EFFECTS OF SOFT FIRST STORY ON SEISMICPERFORMANCE OF RC BUILDINGS ANDSUSTAINABLE APPROACH TO RETROFIT
MD. KAMRUZZAMAN
MEE17712

Supervisor:

Matsutaro SEKIABSTRACTThe structural configuration with a soft first story proved to be very vulnerable and performed poorly
during the past earthquakes. Like other many countries, brick infill masonry is used in Bangladesh as anonstructural element. Its usages in upper stories and keeping building’s ground floor open result inlateral stiffness difference and cause soft first story state. The scarcity of land in Bangladesh hascompelled to construct multi storied RC buildings with an open ground to be used as vehicle parking,stores or other facilities. This research committed to assess the seismic vulnerabilities of RC buildings
with a soft first story, causes behind the collapse of soft first story during earthquakes, seismic
performance difference with bare frames and sustainable approach to retrofit them. Seismic performanceand vulnerabilities of the soft first story were assessed by the JBDPA guidelines of seismic evaluation,
FEMA-356, BNBC-2015 and nonlinear static pushover analysis. Flexural moment magnification at thesoft first story columns during earthquakes was determined. Sustainable retrofitting approaches to
upgrade seismic performance and prevent catastrophe during earthquakes were proposed with cost
analysis. This research found that, seismic behavior, ductility demand, inter story drift pattern anddamage distribution of RC buildings with a soft first story were totally different than the RC buildings
designed by only bare frame analysis. The soft first story suffered huge ductility demand, extreme interstory drift change and concentrated in severe damage. Magnification of flexural moment at soft story
columns was detected as a variable entity. Retrofitting of soft first story was found different fromconventional RC buildings. A combination of RC column jacketing and adding steel bracing proved tobe effective to eliminate stiffness difference and control the excessive inelastic lateral drift.
Keywords: Soft story, Seismic performance, Nonlinear analysis, Retrofit.
1. INTRODUCTION
Bangladesh is one of the most densely populated countries. The scarcity of land has compelled toconstruct multi storied buildings with open ground. Using masonry infills as a nonstructural element in
the upper stories keeping ground floor open results in lateral stiffness difference and cause soft story
vulnerabilities. Many RC buildings with a soft first story collapsed during past earthquakes such as the
1995 Kobe earthquake, 2015 Nepal earthquake. Along with many natural disasters like floods, cyclones
and droughts, Bangladesh is under threat of moderate to strong earthquakes. The three plate boundaries
surrounding Bangladesh are tectonically very active and generates many earthquakes. The common
practice of structural designs in Bangladesh is to design the RC building without considering the effects
of infill masonry. This practice of bare frame analysis leads to inappropriate estimation of structure’s
actual capacity and cannot address the problem of soft first stories. There was no guideline about
consideration of soft story effects in the seismic design code of BNBC-1993, which is now included in
the new seismic design code of BNBC-2015. Many multi storied RC buildings with a soft first story
exist in Bangladesh, and many are under construction.
Public Works Department, Bangladesh.
Visiting Research Fellow, International Institute of Seismology and Earthquake Engineering, BRI, Japan.

