Vol.10, No.1
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
Earthq Eng & Eng Vib (2011) 10: 99-113
March, 2011
DOI: 10.1007/s11803-011-0050-8
A comparison of IBC with 1997 UBC for modal response
spectrum analysis in standard-occupancy buildings
Tariq M. Nahhas†
Umm Al-Qura University, Makkah, Saudi Arabia
Abstract: This paper presents a comparison of the seismic forces generated from a
Modal Response Spectrum Analysis
(MRSA) by applying the provisions of two building codes, the 1997 Uniform Building Code (UBC) and the 2000-2009
International Building Code (IBC), to the most common ordinary residential buildings of standard occupancy. Considering
IBC as the state of the art benchmark code, the primary concern is the safety of buildings designed using the UBC as
compared to those designed using the IBC. A sample of four buildings with different layouts and heights was used for this
comparison. Each of these buildings was assumed to be located at four different geographical sample locations arbitrarily
selected to represent various earthquake zones on a seismic map of the USA, and was subjected to code-compliant response
spectrum analyses for all sample locations and for five different soil types at each location. Response spectrum analysis was
performed using the ETABS software package. For all the cases investigated, the UBC was found to be significantly more
conservative than the IBC. The UBC design response spectra have higher spectral accelerations, and as a result, the response
spectrum analysis provided a much higher base shear and moment in the structural members as compared to the IBC. The
conclusion is that ordinary office and residential buildings designed using UBC 1997 are considered to be overdesigned, and
therefore they are quite safe even according to the IBC provisions.
Keywords:
response spectrum analysis; seismic forces; multi-story buildings; seismic design; building codes; IBC;
UBC
1
Introduction1
The 1997 Uniform Building Code (International
Conference on Building Codes, 1997) was the first
building code that included seismic design provisions
that were significantly based on seismic data collected
in the early 1990’s. This code is usually referred to
as “1997 UBC” and is called “UBC” in this paper. In
this code, the design response spectrum to be used for
a Modal Response Spectrum Analysis (MRSA) was
based on factors such as soil profile and seismic zone
based on fault proximity. UBC was adopted in the
USA and became the basis of the seismic provisions
of the national codes of several developing countries
for the seismic design of buildings. The International
Building Code, first released in 2000 (International
Code Council Inc., 2000), was developed as a collective
effort of various independent code bodies in the USA.
The International Building Code was meant to replace
UBC and all other independent and legacy codes within
the USA and to provide guidelines for codes in other
Correspondence to: Tariq M. Nahhas, Civil Engineering
Department, College of Engineering, Umm Al-Qura
University, PO Box 16222, Makkah, Saudi Arabia
Tel: 00966505525354
E-mail: tmnahhas@uqu.edu.sa
†
Associate Professor
Received May 19, 2010; Accepted November 8, 2010
countries throughout the world. In various parts of the
world including the USA, the IBC has replaced the UBC
and is considered to be a benchmark code. The first
revision to this code was released in 2003 (International
Code Council Inc., 2003), the next in 2006 (International
Code Council Inc., 2006) and the latest revised and
enhanced version was released in 2009 (International
Code Council Inc., 2009). This code referred to as “IBC”
in this paper is scheduled to remain in a revision cycle
with a new release every three years.
Seismic design provisions of the IBC that are
significantly different from the UBC and all the
previous building codes are based in large part on the
recommended provisions for seismic regulations for
new buildings and other structures by the 1997 NEHRP
“National Earthquake Hazards Reduction Program”
(Building Seismic Safety Council, 2004). The ground
motion maps of the 1997 NEHRP provisions adopted by
IBC are based on the 1996 US Geological Service ground
motion maps, which are quite different from 1991
NEHRP provisions used in the previous building codes.
One of the most significant changes in the new maps is
the use of the Maximum Considered Earthquake (MCE)
ground motions to develop design response spectrum.
The MCE ground motions are typically defined as the
maximum level of earthquake ground shaking that is
considered reasonable for typical structures to resist.
The basic approach is to provide an approximately
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EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
uniform margin against collapse in all regions of the
United States. There are several publications and web
pages describing the differences between UBC and IBC.
The reader may refer to Kaplan AEC Engineering Inc
(2000) and Ghosh and Khuntia (1999) and other similar
publications. The UBC provides a complete procedure
and set of formulae for response spectrum analysis
contained in its provisions for seismic design, whereas
IBC (2006 IBC and 2009 IBC) refers to ASCE7-05
(American Society of Civil Engineers, 2006). IBC, in
accordance with ASCE 7-05, requires that in the ranges
of a very long period, the design response spectrum be
modified based on work developed by Newmark and
Hall (1982). For this reason, separate maps to find the
“Long Period Transition Period” called TL have been
introduced. However, this branch of the design response
spectrum will only affect the response spectrum analysis
for buildings with very long fundamental time periods.
