M. S. DONDUREN, A. NAKIPOGLU: COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY ...
575–581
COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY
IRREGULARITY WITH RESPECT TO VARIOUS NATIONAL
BUILDING CODES
PRIMERJAVA NEPRAVILNOSTI NA VE^NADSTROPNIH
BETONSKIH ZGRADBAH GLEDE NA RAZLI^NE NACIONALNE
GRADBENE PREDPISE
Mahmud Sami Donduren, Abdulhamit Nakipoglu
Selcuk University, Faculty of Engineering, Department of Civil Engineering, 42130 Konya, Turkey
sdonduren@selcuk.edu.tr
Prejem rokopisa – received: 2018-02-02; sprejem za objavo – accepted for publication: 2018-03-22
doi:10.17222/mit.2018.015
In the design of earthquake-resistant reinforced-concrete structural systems, the necessity to construct a regular structure is one
of the main principles. Building irregularities generally become obvious with the effect of a seismic load. It is crucial that the
irregularities of structural systems should be considered properly with respect to the conditions determined by the building
codes. In this study, the soft-storey irregularity that causes most of the losses and damage in earthquakes was investigated with
respect to the criterions of various national building codes. Eleven sub-models were produced on the basis of a general building
model, and they were analyzed with respect to the conditions of the codes relating to the soft-storey phenomenon using the
SAP 2000 structural-analysis program. The first-storey heights of the models were different from each other while all the other
parameters and the geometries were the same. In this way, the codes were compared in terms of the effect of the storey height on
the formation of a soft storey. Eventually, it was observed that, especially in the Japanese Seismic Code, the soft-storey
irregularity is handled very sensibly and safely.
Keywords: earthquake, irregularities, codes, soft storey, SAP (structural-analysis program) 2000
Konstruiranje betonsko oja~anih struktur oz. zgradb, odpornih proti potresom, zahteva upo{tevanje nekaterih glavnih principov,
med katere spada tudi obvezna gradnja pravilne oz. simetri~ne strukture. Negativne posledice nepravilnosti v zgradbah se pojav-
ljajo predvsem zaradi u~inkov seizmi~nih bremen. Najbolj pomembno je, da se nepravilnosti na strukturnih sistemih ocenjujejo
na ustrezen na~in glede na stanje, ugotovljeno s pomo~jo gradbenih predpisov. V pri~ujo~i {tudiji avtorji raziskujejo napake na
ve~nadstropnih zgradbah (angl.: soft-storey buildings), ki povzro~ajo ve~ino izgub in po{kodb zaradi potresa, ter jih primerjajo s
kriteriji razli~nih nacionalnih gradbenih predpisov. Na osnovi splo{nega gradbenega modela so pripravili enajst (11) podmode-
lov in jih analizirali s pomo~jo programa za analizo struktur SAP 2000 glede na pogoje, podane s predpisi o ve~nadstropnih
zgradbah (soft-storey buildings). V modelih so izbirali razli~no vi{ino prvega nadstropja, vsi drugi parametri in geometrija
zgradb pa je ostala enaka. Na ta na~in so lahko medsebojno primerjali predpise glede na vpliv vi{ine nadstropja na zgradbo. Na
koncu ugotavljajo, da so {e posebej japonski seizmi~ni predpisi, ki zagotavljajo varnost ve~nadstropnih zgradb pred potresi,
najbolj{i v smislu ob~utljivosti in varnosti le-teh.
Klju~ne besede: potres, nepravilnosti, kode (predpisi), ve~ nadstropne zgradbe, program za analizo struktur – SAP 2000
1 INTRODUCTION
As we all know, a large part of the world is located
along seismic belts. Therefore, the importance of the
accuracy of a seismic analysis is very crucial in civil-
engineering structural projects. If we think that a build-
ing can be subjected to a seismic load in its lifetime, it
becomes crucial that the R/C structural system should be
created with the utmost engineering accuracy. An
earthquake can be described as a failure of the Earth’s
crust at a significant depth, effecting the crust’s tension.
