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Effect of Soft Story on Structural Response of High Rise
Buildings
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Effect of Soft Story on Structural Response of High Rise
Buildings
F. Hejazi11,2 S. Jilani,1,2, J. Noorzaei1,2, C. Y. Chieng1, M.
S. Jaafar1,2, A. A. Abang Ali1 1Department of Civil, Engineering
faculty, University Putra Malaysia, 43300 Serdang, Selangor,
Malaysia 2Institute of Advanced Technology, University Putra
Malaysia, 43300 Serdang, Selangor, Malaysia Email:
[email protected]
Abstract Severe structural damage suffered by several modern
buildings during recent earthquakes illustrates the importance of
avoiding sudden changes in lateral stiffness and strength. Recent
earthquakes that occurred have shown that a large number of
existing reinforced concrete buildings are vulnerable to damage or
even collapse during a strong earthquake. While damage and collapse
due to soft story are most often observed in buildings, they can
also be developed in other types of structures. The lower level
containing the concrete columns behaved as a soft story in that the
columns were unable to provide adequate shear resistance during the
earthquake. Usually the most economical way of retrofitting such as
a building is by adding proper bracing to soft stories. So, in this
paper occurring of soft at the lower level of high rise buildings
subjected to earthquake has been studied. Also has been tired to
investigate on adding of bracing in various arrangements to
structure in order to reduce soft story effect on seismic response
of building. It is lead to assess the vulnerability level of
existing multi-storied buildings so that they can be retrofitted to
possess the minimum requirements. This will help in minimizing the
impending damages and catastrophes.
1. Introduction: A soft story known as weak story is defined as
a story in a building that has substantially less resistance or
stiffness or inadequate ductility (energy absorption capacity) to
resist the earthquake-induced building stresses. Soft story
buildings are characterized by having a story which has a lot of
open space (Figure. 1). Parking garages, for example, are often
soft stories, as are large retail spaces or floors with a lot of
windows. Figure. 2 shows the image of a soft story.
Figure.1. Cross Section of a Soft Story Building
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
Engineering 17 (2011) 012034 doi:10.1088/1757-899X/17/1/012034
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If a building has a floor which is 70% less stiff than the floor
above it, it is considered a soft story building (UBC-1997,
IBC-2003 and ASCE-2002). This soft story creates a major weak point
in an earthquake, and since soft stories are classically associated
with retail spaces and parking garages, they are often on the lower
stories of a building, which means that when they collapse, they
can take the whole building down with them, causing serious
structural damage which may render the structure totally unusable.
The concept of the soft story has been recognized long back by
Fintel and Khan (1969). This concept is an attempt to reduce
acceleration in a building by allowing the first-storey column to
yield during an earthquake and produce energy-dissipation action.
However, excessive drifts in the first story coupled with P-
effects on the yielded columns make buildings collapse. A new
approach was proposed by Mo and Chang (1995) for soft story
building which in this system, Teflon sliders are placed on the top
of the first story reinforced concrete framed shear walls. These
shear walls are framed by columns and beams, and are designed to
carry a portion of the weight of the superstructure and the lateral
load determined by the frictional characteristics of the Teflon
sliders. The remaining first-story columns are designed for ductile
behavior in order to accommodate large drifts. Arlekar (1997)
highlighted the importance of explicitly recognizing the presence
of the open first storey in the seismic analysis of buildings. The
error involved in modeling such buildings as complete bare frames,
neglecting the presence of infills in the upper storeys, was
brought out with different analytical models. Kanitkar and Kanitkar
(2004) studied a five-storey building with soft storey with the
intent of reviewing the new provisions for the earthquake resistant
design of structures addressed in the code IS 1893:2002. Lee and Co
(2007) quantified the effect of the shear walls located in the
lower soft stories on the general seismic response characteristics.
Mastrandrea and Piluso (2009) proposed a design methodology for
development of a collapse mechanism of a global type for
eccentrically braced frames in soft story buildings. Tesfamariam
and Liu (2010) used special index for soft story for seismic risk
assessment of reinforced concrete buildings as classification
vulnerability techniques.