Other than this, the design response spectrum for the
2003 IBC is identical to the 2006 IBC and 2009 IBCs.
Despite the fact that IBC is the prevailing code in
the USA, the UBC is still in wide use in developing
countries. Also, a large number of existing buildings
were designed to satisfy the provisions of UBC. Several
countries that have adopted UBC provisions in their
national building codes require comparative studies of
UBC with IBC. Such comparative studies are necessary
to satisfy the engineers and decision makers to switch
over to using the IBC guidelines. Comparative studies
have been published earlier (Ghosh and Khuntia, 1999;
Adem and Ramazan, 2006; Pong et al., 2006 a, b; Pong
et al., 2007). Among them, a comparative study (Adem
and Ramazan, 2006) between the Turkish Earthquake
Code and the Eurocode 8 with UBC based on MRSA
using the finite element analysis method for structural
analysis was carried out. This is the only publication
that compared two codes using MRSA. In Adem’s
comparative study, the IBC response spectra were
mentioned as being different from UBC and others,
but no MRSA data comparing IBC and UBC were
presented. The comparative study by Ghosh and Khuntia
(1999) is the first publication to compare IBC and UBC.
However, the study was based on the Equivalent Lateral
Force Procedure (ELFP), and not on MRSA. Another
comparative study (Pong et al., 2006 a, b) deals with
the issue of comparing IBC and UBC. It is the most
important paper on this issue and has important findings.
It reports mixed findings for two selected sites in San
Francisco and Sacramento. The results presented in
Pong’s study were also obtained using ELFP instead of
MRSA. This comparative study concluded that UBC is
more conservative in some cases, however, the results
presented are not conclusive enough for structural
designers and officials in various countries who are
evaluating the two codes to determine whether to switch
over to IBC or not and whether existing buildings
designed using UBC are safe or not. Another comparison
by Pong et al. (2007) did not specifically deal with UBC
Vol.10
and IBC, and instead compared IBC with Mexico’s code
and thus is not as relevant to the subject of this paper.
The present paper investigates the forces generated in
the structure through MRSA according to the provisions
of the two codes using finite element analysis to obtain
the internal forces rather than ELFP. Thus, this work is
different from the previous publications, and fills a gap
in the published research on this issue. By investigating
MRSA, the paper addresses the trend in structural design
of using FEM software packages. In IBC, MRSA is
allowed for all cases whereas ELFP is not acceptable in
some cases. This paper focuses on ordinary residential
buildings of Occupancy Category 2, because they are
common throughout the world in much larger numbers
than buildings in other categories such as hospitals and
school buildings, and almost 100% of the population in
developing countries reside in such buildings.
In order to carry out a comparative study, a sample
of four buildings with different layouts and heights is
considered. These buildings are assumed to be located
at each of four arbitrary sample locations shown on a
map of the USA. The sample locations were chosen to
be from the USA spectral acceleration as given in the
IBC because maps of other countries are still being
developed and data are not readily available. The selected
sample locations represent different seismic activities.
Since the sample locations were selected on the map,
they do not relate to any particular buildings at these
locations. For these sample locations, the parameters
SS and S1 are taken from the spectral acceleration map
of the USA as given in the 2009 IBC. Design response
spectra are generated for the four sample points for five
site classes giving twenty cases. Thus, twenty response
spectra are generated for UBC and another twenty
for IBC. For each case, response spectrum analysis is
performed for all four buildings representing a total of
160 analyses. A comparison of the magnitudes of base
shear and maximum moment at the base of the columns
used for column and foundation design for each case
is presented. The effect of scaling required by UBC
and IBC when the internal forces are based on MRSA
and the effect of considering the buildings as hospital
buildings is also presented. The results clearly indicate
which code provides higher values of internal forces.
Since the buildings considered in this paper are not tall
with very long fundamental time periods, the discussion
and results presented herein apply equally to 2006 and
2009 versions of the IBC. All provisions of the IBC used
in this research are the same for both versions of IBC.
2 Modeling and analysis
The research presented in this paper required a 3D
finite element modeling of multi-story building structures.
The software package ETABS (Extended Threedimensional Analysis of Building Systems) (Computers
& Structures Inc., 2008) was used for this purpose.
No.1
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
The 3D building structures used in this research
were modeled as special moment resisting frame
systems, which are a requirement of building codes in
higher seismic zones and is permitted for all zones. Slabs
were modeled using shell elements to represent the real
slab behavior, providing stiffness in all directions and
transfer mass of slab to beams. A rigid diaphragm was
assumed at all floor levels.
The modal combination method used for all models
was the CQC (complete quadratic combination), which
was preferred over SRSS (square root of sum of squares)
because the structural models of the sample buildings
are all three-dimensional with the possibility of closely
spaced modes. It is well known that for structures
with closely spaced modes, CQC results are generally
much more accurate (Gupta, 1992). Actually, for all
the buildings used in this research, the internal forces
obtained using CQC were verified to be about the same
as SRSS. This is because the closely spaced modes for
these structures have very small or negligible modal
mass participation. In any case, the use of a modal
combination method other than CQC will not affect the
comparative results because both IBC and UBC design
spectra are applied to a given structure using the same
modal combination and the results only indicate the
effect of the difference between the design response
spectra.
