International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 15 No: 02
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Comparison of RC Building for Low, Moderate and
High Seismic Categories
Tatheer Zahra1, Yasmeen Zehra2, Noman Ahmed3
Assistant Professor, Department of Civil Engineering, NED University of Engineering and Technology, Karachi,
Pakistan, Email: tatheer@neduet.edu.pk
2
Assistant Professor, Department of Civil Engineering, Sir Syed University of Engineering and Technology,
Karachi, Pakistan, Email: yasmeen_nedian@hotmail.com
3
Design Engineer, Mushtaq and Bilal Consulting Engineers, 304 Noor Estate, Shahrah-e-Faisal, Karachi,
Pakistan, Email: shk_nedian@hotmail.com
1
Abstract--
In this investigation, a study was conducted to
compare the design of a high rise reinforced concrete building in
different seismic zones. A 30 storied building was modelled in
ETABS software and analysis was done for forces in low (seismic
zone 1), moderate (seismic zone 2a, 2b) and high (seismic zone 3,
4) categories and applied forces were compared. The building
had a dual frame comprising of shear walls interacting with
moment resisting frame to provide lateral resistance. All
structural members were designed for moderate zone 2b
(Karachi where the building is situated) and the capacity was
compared for all the above mentioned categories. The results
showed that the members designed for moderate seismic zone
were inadequate for higher seismic zone categories. Some of the
beams and columns which were found adequate in low and
moderate categories were found to be deficient for resisting loads
for high seismic loadings. Similarly shear wall in critically loaded
areas that were performing well in low and moderate zone
needed to be re-designed for high seismic zone categories. The
RC buildings which are analysed and designed to sustain low and
moderate seismic events are not safe for seismic events of higher
category and run the risk human lives and massive devastation.
Index Term-- Buildings, structures & design; Reinforced
concrete structures; Seismic Zones
List of notation
ETABS
SPC
IBC
RC
fc’
fy
R/F
b
h
V
M
Av
s
is the Extended Three Dimensional Analysis
of Buildings Software
is the Seismic Performance Category
is the International Building Code
is the Reinforced concrete
is concrete cylindrical compressive strength
is reinforcement yield stress
is the Reinforcement
is the width of element
is the depth of member
is the shear force
is the bending moment
is the area of shear reinforcement
is the spacing of bars
1. INTRODUCTION
Earthquakes forces are large in magnitude and in short
duration of time creates large amount of displacements and
stresses. These must be resisted by a structure without causing
collapse and preferably without significant damage to the
structural elements. The lateral forces due to earthquakes have
a major impact on structural integrity (Kumar and Papa Rao
2013). For medium to tall buildings, where lateral actions are
predominant, the detailing of elements and joints might be
more critical but could still potentially follow the simplified
design methods outlined by Uniform design building code
(UBC) or International Building Code (IBC) for structures
(Heiza and Tayel 2012). Michael and Majid (2001) and
Taranath (2010) stated the design basic concepts, gravity
systems, lateral loads and dynamic loads affecting the
structural behavior of the high rise buildings. Poor
understanding and design could lead to severe damage
(Haseeb et. al. 2011).
For the design of a high rise reinforced concrete (RC)
building, the International Building Code (IBC), assigns
different level of Seismic Risk or assigned Seismic
Performance Category (SPC) or Seismic Design Category
(SDC), depending upon the seismic zone. The SPC varies
from A to E with SPC of A & B for Seismic Zone 0, 1; SPC of
C for Seismic Zone 2; and SPC of D & E for Seismic Zone 3,
4. Seismic zone 0, 1 are designated as Low; zone 2a, 2b as
Moderate and zone 3, 4 as High seismic risk categories.
Design and detailing requirements differ for each.
A mathematical study was developed to compare the design of
a high rise reinforced concrete building in low, moderate and
high seismic zone. A 30 storied building was modelled in
ETABS software and analysis was done for forces in low
(seismic zone 1), moderate (seismic zone 2a, 2b) and high
(seismic zone 3, 4) categories and results were compared. The
building had a dual frame comprising shear walls interacting
with moment resisting frame to provide lateral resistance. The
strength of concrete was taken as 40 MPa for columns and
shear walls and 27 MPa for slabs and beams. While
reinforcement strength in all the cases was considered to be
413 MPa. All structural members were designed for moderate
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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 15 No: 02
seismic zone 2b and then capacities were compared for all the
above mentioned categories. The results showed that for
members designed for seismic zone 2b category were found
deficient for higher seismic zone categories. Some of the
beams which were found adequate in low and moderate
categories were found to be deficient for resisting loads for
high seismic loadings. Similarly columns and shear walls in
critically loaded areas that were performing well in low and
moderate zone were found deficient and had to be re-designed
in high seismic zone categories. Similar results are obtained
for Indian Standards by Sanghani and Patel (2011), Ram
Kumar et al. (2013), Kumar and Papa Rao (2013) and Sapate
(2012). These results imply that seismic vulnerability
assessment needs to be carried out carefully for areas to
ascertain realistically the seismic zone categories for which
the High Rise RC buildings are to be designed so as to ensure
adequate strength and safety.
