International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 2 - October 2015
Comparative Analysis of Seismic Codes of Nepal and India
for RC Buildings
Er. Pujan Neupane1, Er. Samyog Shrestha2
Department of Urban Development and Building Construction
Government of Nepal
Abstract — Structural design engineers in Nepal use
seismic codes of Nepal and India interchangeably,
although the codes yield different design values. There
exists widespread belief that Indian seismic codes
design for greater seismic forces in the RC frames and
are therefore more conservative. However, there is
little evidence that backs such a broad statement. Any
declaration of that kind could be made only after
analyzing, in each code, all the contributing
parameters that govern the final design seismic loads.
Since, the theory for computation of seismic forces in
the two codes, is reasonably uniform, it allows for a
sound comparative analysis. The outcome of the
analysis provides enough evidence to out-rule such a
general statement that Indian seismic codes are more
conservative than Nepali seismic codes. Results are
not that general; both the codes could be conservative
depending upon conditions- the conditions being
location of site, soil type and number of stories.
Keywords — NBC105:1994, IS1893:2002, NBC vs.
IS, seismic code, seismic code comparison, base shear
coefficient, seismic shear coefficient,
response
spectrum, seismic zoning factor, response reduction,
importance factor
I. INTRODUCTION
Before the introduction of Nepal National Building
Code in 1994 AD, structural design of RC buildings in
Nepal used to be done by referring Indian Standards.
Such reference was relevant as well given the fact that
Nepal borders India in three directions, thus, the
design response spectrum and the diversity of soil type
incorporated in Indian seismic design code IS 1893:
2002 would reasonably be applicable for Nepal.
After 1994, the seismic design code of Nepal NBC
105: 1994 started to come in practice. Since there was
no restriction in the use of Indian Standards in the
government level itself, even after the introduction of
Nepali Standards, the Indian code was equally
popular, if not more. Even as of now, the compliance
of one code would sufficiently ratify earthquake
resistant design; hence depending upon the designer’s
expertise, both codes are widely used and accepted.
As the building code compliance got implemented
more stringently specially in the Kathmandu valley in
the past decade, the awareness and understanding
towards building codes grew among engineers. With
it, emerged a new line of belief that Indian seismic
code is more conservative than Nepali seismic code.
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Although not documented anywhere, the design
engineers presumably expressed such thought as a
generalization of their narrow scope of design
practice.
Most structural engineers in Nepal design
residences 2 to 5 stories, schools 1 to 4 stories,
commercial complexes 4 to 8 stories and apartments 8
to 14 stories and as geotechnical investigation of the
site is often discounted except for tall buildings, the
soil type II: Medium soil, is commonly adopted for
design purpose. It is quite reasonable to assume that
based on such narrow scope of design practice,
engineers could have made a doubtful generalization.
To declare that IS 1893: 2002 gives conservative
result or NBC 105: 1994 yields less exaggerated
result, a very broad set of parameters needs to be
analyzed.
II. METHOD
All building codes have their own principles, so it is
not wise to mix the requirements of one code with
another. Indian seismic code was prepared on the basis
of deterministic seismic hazard analysis from
historical data of past earthquakes whereas Nepali
seismic code was prepared on the basis of
probabilistic seismic hazard analysis of all faults
within 150 km boundary of Nepal. Keeping the same
into consideration, the comparative analysis has been
conducted by treating the two codes independently
throughout and tallying the final design results of the
two.
Both codes have their own design response
spectrum. The nature and essence of the spectrum are
similar in the two codes but they differ in
normalization of the values of what has been termed
as Spectral Acceleration Coefficient (Sa/g) in IS1893:
2002 and Basic Seismic Coefficient (C) in NBC105:
1994 as given in Fig. 1 and 2. There are three
spectrums for three types of soil; Type I: Stiff soil,
Type II: Medium soil and Type III: Soft soil. The
definition of these types match in the two codes, so a
particular site that would fall under Type I as per NBC
would also fall under Type I as per IS and so on.