3 Sample locations
Four sample locations were selected rather
arbitrarily. These locations are located on the IBC map
of “maximum considered earthquake accelerations” as
shown in Fig. 1, while the same geographical locations
101
are located on the UBC Seismic Zones map as shown in
Fig. 2. The exact position of these locations in terms of
latitude and longitude are given in Table 1, which also
shows the UBC zones and IBC spectral accelerations
for short and long periods. Sample Locations 1, 2, 3
and 4 are randomly selected points in UBC Zone 2A,
Zone 2B, Zone 3 and Zone 4, respectively. Note that
these locations cover four seismic zones but are arbitrary
and do not represent any specific buildings in these
locations.
4 Design spectra cases
For each sample location described in the previous
section, all five IBC/UBC site classes representing
different soil types are considered. This amounts to 20
different cases for each building. A summary of these
twenty cases is shown in Table 2. The first column in
this table has the location ID and refers to the sample
locations described in the previous section. For all
twenty cases, the table shows the UBC zone and the
seismic coefficients Ca and Cv as well as the IBC
parameters SS, S1 and TL.
For each case shown in Table 2, the design response
spectrum is generated for the UBC as well as the IBC.
Each of these design response spectra is applied to
the four different multi-story building structures as
described in the next section. Figures 3(a) and 3(b) show
a comparison between IBC and UBC design response
spectra for all 20 cases, where the darker line indicates
UBC and the lighter line indicates IBC. It presents a
good comparison of IBC vs. UBC design response
spectra. For the selected sample points, it is obvious
from the figures that the upper curve is due to the UBC
Fig. 1 Sample locations shown on IBC 2009 spectral acceleration map
102
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
Vol.10
Fig. 2 Sample locations shown on UBC 1997 seismic zoning map
Table 1 Sample geographical locations data
Geographical
location
ID
1
2
3
4
Location
UBC 1997
Latitude
Longitude
34º
37º
43º
34º
98º
114º
123º
119º
30’
30’
00’
00’
00’
30’
00’
00’
IBC 2006
Zone
Ss %
2A
2B
3
4
40
70
100
200
Table 2 Design spectra cases
LOC
ID
1
2
3
4
Site class
A
B
C
D
E
A
B
C
D
E
A
B
C
D
E
A
B
C
D
E
Zone
2A
2A
2A
2A
2A
2B
2B
2B
2B
2B
3
3
3
3
3
4
4
4
4
4
UBC 1997
Ca
0.12
0.15
0.18
0.22
0.30
0.16
0.20
0.24
0.28
0.34
0.24
0.30
0.33
0.36
0.36
0.48
0.60
0.60
0.66
0.54
Cv
0.12
0.15
0.25
0.32
0.50
0.16
0.20
0.32
0.40
0.64
0.24
0.30
0.45
0.54
0.84
0.64
0.80
1.12
1.28
1.92
Ss
0.4
0.4
0.4
0.4
0.4
0.7
0.7
0.7
0.7
0.7
1
1
1
1
1
2
2
2
2
2
IBC 2006
S1
0.095
0.095
0.095
0.095
0.095
0.15
0.15
0.15
0.15
0.15
0.4
0.4
0.4
0.4
0.4
1
1
1
1
1
TL
12
12
12
12
12
6
6
6
6
6
16
16
16
16
16
8
8
8
8
8
S1 %
9.5
15
40
100
No.1
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
0.35
0.30
0.25
Case 1:
Location ID: 1
Site class: A
0.20
0.15
0.10
0.05
0
0
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
00
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
10
2
3
4
5
0.4
Case 3:
Location ID: 1
Site class: C
6
7
8
9
10
Case 4:
Location ID: 1
Site class: D
0.3
0.2
0.1
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
0
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
0
0
0.5
0.6
0.4
2
3
4
5
6
1
2
3
4
5
6
0.5
Case 7:
Location ID: 2
Site class: B
0.3
1
8
9
10
7
8
9
10
Case 8:
Location ID: 2
Site class: C
0.4
0.3
0.2
7
Case 6:
Location ID: 2
Site class: A
0.7
0.2
0.1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
00
1
0.5
0.6
0
0
Case 2:
Location ID: 1
Site class: B
0.6
Case 5:
Location ID: 1
Site class: E
0
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
00
103
0.1
1
2
3
4
5
6
7
8
9
10
00
10
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
Case 9:
Location ID: 2
Site class: D
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
10
Case 10:
Location ID: 2
Site class: E
1
2
Fig. 3(a) Response spectra cases (Cases 1 to 10)
3
4
5
6
7
8
9
10
104
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
10
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
00
10
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
00
0.7
0.6
0.5
Case 11:
Location ID: 3
Site class: A
0.4
0.3
0.2
0.1
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
2
3
4
5
6
7
8
9
Case 13:
Location ID: 3
Site class: C
0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
00
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1
1
2
3
4
5
6
7
8
9
Case 12:
Location ID: 3
Site class: B
1
2
3
4
5
6
7
8
9
10
Case 14:
Location ID: 3
Site class: D
1
2
3
4
5
6
7
8
9
10
1.4
1.2
1.0
Case 15:
Location ID: 3
Site class: E
Case 16:
Location ID: 4
Site class: A
0.8
0.6
0.4
0.2
1
2
3
4
5
6
7
8
9
10
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
0
10
10
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
Case 17:
Location ID: 4
Site class: B
1
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
Case 19:
Location ID: 4
Site class: D
0
Vol.10
1
2
3
4
5
6
7
8
9
10
Case 18:
Location ID: 4
Site class: C
1
2
3
4
5
6
7
8
9
10
Case 20:
Location ID: 4
Site class: E
1
2
Fig. 3(b) Response spectra cases (Cases 11 to 20)
3
4
5
6
7
8
9
10
No.1
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
and the lower is due to the IBC, which clearly indicates
that the design response spectra represent more severe
seismic loads for the UBC.