2. ANALYSIS OF BUILDING FOR DIFFERENT SEISMIC ZONES
A reinforced concrete high rise building of 30 stories was
modelled in ETABS software which comprised of dual system
of moment frame interacting with shear walls. The slab
system was solid slab supporting on prismatic rectangular
sections. The building was analysed and designed according to
ACI 318-02. The applied loads, Loads combinations and
seismic data were taken from IBC-2003 specifications. The
building is a real proposed project to be used as commercial
cum residential building in Karachi. The modelled plan and
elevation of the building is shown in Figure 1. The input of the
software and general information of the building is given in
Table I.
2
Analysis was performed for gravity and lateral seismic loads
and critical members were found. The members considered for
comparison includes Beam, Column and Shear wall. For most
critical members internal forces like shear, moment and axial
forces were determined.
3. DESIGN OF MOST CRITICAL MEMBERS FOR MODERATE
ZONE
After determining the most critical beam, column and shear
wall under lateral loading, the same were designed and
proportioned for moderate Zone 2b forces. For designing of
members, specifications of ACI-318-02 were followed. Zone
2b is selected, since this building has been proposed for
Karachi region which comes under seismic risk zone 2b
(UBC-97). The axial, shear and moment capacities of the
designed critical elements were estimated.
The capacities were then compared with the forces developed
on the same members in each seismic zone. For beams, shear
force and negative - positive bending moments were
compared. For columns and shear walls, load moment
interaction curves were developed for the designed sections.
Applied axial and bending forces for each zone were then
compared with the load-moment interaction diagrams of
column and shear wall. For shear capacity of shear wall,
graphs have been plotted for comparison between each zone.
Table I
Building Information and Input data
No. of stories
G+30
Floor Height
3.2 m (10’-6”), 2.7 m (9’) for parking & 3.35 m (11’) for amenity
Slab thickness
175 mm (7 in)
300 mm × 750 mm (12 in × 30 in) at amenity &
Beam size
200 mm × 900 mm (8 in × 36 in) other levels
Exterior Column size
525 mm × 525 mm (21 in × 21 in)
Interior Column size
750 mm × 1350 mm (30 in × 54 in)
Shear wall thickness
375 mm (15 in)
Slab & Beam - concrete strength (fc’)
27 MPa (4 ksi)
Column & Shear wall - concrete strength 40 MPa (6 ksi)
(fc’)
Steel Strength (fy)
415 MPa (60 ksi)
Seismic Zone
1, 2a, 2b, 3, 4
Soil Type
SC (Very dense soil and soft rock)
Over strength factor (R)
5.5 (Zone 1, 2a, 2b) & 8.5 (Zone 3, 4)
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Fig. 1. Building plan and elevation
4. RESULTS AND D ISCUSSION
The analysis of the most critical beam, column and shear wall
in terms of developed axial force, shear force and bending
moment is presented in this section. The design of each of
member for the forces of Zone 2b has also been given in the
following subsections.
4.1 Critically loaded Beam
The most critically loaded beam was found at amenity level
(9th floor) having span of 13.6 m and rectangular cross section
of 300 mm × 750 mm using ETABS. The maximum shear
force and bending moment on this beam for each seismic zone
are depicted in Table II.
Table II
Beam analysis results for each zone
Seismic Zone
Max. Shear
Force (kN)
Max. Negative
Moment (kN-m)
Max. Positive
Moment (kN-m)
Zone 1
363
695
464
Zone 2a
386
832
464
Zone 2b
392
912
464
Zone 3
406
1032
464
Zone 4
430
1136
464
It can be observed from Table II that with the increase in
seismic risk the shear force and negative moment is
increasing. It is happening since for both of these forces
seismic load combination is governing. On the other hand
positive moment in each case is constant revealing that for this
case gravity load combination is the governing one.