The magnitude of an earthquake indicates the level of the
failure, thereby also indicating the amount of the re-
leased energy.
1
Regular structural systems are practical, economical
and, most importantly, safe with respect to the structural
analysis, application, dimensioning, etc. The buildings
that need to be avoided during design and construction
work due to the risks regarding their seismic behavior
are described as irregular buildings by the seismic codes.
Interstorey-rigidity irregularity or, in other words, a soft
storey is a vertical structural irregularity, which causes
the most significant loss and damage among all the
structural irregularities. So, we need to pay a lot of
attention to this irregularity when designing a structure.
2
The irregularity conditions from different codes
differ from each other with respect to analyses, calcul-
ations and/or approaches.
3,4
These differences between
the codes are the results of the seismicity and different
soil conditions of a country or region where the code is
used.
5
Similar studies were conducted by other research-
ers.
6–9
In this study, various national building codes were
compared regarding the structural-system irregularities
to reveal the differences.
Materiali in tehnologije / Materials and technology 52 (2018) 5, 575–581 575
UDK 550.34:699.841:69.032.2:620.19:691.32 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(5)575(2018)

1.1 Soft storey (interstorey-rigidity irregularity)
A soft storey is a phenomenon indicating that a
storey’s rigidity is higher or much lower than that of the
storey above or below. In this case, abrupt changes in the
amount of relative storey displacements of the adjacent
storeys occur. Thereby, the storey that has less rigidity
and more displacements is defined as a soft storey. The
existence of a soft storey in a building can cause great
damage as shown in Figure 1.
The most important reasons of the formation of the
soft-storey phenomenon are:
• The building’s first-storey height is much greater than
the second-storey height (or the height of any storey
is much greater than that of the storey above).
• The first storey has very few or no infill walls in
comparison to the second storey.
2 ANALYTICAL PART
When comparing the building codes, the best results
were obtained with the help of analytical calculations. In
this stage, one of the most critical irregularities, the soft
storey, was handled with the SAP2000 program. The
criterions for a soft storey in the Turkish Seismic Code
(TSC 2007), American Code of Minimum Design Loads
for Buildings and Other Structures (ASCE 7-2002),
Indian Seismic Code (IS 1893-1, 2002) and Japanese
Building Code were taken into consideration. The calcul-
ation methods of these codes were compared. Within this
scope, a building model, which was symmetrical with
respect to the plan dimensions, was converted into seve-
ral sub-models, based on the condition that all the para-
meters were the same and constant except for the
changes in the height of the first storey. The main
objective was to obtain the limit values of these codes
regarding the soft-storey issue by changing the model’s
first-storey height to form a soft storey. The heights,
which result in a soft storey, were compared and the data
obtained with analytical calculations allowed a
comparison of the storey-irregularitiy criterions included
in different codes.
2.1 Information about the model
The general model is an R/C frame system with co-
lumns and beams. The plan and 3D views of the model
are given in Figure 2.
The parameters used in the analyses are given in
Table 1.
Table 1: Parameters of the general model
Type of buiding’s structural
system:
Frame with beams and
columns
Total distance in the x direction: 9 m
Total distance in the y direction: 9 m
Span length between the axes: 3 m
Storey heights: 3 m
First-storey height: h = 3 m (changes)
Number of storeys: 4
Total building height: 12–14 m (changes)
Seismic zone:
1.seismic zone (most
severe)
Soil class: Z3 (medium firm)
Building importance factor: I=1
Response-reduction factor: R=8
Concrete strength:
C25 (25 MPa compressive
strength)
Rebar strength:
S420 (420 MPa yield
strength)
Column dimensions: 30 cm × 30 cm
Beam dimensions: 30 cm × 50 cm
Slab thickness: 15 cm
Beam loads: G = 6 kN/m
Slab loads:
G = 1.5 kN/m
2
, Q=2
kN/m
2
Seismic load:
1999 Düzce earthquake,
east-west direction (time
history)
The first-storey heights of the models are given in
Table 2.