The building structures for each of the four buildings
are described as follows. The basic structural data
concerning all four buildings including the section sizes
are given in Table 4. Note that the effect of the detailed
design of an individual building is not an issue in this
comparative study. Even if the building structures
had been designed with dimensions and cross-sections
entirely different from those assumed in this study, it
will not affect the comparative results presented in this
paper. This is because for a given building, two different
design response spectra are applied and then the results
are compared. The results show only the difference due
to the design response spectra. The actual magnitude
of internal forces is not the same for two buildings
designed differently but the comparative effect does
not alter the results. This point has been proven through
testing one of the buildings (Building Structure A) as
described below.
5 Sample buildings
A sample of four buildings, ranging from four to
six floors, is used in this study. These buildings are
assumed to be of Occupancy Category II as defined
in IBC2009, and Occupancy Category 4 in UBC97,
and situated at the four sample locations described in
Section 3. Hospital, schools and other critical buildings
are excluded. Instead, it includes ordinary residential
and office buildings, which according to this occupancy
category, have an importance factor equal to 1 in both
codes. This occupancy category represents the seismic
design based on structural properties and the structural
response to seismic design response spectra. In contrast,
the other occupancy categories represent seismic design
requirements with extra precautions for public safety
and are not based on structural considerations alone.
All four buildings are assumed to have a special
moment resisting frame system. For such frames, the
IBC response modification factor (R) is 8.0 and the
UBC response modification factor is 8.5. Concrete
compressive strength of 42 MPa and modulus of
elasticity equal to 30459.4813 MPa was assumed with
reinforcing steel having a yield strength of 420 MPa.
The fundamental time periods of the four buildings, as
shown in Table 3, were obtained by the modal analysis
using ETABS software and were also calculated using
ELFP.
Building Structure A
Building Structure A is shown in Fig. 4. This
building has greater academic interest and it was used
to perform most of the experimentation using ETABS
to gain insight on the effect of various parameters and
to verify the results using hand calculations. It is a 4story reinforced concrete building with all columns and
beams of the same cross-section of 250 mm × 400 mm.
The earthquake excitation direction is along the Y-axis.
Its fundamental mode of vibration was found to have the
time period of 0.779 s. This time period is higher than
the ELFP fundamental time period shown in Table 3
(UBC: 0.585, IBC: 0.565). Note that IBC does not allow
Table 3 Fundamental time periods
Building
A
UBC (ELFP)
0.585
IBC (ELFP)
0.565
FEM (ETABS)
B
0.585
0.565
0.761
C
0.557
0.533
0.558
D
0.639
0.628
0.954
0.779
Table 4 Data for building sample building structures
Data item
No. of floors
Story height (m)
Building A
4
Building B
4
Building C
5
Building D
6
4
4
3
3
Beams section (mm)
250 × 400
B1: 200 × 500;
B2: 200 × 400
B1: 200 × 750;
B2: 200 × 500
Column section (mm)
250 × 450
C1: 200 × 500;
C2: 200 × 400
200 × 500
175
B1: 200 × 800;
B2: 200 × 600
B3: 200 × 400;
B4: 120 × 500
C1: 200 × 600;
C2: 200 × 500
C3: 200 × 400
175
175
175
Concrete
Concrete
Concrete
Concrete
Y- direction
X- direction
X- direction
X- direction
Slab thickness (mm)
Material
Excitation direction
105
106
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
6000
Beam 250×450
6000
Beam 250×450
2
Beam 250×450
Beam 250×450
Beam 250×450
4000
Beam 250×450
1
C
B
A
Vol.10
Beam 250×450
Y Z X
Unit: mm
(a) Plan view
(b) Three-dimensional model
Fig. 4 Building A
the beams and columns. The cross section set (columns
= 450 mm × 450 mm and beams = 200 mm × 600 mm)
was used. For this test, geographical location 3 was
assumed. The results are given in Fig. 5 showing the
Building A - Sample point 1 (Base shears in Y-direction)
1.4
Base shear (102kN)
the use of ELFP for calculating the base shear and other
internal forces for buildings in Seismic Design Category
D and above if the modal fundamental time period of
a structure calculated by FEM is larger than that by
ELFP. In such cases, methods like the modal response
spectrum analysis, linear response history, nonlinear
static procedure or nonlinear response history analysis
must be used (Ghosh et al., 2009). In such cases, the
most commonly used method is the response spectrum
analysis method (ASCE 7-05 Section 162), which is
also allowed by IBC and has been adopted for all cases
in this paper. Using this method, the time periods and
modal participating mass ratios are calculated using
FEM as shown in Table 5 for each of the 10 modes.