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2. THEORY AND METHODOLOGY
A six (06) storied RC building (Figure 1) in Bangladesh having soft first story designed by only bareframe analysis following the seismic design code of BNBC-1993 (Bangladesh National Building Code)is selected. Seismic evaluation and structural performance level checking by JBDPA (Japan Building
Disaster Prevention Association) guidelines of seismic evaluation, ATC-40 (Applied Technology
Council), FEMA-356 (Federal Emergency Management Agency) and BNBC-2015 are conducted.
Nonlinear static pushover analysis is done to understand the progressive damage pattern and to estimate
the structure’s capacity by capacity spectrum method. Sustainable and cost effective retrofitting methods
are proposed and reevaluation is conducted to check the structural safety of soft first story. The major
theory and concepts used in this study are summarized below:a) In RC frame structures, discontinuity of walls in some floors causes stiffness differences.
According to the definition of BNBC-2015 and ASCE7-05 (American Society of Civil Engineers), a
soft story is one in which the lateral stiffness is less than 70% of that in the story above or less than 80%of the average lateral stiffness of the three stories above irregularity.
b) Any structure will not only perform within linear range but also in inelastic range after
yielding when subjected to earthquakes. So, inelastic analysis is needed to understand the modes offailure and sequence of collapse. Seismic performance criteria of FEMA-356, ATC-40, BNBC-2015 arefollowed in this research. Demand, capacity and performance of the structure are obtained by “Capacity
Spectrum Method”, and ADRS (Acceleration Displacement Response Spectrum) format described in
BNBC-2015 and ATC-40.
c) ETABS-2015 is used in this research for nonlinear analysis. An equivalent diagonal strut
is used to represent infill masonry. The equivalent strut width (a) and compressive strength (f’m) is
calculated as per the formula proposed by Pauly and Priestly. Alchaar equation is used to calculate thestrength reduction due to the presence of openings in the infill masonry.
d) Seismic demand index, Iso which is the level of seismic capacity needed for a structure
to remain safe against a certain ground motion or code defined ground motion. Seismic demand index
for the RC (Reinforced Concrete) buildings in Bangladesh is calculated as per the proposal of the“Seismic Assessment Manual” prepared by CNCRP (Project for Capacity Development on Natural
Disaster-Resistant Techniques of Construction and Retrofitting).
e) The seismic evaluation standard proposed by JBDPA consider only RC walls. In
Bangladesh, clay bricks are used widely as the infill elements in the RC frame. Shear strength andductility index is calculated on the basis of behavior of infill masonry within RC frame. Seki et al.
proposed the shear strength of infill panel as 0.2 Mpa. Alwashali et al. proposed R-max (maximum drift
angle) for masonry infill as 1% of story drift. Ductility index based on this proposal is calculated as 1.5.
f) The strength index and ductility index of the jacketed column by RC jacketing and FRP(Fibre Reinforced Polymer) wrapping and retrofitted by steel bracing are calculated by the guideline of
JBDPA seismic evaluation. For ETABS modeling of FRP wrapped columns, “Lam and Teng’s stressstrain model for FRP confined concrete” is used.

3. SEISMIC PERFORMANCE OF SOFT FIRST STORY
3.1. Outline of the target building
The target building is a six (06) storied RC building with soft first story located in seismic zone III(BNBC-2015) and designed by following the building design code BNBC-1993. The open ground isused for parking and brick infill masonry with various opening is present in the upper floor. The building
is designed by analyzing only bare frames. Individual footings are used as foundation. The soil type is
SC. Architectural plan of ground floor and elevation A-A is presented in Figure 1 and Figure 2
respectively. As, the building is an office building, its occupancy category is IV.

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Figure 1. Ground floor plan.

Figure 2. Elevation A-A.

Stiffness difference (%)

3.2. Lateral stiffness
Lateral stiffness of any story is the ratio of story
shear force to story drift displacement. This is thecriteria to define a soft story. The stiffness
difference with upper floors for bare frames
without and with considering infills is shown in
Figure 3. As per the definition of BNBC-2015 andASCE7-05, the first floor (when masonry infill inthe upper floors is considered) has verticalirregularity and can be called as a “soft first story”.
But in case of bare frame analysis no story has
experienced such lesser stiffness difference.
Seismic code of Japan defines lateral stiffness as
the ratio of story height to story drift. If the ratioof any story’s stiffness to average of all story isless than 0.6, then vertical irregularity or soft story
is present. The first floor is a soft story when infill
is considered in upper floors. But no suchcondition is observed when bare frame analysis isdone (Figure 4).

Stiffness difference (%) with upper floor (X dir.)
60.00
Infill Masonary
35.00
Bare Frame
10.00
5th

-15.00

4th

3rd

2nd

1st

-40.00-65.00

Ratio

Figure 3. Stiffness difference (%).
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0

Ratio of lateral stiffness (X-direction)
Infill Masonary
Bare Frame

Roof

5th

4th
3rd
Story ID

2nd

1st

Figure 4. Stiffness ratio.
3.3. Story displacement and inter story drift

3

Allowable drift 0.01

Building height (m)Story drift at performance point
Figure 5 displays that, in the case of soft first story
25
structures, soft first story undergoes large
Soft First Story
Bare Frame
20
deformation beyond the elastic limit, and upper
floors experience very subtle inter story
15
displacement. Soft first story exceeds the allowable
10
drift limit (1%) as mentioned in BNBC-2015 andSoft story level
FEMA-356 for occupancy category IV structures.
5
So, soft first story columns are very vulnerable to0earthquakes if they don’t have adequate ductility
0
0.005
0.01
0.015
0.02
Story drift
and strength to meet the high ductility demand. As
Figure 5. Inter story drift.
a sudden change of story drift occurs in the soft firststory, it enhances the probability of forming the non
uniform plastic hinge in soft first story columns and severe damage or even collapse during earthquakes.
But in case of bare frames, uniform change of the inter story drift is observed. If a soft first story building
is designed by only bare frame analysis, it may suffer severe damage or collapse during an earthquake.