Figure.2. Example of a soft story at the ground floor
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
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It was observed that although the new soft storey provisions of
IS 1893 are a step in the right direction, more investigations are
required to completely define the resulting buildings in terms of
their ductility capacities and stable inelastic action under the
expected seismic loads. The objective of this paper is to study the
effect of soft story on structural behavior of high rise buildings
and retrofitting and seismic rehabilitation of soft story
structures through bracing devices. Also compare the soft story
structural response of high rise building with various type of
bracing arrangement on building and finding of optimum design of
earthquake resistance soft story buildings by considering of
required performance level. 2. Building Analyzed The intention to
study a 12 reinforced concrete framed storey building is to see the
effect of high rise building on the earthquake and the effect of
soft story which are designed at the bottom floor of the building.
Application of building is consider as Hotel and three dimension
view of structure are showed in Figure. 3.
Figure.3. Three dimensional view of considered model
Stories height is in 3.3m except first and top floors. First
story height is 5m and for top floor, the storey height is only
2.8m. The column size is increasing from top floor to bottom due to
the increase in loading that the columns have to support. The frame
members are modeled with rigid end zones, and the floors are
modeled as diaphragms rigid in-plane. 3. Loading The loading
calculation was made based on BS6399 CODE of practice. Table (1)
shows the detail calculation of weight of building due dead and
live load. The dead load and live load and calculation of weight of
floors are tabulate on Table (1). Based on the code just 20% of
live load is considered for calculation of weight of building in
earthquake excitation.
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Table 1. Total weight for each floor
Describe
Unit
area
m2
No
Dead load kN/m2
Live load
kN/m2
Total weight kN
Bedroom 36
14
4.7651
20.2= 0.4
2603.2
Corridor 90 1
4.7651
40.2= 0.8 500.8
staircase 18 2 6.376
40.2= 0.8 258.3
roof 18 4 4.605 0.750.2= 0.15
342.3
External wall
3.0* -- 2.6 --
internal wall
3.0* -- 1.373 --
1012.9
*height of wall So total weight of building is determined as
follow:
Total weight for each floor:
w1 = 2603.2 + 500.8 + 258.3 + 1012.9
= 4375.3 kN
Total weight for 11th floor:
w11 = (3614)4.755+ 500.8 + 258.3 + 012.9
= 4168.6
Total weight for whole building:
W = 10 w1+ w11+ Roof load
= 10 4375.3 + 4168.6+342.3= 48264 kN
The condition of earthquake load calculation in this design is
assumed that the location of building is in medium zone of risk for
earthquake to happen. Also the soil condition is assumed as beds of
gravel and sand with weak cementation (soil type III). Equivalent
static base shear force due to earthquake load is calculated as
follow:
Vb = c w Where Vb is shear force due to earthquake load, c is
earthquake coefficient and w is total weight of building with
consider of dead load and 20% of live load. Base on the code,
earthquake coefficient for considered building is 0.0821 in mention
condition. So earthquake equivalent shear force will be determined
as:
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
Engineering 17 (2011) 012034 doi:10.1088/1757-899X/17/1/012034
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Vb = 0.0821 48264 = 3962 kN After calculate the base shear, by
using equation below, the lateral force due earthquake imposed on
each floor are calculated and tabulated in table (2).
Table 2. Lateral force for each storey
Floor Storey height
m
Height above base
m
Weight of
storey kN
Lateral force kN
1 5 5 4375.3 83.27139
2 3.3 8.3 4375.3 138.2305
3 3.3 11.6 4375.3 193.1896
4 3.3 14.9 4375.3 248.1487
5 3.3 18.2 4375.3 303.1079
6 3.3 21.5 4375.3 358.067
7 3.3 24.8 4375.3 413.0261
8 3.3 28.1 4375.3 467.9852
9 3.3 31.4 4375.3 522.9443
10 3.3 34.7 4375.3 577.9035
11 3.3 38 4168.6 602.9646
12 2.8 40.8 342.3 53.16001
4. Retrofit models Usually the most economical way of
retrofitting of soft story building is by adding proper bracing to
soft stories. For investigation on effect of different bracing
installation arrangement on building in seismic response of
structure with soft story at bottom, 6 models are designed with
different condition: 4.1. Model 1 The structure with out any
lateral load resistance system is called model 1 in this paper and
elevation view of the building is showed in Figure. 4.