Table 2: First storey heights of the models
Model number
First storey
height
Model number
First storey
height
1 3.00 m 7 4.30 m
2 3.30 m 8 4.35 m
3 3.70 m 9 4.40 m
4 3.85 m 10 4.50 m
5 4.00 m 11 5.00 m
6 4.25 m - -
M. S. DONDUREN, A. NAKIPOGLU: COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY ...
576 Materiali in tehnologije / Materials and technology 52 (2018) 5, 575–581
Figure 2: General model’s plan and 3D views
Figure 1: Soft-storey damage
1

2.2 Load conditions
The calculations were done with three seismic-anal-
ysis methods including an equivalent static load, a modal
analysis and time history. It was found that the maximum
displacement values for the storeys occured during the
analyses using the time-history method. Hence, the
time-history method was chosen for the analysis. For the
seismic load, the acceleration records of the 1999 Düzce
Earthquake in the east-west direction were used. Fig-
ure 3 shows the displaced view of the general model
after the loading.
2.3 Storey displacements
To get the limit values of the codes regarding the soft
storey, eleven sub-models were designed. Lateral storey
displacements of each model were obtained with
SAP2000 and tabulated for the calculations of storey
rigidities.
3 CALCULATIONS AND RESULTS
3.1 Calculations according to the Turkish Seismic
Code (TSC 2007)
With respect to the TSC 2007, the soft-storey irregu-
larity plays an important role when choosing the
seismic-analysis method different from the other modern
countries’ codes. The soft-storey conditions from the
TSC 2007 are expressed with Equations (1) and (2):
 ki
=(  i
/h
i
)
avg
/(  i+1
/h
i+1
)
avg
> 2.0  Soft Storey (1)
 ki
= ( i
/h
i
)
avg
/(  i–1
/h
i–1
)
avg
> 2.0  Soft Storey (2)
where  is the storey displacement, and h is the storey
height.
10
According to the storey-displacement values obtained
with the analyses, Equations (1) and (2) were applied
and the critical first-storey height was determined. With
regard to the calculations, Model 8 was found to have the
critical height of the soft-storey formation of 4.35 m.
Figure 4 shows joint displacements of Model 8.
Lateral-displacement values for Model 8 are tabu-
lated in Table 3.
Table 3: Displacement values for Model 8
h = 4.35m
Joint
Displacement-x
(mm)
41 11.97
29 16.06
17 18.94
1 20.54
The calculation for Model 8 is shown in Equation
(3):
h
h
h
k1
1
1
2
2
1197
435
1606 1197
3
==
−
Δ
Δ
.
.
(. .)
(3)
 k1
= 2.02 > 2.0  Critical – Soft storey formation with
a minor difference
In Table 4, the results of all the models are given.
Based on the observations of all the calculations, we can
say that the soft-storey irregularity was first formed in
Model 8 and then in the other models where the first-
storey height was more than 4.35 m. The value of  ki
=
2.032 > 2.0 was confirmed to be the critical value of the
soft-storey formation according to the TSC 2007. In
other words, the minimum height of the first storey,
M. S. DONDUREN, A. NAKIPOGLU: COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 575–581 577
Figure 4: Joint displacements of Model 8
Figure 3: General model’s displaced view after the loading

which made this building form a soft storey with respect
to the TSC 2007, was 4.35 m.
Table 4: Models’ soft-storey-irregularity situations according to TSC
2007
Model
number
h, height of
the first storey
(m)
 ki
(rigidity-irregularity
index)
Soft-storey
irregularity
situation
1 3.00 1.05 < 2.0 Regular
2 3.30 1.12 < 2.0 Regular
3 3.70 1.53 < 2.0 Regular
4 3.85 1.57 < 2.0 Regular
5 4.00 1.75 < 2.0 Regular
6 4.25 1.94 < 2.0 Regular
7 4.30 1.98 < 2.0 Regular
8 4.35 2.03 > 2.0
Critical -
Irregular
9 4.40 2.05 > 2.0 Irregular
10 4.50 2.12 > 2.0 Irregular
11 5.00 2.48 > 2.0 Irregular
3.2 Calculations according to the Indian and Ameri-
can Seismic Codes (IS 1893-1, ASCE 7)
The American Code (ASCE 7) and Indian Code (IS
1893-1) treat a lot of formulations and obligations
regarding the soft-storey conditions in the same way.