The coefficients are read out from the spectrums
against the period of the structure (T) which is given
by T = 0.075 h0.75 in IS and T = 0.06 h0.75 in NBC,
where h is the total height of the structure.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 2 - October 2015
Fig. 1: Response spectrum for NBC 105
Fig. 2: Response spectrum for IS 1893
So, the total height of the structure is an important
parameter which can be substituted by the more easily
perceived variable, the number of stories in the
building. The height of typical story in RC buildings
in Nepal varies among 2.7m, 3m, 3.3m and 4m
depending upon the location and architectural
requirement. Different story heights could also yield
different results and hence, needs to be accounted for.
Symbols
Basic
coefficient
Time
period
C
T
Seismic
zoning
Z
Reduction
factor
Importance
factor
Base shear
coefficient
Load comb.
factor
K
I
NBC
IS
Values
Symbols
Values
Based on T &
Based on T &
Sa/g
soil type
soil type
0.06 h0.75
0.9 for Zone A
1.0 for Zone B
1.1 for Zone C
1.0 for SMRF
1.0 for Dual
1.0 Normal
1.5 Important
T
0.075 h0.75
Z
0.36
R
I
5.0 for SMRF
5.0 for Dual
1.0 Normal
1.5 Important
Cd
CZIK
Ah
Sa/g*I/R*Z/2
LCF
1.25
LCF
1.5
Table 1: Values of essential parameters
Both codes have a seismic zoning factor (Z). The
whole of Nepal falls under Zone V (Z = 0.36) based
on categorization of the Indian Standard whereas
according to Nepali Standard, the country is divided
into three zones which, for simplicity, will be called
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Zone A (Z = 0.9), Zone B (Z = 1.0) and Zone C (Z =
1.1).
The importance factor (I) considered in the two
codes is identical and need not be considered as a
governing variable in this analysis; important
buildings like hospitals, schools, fire stations,
cinemas, power stations etc. are designed for 50%
greater seismic forces than normal buildings.
Basically seismic codes have another key aspect
which accounts for the ductility, redundancy and overstrength of the RC members. In IS1893: 2002, this
aspect is dealt by Response reduction factor (R) which
reduces the design elastic seismic forces by an amount
based on the structural system of the building; so
higher R values would mean lesser design seismic
forces and greater reliance on redundancy, overstrength and ductility. However, in NBC105: 1994,
Structural performance factor (K) is used which, on
contrary, is a multiplier and therefore greater K value
means larger design seismic forces.
Another factor that needs to be accounted is the load
combination factor since in NBC, in all load
combinations, a factor of 1.25 is used for the
earthquake loads whereas in IS, a factor as high as 1.5
is used for the earthquake loads. All these parameters
(refer Table 1) receive different values in the two
codes, but when all of them are considered, the
combined effect gives a base shear coefficient which
is rather comparable.
SN
1
2
3
4
Basic variables Symbol
Range
Storey height
hst
2.7m, 3.0m, 3.3m, 4.0m
No. of story
n
1, 2, 3, 4, 5….15
Soil type
ST
I, II, III
Zone within Nepal Znep
A, B, C
Table 2: Variables in analysis
It is also significant to note that the base shear
distribution to the floor levels in NBC is linear
compared to parabolic distribution in IS, which if left
unaddressed could yield misleading results. IS code
assumes parabolic distribution of base shear to the
floor levels, so base shear is distributed in proportion
to the product of seismic weight of the floor and
square of the height of the floor from ground, whereas
NBC assumes linear distribution, so base shear is
distributed in proportion to the product of seismic
weight of the floor and the height of the floor from
ground.
III. RESULT
It was observed that the story height of the RC
building does not play any major role in analyzing the
differences in the design seismic forces of the two
codes, so the results for the most popularly adopted
story height of 3m, have only be presented.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 2 - October 2015
in the structural members rather than the base shear as
a whole.
