SEISMIC DESIGN OF THE MASONRY INFILL RC
FRAME BUILDINGS WITH FIRST SOFT STOREY
Prakirna TULADHAR *Supervisor: Dr. Koichi KUSUNOKI**

MEE07158
ABSTRACTThe main aim of this study was to investigate the seismic performance and design of the masonry
infill Reinforced Concrete (R/C) frame buildings with the soft first storey under a strong groundmotion. The study also highlighted the error involved in modeling of the infill RC frame building ascompletely bare frame neglecting stiffness and strength of the masonry infill wall in the upper floors.
The envelope curve for representing non-linear behaviors of masonry infill wall was determined fornumerical analysis. The attempt was made to determine the strength increasing factor (η) to accountthe effect of the soft storey through various 2D analytical models using Capacity Spectrum Method(CSM) and established its relationship with initial stiffness ratio (Ko2/Ko1). In this study, the nonlinear dynamic time history analysis was also carried out with a three dimensional practical model inorder to verify the proposed strength increasing factor (η).
Κey words: First soft storey, Capacity Spectrum Method, Strength increasing factor, Storey drifts
INTRODUCTION
In urban and semi-urban areas of Nepal, like several other developing countries around the world,
reinforced concrete frame structure is very popular for the building construction. Usually, the RC
frame is filled with burnt clay bricks masonry as non-structural wall for partition of rooms in thefloors which is called infill wall. The reason behind the popularity of using brick masonry indeveloping countries may be due to economy, architecture; aesthetic, locally available skill andmaterial, good for water proofing, heat and sound insulation, and better security. Similarly, open first
storey is nowadays as unavoidable features for the most of urban multistory buildings for vehicles
parking, shop, reception, and lobby etc. In building with the first soft story, the upper storey being
stiff undergoes less inter-storey drift, while the displacement will be concentrated at the first storey
during seismic events. Hence, an abrupt change in the stiffness in first storey have adverse effect on
the performance of the building during ground shaking which is known as first soft storey effect.
Problem statement
In current practice of structural design of the buildings in Nepal, masonry infills are treated as nonstructural element and the frame is modeled as a bare frame. The strength and stiffness contribution of
infill walls are often neglected and self weight of infill wall is modeled as uniformly distributed load.
The entire lateral load is assumed to be resisted only by the moment resisting frame. In reality, thepresence of the infill wall in the frame changes the lateral load transfer mechanism into truss actions
leading to stress concentration in the ground floor. Hence, the effect of the first soft storey is failed tocapture in the analysis. This error in modeling may lead to damage in the first soft storey during astrong earthquake.

* Engineer, Dept. of Urban Development & Building Construction, Govt. of Nepal** Associate Prof., Yokohama National University, Yokohama, Japan1

Macro modeling for brick infill masonry wall
The contribution of the masonry infill wall to represent the infill frame can be modeled by replacing
the panel by a system of equivalent horizontal shear spring connected to the main frame vertically.
The non-linear properties of the equivalent shear spring can be determined by the envelope curve. It
can be represented by poly-linear strength envelop with initial elastic stiffness until the cracking force
Qc , post cracking degraded stiffness until the maximum force Qy and post peak residual force Qu.
The corresponding lateral displacements are γc, γy and γυ respectively (Ref. Figure 1).
400.00Q in K N

300.00200.00100.00
0.00
0

10

20

30

40

50

60

70

Displacement in mm
Figure 1 Skeleton for masonry infill wallcontext.