These codes scrutinize the soft-storey issue for two
cases.
11,12
1. Normal Soft Storey: If a storey’s lateral stiffness is
less than 70 % of the stiffness of the storey above it
or less then 80 % of the average stiffness of the above
three storeyes, this storey is a soft storey.
2. Extreme Soft Storey: A storey’s lateral stiffness is less
then 60 % of the above storey’s stiffness or less then
70 % of the average stiffness of the above three sto-
reyes.
Equations (4) and (5) show the conditions of a soft-
storey formation.
k
i
< 0.7k
i+1
 Soft storey (4)
or
k
i
< 0.8
kkk
iii +++
++ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 123
3
 Soft storey (5)
where k is the lateral-storey stiffness.
11,12
To find storey
stiffness k, Equation (6) is used.
F=k×u (6)
where F is the storey’s equivalent static seismic load
and u is the lateral displacement value in the seismic
direction. The condition for the formation of a soft
storey according to the ASCE/Indian codes is also given
in Figure 5.
To obtain more realistic results and determine the
equivalent static seismic loads of the storeys, it is more
appropriate to calculate the loads on the basis of the
Indian equivalent static seismic load method. These cal-
culations were done for eleven models. The results of the
analyses showed that Model 10 was the critical model
according to the Indian Seismic Code. The first-storey
height of this model was h = 4.50 m. The lateral-dis-
placement values for Model 10 are given in Table 5.
Table 5: Displacement values for Model 10
h = 4.50 m
Joint
Displacement-x
(mm)
41 13.30
29 16.91
17 20.25
1 21.91
The calculations of the equivalent static seismic load
and lateral-acceleration spectrum were done using
Equations (7) and (8), respectively.
V
B
= A
h
× W (7)
A
ZIS
Rg
h
a
=
2
(8)
where V
B
is the total equivalent static seismic load, A
h
is
the lateral acceleration spectrum and W is the total
weight of the building in Equation (7). In Equation (8),
Z is the seismic zone (from the table), I is the building
importance factor (from the table), R is the response
reduction factor (from the table) and S
a
/g is the average
response-acceleration coefficient.
12
The calculation formula for the equivalent static
seismic load applying to any storey is given in Equation
(9).
QV
wh
wh
iB
ii
jh j
n
=
=
∑
2
2
1
(9)
The natural-period calculation for RC-frame build-
ings is done with Equation (10) where h is the building’s
total height in meters.
T
a
= 0.075 h
0.75
(10)
The above equations were applied to all the models
and the calculations for the critical one (Model 10) are
given below.
M. S. DONDUREN, A. NAKIPOGLU: COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY ...
578 Materiali in tehnologije / Materials and technology 52 (2018) 5, 575–581
Figure 5: Condition for a soft storey
12

Q
1
2
22
7773
14302 450
3 12506 3 14302 450
=×
×
××+×
= .
..
(.)..
138.9 kN
Q
2
2
22
7773
12506 3
3 12506 3 14302 450
=×
×
××+×
= .
.
(.)..
139.5 kN
k
F
u
1
3589
132985
==
.
.
= 26.99 kN/mm
k
F
u
2
1395
169111 132985
==
−
.
(. . )
= 38.61 kN/mm
26.99 < 0.7 × 38.61 = 27.027  Critical – Soft storey
formation with a minor difference
In Table 6, soft-storey results for all the storeys are
given. The outcomes of the analyses show that Model 10
is the critical one according to the Indian Seismic Code.
It was found that the models with a first-storey height of
4.50 m or more exhibited a soft-storey irregularity.