Since more than 98% mass participation occurs in the
direction of earthquake excitation, the number of modes
considered is sufficient.
Since this structure was used to verify several aspects
of the problem, the effect of variation in structural design
on the comparative results was also investigated. For this
purpose, a test was performed with Building A designed
in two different ways. One design was as described
above and the other design had different dimensions of
IBC
UBC
1.2
1.0
0.8
0.6
0.4
0.2
0
A
B
C
D
E
A
B
C
Period(s)
1
2
3
4
5
6
7
8
9
10
0.778839
0.718533
0.531021
0.25301
0.228186
0.168369
0.148673
0.129074
0.1105
0.09514
E
Fig. 5 Comparison of base shear for two different designs of
Building A
Table 5 Modal periods & participating mass for Building Structure A
Mode
D
Sample Point 3 (Cross Sections Set 1) Sample Point 3 (Cross Sections Set 2)
Individual participation (%)
Along X
Along Y
Along Z
85.5258
0
0
0
84.0825
0
0
0
0
10.1768
0
0
0
11.1355
0
0
0
0
3.3889
0
0
0
3.7229
0
0.9085
0
0
0
0
0
Cumulative participation (%)
Along X
Along Y
Along Z
85.5258
0
0
85.5258
84.0825
0
85.5258
84.0825
0
95.7026
84.0825
0
95.7026
95.218
0
95.7026
95.218
0
99.0915
95.218
0
99.0915
98.9409
0
100
98.9409
0
100
98.9409
0
No.1
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
base shear comparisons. In Fig. 5, Cross-section Set 1
refers to the original design of Building A and Crosssection Set 2 refers to the second design. Similar results
are obtained for the maximum moment comparison.
This clearly proves the point that though the structural
response itself differs and depends on the structural
design, the structural design will not have any significant
effect on the comparative results.
Building Structure B
The floor plan of Building Structure B is shown
in Fig. 6. It is a four-story concrete building. Table 6
shows the time periods and modal participating mass
ratios for each of the first ten modes. The fundamental
time period of this structure is 0.761 s. Again for this
building, the fundamental time period is higher than the
ELFP time period given in Table 3 (UBC: 0.585, IBC:
0.565) and the same comments made in the discussion
of Building Structure A concerning the analysis method
are applicable here. The first ten modes considered for
the modal analysis give 99% mass participation in the
direction of excitation of the earthquake.
B4
C2
B4
B4
(a) Plan
The floor plan of Building Structure D is shown in
Fig. 8. It is a six story concrete building. Table 8 shows
the time periods and modal participating mass ratios for
each of the first ten modes. The fundamental time period
of this structure is 0.945 s and more than 99% mass
participation occurs in the direction of excitation of the
earthquake. As with the other buildings, the fundamental
time period of 0.954 s calculated by modal analysis using
B2
B3
C1
B2
B3
B3
C2
B3
Building Structure D
B3
C1
B2
B2
C2
The floor plan of Building Structure C is shown
in Fig. 7. It is a five story concrete building. Table 7
shows the time periods and modal participating mass
ratios for each of the first ten modes. The fundamental
time period of this structure is 0.558 s. For this building,
the fundamental time period is also higher (though very
slightly) than the ELFP time period given in Table 3
(UBC: 0.557, IBC: 0.533). The ten modes considered
make the mass participation greater than 98% in the
direction of excitation of the earthquake.