Figure 2 Skeleton curve for masonry wall in Nepalese

In this study, the brick masonry wall in Nepalese context was considered to determine theskeleton curves with the given parameters: Size of brick = 230x110x57mm, thickness of mortar=
10mm and mortar ratio 1:5. The compressive strength of the burnt brick, fcb= 7.5 N/mm2, compressive
strength of mortar , fj= 5.0 N/mm2 and tensile strength of brick, ftb = 0.1fcb = 0.75 N/mm2. The
computer program was written in FORTRAN to obtain strength envelope (Skeleton) curve as shownin figure 2.
NUMERICAL MODLING FOR THE STRUCTURAL ELEMENTS
The structural elements were modeled by STERA3D (Dr. Taiki Saito, the chief researcher of IISEE,
BRI) for non-linear structural analysis. All the structural members were considered as line elementswith non linear springs to represent the non-linear behavior of the structure. The cross sections wereconsidered to be rigid. The details of modeling are discussed below.
Column: The structure to be analyzed being first soft storey and often subjected to large fluctuating
axial load due to overturning moment, the fiber model (multi spring model) was used for the column
element to take into account the interaction between bending moment and axial force. Column
element was idealized with the line element with fiber slice model at each ends consisting of steel andconcrete multi axial springs elements in order to represent flexural and axial deformation to takeaccount of the interaction between bi-directional bending moments and axial load. The bi- axial shear
springs were also considered to represent the non linear shear deformation of the column. An originoriented poly-linear hysteresis model was used for representing non-linear shear spring.
Beam: The beam elements were represented as a uni-axial model with non-linear rotational spring at
two critical ends and non-linear shear spring at the middle of the beam for shear deformation. The
hysteresis tri-linear degradation modified Takeda model was applied for non-linear rotational spring
model
Masonry infill wall: Masonry walls were represented with a line element with non-linear shear
spring at the mid of element. The poly-linear slip model was used as the hysteresis model.

2Uni-axial
shear
springBending
springShear
springMulti-axial
Springelement(MSa)Ms column model

Rigid zoneb) Non-linear beam modelc) Infill masonry model

Figure 3 Structural Elements Models

DETERMINATION OF STRENGTH INCREASING FACTOR (η) FOR FIRST SOFTSTOREY BY CAPACITY SPECTRUM METHOD
It was already discussed about soft storey effect in first storey due to presence of infill wall in theupper floors. However, the designers do not consider the strength and stiffness of the masonry wall in
their analysis due to limitation in their knowledge. The performing the non-linear analysis is also very
difficult for many engineers. Hence, the attempt was made to determine the strength increasing factor(η) to account for the soft storey effect due to infill wall and tried to establish its relationship withinitial stiffness ratio between two floors and height of the structure so that designer can increase thelateral strength of the columns subjected to first soft storey to meet the drift demand due to the soft
storey.
Model studied:
A prototype apartment building with typical floor plan was considered. The building was symmetricalin plan and the plan dimension of the building was 20m x 12m. The span length along both the axis
was 4m c/c and floor height 3m c/c. The structural frame elements were designed for six storeys
according to current practice with critical load combination prescribed in the prevailing code.
Capacity Spectrum Method:
Demand Spectrum Curve for Nepal
1
5% Damping
10% Damping
15% Damping
20% Damping
30% Damping

0.8
S a m /s e c2

Capacity Spectrum Method (referred as to CSM hereafter)
can provide the realistic representation of the structure
subjected to a severe earthquake and can be established asa rational alternative to nonlinear analysis for assessing
seismic demands of the MDOF structure. The seismic
demand in form of response spectrum and capacity in formof pushover analysis is plotted in ADRS format. The pointof the intersection of two curves is the response ofstructure for the demand and is called the performancepoint.
Demand Spectrum Curve: Unfortunately, in Nepalesecode, only design spectra are given. Hence, based on
Indian code IS1893:2002, The demand spectrum curve for
Nepal was plotted for different damping factor considering
0.36g PGA as maximum considered earthquake(MCE) of
return period 500 years.

0.6
0.4
0.2
0
0

5

10

15

20

25

30

Sd in cm

Figure 4 Demand Spectrum Curve

31This simple static non-linear analysis method was used
to determine the strength increasing factor (η). Firstly, thepushover analysis was carried out for the bare frame by applying
incremental lateral loading according to the Ai distribution up to
drift angle of 1/50 as the life safety limit. Similarly, analysis of
the models by considering infill wall with first soft storey was
carried out and it could not meet the demand. Then, the lateralstrength of column in terms of size and reinforcement wereincreased gradually so that the strength could meet the demand.
Therefore the strength increasing factor (η) is the ratio of thelateral strength of the strengthened frame structure subjected tosoft storey to the lateral strength of the bare frame structure.
Strength increasing factor (η) is function of initial storey
stiffness ratio (Ko2/Ko1).