Table 6: Models’ soft-storey-irregularity situations according to
Indian Seismic Code (IS 1893-1)
Model
number
h, height
(m)
k
1
/k
2
(Storeys’
rigidity ratio)
Soft-storey-irreg
ularity situation
1 3.00 0.90 > 0.7 Regular
2 3.30 0.86 > 0.7 Regular
3 3.70 0.81 > 0.7 Regular
4 3.85 0.79 > 0.7 Regular
5 4.00 0.77 > 0.7 Regular
6 4.25 0.74 > 0.7 Regular
7 4.30 0.73 > 0.7 Regular
8 4.35 0.72 > 0.7 Regular
9 4.40 0.71 > 0.7 Regular
10 4.50 0.69 < 0.7
Critical –
Irregular
11 5.00 0.63 < 0.7 Irregular
3.3 Calculations according to the Japanese Seismic
Code
The Japanese Seismic Code determines the soft-sto-
rey irregularity using a much more different approach, as
shown below.
R
r
r
s
s
s
= , R
s
< 0.6  Soft storey (11)
where R
s
is the lateral-stiffness ratio and r
s
is the lateral
stiffness. The lateral stiffness is defined as the storey
height divided by the storey drift caused by the lateral
seismic shear for moderate earthquake motions. r
s
is the
avearage lateral storey stiffness.
13
Table 7: Displacement values for Model 2
h = 3.30 m
Joint Displacement-x (mm)
41 6.12
29 11.11
17 14.39
1 16.32
In the results of the analyses, Model 2 was deter-
mined as the critical one according to the Japanese
Seismic Code. This model’s first-storey height was 3.30
m. The resulting displacement values for Model 2 are
given in Table 7.
Calculations were done for all the models and the
solution for Model 2 is given below:
r
h
s1
==
Δ
33
612
.
.
= 0.54 m/mm
r
h
s2
==
− Δ
3
1111 612 (. .)
= 0.60 m/mm
r
h
s3
==
− Δ
3
1439 1111 (. .)
= 0.92 m/mm
r
h
s4
==
− Δ
3
1632 1439 (. .)
= 1.55 m/mm
r
rrrr
s
s1 s2 s3 s4
=
+++
=
+++
4
054 060 092 155
4
....
=
=0.902 m/mm
R
r
r
s
s1
s1
===
054
092
0599
.
.
.
0.599 < 0.6  Critical – Soft storey formation with a
minor difference
Table 8 shows the soft-storey results for all the mo-
dels. According to the Japanese Seismic Code, the
critical model was Model 2 where the first-storey height
was 3.30 m.
Table 8: Models’ soft-storey-irregularity situations according to
Japanese Seismic Code
Model
number
h, height
(m)
R
s
(Lateral-
stiffness ratio)
Soft-storey-
irregularity situation
1 3.00 0.654 > 0.6 Regular
2 3.30 0.599 < 0.6 Critical – Irregular
3 3.70 0.429 < 0.6 Irregular
4 3.85 0.416 < 0.6 Irregular
5 4.00 0.398 < 0.6 Irregular
6 4.25 0.370 < 0.6 Irregular
7 4.30 0.366 < 0.6 Irregular
8 4.35 0.362 < 0.6 Irregular
9 4.40 0.358 < 0.6 Irregular
10 4.50 0.350 < 0.6 Irregular
11 5.00 0.310 < 0.6 Irregular
3.4 Soft-storey conditions from some other national
building codes
Eurocode-8
This code recommends an increase in the load-bear-
ing capacities of columns to compensate for the rigidity
loss due to the lack of infill walls in some storeys. The
European Standard also states that even if there are the
infill walls of a building’s first storey and if the building
is a residential one, there should be stirrups along all the
first storey’s columns, forming a stirrup-densification
M. S. DONDUREN, A. NAKIPOGLU: COMPARISON OF R/C BUILDINGS WITH A SOFT-STOREY ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 575–581 579

zone. This makes it possible to use the building as an
office or to remove the infill walls.
