a comparative study of is 1893:2016 and draft nbc105:2019...

A Comparative Study of IS 1893:2016 and Draft NBC105:2019 in the context

of Seismic Design of the Dharahara, a Historic Monument

Manohar Rajbhandari*a, Nima Sthapitb & Srijana Sthapitc

Abstract

“Dharahara” is the historical monument of 61.88 m height which was destroyed by the April 2015 Gorkha

Earthquake. With the aim of re-establishment of the monument, new planning with 72.96 m height

monument was proposed. Numerical modeling was performed in SAP 2000:20:2 considering Indian

Standard Codes. Nepal National Building Code was newly introduced as “Draft NBC 2019” during the

construction process of Dharahara. In this study, Dharahara structural outputs of the monument is analyzed

and compared in two different codes i.e. IS 1893:2016 and Draft NBC 2019 which includes also site-

specific response of the dharahara site. The result of the study shows that the analysis performed in Draft

NBC 2019 gives substantially higher values in Base Shear, Deflection, Drift and Required Area of the Steel

for Structural Elements compared to those checked in IS 1893:2016 code. The Drift, Deflection and Base

Shear values obtained in Draft NBC 2019 are higher by 153.3%, 152.1%, 125.8% respectively.

Keywords: Base Shear, Drift, Deflection

*a Earthquake Engineer, MRB& Associates, Nepal, [email protected] b Earthquake Engineer, MRB& Associates, Nepal, [email protected] c Senior Civil Engineer, MRB& Associates, Nepal, [email protected]

mailto:[email protected]



1. INTRODUCTION

The analysis of this historic monument has been carried out under the usage of the two different

codes, namely, IS 1893:2016 and Draft NBC 2019. Tube in tube structural system with tapered

Column system was used in the design of the structure and same model is used for this case study.

For dynamic analysis of the structure, Response Spectrum curves provided in the respective codes

are used as well as Site Specific Response carried out at Dharahara site has been used.

Figure 1: Design Acceleration coefficients (Sa/g) (corresponding to 5 percent damping)

(Code: IS 1893:2016)

Figure 2: Spectral Shape factors (Code: NBC105:2019), Ch(T) for Modal Response Spectrum

Method, Nonlinear Time History Analysis, Vertical Loading and Parts and Components

Figure: 3 Site Specific Response of Dharahara Site (Availed on December 2018)

2. METHODOLOGY

SAP 2000:20.2 was used to model the structure which is a stand-alone finite-element-based

structural program for the analysis and design of civil structures. The model studied is 22 stories

historical monument build with height of 72.96 m above the ground level and with two basements,

with total of 8.4m below the ground level (Figure 4). The story height varies from 2.65 m to 4.2

m. Length and breadth of the basement is 60.2m and 27.392m respectively. The total area of lower

basement is 1908.43 square meters. The column and shear wall layout plan of tower area has been

shown in Figure 5. Column A, B, C, D, E, F, G & H are the columns under study for reinforcement

demand check whereas for drift and deflection column A is considered rather than for whole

structure.

0

500

1000

1500

2000

0 0.5 1 1.5 2 2.5 3

SP

EC

TR

AL

AC

CE

LE

RA

TIO

N

(GA

L)

PERIOD (SECS)

SITE SPECIFIC RESPONSE SPECTRA

RT 100 years RT 475 years RT 975 years

Figure 4: 3D building elevation Figure 5: Column & shear wall layout plan

Figure 6: Elevation of the model Figure 7: Plan of the model

Mainly four different models have been created using different response spectrums and codes.

They are named as Model 1, Model 2, Model 3 and Model 4 which are elaborated as follows,

Model 1

Design of the monument ‘Dharahara’ using Code IS 1893:2016 with Response Spectra provided

in the same code.

Model 2

Design of the monument ‘Dharahara’ using Draft Nepal National Building Code (NBC 105:2019)

with Response Spectra provided in the same code.

Model 3

Design of the monument ‘Dharahara’ using Code IS 1893:2016 with Site Specific Response at

Dharahara Site.

Model 4

Design of the monument ‘Dharahara’ using Draft Nepal National Building Code (NBC 105:2019)

with Site Specific Response at Dharahara Site.

Seismic load parameters taken for different models:

Table 1: Seismic parameters for 4 different cases.

Input parameters Model 1 Model 2 Model 3 Model 4

Seismic zone factor

(Z) 0.36 0.35 0.36 0.35

Importance factor (I) 1.5 1.25 1.5 1.25

Soil Type III III III III

Response Reduction

Factor (R) 4 4 4 4

Design Seismic

Horizontal

Coefficient

0.071 0.164 0.071 0.164

3. RESULT

Top Storey Deflection and drift has been considered in single column element to see the variation

in the same point all 4 cases.

The table below shows the increase in percentage keeping the values obtained from model 1 i.e.

values obtained from Response Spectra Method for dynamic analysis using code IS 1893:2016 as

the reference.

a. Base Shear

Table 2: Base Shear Comparison Table

Base Shear (kN) Percentage Increase

Model 1 X-direction 6446.089

Y-direction 6614.474

Model 2 X-direction 14559.366 125.8

Y-direction 14559.219 120.1


Y-direction 6612.638 -0.01


Y-direction 14562.138 120.1

From the above table, it is observed that the increase in base shear obtained from Model 2, i.e.

using NBC 2019/Code spectrum has increased by 125.8% in x-direction and by 120.1% in

y-direction compared to the base shear obtained from Model 1, i.e. using IS 1893:2016/Code

spectrum.