The beam reinforcement was designed using ETABS for
forces of zone 2b. The required area of steel for shear force
and bending moment with their respective capacities
(resistance) are given in Table III.
Table III
Beam design for Zone 2b and capacities
Shear R/F [Av/s]
1.2
Shear Capacity (kN)
(mm2/mm)
Negative R/F (mm2)
4375
Positive R/F (mm2)
1975
396
Negative Moment Capacity
(kN-m)
Positive Moment Capacity
(kN-m)
954
485
The difference of shear and moment capacities of the designed
beam with the respective forces obtained for each seismic
zone model in terms of percentages is shown in Figure 2. It is
evident from comparison that this beam can sustain only the
positive moment in all the zones adequately. It is because
maximum positive moment is governing due to gravity
loading which is not changing with change in the seismic risk.
While shear force has increased about 9% in zone 4 as
compared to the shear capacity of the beam with the increase
in the seismic zone. Hence, the beam has a lesser capacity to
withstand seismic shearing force of zone 3 and 4.On the other
hand the same beam will also not be able to resist the negative
moment in both zone 3 and 4 and would fail in higher seismic
event. The increase in negative applied moment in zone 4 is
about 19% in comparison to the designed negative moment
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resistance of the beam. Consequently, the beam designed for a
moderate seismic event can perform well in low and moderate
earthquake categories but not adequate in high seismic risk
events (Zahra and Zehra 2012).
For zone 1, 2a and 2b, the building was designed as
intermediate sway as per ACI Code. While for zone 3, 4 (as
specified in Code) special sway option was adopted and
design was carried out accordingly using ETABS. From Table
4, it is evident that as the seismic risk increasing the forces are
also increasing progressively. The applied forces are
maximum for the high seismic zone 4.
4.2 Critically loaded Column
The most critical column was at ground floor level from
ETABS of cross section 750 mm × 1350 mm. The summary
of the applied forces in each seismic zone is shown in Table
IV.
The column reinforcement was designed for the moderate
zone 2b using ETABS. The required longitudinal
reinforcement ratio was determined to be 2.05% [42 bars of
#25 (soft metric size)]. To compare the resistance of column
with the applied forces in each zone, load-moment interaction
diagram was developed for the major and minor axis of the
designed column (Figure 3, 4). It can be observed form load
moment interaction diagram that column has adequate
resistance against seismic forces of the zones 1, 2a and 2b
about its major and minor both the axes. But it is deficient to
take seismic forces developed in zone 3 and 4. As a result, the
column designed for moderate resistance is deficient and
inadequate to undergo forces of high seismic event. Similar
results were obtained by Zahra and Zehra (2012) for
comparison between zone 2b and 3.
Table IV
Column analysis results for each zone
Seismic Zone
Axial Force
(kN)
Major axis
Moment (kN-m)
Minor axis
Moment (kN-m)
Zone 1
20338
346
775
Zone 2a
21404
816
821
Zone 2b
22127
1156
843
Zone 3
23190
1630
879
Zone 4
23995
1998
944
2
Fig. 2. Percentage difference of Beam capacity with different seismic zone force
s
Fig. 3. Load-Moment Interaction of Column (Major Axis)
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Fig. 4. Load-Moment Interaction of Column (Minor Axis)
General trend is that with increase in seismic risk activity the
forces have also increased which should be likely the case.
The shear wall was designed using ACI-318-02 specifications.
4.3 Critically loaded Shear Wall
The 375 mm thick by 7200 mm long shear wall was found to
have maximum loading in each seismic zone. The forces on
the wall are summarized in Table V.
Table V
Shear wall analysis results for each zone
Seismic
Zone
Axial Force
(kN)
Major Axis
Moment
(kN-m)
Minor Axis
Moment
(kN-m)
Shear
Force
(kN)
Zone 1
19171
1994
86
4137
Zone 2a
22467
4930
167
6222
Zone 2b
28736
63473
73
7615
Zone 3
29102
83151
100
10221
Zone 4
29134
100296
166
12319
The required longitudinal steel reinforcement in zone 2b was
calculated as 1% of the gross wall area [56 bars of #25 (soft
metric size) @ 275 mm on each wall face]. Comparison was
made for each zone forces for the load and moment capacities
of the shear wall along major and minor axes using loadmoment interaction diagrams (Figure 5, 6). For major axis,
significant increase in the applied moment can be seen from
Table 5 with increasing seismic zone and the same is also
revealing from Figure 5. The shear wall is inadequate to
provide resistance in zones 3 and 4.