On investigating particularly for soil type III, it is
observed that the cumulative story shears for most of
the floor levels of a 15 story building, come out to be
higher as computed using NBC than IS code (Refer
Fig. 6).
Fig. 3: Base shear coefficients for ST I
Fig. 6: Factored story shear coefficients for N=15 (Soil Type III)
Similarly, an obvious inference can be made
comparing Fig. 5 and Fig. 7, that the parabolic
distribution of base shear in IS code yields greater
structural demand than the linear distribution in NBC.
The factored base shear coefficient for IS, referring
Fig. 5, is lesser than the corresponding values in NBC
Zone B and NBC Zone C for a 10 story building. So,
obviously, if the two codes followed the same base
shear distribution pattern, for all floor levels of a 10
story building, the seismic shears should have been
lesser in IS which is, evidently not the case as can be
seen in Fig. 7.
Fig. 4: Base shear coefficient for ST II
Fig. 5: Base shear coefficients for ST III
Fig. 7: Factored story shear coefficients for N=10 (Soil type III)
It is worth noting that for soil types I and II, the
factored base shear coefficients obtained from Indian
Standard more or less exceed those obtained from
Nepali Standard, but for soil type III, the factored base
shears obtained from Indian Standard is comparatively
lesser when the building is more than 8, 9 and 10
stories tall (and up to 15 story) respectively in case of
buildings in Zone A, B and C. However, as the graphs
of NBC Zone A, NBC Zone B and NBC Zone C in
Fig. 5 will decline exponentially after the 15 story
height, further inferences require further analysis.
As the base shear gets distributed in the floor level
differently in the two codes, the effect of such
difference is also of concern because it is the seismic
shear forces in the floor level that governs the stresses
Based on this, it is also true that the story shears for
each floor computed using IS will be higher when the
building is lesser than 8 stories (about 25 meters tall)
in a site having soft soil (Soil type III) as well as when
the building is of any story but in a site with medium
or stiff soil.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 2 - October 2015
REFERENCES
[1]
[2]
[3]
[4]
Fig. 8: Factored story shear coefficients for N=5 (Soil type III)
Also, as in Fig. 8, for a 5 story building in a site
having soil type III, the gap between factored story
shear coefficient for IS and NBC works out to be
larger than when the building is 8 story tall.
IV. CONCLUSION
Many factors play part in determining the seismic
demands on the structural members of an RC building.
After accounting all such factors, it can be concluded
that for RC buildings resting on stiff or medium soil,
the seismic demand as computed using IS 1893 is
always higher than NBC 105. But, this should, strictly,
not be interpreted as any one code being faulty, rather
both codes have their own design principles and
assumptions which considerably differ the seismic
capacity of the building being designed.
There are also cases when NBC can yield
conservative outcomes. This depends on three major
factors- the location of site, the soil type at site and the
number of story of the building. Typically, for high
rise buildings more than 10 to 12 stories tall (30 to 36
meters) in soft soil (Soil type III), it becomes difficult
to generalize which code gives more conservative
results and when it is even taller, NBC 105 will yield
higher seismic demand and become more conservative
than IS 1893.
More importantly, these findings outline the lack of
harmony between the two codes which builds
skepticism on believing the numbers that the codes
prescribe. In a seismically active nation like Nepal, it
is a challenge to urgently stipulate unambiguous rules
and coherent code provisions regarding earthquake
resistant design, so as to reduce earthquake related risk
in the country. Deeper research to make revisions if
needed, and implement a single well-justified seismic
code in Nepal without giving any place to other codes,
must be a top-priority in the policy level.
Further, major differences and inconsistencies in the
end results of the two codes developed by experts in
the field of seismicity and structural design of each
country, have created room for uncertainty especially
when the subject being dealt is quite unpredictable, so
the structural engineers should not just stick to code
compliance but should start designing more resilient,
redundant, collapse preventive and better performing
structures in future.
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