3.4. Hinge mechanism and damage distribution
In Figure 6 and 7, hinge mechanisms at
performance points of the building’s elevation
04-04 considering a soft story and bare frame areshown. The bare frame did not reach collapse
prevention (CP) or life safety (LS) state, and only
damage of immediate occupancy level (IO) is
distributed all through the structure. But soft story
columns suffered and concentrated in collapse
prevention level damage. Total 27 CP and 6 LS
hinge were formed in soft first story columns, but
only 24 IO hinges were formed when only bareframe was considered.

Figure 6. soft story case. Figure 7. bare frame case.

3.5. Seismic evaluation by JBDPA standard
1st level screening, X- direction

4

Iso = 0.58

5

5
4

3

Story IDIs

2nd level screening, X-direction

6

Iso = 0.48

6

Story ID

1st level screening
for bare frameshows the lack ofseismic capacity inall floors. 2nd levelscreeningshows,
when masonry is
considered, all thefloors except soft
first story haveadequate capacity.

22

Is BareFrame
Is withInfill3

1
0.00

Unsafe

Safe

1

0.20

Is 0.40

0.60

Figure 8. First level screening.

0.25

0.45

0.65 Is

0.85

1.05

nd

Figure 9. 2 level screening.

4. RETROFIT OF SOFT FIRST STORY
4.1. The retrofit strategy of the soft first story
Strategies of soft first story retrofitting are rather
different from conventional retrofitting of RC buildings.
The major objectives of a soft first story retrofit are toeliminate the extreme stiffness difference and control
excessive story drifts. Using column jacketing, steel
plate jacketing or FRP alone may not eliminate theextreme stiffness difference. Using only steel bracing
can provide stiffness to the frame but may require a large
number of bracings. So, a combination of these retrofit
methods can be a sustainable and cost effective solution
Figure 10. Retrofit option-2.
for retrofitting a soft first story. In option1, only columns
which are not adjacent to steel brace system are jacketed with 100 mm thick RC, and required numberof steel bracings are placed in outer frames. In option 2, column jacketing is done for columns adjacentto the steel bracing, which provides more inner spaces, performed better and recommended by this study.
In Figure 11 and 12, seismic index and inter story drift after retrofitting are presented which are within
the allowable limit. Improvement in capacity of spectral acceleration and elimination of CP hinges in
the retrofitted soft first story is presented in Figure 13 and 14 which ensures the structural safety.

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4
3

Building height (m)Iso = 0.48

Story ID

5

Is after Column
Jacketing(Op-2)
Is(original structure)
Final Is

2

Unsafe

Safe

1
0.25

0.40

0.55

0.70

0.85

20
15
10
Soft story levelRF( op-2)
Soft 1st Story
ColumnJacketing (op-2)

5
0

1.00

0

Is

Figure 11. Is of retrofitted structure.

0.005Story drift0.01

0.015

Figure 12. Inter story drift.

Performance point by CSM
Demand spectrum, BNBC2015
Bare frame

1
0.8

Sa (g)

Story drift at performance point

25

Allowable drift 0.01

Is of retrofitted structure

6

Soft story

0.6

RF Op-1

0.4

RF op-2
0.2
0
0

75

Sd (mm) 150

225

300

Figure 14. Hinge formation.

Figure 13. Performance point by CSM.
4.2. Retrofit with FRP and steel bracing: A-case study

FRP wrapping is an effective way to retrofit soft story because retrofitted columns have almost the same
size as before, do not add any weights and strength and ductility of existing RC columns can be upgraded.
FRP wrapping does not increase flexural strength much. So, the columns have shear failure mode canbe retrofitted by FRP. This can alter the failure mode from shear failure to flexural failure. Shear failure
is not desirable in RC structure as it causes sudden collapse without warning. The soft first story columns
ofthetargetbuilding
have Table 1. Enhanced of ductility index, strain and strength of FRP wrapped column.
ColumnF
Failure mode
F (after
Failure mode
Enhanced
Enhanced
flexuralfailureID(before retrofit)(before retrofit)
retrofit)(after retrofit)
strainstrength (Mpa)
C1
1.12
Shear
2.30
Flexural
0.010223.79
mode. For a case
C2
1.18
Shear
2.01
Flexural
0.012122.97
study, the shear
C4
1.03
Shear
2.95
Flexural
0.015923.48
reinforcement of
the target building ground floor columns are considered as d-10 mm having 250 c/c spacing to ensure
shear failure. Three layers of FRP having thickness 0.167 mm of each layer are used for retrofitting inthis case study. The upgraded ductility index and the changed failure mode is presented in Table 1.
Using Lam and Teng’s model the enhanced compressive strength and strain of FRP wrapped columns
are calculated and presented in Table 1.
4.3. Bracing connection design and cost analysis
Proper connection between steel
bracing and existing RC frames isvery important. RC shear walls
should be constructed beneath thesteel bracing to ensure safer
transfer of shear force. Summaryof the connection design isTable 2. Summary of bracing connection design.
Panel IDA(03-04)1(B-C)
1(C-D)
5(B-D)

