building is carried out
using ETABS Nonlinear v9.2.0 software package. ETABS is
a powerful program developed by Computers and
Structures Inc, Berkeley, California, USA which can greatly
enhance an engineer's analysis and design capabilities for
structures. Part of that power lies in an array of options
and features. The other part lies in how simple it is to use.
The basic approach for using the program is very straight
forward. The user establishes grid lines, defines material
and structural properties, places structural objects relative
to the grid lines using point, line and area object tool. All
the types of loads that the structure is subjected to, can be
defined and assigned to the appropriate structural
components. Dynamic analysis properties like mass
source, total number of mode shapes and its directions can
be defined. The following topics describe some of the
important areas in the modelling. Finally, the analysis can
be performed and the results are generated in graphical or
tabular form that can be printed to a printer or to a file for
use in other programs. The following topics describe some
of the important areas in the modelling.
MODELLING OF REGULAR BUILDINGS:
Details of regular buildings considered in this work are as
follows:
Column size 230X600 mm
Beam size is 230X450 mm
Slab size is 150mm thick
Height of the floor 3m
Live Load on roof slab 1.5 KN/m2
Live Load on floor slab 3KN/m2
Floor Finish on roof slab 1.5 KN/m2
Floor Finish on floor slab 1KN/m2
All the columns are assumed to be fixed at their base
Characteristic compressive strength of concrete in slabs is
M25
Characteristic compressive strength of concrete in
Columns and Beams is M30
ETABS model screen shot of a regular 10 storied building
The similar models are generated using ETABS with &
without irregularities are for 10, 15 & 20 storey’s
MODELLING IN IRREGULAR BUILDINGS:
Non parallel Irregularity
In this irregularity the changes with respect to Regular
building is that the Shear Walls has been provided which
are not parallel compared to that of regular building as
per the IS 1893-2002 part I.

ETABS model screen shot of a Non Parallel irregular
10 storied building
Offset Irregularity
In this irregularity the changes with respect to Regular
building is that the Shear Walls have been provided in the
global Y direction as shown below but the shear walls have
been removed from the ground floor in alternate grid lines
with respect to the regular buildings as per IS 1893-2002
part I.
ETABS model screen shot of a offset irregular 10
storied building
SEISMIC ANALYSIS OF STRUCTURES
DESIGN LATERAL FORCE
The procedure recommended for the determination of
lateral force in IS:1893-2002(Part 1) performing are based
on the approximation that effects of yielding can be
accounted for by linear analysis of the building using
design spectrum. This analysis is carried out by either
equivalent lateral force procedure or dynamic analysis
procedure given in the clause 7.8 of IS:1893-2002 (Part 1).
The main difference between the two procedures lies in
the magnitude and distribution of lateral forces over the
height of the building. In the dynamic analysis procedure,
the lateral forces are based on properties of the natural
vibration modes of the building which are determined by
distribution of mass and stiffness over the height. In the
equivalent lateral force procedure the magnitude of forces
is based on an estimation of the fundamental period and
on the distribution of forces as given by a simple emperical
formula that is appropriate only for regular buildings. In
the analysis of irregular buildings, equivalent lateral force
method fails to capture the actual three dimensional
behaviour of the structure. The following sections will
discuss in detail the above mentioned procedures of
seismic analysis.
Equivalent Lateral Force Method
The total design lateral force or design base shear along
any principal direction is given in terms of design
horizontal seismic coefficient and seismic weight of the
structure. Design horizontal seismic coefficient depends on
the zone factor of the site, importance of the structure,
response reduction factor of the lateral load resisting
elements and the fundamental period of the structure. The
procedure generally used for the equivalent static analysis
is explained below:
(i) Determination of fundamental natural period (Ta)of
the buildings
0.75
aT =0.075h Moment resisting RC frame
building without brick infill wall
0.75
aT =0.085h Moment resisting steel frame
building without brick infill walls
a dT =0.09h/ All other buildings including
moment resisting RC frame
building with brick infill walls.
Where,
h -is the height of building in m
d - is the base dimension of building at plinth level
in m, along the considered direction of lateral
force.
(ii) Determination of base shear (VB)n of the building
B hV A W 
Where,

