Reinforced concrete (RC) framed buildings consist of several vertical load bearing members (columns and walls). The structural engineers have a tendency to model all vertical load bearing members as columns irrespective of their aspect ratio (breadth to thickness ratio). While using commercially available software package, if the structural walls (wide thin columns) are also modelled as columns, undesirable amount of longitudinal reinforcing steel becomes necessary. Hence a comparison has been made to study the savings in longitudinal reinforcing steel if the walls are modelled as surface elements (area elements) instead of line element. The incorrect modelling may lead to shear failure of beams during earthquakes which will trigger the collapse of building in an unexpected manner. It may cause loss of life. In this work, a typical symmetric RC building has been considered to study its behaviour, if the vertical load bearing members are modelled as surface/line elements. From the study it is concluded that attention should be paid while modelling the vertical load bearing members based on their aspect ratio. The effect of force distribution, time period, base shear and amount of reinforcement is studied in detail

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 8, Issue 1, January 2017, pp. 695–703 Article ID: IJCIET_08_01_081
Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=1
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication Scopus Indexed
BEHAVIOUR OF RC MULTI-STOREY FRAMED
BUILDINGS WITH WIDE THIN COLUMNS
Vaishali G Ghorpade
Professor, Department of Civil Engineering,
JNTU College of Engineering, Ananthapuramu, Andhra Pradesh, India
H. Sudarsana Rao
Professor, Department of Civil Engineering,
JNTU College of Engineering, Ananthapuramu, Andhra Pradesh, India
R.K. Tharun Thej
PG student, Department of Civil Engineering,
JNTU college of Engineering, Ananthapuramu, Andhra Pradesh, India
ABSTRACT
Reinforced concrete (RC) framed buildings consist of several vertical load bearing
members (columns and walls). The structural engineers have a tendency to model all vertical
load bearing members as columns irrespective of their aspect ratio (breadth to thickness
ratio). While using commercially available software package, if the structural walls (wide thin
columns) are also modelled as columns, undesirable amount of longitudinal reinforcing steel
becomes necessary. Hence a comparison has been made to study the savings in longitudinal
reinforcing steel if the walls are modelled as surface elements (area elements) instead of line
element. The incorrect modelling may lead to shear failure of beams during earthquakes which
will trigger the collapse of building in an unexpected manner. It may cause loss of life. In this
work, a typical symmetric RC building has been considered to study its behaviour, if the
vertical load bearing members are modelled as surface/line elements. From the study it is
concluded that attention should be paid while modelling the vertical load bearing members
based on their aspect ratio. The effect of force distribution, time period, base shear and
amount of reinforcement is studied in detail.
Key words: Framed buildings, thin wide column, Analysis, Time period, Base shear,
Conventional RC Frame
Cite this Article: Vaishali G Ghorpade, H. Sudarsana Rao and R.K. Tharun Thej, Behaviour
of RC Multi-Storey Framed Buildings with Wide Thin Columns. International Journal of Civil
Engineering and Technology, 8(1), 2017, pp. 695–703.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=1
1. INTRODUCTION
In developing countries like India, demand of high rise buildings is increasing day by day due to
growing population and lack of land in urban areas [1]. From structural engineer’s point of view high