Bolt
Total
no.
8
8
8
8

Dia
(mm)
12
16
16
12

5

Headed stud
S
Total
(mm)
no.
200
63
150
29
150
40
200
60

Dia
(mm)
16
16
16
16

Anchor bolt
S
Total
(mm)
no.
200
31
100
21
100
29
200
29

Ladder rebar
Spacing
Dia
(mm)
(mm)
200
12
200
12
200
12
200
12

presented in Table 2. By cost
analysis (Table 3), it isunderstood that retrofitting by
FRP is a costly option thanother retrofit methods.

Table 3. Cost analysis.
Retrofit option-1

Retrofit option-2

FRP + bracing

Column jacketing ($)$3056

Column jacketing ($)$3768

FRP ($)$11694

Steel bracing ($)$1149

Steel bracing ($)$11493

Steel bracing ($)$11493

Total cost ($)$14550Total cost ($)$15262

Total Cost ($)$23188

5. MAGNIFICATION FACTOR FOR SOFT FIRST STORY COLUMN

Magnificaion factor

Magnification factor
Magnification factor of flexural moment is the
3.00
ratio of flexural moment induced at soft first story
N=6
column to that of full infill column. This research
2.50
suggests that, magnification factor to design soft
N=4
story columns should not be any constant entity 2.00
which is mentioned by BNBC and ASCE as 2.5. It
N=2should vary with the number of stories and amount
1.50
of infill presents in the upper floors and stiffness
difference of soft story with upper floors. Three
1.00
equations are proposed by this research to
30
35
40
45
50
55
(-) Stiffness difference (%) of first story
calculate magnification factors of soft first story
columns with two, four and six storied buildings. Figure 15. Flexural moment magnification factor.

6. CONCLUSIONS AND RECOMMENDATIONS
Any RC building with a soft first story will suffer severe damage even collapse if designed by only bareframe analysis. Regular RC building designed by only bare frame analysis is proved to be safe in
Bangladesh. The soft story suffers large displacement during earthquake. Sudden extreme change ofinter story drift is one of the main causes of soft first story damage. By retrofitting, collapse or damageof soft first story can be minimized. Combination of RC column jacketing and adding steel bracing
which can eliminate stiffness difference and control excessive lateral drift is found sustainable to retrofit
a soft first story. Using FRP, column failure mode can be altered, and shear failure can be prevented.
The magnification factor to design soft first story column should not be a constant entity. It should varywith the number of story above soft first story and stiffness difference with the upper floors.
The exact shear strength and ductility index should be investigated for the masonry used in
Bangladesh. Only diagonal compression failure of masonry infill was considered, therefore, sliding
shear and out of plane failure should be incorporated in future research. To calculate flexural moment
magnification factor, full building with more stories should be analyzed to determine it more precisely.
7. ACKNOWLEDGEMENTS
I would like to express my sincere and heartfelt gratitude to my supervisor Dr. Eng. Matsutaro Seki,
advisor Dr. T Kashima and Dr. T Azuhata for their valuable guidance, suggestions and benevolence.
REFERENCES
Al-Chaar G., Lamb G.E., Issa M.A., 2003, ACI Special Publication. SP-211-12; 247–261.
AlWashali, H., Torihata, Y., Jin, K., and Maeda, M., 2018, 16th European Conference of Earthquake Engineering.
Applied Technology Council (ATC), 1996, Redwood City, California.
BNBC, 2015, Final Draft Version Bangladesh National Building Code 2015.
FEMA-356, 2000, Federal Emergency Management Agency, Washington, USA.
Japan Building Disaster Prevention Association (JBDPA), 2001.
Seki M., 2017, 16th WCEE, Santiago, Chile, Paper No. 566.

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