Z I Sa
Ah
2 R g
 is the design horizontal seismic coefficient,
which depends on the seismic zone factor (Z), importance
factor (I), response reduction factor (R) and the average
response acceleration coefficients (Sa/g). Sa/g in turn
depends on the nature of foundation soil (rock, medium or
soft soil sites), natural period and the damping of the
structure.
(iii) Distribution of design base shear
The design base shear VB thus obtained shall be distributed
along the height of the building as per the following
expression:
2
i i
i B n
2
i i
i 1
W h
Q V
W h



Where, Qi is the design lateral force, Wi is the seismic
weight, hi is the height of the ith floor measured from base
and n is the number of stories in the building.
RESULTS AND DISCUSSIONS
The results of each building models are presented in this
chapter. The analysis carried out are equivalent static and
seismic analysis, the results are obtained for ten, fifteen
and twenty story buildings The result of Torsional
moment, Fundamental time period and Base shear for
different models are presented and compared to with
different models for different irregularities, In addition to
this all models are evaluated with different type of soils, all
the zones, mass, rigidity and performance of different type
of irregularities in different soil and zone types are
presented.
Note: The notations used below are as follows
i) R= REGULAR
ii) IR= IRREGULAR
iii) REGS1= REGULAR SOIL TYPE 1
iv) REGS2= REGULAR SOIL TYPE 2
v) REGS3= REGULAR SOIL TYPE 3
vi) IREGS1= IRREGULAR SOIL TYPE 1
vii) IREGS2= IRREGULAR SOIL TYPE 2
viii) IREGS3= IRREGULAR SOIL TYPE 3
NON PARALLEL OFFSET IRREGULARITY
TORSIONAL MOMENT
0
0.02
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone II
RegS1
RegS2
RegS3
0
1
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone II
IRegS1
IRegS2
IRegS3
0
0.02
0.04
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone III RegS1
RegS2
RegS3
0
0.5
1
1.5
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone III
IRegS1
IRegS2
IRegS3
0
0.05
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone IV
RegS1
RegS2
RegS3

BASE SHEAR
OFFSET IRREGULARITY
TORSIONAL MOMENT
0
2
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone IV IRegS1
IRegS2
IRegS3
0
0.1
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone V RegS1
RegS2
RegS3
0
2
4
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone V
IRegS1
IRegS2
IRegS3
0
200
400
0 10 20 30
BaseShear
No Of Storeys
Zone II RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
200
400
600
0 10 20 30
BaseShear
No Of Storeys
Zone III RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
500
1000
0 10 20 30
BaseShear
No Of Storeys
Zone IV RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
500
1000
1500
0 10 20 30
BaseShear
No Of Storeys
Zone V RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
0.01
0.02
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone II
RegS1
RegS2
RegS3
0
0.05
0.1
0 10 20 30
Torsional
Moment
No Of Storeys
IR ZoneII
IRegS1
IRegS2
IRegS3
0
0.01
0.02
0.03
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone III
RegS1
RegS2
RegS3

BASE SHEAR
CONCLUSION
From the study on Non-parallel irregularity following are
concluded:
 The torsional moments, fundamental period &
base shear increases with the increase in the
height of a regular building.
0
0.1
0.2
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone III
IRegS1
IRegS2
IRegS3
0
0.2
0.4
0 10 20 30
Torsional
Moment
No Of Storeys
IR Zone IV IRegS1
IRegS2
IRegS3
0
0.05
0.1
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone V
RegS1
RegS2
RegS3
0
0.2
0.4
0 10 20 30
Torsional
Moment
No Of StorEys
IR Zone V
IRegS1
IRegS2
IRegS3
0
200
400
0 10 20 30
BaseShear
No of Storys
Zone II
RegS1
IRegS1
RegS2
IRegS2
RegS3
0
500
0 10 20 30
BaseShear
No of Storys
Zone III RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
1000
2000
0 10 20 30
BaseShear
No of Storys
Zone V RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
500
1000
0 10 20 30
BaseShear
No of Storys
Zone IV RegS1
IRegS1
RegS2
IRegS2
RegS3
IRegS3
0
0.05
0 10 20 30
Torsional
Moment
No Of Storeys
R Zone IV
RegS
1
RegS
2