C1
C1
B3
B2
C2
B3
C2
B4
B3
C1
B3
B3
C3
B4
B3
B4
4000
C2
B3
C2
C3
B4
B3
C2
Building Structure C
F
4800
B4
B3
B4
C1
B3
C1
E
2500
B3
C2
C2
B4
B3
B1
4000
B3 B3
B1
1500
D
C2
C3
B3
B1
1500
4
B4
C3
B3
4000
5
C2
B2
B4
B4
C1
4000
B4
B3
C2
B4
2
C
5500
B4
B3
C1
4000
3
B
5500
B4
B2
1
B3
A
C2
C2
B3
C1
Y Z X
B4
Unit: mm
(b) Three-dimensional model
Fig. 6 Building B
Table 6 Modal periods & participating mass for Building Structure B
Mode
Period (s)
1
2
3
4
5
6
7
8
9
10
0.776387
0.761277
0.652396
0.251660
0.247979
0.211274
0.146275
0.144426
0.122442
0.106966
107
Individual participation (%)
Along X
Along Y
Along Z
85.1174
0.0498
0
0.0670
74.8168
0
0.0031
10.3425
0
10.2617
0.0115
0
0.0149
9.0406
0
0.0004
1.2828
0
3.5142
0.0049
0
0.0064
3.0557
0
0.0002
0.4072
0
1.0134
0.0011
0
Cumulative participation (%)
Along X
Along Y
Along Z
85.1174
0.0498
0
85.1843
74.8665
0
85.1874
85.2091
0
95.4491
85.2206
0
95.4640
94.2612
0
95.4644
95.5440
0
98.9786
95.5490
0
98.9849
98.6046
0
98.9851
99.0118
0
99.9985
99.0129
0
108
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
F
C
500
C1
B1
C1
B1
C1
B1
C2 B1 C2
B1
B1
C2
B1 C2
B1
C1
5
B1
B1
C1
B1
C1
C2 B1
C2
B1
C1
C2
B1
C2
C2
B2
C2
2500
C1
B2
B1
B1
B1
B2
4
C1
B2
B1
1500
B1
B1
3000
3
C1
3000
B1
C1
2000
B2
B1
2
B1
B2
2500
H
G
500
4000
B1
2000
B1
3000
1
E
D
C1
B1
B1
B
B1
A
Vol.10
C1
B1
Y Z
C1
X
Unit: mm
(a) Plan
(b) Three-dimensional model
Fig. 7 Building C
Table 7
Cumulative participation (%)
Period (s)
Along X
Along Y
Along Z
Along X
Along Y
Along Z
1
0.555816
0
85.4394
0
0
85.4394
0
2
0.488576
83.5009
0
0
83.5009
85.4394
0
3
0.429375
0.96550
0
0
84.4664
85.4394
0
4
0.185058
0
10.0069
0
84.4664
95.4463
0
5
0.159820
10.0356
0
0
94.5019
95.4463
0
6
0.141734
0.09550
0
0
94.5975
95.4463
0
7
0.111193
0
3.1075
0
94.5975
98.5537
0
8
0.093398
3.49090
0
0
98.0883
98.5537
0
9
0.084234
0.02350
0
0
98.1118
98.5537
0
10
0.082548
0
1.1627
0
98.1118
99.7164
0
B2
D
4200
4200
B2
1800
4200
B2
B2
B2
B1
B1
B1
B2
F
E
G
B1
B2
C
B2
1800
B2
B
4200
4200
2
Individual participation (%)
Mode
A
1
Modal participating mass for Building Structure C
B2
B2
2600
3
6
B1
B2
B2
B2
B2
B2
B2
B1
B2
B1
(a) Plan
B2
B2
B2
B2
B2
B1
B1
B1
5
4200
B2
B1
B2
5200
B2
B1
B1
4
B1
4900
B1
B1
B2
Y Z
X
B1
Unit: mm
(b) Three-dimensional model
Fig. 8 Building D
No.1
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
109
Table 8 Modal participating mass for Building Structure D
Mode
Period (s)
1
0.944924
2
3
Individual participation (%)
Cumulative participation (%)
Along X
Along Y
Along Z
Along X
Along Y
Along Z
84.2847
0
0
84.2847
0
0
0.710861
0
84.3473
0
84.2847
84.3473
0
0.690169
0.1712
0
0
84.4559
84.3473
0
4
0.312748
9.7628
0
0
94.2187
84.3473
0
5
0.234799
0
9.8149
0
94.2187
94.1622
0
6
0.228175
0.0147
0
0
94.2334
94.1622
0
7
0.185529
3.3580
0
0
97.5914
94.1622
0
8
0.138858
0
3.4090
0
97.5914
97.5712
0
9
0.135439
0.0872
0
0
97.6786
97.5712
0
10
0.132930
1.4573
0
0
99.136
97.5712
0
6 Scaling of base shear
IBC and UBC require that the base shear obtained
by code-compliant Modal Response Spectrum Analysis
(MRSA) be scaled up by a factor of equivalent static
base shear calculated using ELFP. The IBC refers to
ASCE7-05 for response spectrum analysis of building
systems, where clause 12.9.4 states that the MRSA shall
not be less than 85% of the ELFP base shear. Similarly,
according to UBC clause 1631.5.4, the MRSA base
shear shall not be less than 100% of the ELFP base shear
for irregular buildings and not less than 90% for regular
buildings. Both codes define their own methods for
determining ELFP base shears.
To understand the effect of scaling, the base shear is
computed in three ways: ELFP, MRSA without scaling
and MRSA with scaling. The results are shown in Fig.