Fig 1: Comparative chart showing base shear in different seismic conditions

The chart above shows the comparison between the base shear obtained from the Model 1, Model

2, Model 3 and Model 4. The base shear using NBC 2019/code spectrum is significantly increased

as compared to the base shear using IS 1893:2016/code spectrum. The base shear using IS

1893:2016/code spectrum is found 6446.08kN and 6614.47kN, whilst the base shear has increased

to 14559.36kN and 14559.21kN in x and y-directions respectively. using NBC 2019/code

spectrum.

b. Top Storey Deflection

Table 3: Top Displacement Comparison

Top Storey Displacement (in mm) Percentage Increase


Y-direction 45.53448


Y-direction 113.882117 150.1


Y-direction 49.923778 3.8


Y-direction 110.43519 142.5

02000400060008000

10000120001400016000

IS 1893:2016/CODESPECTRUM

NBC 2019/ CODESPECTRUM

IS 1893:2016/SITESPECIFIC SPECTRUM

NBC 2019/ SITESPECIFIC SPECTRUM

BA

SE S

HEA

R (

kN)

CODE PARAMETER

COMPARATIVE CHART SHOWING BASE SHEAR IN DIFFERENT SEISMIC CONDITIONS

BASE SHEAR IN X - AXIS BASE SHEAR IN Y- AXIS

From the above table, it is observed that the increase in top story displacement obtained from

Model 2, i.e. using NBC 2019/code spectrum has increased by 152.1% in x-direction and by

150.1% in y-direction compared to the top storey displacement obtained from model 1, i.e. using

IS 1893:2016/code spectrum.

Fig 2: Comparative chart of deflection in various codes

The plot above shows the comparison between the stories deflections obtained from the four

models discussed above. The permissible top story deflection is 277.6 mm in both x and y

directions. It can be observed that the deflection using NBC 2019/code spectrum is considerably

increased as compared to the deflection using IS 1893:2016/code spectrum. Even though all the

model is within permissible deflection, top story deflections found out to be 49.17mm and

45.53mm using IS 1893:2016/code spectrum, whilst the same using NBC 2019/code spectrum are

123.98mm and 113.88mm in x-and y-directions respectively.

c. Drift

Table 4: Drift Comparison

Drift (in mm) Percentage Increase


Y-direction 0.0014


Y-direction 0.0035 150





From the above table, it is observed that the increase in drift obtained from Model 2, i.e. using

NBC 2019/code spectrum has increased by 153.3% in x-direction and by 150% in y-direction

compared to the base shear obtained from model 1, i.e. using IS 1893:2016/code spectrum.

Fig 3: Comparative chart of drift in various codes

The chart above shows the comparison between the drifts obtained from the four models discussed

above. The drift is found out to be 0.0015 and 0.0014 in x and y directions using IS 1893:2016/code

spectrum, whilst the drifts have increased to 0.0038 and 0.0035 in x and y directions using NBC

2019/code spectrum. Both are within permissible value.

Also, a table showing the increase in percentage of the top story displacement of the overall

monument keeping the values obtained from model 1 i.e. values obtained from Response Spectra

Method for dynamic analysis using code IS 1893:2016 as the reference is shown below.

Table 5: Top Displacement Comparison

Top Story Displacement (in mm) Percentage Increase

Model 1 X-direction 71

Y-direction 68.17



Model 3 X-direction 71 0.00




d. Column Reinforcement Demand

Above chart shows the demand of area of steel increment using NBC 2019/code spectrum

compared to demand of area of steel obtained IS 1893:2016/code spectrum.

83

84

85

86

87

88

A B C D E F G H

Per

cen

tage

incr

emen

t of

Rei

nfo

rcem

ent

Column

Average Reinforcement Increment In

Selected Columns

4. DISCUSSION

Prior to the Draft NBC 2019, the most designers followed NBC 105:1994 and IS 1893(2002)

codes. Same codes were followed during the design period of Dharhara. Afterwards IS

1893:2016 and Draft NBC 2019 was introduced during the construction period of Dharahara. In

the process of designing, we took required consideration of other variables and coefficient, as

design of Dharahara is of utmost national interest and importance. In this study, the model was

analyzed in both codes. The result of the study shows that the analysis performed in Draft NBC

2019 gives substantially higher values in Base Shear, Deflection, Drift and Required Area of the

Steel for Structural Elements compared to those checked in IS 1893:2016 code. The Drift,

Deflection and Base Shear values obtained in Draft NBC 2019 are higher by 153.3%, 152.1%,

125.8% respectively. We are satisfied that our design meets the latest Draft NBC 2019.

5. CONCLUSIONS AND RECOMMENDATIONS

• Base shear, Drift and Deflection values obtained from models analyzed in two different

codes; IS 1893:2016 and Draft NBC 2019 are significantly different.

• Base shear, Drift and Deflection value are higher by 125.8%, 153.3% and 152.126%

using Draft NBC 2019 compared to IS 1893:2016.

• Reinforcement percentage demand in considered columns using Draft NBC 2019 is 86%

(Average) higher than demand in IS 1893:2016.

• Draft NBC 2019 and IS 1893:2016 Codes were taken into consideration for checking the

final design, but when these two codes are applied, the results are considerably different.

Thus, we recommend international review to be taken in this regard.

Acknowledgements

The writers would like to express their gratitude towards NRA and members of organization for

their kind cooperation and encouragement which help us in completion of this paper. We would

also like to express our special thanks to Mr. Druba P Paudyal for giving us this opportunity to

prepare this paper on the project, in which we are involve for design too.

References

Gaurav Charavande,Savita Maru,2019,Earthquake Analysis, of RC Structure using Different

Codes and Different Countries

S.H.C.Santos,2013,Comparative Study of Codes for Seismic Design of Structures

a comparative study of is 1893:2016 and draft nbc105:2019...

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