14,15
Bulgarian Seismic Standard
If the rigidity ratio of the adjacent storeys is less then
50 %, the less rigid storey is defined as a soft storey
according to the Bulgarian Seismic Standard. It is ad-
vised that the seismic load affecting the soft storey
should be calculated when determining the seismic-load
design of a building. It is desired that a building with a
soft storey should have a lateral-load capacity that is
three times larger than the potential load. Apart from
determining this situation, the standard does not make
any other calculation suggestions.
6
New Zealand Seismic Code (NZS 4203:1992)
The New Zealand Seismic Code states that to fulfil
the vertical-regularity requirement, when using the equi-
valent static method, the lateral displacements of indi-
vidual storeys should be reasonably close.
16
However, it
does not explain what reasonably close means.
Israeli Seismic Standard (SI-413, 1995)
When a storey’s lateral stiffness is less then 65 % of
the above storey’s stiffness or when a storey’s lateral
stiffness is less then 70 % of the average stiffness of the
above three storeys, a soft-storey irregularity occurs.
17
The calculation of the stiffness is not explained in the
standard.
4 CONCLUSIONS
In this study, the soft-storey irregularity, formed
because of various reasons was investigated. Various
national building codes were compared. Analytical com-
parisons were made between the Turkish Seismic Code
(TSC 2007), American Code of Minimum Design Loads
for Buildings and Other Structures (ASCE 7-2002),
Indian Seismic Code (IS 1893-1, 2002) and Japanese
Building Code. Additionally, some information was
given about the soft-storey irregularity according to
Eurocode-8, Bulgarian Seismic Standard, New Zealand
Seismic Code and Israeli Seismic Standard. The analyses
were done using the SAP2000 structural-analysis prog-
ram. In this context, in a building model, which was
symmetrical with respect to the plan dimensions, a soft-
storey irregularity was deliberately formed. Eleven sub-
models were formed by changing the general model’s
first-storey height to form different soft storeys. These
models were completed and then compared according to
the seismic codes’ conditions and limiting values with
regard to the soft-storey formation. In the eleven models,
the storey heights were 3 m, except for the first storey.
All the other parameters were the same.
According to the results obtained, the following
conclusions can be made:
With respect to the TSC 2007, the condition for a
soft-storey formation is n
ki
> 2.0. The calculations
showed that in Model 8, this parameter was exceeded
with a minor difference of 2.035 and a soft-storey irregu-
larity occured. For this model, the first-storey height was
4.35 meters.
As regards the Indian and ASCE Seismic Standards,
the k
1
/k
2
storey-stiffness ratio was at the limit value in
Model 10. The ratio for this model was obtained as
k
1
/k
2
= 0.699 < 0.7 and so a soft-storey irregularity was
formed. The first-storey height of Model 10 was 4.50 m.
The calculations of the Japanese Seismic Code are
very different from those of the other codes. When the
results were examined, it was found that the R
s
late-
ral-rigidity ratio was at the limit in the calculations for
Model 2. In consequence, the R
s
= 0.599 < 0.6 value
made the first storey into a soft storey. For Model 2, the
first-storey height was h = 3.30 m.
When all the conclusions are considered, it is obvious
that in the Japanese Seismic Code, the soft-storey
phenomenon is handled much more carefully. With
respect to this issue, the Japanese Code stays much more
on the safe side in comparison with the other seismic
codes. This can be thought of as reasonable if we think
about the seismicity and soil conditions of Japan.
On the other hand, in the ASCE, Indian Seismic
Code and Turkish Seismic Code, the soft-storey irregu-
larity is generally tolerated. Also, in the Bulgarian, New
Zealand and Israeli Seismic Codes, the soft storey is
mentioned but these codes include no detailed numerical
and analytical formulations or conditions regarding this
critical issue. In these codes, the soft-storey conditions
are more superficially treated.
The soft-storey irregularity, which causes great losses
under the effect of a seismic load, should be determined
with much more sensitive numerical calculation methods
in the national seismic codes, especially in the earth-
quake-prone countries.
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