Capacity
Demand curve
Demand Before

0.8

Capacity Before

S a m /s e c2

Bare frame capacity

0.6
`

0.4
0.2
0
0

5

10

15 20
Sd in cm

25

30

Figure 5 Capacity Spectrum Curves

Strength Increasing Factor (η) Vs Storey Stiffness Ratio

M O D E L II
2 3 0 m m w a ll

M O D E L IV

M O D E L III
1 1 0 m m w a ll

M ODEL V

2 3 0 m m w a l l w i t h 3 0 % o p e n i n g 1 1 0 m m w a l l w i th 3 0 % o p e n i n g

M O D E L V II
1 1 0 m m w a ll

M O D E L V III

M ODEL VI
2 3 0 m m w a ll

M O D E L IX

2 3 0 m m w a l l w i t h 3 0 % o p e n i n g1 1 0 m m w a l l w i th 3 0 % o p e n i n g

Figure 5 First soft storey models
Table 1 Table 1 Initial Stiffness Ratio

Figure 7 Storey drift angle

Figure 8 ƒÅ ? ko2/ko1 curve

Story- Drift

Strength Increasing Factor Vs Stiffness Ratio

6
model I
model II

5

model IIImodel IV
model V

4model VI

S t o rymodel VIImodel VIII

3

model IX2

2.5
S t r e n g t h In c r e a s in g f a c t o r

M OD EL I
B a reAs the soft storey means the abrupt change instiffness along height of the building, the strength
increasing factors depends upon storey initialstiffness ratio of the soft storey to the adjacent
storey above it is a called initial stiffness ratio(Ko2/Ko1). Therefore, 2D models of six storiedbuilding with the most practical configurations ofinfill walls as shown in figure 5 were considered sothat rational relationship could be establishedbetween Strength increasing factor (η) and initialstiffness ratio (Ko2/Ko1).
The pushover analysis for each of themodels by considering the stiffness and strength of
the infill wall (except Model I) was carried out insuch a way that capacity curve of the all models
would meet the demand by increasing its lateralcapacity. The strength increasing factors were
calculated for each model as a ratio of the lateralcapacity of the strengthened to bare (ref figure.6).
The strength increasing factor (η) withinitialstiffness ratio (Ko2/Ko1) is tabulated in the table1

2

1.5
1

0.5

1

0
0
0.00%

1
0.05%

0.10%
Drift %

4

0.15%

0.20%

1.5

2

2.5

Intial Stiffness Ration (K2/K1)

3

Result and Discussion:
The storey drift ratio for each models showed that the storey drift demand increased with the increasein the stiffness ratio (Ko2/Ko1). Hence, the relationship between strength increasing factor (η) andInitial stiffness ratio (Ko2/Ko1) could be established and plotted in the figure 7.
CASE STUDY: PRACTICAL VERIFICATION OF STRENGTH INCREASING FACTOR (η)
5000

5000

5000

5000

5000

5000

5 00 200
200

9 00

1900
200 500900

500 200
200

1900
200 5 00

5 00 05000

5 0 005 0 005000

1 9 001 3 001900

1100

Design of the Structural Elements

200

2 00

1300

1100

TYPICAL FLOOR PLANGROUND FLOOR PLAN

The structural design of the building was carried out to resist the
Figure 8 floor plans
seismic load as prescribed in Nepalese code and designed .
Detailing of the members was done according to the Indian building code. The size of the columns
were calculated as two type, peripheral columns were 500X500mm with 8# 20φ + 4# 25φ andsimilarly, inner columns were 500X500mm with 8# 25φ +4# 20φ. The 8φ ties were provided as
100c/c. Grade of concrete and reinforcement were 16MPa and 415Mpa respectively.
Table 2: Determination of η

According to the graph of figure 8, the strength increasing
factor (η) corresponding to Initial stiffness ratio 1.73 is
1.53. Hence, the columns of the building were redesigned
for the base shear 1.53 times than that of the previous
calculated base shear. The size and the reinforcement were
calculated as 575 X 575mm and 12# 25φ respectively.