 The effect of variation in the Torsional moments
are high in 10 to 20 storied building in all seismic
zones & all soil types.
 Base shear varies linearly in 10 to 20 storied
building in all seismic zones & all soil types.
The effect of variation in the fundamental time period is
high in a 10 storied building compared to a 15 or a 20
storied building
From the study on Offset irregularity following are
concluded:
 The torsional moments, fundamental period &
base shear increases with the increase in the
height of a regular building.
 The effect of variation in the Torsional moments
are high in 10 to 20 storied building in all seismic
zones & all soil types.
 Base shear varies linearly from 10 to 20 storied
building in all seismic zones & all soil types.
 The effect of variation in the fundamental time
period is high in a 20 storied building compared to
a 10 or a 15 storied building.
REFERENCES
1. Ahmad J. Durrani, S.T. Mau, Amr Ahmed
AbouHashish and Yi Li “EarthQuake Response of
Flat-Slab Buildings” Vol.120 No. 3, March, 1994.
©ASCE,ISSN 0733-9445/94/0003-0947
2. Anil K. Chopra and Chatpan Chintanapakdee
“Seismic Response of Vertically Irregular Frames:
Response History and Modal Pushover Analysis”
Vol. 130, No. 8, August 1, 2004. ©ASCE,ISSN 0733-
9445/2004/8-1177–1185
3. Babak Rajaee Rad and Perry Adebar “Seismic
Design of High-Rise Concrete Walls: Reverse Shear
due to Diaphragms below Flexural Hinge”(2009)
Vol. 135, No. 8, August 1, 2009. ©ASCE, ISSN
0733-9445/2009/8-916–924
4. Devesh P. Soni and Bharat B. Mistry “Qualitative
Review Of Seismic Response Of Vertically
Irregular Building Frames”, Vol. 43, No. 4,
December 2006, pp. 121-132
5. Dhiman Basu and Sudhir K. Jain “Seismic
Analysis of Asymmetric Buildings with Flexible
Floor Diaphragms”, Vol. 130, No. 8, August 1,
2004. ©ASCE, ISSN 0733-9445/2004/8-1169–
1176
6. Eduardo Miranda and Carlos J. Reyes
“Approximate Lateral Drift Demands in Multistory
Buildings with Non uniform Stiffness” Vol. 128,
No. 7, July 1, 2002. ©ASCE, ISSN 0733-
9445/2002/7-840–849
7. IS:1893-2002(Part 1) Criteria for Earthquake
Resistant Design of Structures, part 1-General
provisions and buildings, fifth revision, Bureau of
Indian Standards, New Delhi, India
8. IS: 456-2000, “Code of Practice for Plain and
Reinforced Concrete”, Bureau of Indian Standards,
New Delhi, India.
BIOGRAPHIES
Lohith Kumar B C
Asst.Professor Department of
Civil Engineering MWU
Bale Robe Ethiopia
Having 6 years of teaching
Experience
Manjunath N K
Asst Professor Department of
Civil Engineering MWU
Bale Robe Ethiopia
Having 5 Years of Teaching
Experience
“Gangadhar N
Asst.Professor Department of
Civil Engineering RRCE
Bangalore
Having 6.5 years of teaching
Experience “

Study On Seismic Behaviour of Tall Irregular Buildings Under Influence of Non Parallel And Offset Irregularities

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