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Figure. 1. Model 1 - Soft storey at bottom floor of structure
4.2. Model 2 The model 1 (Soft story at bottom) is modified into
this model with adding the bracing at the lowest floor. The bracing
is added at all the first floor as shown in Figure. 5. So Model 2
is a Model that tries to eliminate the effect of soft story at the
lowest floor and reduce soft story effect on seismic response of
retrofitted building. 4.3. Model 3 Model 3 is also a bracing
building. The bracing is only added at the centre bay of the
building. In this Model, the soft story at the lowest floor has
been added the bracing in center bay too. Elevation view of this
model is shown in Figure. 6.
Figure 2. Model 2 Add bracing in first floor
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Figure. 3. Model 3 Bracing at the center bay 4.4. Model 4 As
shown in Figure. 7, in this model, bracings are added from second
floor to top floor but not all the members are braced. This model
has a soft story at the bottom floor too. Include this model; there
are a total number of 5 models of bracing building.
Figure. 4. Model 4 Bracings are added to some bays from second
floor to top floor 4.5. Model 5 The difference of model 5 with
model 4 is bracing added in all bay from second floor to the uppest
floor of the building. So as expected the soft story will happened
at first floor. This model is shown in Figure. 8.
Figure. 5. Model 5 All bays bracing from second floor 4.6. Model
6
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In order to see the effect and the use of bracing in the whole
bay, model 6 is prepared and all bays in all floors are braced in
both direction. However, this type of model is seldom seen in real
live as it might affect the aesthetically of the whole
building.
Figure. 6. Model 6 Bracing installed at all bays and all floors
5. Concept analysis of building Linear and nonlinear static
analyses are performed for the all models of the building which
described before, by SAP2000 software package. After preparing the
physical model of building and define of material properties and
beam, columns sections, live and dead load applied to the structure
and earthquake static equivalent coefficient have been determined
and imposed to the model. Then different load combination cases are
considered in order to get the critical result. Stiffness of each
stories are calculated and tabulated in order to recognize of soft
story in different models. The response of structures in term of
displacement of model in height of structure is evaluated and
plotted into graph for comparison. 6. Result and discussion Table
(3) and Table (4) show the stiffness in X and Y direction
corresponding for different type of bracing buildings. Generally,
the stiffness is decrease while increase the number of floors. That
is because for upper floor, the loading that should be carried by
the column is lower as at upper floor, only few floors of weight
need to be carried. However, the stiffness at the bottom floor is
still lower than second although bracing is added on the lowest
floor for Model 2, Model 3 and Model 6. The stiffness of floors for
model 1 to 6 are calculated and tabulated in Table (3) for X
direction and Table (4) for Y direction. All models except model 1
are bracing building which braces are added in different
arrangement. As seen in the table, from out of 6 models, 5 models
have the soft story at the lowest floor. Relative stiffness
(stiffness of each story compare of above story stiffness) of first
story in model 1 is below 70% and it is shown that the soft story
happened in this floor. For solving of soft story problem, bracings
are added to this floor in model 2 on both side of building.
Although the relative stiffness is still below 1 in X and Y
direction but it is not consider as a soft story as its relative
stiffness is greater than 70%. In model 3, where bracing is only
added at the center bay of the building, base on the relative
stiffness values for this building, soft story is happened in first
floor. Model 4 and Model 5 shows a very soft story in the lowest
floor as the relative stiffness to the storey above is less than
25% in X direction and 20% in Y direction.