For minor axis, on the other hand, due to insignificant
magnitude of applied moment in different seismic zones, shear
wall is performing quite safely in each zone (Figure 6).
Fig. 5. Load-Moment Interaction of Shear wall (Major Axis)
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Fig. 6. Load-Moment Interaction of Shear wall (Minor Axis)
Fig. 7. Comparison of shear wall transverse (shear) resistance
The transverse reinforcement for shear wall was also
determined to resist the shear forces in zone 2b and was found
to be #13 (soft metric size) @ 175 mm on each wall face. The
shear resistance of the wall for the provided transverse
reinforcement was found as 8400 kN. The comparison of
shear capacity of the shear wall with the shear forces in each
seismic zone is shown in Figure 7. The increase in shear
forces in zone 3 and zone 4 are 34% and 62% respectively in
contrast to that of in zone 2b which is very significant increase
and thus shear wall will be deficient to take shearing force
developed in high earthquake event.
5. CONCLUSIONS
In this study, a high rise building was analysed in different
seismic zone risk categories and was designed for moderate
zone 2b (Karachi region). The capacity of the most critically
loaded beam, column and shear wall was compared in all the
seismic categories and following conclusions are drawn:
1. The beam designed for moderate seismic risk (zone 2b) is
inadequate to resist high seismic zone forces (zone 3 and
4).
2.
The column in its major and minor axis both,
proportioned to resist zone 2b forces is found
deficient in high seismic risk activity.
3.
The shear wall designed in zone 2b, is also found
unable to resist the shearing force, axial load and
bending moment caused by higher seismic event.
4.
Thus, a building which is designed to withstand
moderate seismic forces can exhibit failure in case of
high earthquake event.
[1]
[2]
[3]
[4]
REFERENCES
Kumar K and Papa Rao G (2013). Comparison of percentage steel
and concrete quantities of a R.C building in different seismic zone,
International Journal of Research in Engineering and Technology,
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Heiza KM and Tayel MA (2012). Comparative study of the effects
of wind and earth quake loads on high rise buildings, Concrete
Research Letters, 3(1), pp. 386-405.
Michael R L and Majid B (2001). Seismic design of building
structures. Professional publications, Inc. Belmont, CA, USA,
Eighth edition.
Taranath SB (2010). Reinforced concrete design of tall buildings.
CRC press Taylor & Francis Group.
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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 15 No: 02
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[5]
Haseeb M, Xinhailu, Bibi A, Khan JZ, Ahmad I and Malik R
(2011). Construction of Earthquake Resistant Buildings and
Infrastructure Implementing Seismic Design and Building Code in
Northern Pakistan 2005 Earthquake Affected Area, International
Journal of Business and Social Science. 2(4), pp. 168-177.
[6] Sanghani KB and Patel PG (2011). Behavior of Building
Component in Various Zones, International Journal of Advances
in Engineering Sciences, 1(1), pp. 69-74.
[7] Ram Kumar NV, Satyanarayana SV and Usha Kranti J (2013).
Seismic Behavior of Multi-Storied Buildings, International
Journal of Engineering Research and Applications (IJERA), 3(4),
pp.2076-2079.
[8] Sapate OV (2012). Inter-relationship between moment values of
columns in a building with different architectural complexities and
different seismic zones, International Journal of Engineering
Research and Development, 5(2), pp. 55-59.
[9] ACI (American Concrete Institute) 2002 - Building Code
Requirements for Reinforced Concrete (ACI 318-02) – Detroit,
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[10] UBC (Uniform Building Code) (UBC-1997); Seismic Design (Div.
I & IV)
[11] IBC (International Building Code) (IBC-2003); Seismic Design
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[12] Zahra T and Zehra Y (2012). Effect of rising seismic risk on the
design of high rise buildings in Karachi, International Journal of
Civil & Environmental Engineering IJCEE-IJENS, 12(6), pp. 4245.
PRACTICAL RELEVANCE AND POTENTIAL APPLICATIONS
High rise buildings are urban contrition and their design
should be safe against seismic risk. This research was
commenced to study the impact of changing seismic zone
intensity on the load carrying members (beam, column, shear
wall) of a high rise building. It was found that a building
which is designed for moderate seismic risk cannot withstand
high seismic events. This work could be beneficial in seismic
risk vulnerability assessment of high rise buildings.
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