2. Vaishali G Ghorpade, H. Sudarsana Rao and R.K. Tharun Thej
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rise multi-storeyed building is the 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 [2]. In this type of buildings, wind and
seismic loads play a predominant role. Hence the lateral stability is most important for high rise
buildings [3].Earthquakes are one of the nature’s greatest hazards to life on this planet. They are the
least understood of the natural hazards and in the early days were looked upon as supernatural events.
The annual losses due to earthquakes are very large in many parts of the world. The human and
economic losses resulting from severe earthquakes are due to failure of human-made facilities such as
buildings, bridges etc [4].
In recent years a powerful earthquake was struck the Kutch region of the province of Gujarat at
8:46 a.m. on 26th
of January 2001, with a magnitude of 6.7. The epicentre of the earthquake was
located at 50 km northeast of the town of Bhuj. This earthquake ranks as one of the most destructive
events recorded so far in India in terms of death, damage to infrastructure and devastation in last ten
years. After investigation it was revealed that damage of various structures is due to deficiency in
design, planning practice, analysis, and even poor quality of construction and poor detailing. Usually
while analysing a building, the vertical load bearing members are treated as columns irrespective of
their breadth to thickness ratio. But if a structural wall is modelled as a line element (column) the
behaviour of the building differs from that of actual behaviour of the building. Also higher percentage
of reinforcement is to be provided if a vertical load bearing member is modelled as line element
(column) though it is a surface/area element (structural wall). Hence a typical multi-storey building is
considered to quantify the saving in the percentage of steel in vertical load bearing member if analysed
by following the guidelines of Bureau of Indian Standards [5].
2. OBJECTIVES
Objective of the present work is limited to (i) study the effect of earthquake forces on vertical load
bearing members and beams. (ii) to study the effect of incorrect modelling of vertical load bearing
members by using ‘line element’ for ‘structural walls’ instead of adopting ‘surface element’ (area
element) in various seismic zones [6].
The aim of the present work is to conduct analysis of residential buildings, when vertical load
bearing members are treated as columns irrespective of their breadth to thickness ratio in various
seismic zones [7].
3. METHODOLOGY
3.1. Nonlinear Dynamic Analysis
It is known as Time history analysis. It is an important technique for structural seismic analysis
especially when the evaluated structural response is nonlinear. To perform such an analysis, a
representative earthquake time history is required for a structure being evaluated [8]. Time history
analysis is a step by step analysis of the dynamic response of a structure to a specified loading that
may vary with time. Time history analysis is used to determine the seismic response of a structure
under dynamic loading of representative earthquake.
3.2. Nonlinear Time History Analysis
NLTHA is one of the methods and the most accurate method available to understand the behaviour of
structures subjected to earthquake forces. As the name implies, it is the process of finding out the
history of response responses throughout the life span of the dynamic loading like an earthquake
ground acceleration record until the structure reaches a limit state. The dynamic loading consists of
applying earth quake ground acceleration record of lateral loads to a model which captures the
material non- linearity of an existing or previously designed structure, and monotonically increasing
those loads which vary with time so that the peak response of the structure is evaluated.

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4. STRUCTURAL MODELLING
The ETABS software is used in the present study to develop RC frame Models and to carry out the
analysis. Linear dynamic analysis of the building models is performed on ETABS. The buildings
considered are RC frame model; infill and diagrid models of 30 storied structures have been done [9].
An RC two-bay building having 7, 10, 13 and 16 storeys have been considered to study the effect of
vertical load bearing member by modelling it either as line element or surface element. The storey
height is kept uniform of 3m for all building models which are shown in the Table. 1.The columns
dimensions and dimension ratios are shown in Table 2.The lateral loads generated by ETABS with
respect to the seismic zone V and the 5% damped response spectrum are given in code IS: 1893-2002
Table 1 Building data sheet
S.No Description Information Notes
1 Use of building Residential
2 Number of storeys 7
3 Type of structure RC frame
4 Horizontal floor system Beams and slabs
5 Seismic zone III IS 1893:2002
6 Important factor I 1 IS 1893:2002
7 Seismic zone factor Z 0.16 IS 1893:2002
8 Response reduction factor R 5 IS 1893:2002
9 Fundamental natural period Ta 0.34s,0.28s IS 1893:2002
10 Grades of concrete used in different
parts of building
M45 columns M30 slabs
and beams
11 Thickness of slabs used in
buildings(mm)
150,115,100
12 Type of reinforcement used 415 N/mm2
13 Method of analysis Equivalent static analysis
14 Storeys height (m) 3
15 Computer software used ETABS
16 Dead loads(unit weight adopted)
Water 10 KN/mm2
IS 875 Part 1Brick masonry 20 KN/mm2
Plain concrete 24 KN/mm2
17 Live loads
IS 875 Part 2Floor loads 2.3 KN/mm2
Roof loads 1.5 KN/mm2