9 for Building A, which is assumed to be located at
Sample Location 1 for all the five site classes. As seen
in the figures, UBC is over conservative. Similar results
have been presented (for ordinary residential buildings)
by Ghosh and Khuntia (1999) and Pong et al. (2006
a, b). Qualitatively, the results concerning the safety
of the design are the same, i.e., UBC generates overconservative designs and therefore the structures are
safer. However, for certain conditions, scaling is required
by both codes if MRSA is used instead of ELFP. It is
obvious from Fig. 9 that though the base shear increases
after scaling and affects the design of a building for any
of the two codes, the difference between the base shear
due to the two codes remains approximately the same
in both cases with or without scaling. This proves that
scaling (whenever required by the code provisions)
does not affect the results of comparison between the
two codes. Therefore, all comparisons in this paper are
presented without using the scaling.
Since the results presented in Fig. 9 assume that
the building falls in a standard occupancy category
including only the residential and office buildings, the
same exercise was repeated for a hospital building. For
this purpose, to observe the effect of scaling, building
B was assumed to be a hospital building located at
geographical Sample Location 4 with Soil Type D.
Figure 10 shows the base shear obtained for both UBC
and IBC using ELFP, MRSA without scaling and MRSA
with scaling. It is obvious and interesting to note that
the results are not the same as for ordinary occupancy
buildings but instead, IBC and UBC both generate about
the same base shear. For MRSA without scaling, UBC
generates significantly higher base shear than IBC but
Building A - Sample Point 1 (base shears in Y-direction)
1.2
Base shear (102kN)
ETABS is higher than the ELFP time period given in
Table 3 (UBC: 0.639, IBC: 0.628) for this building. This
building has the Modes 8, 9 and 10 closely spaced as is
obvious from Table 8. However, the modal participation
is insignificant and therefore the results obtained by
using modal combination methods CQC or SRSS are
expected to be the same. This was verified numerically
by performing ETABS analysis using CQC and SRSS.
Similar verifications were made for all the buildings
and it was found that the use of CQC or SRSS provide
almost the same results for all buildings classified as
standard occupancy.
1.0
IBC
UBC
0.8
0.6
0.4
0.2
0
A B C D E A B C D E A B C D E
ELFP base shears
MRSA base shears
(without scaling)
MRSA base shears
(with Scaling)
Fig. 9 Comparison of ELFP and MRSA base shears (Building A)
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
25
Base shear (102kN)
20
Building B - Sample Point 4 - Soil Type D (base shears in X-direction)
IBC
UBC
15
10
5
0
ELFP base shears MRSA base shears
MRSA base shears
(without scaling)
(with scaling)
Fig. 10 Building B considered as hospital - comparison of
ELFP and MRSA base shear
Base shear (102kN)
when the scaling is applied, the results as shown in Fig. 10
indicate that the qualitative comparison for MRSA with
scaling is about the same as for ELFP. These results
depend on the structural characteristics and the location
of the buildings and in some cases for hospital buildings,
IBC may generate larger values for the base shear than
UBC as reported in Pong et al. (2006 a, b).
Building A
2.0
IBC
1.8
UBC
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 11 Comparison of MRSA base shear for Building A
7
8
6
4
2
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 12 Comparison of MRSA base shear for Building B
6
Building C
IBC
UBC
5
4
3
2
1
0
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 13 Comparison of MRSA base shear for Building C
IBC
UBC
10
0
8
Base shear (102kN)
Base shear (102kN)
12
The maximum base shear using MRSA for each case
was obtained by the response spectrum analysis runs of
ETABS. For Building A, the results are shown in Fig. 11,
where the maximum base shears due to IBC and UBC are
compared for Soil Types A to E assuming the building
located at all four sample geographical locations. The
results actually verify the expected results based on the
design response spectra comparison shown in Figures
3(a) and 3(b). However, from the design spectra, the
large difference that is now obvious between IBC and
UBC could have not been guessed. Similar results were
obtained for Buildings B, C and D as shown in Figs. 12,
13 and 14, respectively. Quantitatively, the results vary
from building to building but qualitatively remain about
the same. Note that the maximum base shear varies from
Sample Location 1 (area of low seismicity) to Sample
Point 4 (area of high seismicity) in a logical manner.
Also, the maximum base shear values increase with
varying site classes from A to E for each sample location
except for Sample Location 4, when for all buildings,
the maximum base shear for Site Class E is significantly
less than for Site Class D. This is actually related to the
modal contribution and the design response spectrum
for the Sample Point 4. The design response spectrum
Building B
14
Vol.10
7 Results
Base shear (102kN)
110
20
18
16
14
12
10
8
6
4
2
0
Building D
IBC
UBC
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 14 Comparison of MRSA base shear for Building D
Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings
(1)
.
where, VUBC and VIBC are the maximum base shears in
the building due to UBC and IBC, respectively. Note
that the results are more quantitative. First, it shows
that the difference between the values of maximum
base shear are much higher for Sample Locations 1
and 2 as compared to the areas of higher seismicity
(Locations 3 and 4), and it also shows that the UBC
provides a drastically higher maximum base shear in
the range of 40% to 60% for Sample Locations 1 and 2.