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
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Table 3. Relative stiffness of stories X direction for Model 1
to Model 6
Relative stiffness of stories in X
direction No. of floors Model 1 Model 2 Model 3
1 0.335682 0.775756 0.362097 2 1.135028 1.135028 1.123185 3
1.189777 1.189777 1.170297 4 1.530209 1.530209 1.451228 5 1.085898
1.085898 1.072179 6 1.222282 1.222282 1.180376 7 1.348433 1.348433
1.265316 8 1.53476 1.534759 1.361131 9 1.839861 1.839863
1.445646
10 6.244613 6.244577 1.803904 11 1.221694 1.221701 0.941562
No. of floors Model 4 Model 5 Model 6
1 0.250729 0.210731 0.461593 2 1.097524 1.080711 1.080711 3
1.130203 1.10539 1.10539 4 1.311861 1.238269 1.238269 5 1.048798
1.036858 1.036858 6 1.115215 1.084639 1.084639 7 1.154647 1.109128
1 8 1.182938 1.122495 1.244991 9 1.185058 1.116775 1.11684
10 1.227082 1.132214 1.132148 11 0.914329 1.570384 1.570384
Because in this models by adding the bracing on all floors
except bottom floor, effect of soft story is intensified. All bays
in all floors are equipped bracing in model 6 on both side.
Although the stiffness of floors is so high, but as seen in Table
(3) and (4) soft story happened in first floor on both direction
again because of first story height. Displacement of different type
of bracing buildings against storey height in horizontal direction
(horizontal movement) under earthquake excitation is showed in
Figureure 10. As seen in the plot displacements are increasing when
the storey height increased. This graph is plotted base on
nonlinear response of reinforced structure response and that it is
easy to make the comparison and the effect of using different type
of bracing in building.
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
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Table 4. Relative stiffness of stories Y direction for Model 1
to Model 6
Relative stiffness of stories in Y direction No. of floors
Model 1 Model 2 Model 3 1 0.301 0.958 0.338 2 1.219 1.219 1.155
3 1.099 1.099 1.068 4 1.147 1.147 1.097 5 1.023 1.023 1.015 6 1.065
1.065 1.042 7 1.316 1.316 1.181 8 1.461 1.461 1.221 9 1.642 1.642
1.230
10 2.791 2.791 1.299 11 1.222 1.222 7.329
No. of floors Model 4 Model 5 Model 6
1 0.172 0.144 0.459 2 1.114 1.094 1.094 3 1.050 1.040 1.040 4
1.068 1.055 1.055 5 1.011 1.008 1.008 6 1.029 1.023 1.023 7 1.119
1.093 1.093 8 1.135 1.102 1.102 9 1.129 1.095 1.095
10 1.149 1.105 1.105 11 14.719 2.356 2.356
Since all the models are fixed at the ground floor, there is no
displacement at ground floor. Model 1, 4 and 5 states a maximum
displacement in horizontal direction at the soft story (First
story) about 11.085, 9.504 and 8.822mm respectively. It is because
there is no bracing at the soft story of these models and the
lateral strength become very weak in compare of other models. The
maximum displacement at top floor is cause by Model 1 with a
displacement of 148.57 mm. Also, model 2 have a maximum
displacement in top story (139.131mm) in compare of other bracing
buildings. As describe before only in this model soft story problem
is solved by adding bracing. For this reason horizontal movement of
this model at first story is less than model 3 to 5. But because of
install bracing just at first floor of model 2, stiffness of other
floors are less in compare of other models, so relevant
displacements is so more. Stiffness of structure stories in Model
3, 4, 5, 6 respectively are increased because of bracing arrange
format on buildings.
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
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Bracing are added just in 1 bay in model 3, 4 bay in model 4, 7
bay from second floor up to top in model 5 and in all bays on model
6, therefore number of using braces and stiffness of buildings are
increasing respectively in models. Top floor horizontal
displacements of model 3, 4, 5, 6 are evaluated about 101.59,
59.775, 28.068 and 21.422 respectively.