4. Vaishali G Ghorpade, H. Sudarsana Rao and R.K. Tharun Thej
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Table 2 Columns dimensions and dimension ratios
Column
Number
Size (mm)
(b xt)
b/t Column
Number
Size(mm)
(bxt)
b/t
C1 1250x200 6.25 C13 1000x200 5
C2 1100x200 5.5 C16 1350x200 6.75
C3 1745x200 8.725 C17 1200x200 6
C4 1100x200 5.5 C18 1200x200 6
C5 1200x200 6 C19 1700x200 8.5
C6 1300x200 6.5 C20 2250x200 10.5
C7 1000x200 5 C21 1100x200 5.5
C8 1500x200 7.5 C22 1100x200 5.5
C9 1000x200 5 C23 1400x200 7
C10 1300x200 6.5 C24 1475x200 7.25
C11 1000x200 5 C31 750x200 3.75
C12 900x200 4.5 C33 1000x200 5
These buildings have been analysed for various seismic zones as per IS 1893(part1):2002. Height
of the building from foundation to ground floor is 2 m and the height of each floor is 3 m. Size of
beams for all storeys are 200×600mm. The plan of the building is shown Fig 1.
Table 3 Size of columns /structural walls in building
Storeys Height(m) Size of column/structural wall
Breadth(b) x Thickness(t)
(mm)
b/t
7 23 1000x200 5
10 32 1200x200 6
13 41 1400x200 7
16 50 1600x200 8
Three different Columns/ Structural walls (A1, A2, B1) and one internal Column/Structural wall
(B2) as shown in Fig 1and Fig 2 were considered to study the effect of incorrect modelling.
Figure 1 Typical plan of building at ground level

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Figure 2 Elevation of 7 storey Building
Isometric view of the 7 storey structure modelled as line element and surface element is shown in
the Figs 3 and 4.
Figure 3 Isometric view of 7 storey building Figure 4 Isometric view of 7 storey building
Vertical load bearing members modelled as vertical load bearing members modelled as
line elements surface elements.
5. RESULTS AND DISCUSSION
The comparative analysis of RC Frame model, Isometric model in terms of time period, Base shear are
presented in this section
5.1. Time Period
The approximate fundamental natural period of vibration Ta in seconds, of a moment resisting frame
building without brick infill panels may be estimated by the following empirical formula (IS 1893
(Part 1):2002 (Clause 7.6.1))

6. Vaishali G Ghorpade, H. Sudarsana Rao and R.K. Tharun Thej
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Ta = 0.075h 0.75
for RC frame building (1)
The approximate fundamental natural period of vibration in seconds of all other, buildings
including moment resisting frame buildings with brick infill panels may be estimated by the following
expression. (IS 1893 (Part 1):2002 (Clause 7.6.2))
Ta =0.09h/√d (2)
Where,
H = Height of building in meters. (This excludes the basement stories where basement walls are
connected with the ground floor deck or fitted between the columns. But it includes the basement
stories, when they are not connected)
d = base dimensions of the building at the plinth level, in m, along the considered direction of the
lateral force.
Time period is obtained by using Equation 1 and 2.Table 4 suggests time period with vertical load
bearing member modelled as surface element. Table 5 shows time period with vertical load bearing
member modelled as line element. Percentage increase in time period with vertical load bearing
member modelled as line element is shown in Table 6.
Table 4 Time period with vertical load bearing member modelled as surface element
Number of storeys 7 10 13 16
Time period from Empirical Formulae (sec) 0.59 0.83 1.06 1.29
Time period from model analysis (sec) 1.23 1.65 2.05 2.4
Table 5 Time period with vertical load bearing member modelled as line element
Number of storeys 7 10 13 16
Time period from Empirical Formulae (sec) 0.78 1 1.22 1.41
Time period from model analysis (sec) 1.38 1.9 2.4 2.9
Table 6 Increase in time period with vertical load bearing member modelled as line element
Number of storeys 7 10 13 16
Increase in time period from Empirical Formulae
(%)
32 20 15 9.5
Increase in Time period from model analysis (%) 12 15 17 20
From tables 4 to 6 it is seen that time period is reduced when vertical load bearing member
modelled as surface element. For example the time period of the nine storey building is reduced from
1 s (4.3) to 0.83 s (4.5), when vertical load bearing member modelled as structural member.
5.2. Base Shear
The seismic constrain at base of the building is known as the base shear. Tremors regularly harm
structures at this level. Weight of the working over the breaking is the shear drive that broke the
building. Commonly earthquake harm happens at base of building. Base shear obtained for vertical
load bearing modelled as surface element is given in Tables 7&8. The value of base shear values for
vertical load bearing modelled as line element is shown in table 8. The percentage of decrease in base
shear when vertical load bearing member is modelled as line element is given in Table 9.