60
Percentage difference of base shear
Building A
Building B
Building C
Building D
50
40
30
20
10
0
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Maximum moment in columns (102kN.m)
Fig. 15 Conservativeness of UBC spectrum base shear over IBC
Building A
1.2
1.0
IBC
UBC
0.8
0.6
0.4
0.2
0
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 16 Comparison of maximum moments at base for Building A
Maximum moment in columns (102kN.m)
D = 100×(VUBC – VIBC) / VIBC
111
This implies that UBC was too conservative for areas of
low seismic activity and IBC corrected this. Also, the
percentage difference between IBC and UBC base shear
is prominently higher for Class E in Sample Location 3.
For Sample Location 4, however, the difference between
the two codes is lower for Site Class E as compared to
Site Class D.
Figures 16 to 19 summarize the corresponding
results of the maximum moment at a column-base
Building B
2.5
2.0
IBC
UBC
1.5
1.0
0.5
0
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 17 Comparison of maximum moments at base for Building B
Maximum moment in columns (102kN.m)
for Soil Types D and E at Sample Point 4 have a large
difference in the peak spectral acceleration. For Site
Class D it is close to 1.7 g, where for Site Class E it is
about 1.4 g. This discrepancy does not exist for other
sample locations and is related to how the code has been
developed for this area of high seismicity. However, it
does not affect the main issue addressed in this paper
and the comparative results between IBC and UBC
even for this case are the same, i.e., UBC gives higher
base shear than IBC. To show the comparison in a more
quantitative way for all four buildings at a glance, the
percentage difference in base shear between the IBC and
UBC is plotted in Fig. 15 and is defined as follows:
Building C
1.0
IBC
0.9
UBC
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 18 Comparison of maximum moments at base for Building C
Maximum moment in columns (102kN.m)
No.1
Building D
3.0
2.5
IBC
UBC
2.0
1.5
1.0
0.5
0A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 19 Comparison of maximum moments at base for Building D
112
EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION
60
Percentage difference of maximum moments
Building A
Building B
Building C
Building D
50
40
30
20
10
0
A B C D E A B C D E A B C D E A B C D E
Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4
Fig. 20 Conservativeness of UBC spectrum moments over IBC
junction. Figure 20 shows the percentage difference of
maximum moments. Again, this shows the quantitative
nature of the difference between IBC and UBC and the
results are about the same as for similar plots of the
maximum base shear shown in Fig. 15.
8 Conclusions
A comparison of the 2009 IBC with the 1997 UBC
has been presented focusing on the specific provisions
of the two codes for Modal Response Spectrum Analysis
(MRSA) of residential and office buildings in the
standard occupancy category. The results are needed by
structural engineers and code development authorities in
developing countries throughout the world that have a
large number of existing buildings designed according
to the 1997 UBC and need to evaluate their seismic
safety. To make valid comparisons in general terms,
four geographical sample locations in four different
seismic activity regions of the USA have been arbitrarily
selected. The locations are not associated with particular
buildings. Four buildings ranging from four to six floors
were chosen to represent the bulk of residential and
office buildings in developing countries and were used
for comparison purposes. The design response spectra
for all five site classes at all four geographical locations
were generated both for the IBC and the UBC. The design
response spectra clearly indicate that the UBC design
response spectra are always more conservative than
the IBC. The structural models for the sample buildings
were created using ETABS software and the results for
maximum base shear and maximum bending moment
were obtained using MRSA. The effect of scaling of the
base shear according to the two codes was shown for both
ordinary residential and hospital buildings. It was shown
that scaling as required by both codes does not affect
the results and in all cases, the UBC was found to be
over conservative as compared to the IBC. It was found
that the maximum base shear and the maximum internal
moments generated by the UBC in areas of lower seismic
activity are much higher than IBC when compared to
Vol.10
the more seismically activity areas. It seems the IBC is
successful in correcting the over-conservativeness of the
UBC in areas of lower seismic activity. Furthermore, the
UBC yields higher base shear and internal moments for
all cases. Therefore, the buildings designed using the
1997 UBC can be considered safer than the buildings
designed using the IBC. The results presented apply to
residential and office buildings. For hospital buildings,
the two codes produce very close results. It is possible
that a hospital building designed using the UBC may not
satisfy all the provisions of the IBC as demonstrated in a
previous publication.
The study presented in this paper increases the
understanding of an important earthquake engineering
research issue concerning the IBC and the UBC dealing
with the safety of structural designs using the UBC.
Since the UBC is still widely used among structural
designers in developing countries, the question about
comparing the two codes and obtaining conclusive
results is an important research issue and a design
concern. The results presented herein will help structural
designers as well as the authorities responsible for the
development of building codes in various countries
throughout the world.
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