Figure. 7 Displacement of building in X direction against Storey
height for different type of bracing
buildings
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100 120 140 160
Stor
ey h
eigh
t (m
)
Displacement (mm)
Model 1 Model 2 Model 3 Model 4 Model 5 Model 6
Figure. 8 Displacement of building in Y direction against Storey
height for different type of bracing
buildings
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250
Stor
ey h
eigh
t (m
)
Displacement (mm)
Model 1 Model 2 Model 3 Model 4 Model 5 Model 6
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From this graph, the best model to resist earthquake is Model 6
with the minimum displacement for overall buildings although it is
a soft story model. In actual buildings, it is impossible to add
bracing in all bays but from the result, it is observed that effect
of soft story can reduced by adding some earthquake resistant
method such as bracing. Figure. 11 shows vertical displacement of
buildings against the storey height for various modeled which
explained as above. As seen in this plot, although Model 2 is not a
soft storey, it has a maximum displacement in bracing buildings at
each floor unless the first floor. This is because the bracing is
only be added at the first floor, so the displacement in the first
floor is relatively small compare with others. After the first
floor, it starts to deviate much as the no bracing at all start
from second floor. Model 3 also has great effect during earthquake
although the entire stories are braced. This is due the bracing
Model 3 only added at the centre of the building. Earthquake is a
vibration in the whole building so that if the braced is only added
at the centre of the building, it will not give good effect on
earthquake resistant. Model 4 and Model 5 state a maximum
displacement at the lowest floor as the relative stiffness is too
low compare with others. Model 6 shows a most ideal condition
during earthquake, the maximum displacement is only 20mm while
other gives a larger value for this. From these few Models, bracing
is one of the method that can be used to resist earthquake compare
to moment resisting frame. It is because beside increase the
strength in the member, it also increases the overall stiffness in
the building. However, the bracing must be added at the correct
position in order to get good effect on earthquake resistance. With
evaluation of various bracing arrangements on structure,
vulnerability level of existing multi-storied buildings is assess
and it helped for design new and exist building retrofit plan on
consider level of operation and safety with minimum requirements.
6. Conclusion RC frame buildings with open first storeys are known
to perform poorly during in strong earthquake shaking. The large
opening on the lowest floor causing the stiffness is relative low
compare to the stiffness at the storey above. The stiffness at
lower floor is 70% lesser than stiffness at storey above it causing
the soft storey to happen. The lateral strength of the building is
related to the stiffness. The lateral strength of a building is sum
of all the stiffness from column, shear wall and bracing are added
at each storey. So the low strength in the lowest floor causing the
failure occurs especially during earthquake. For a building that is
not provided any lateral load resistance component such as bracing
or shear wall, the strength is consider very weak and easy fail
during earthquake. In this paper it has been tried to investigate
on adding bracing to the building in different arrangement in order
to reduce soft story effect on structural seismic response. It was
found that location and numbering of bracing acts an important
factor for the soft story structures to displace during earthquake.
Also the soft story has been eliminated as the bracing is added to
the consider floor, although the displacement at the top floor is
still very high because there is no bracing at the top floor. So,
result show that a bracing will only makes a different result for
the storey that equipped with bracing. The horizontal and vertical
movements of building which bracing installed in most bays are much
reduced during earthquake compare with other models. So it shows
that the use of bracing is effectively reduced effect of soft story
on structure response in earthquake excitation. In the other hand,
vulnerability level of existing multi-storied buildings is assess
by analysis of different arrangement of bracing on building and it
helped for retrofitting of structure on consider
CAMAN IOP PublishingIOP Conf. Series: Materials Science and
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level of operation and safety with minimum requirements.
Acknowledgment This work has received financial support from
Ministry of Science, Technology and Innovation of Malaysia under
Research Project No. 5450366 and supported by Institute of Advance
Technology of University Putra Malaysia and gratefully
acknowledged. References [1] M. Fintel and F. Khan (1969) "
Shock-absorbing soft story concept for multistory earthquake
structures ". Proceeding. [2] Farzad Naeim (2001), The seismic
design handbook, 2nd Ed. Springer, 304p-306p [3] James E. Ambrose,
Dimitry Vergun (1995), Simplified Building Design for Wind and
Earthquake Forces, John Wiley and Sons. [4] Brayan Stafford Smith
(1991), Tall Building Structures: Analysis And Design, John Wiley
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Application of base isolation concept to soft first story buildings
", journal of Computers and Structures, Vol. 55, pp 883-896 [7]
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Building [8] American Society of Civil Engineers (2002). Seismic
Evaluation of Existing Buildings, American Society of Civil
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[10] International Building Code (2003). International Code
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performance of conventional multi-story building with open ground
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