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Table 7 Base shear calculations for vertical load bearing member modelled as surface element
Number of
storeys
Seismic
weight
(KN)
Horizontal seismic coefficient Base shear(KN)
Zone -3 Zone -4 Zone -5 Zone -3 Zone -4 Zone -5
7 12751 0.0368 0.0552 0.0828 469 701 1052
10 18890 0.0262 0.0393 0.0589 495 742 1113
13 24782 0.0205 0.0307 0.0461 508 763 1144
16 30834 0.0168 0.0252 0.0379 519 779 1169
Table 8 Base shear calculations for vertical load bearing modelled as line element
Number of
storeys
Seismic
weight
(KN)
Horizontal seismic coefficient Base shear(KN)
Zone -3 Zone -4 Zone -5 Zone -3 Zone -4 Zone -5
7 12751 0.02676 0.0414 0.0621 352 528 792
10 18890 0.02176 0.0326 0.0489 409 616 924
13 24782 0.0177 0.0267 0.0401 438 662 994
16 30834 0.0153 0.0231 0.0347 471 713 1070
Table 9 Percentage of decrease in base shear when vertical load bearing member modelled as line element
Number of storeys % of decrease in base shear
Zone -3 Zone - 4 Zone -5
7 25 25 25
10 17 16 16
13 13 13 13
16 9 8 8
From above tables 7 to 9 it observed that there is an increase in base shear value when the vertical
load bearing member is modelled as surface element. For example the base shears for six storey
building are 352 KN and 469 KN respectively if the vertical load bearing member is modelled as line
element and surface element (for zone-3).
Figure 5 presents the comparison of Time period for Empirical Formula and Model Analysis
Figure 5 Comparison of Time period for Empirical Formula and Model Analysis
32
20
15
9.5
12
15
17
20
0
5
10
15
20
25
30
35
7 10 13 16
%IncreaseinTimePeriod
No of Storeys
% Increase in Time
Period from
Empirical Formula
% Increase in Time
Period from Model
Analysis

8. Vaishali G Ghorpade, H. Sudarsana Rao and R.K. Tharun Thej
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From Table 6, the percentage increase in time period when vertical load bearing member modelled
as line element is 20% in nine storey building.
The percentage decrease in base shear values and its comparison for different seismic zones is
shown in Fig. 6
Figure 6 comparison of base shear in zones 3, 4 and 5
From Table 9, the percentage decrease in base shear is found as 25 for six storey building for all
zones. If the number of storeys increases, the percentage decrease of base shear will increase
accordingly.
6. CONCLUSION
The following conclusions are drawn from the present study:
• The fundamental time period of the buildings (ranging from seven to sixteen storeys) with vertical load
bearing elements modelled as line elements gets over-estimated in the range of 9 to 32% when
compared to that with vertical load bearing elements modelled as surface elements. This results in
underestimation of design base shear (about 9 to 25%) in a building where vertical load bearing
elements are modelled as line elements instead of surface elements leading to possible shear failure.
• The longitudinal reinforcement required for a structural wall and a column will be different due to the
difference in their structural behaviour. The structural wall has negligible flexural strength in the out-of-
plane direction and hence is designed only to resist in-plane-bending, combined with axial compression
and shear force. However, by incorrectly modelling structural wall as a line element (column), the
design has to account for bi-axial bending. This results in excessively high longitudinal reinforcement
than required.
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%Decreaseinbaseshear
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9. Behaviour of RC Multi-Storey Framed Buildings with Wide Thin Columns
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