Initial stiffness
ratio
In X
Dir.
1.73

Strengthincreasing
factor
In X
In Y
Dir.
Dir.
1.53
1.5

In Y
Dir.
1.65

2

2

1.5

1.5

1.5

1

0.5

Sa m/sec 2

2

Sa m /sec 2

Sa m /sec 2

Non-linear Time History Dynamic Analysis
Non-linear dynamic time history analyses were carried out separately for bare, infill and strengthened
infill frame with Kobe, Hachinohe and Chamauli (India).All the earthquakes were normalized to theirmaximum velocity of 50cm/sec. The Demand Spectrum for the response of all three normalized input
ground motion were plotted in ADRS format. Similarly, the demand required by elastic response
spectra was also plotted. Then, comparisons were done among them for all the three cases.

1

0.5

0.5

0

0

0

0

5

10

15
Sd in cma) Kobe

20

25

30

1

0

5

10

15

20

25

30

35

40

45

Sd in cmb) Hachinohe
Figure 9 Demand Spectrum Curves

5

50

0

5

10

15

20

Sd in cmc) Chamauli (India)

25

30

5000

A typical six story high apartment typed building withmasonry infill wall with open soft first storey wasconsidered which is very common in the urban areas of
Nepal as shown in figure 8. The plan of the building issymmetrical with both directions and grid of the column
layout is 5m c/c in each direction and the storey heightof the building is 3m.

Result and Discussion
All the three non linear dynamic time history analysis showed the development of soft storey in thefirst floor . By strengthening columns with the strength increasing factor (η), drift demand at firstfloor were considerably decreased. The result obtained from Chamauli earthquake which agrees withthe demand spectrum showed that the drift ratio obtained after the strengthening the columns withstrength increasing factor were almost 2% (Ultimate drift ratio). Hence, the proposed strength
increased factor was well agreed.
Drift in x dirc tion for
Hac hinohe

Drift in x dirc tion for K obe
66

B are F rame
Infill R C F rame5

S trengthen R C
F rameS to ry

S to ry

4

3

3

2

2

2

1

1

1

0

0
0

0.01

0.02

0.03

0.04

0

0

0.005

0.01

0.015

0.02

0.025

Drift Ang le

Drift Ang lea) KobeInfill R C F rame5

S trengthen R C
F rame4

3

B are F rame

Infill R C F rame

S trengthen R C
F rameS to ry

6

B are F rame

5

4

Drift in x dirc tion for
C hamaulib) Hachinohe

0

0.01

0.02

0.03

0.04

0.05

Drift Ang lec) Chamauli (India)

Figure 10 Max storey drift ratio

CONCLUSIONS
•
•
•

•

The effect of infill wall predominately changes the behavior of the structure and it is essential
to consider infill walls for seismic evaluation of the structure.
NBC105 underestimates the base shear by around 10%. Hence, the immediate review of thecode is required.
Arrangement of infill wall in the frame affects the behavior of the structure. A relationshipcan be established between strength increasing factor(η) and initial stiffness ratio. Designer
can use strength increasing factor (η) to take care of soft storey effect without carrying out
non-linear analysis
The non-linear dynamic time history analysis for 3d models proved that the demand of thesoft first storey can be met by application of strength increasing factor (η).
ACKNOWLEGDEMENT

I would like to extend my sincere gratitude to my advisor Dr. Taiki Saito, the chief researcher andEngineering course leader of IISEE for letting me to use his software STERA3d, continuously
supporting and encouraging me during my study.
REFERENCES
A. Madan,A.M. Reinhorn, ,J. B. Mandar, R.E. Valles, 1997, ASCE, Vol.114, No.8, pp.1827-1849
T. Pauley, M.J.N. Pristley, 1992, Seismic Design of Reinforced Concrete and Masonry building,
JOHN WILEY & SONS, INC
ATC, Seismic Evaluation and retrofit of concrete buildings, Rep ATC 40, Applied Techonology
Council, Redwood City, California (1996)
Dr. T. Saito, 2005, Structural Earthquake Response Analysis 3D (STERA 3D Ver. 4.2) Manual,
Building Research Institute, Japan
NBC-105, 2004, Nepal National Building Code for Seismic Design of Buildings in Nepal,

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