The analytical expression of connected structures subjected to earthquake ground motions are derived and solve using step-by-step procedures. The seismic response of connected twin towers is compared with those of un-connected structures. It is observed that the base isolated sky corridor reduces displacement demand, base shear and absolute acceleration significantly.

1. SEISMIC MITIGATION OF TWIN TOWER STRUCTURES USING AN
ISOLATED CORRIDOR
1Harshil J Shah, 2Devesh P Soni
1
PG Student 2
Head of Department Civil Engineering,
Sardar Vallabhbhai Institute of Technology, Vasad-388306, Gujarat, India
Email:1
harshilshah6512@gmail.com, 2
devesh18@gmail.com
ABSTRACT
The concept of connecting two tower structures with a view to reduce displacement
demands in the structures by means of base isolated corridor is examined in this paper.
The analytical expression of connected structures subjected to earthquake ground
motions are derived and solve using step-by-step procedures. The seismic response of
connected twin towers is compared with those of un-connected structures. It is
observed that the base isolated sky corridor reduces displacement demand, base shear
and absolute acceleration significantly.
KEYWORDS: Base-isolation, Seismic mitigation,
1.INTRODUCTION:
Twin towers connected by a sky corridor are extensively used to their own
benefits in terms of aesthetic view, convenient communication, and capability to
provide escape channel at the time of fire emergencies or any other emergencies. In
present days, the design of any special structures lateral loads plays an important role.
From structural point of view, we can appreciate that lateral stiffness of these structures
needs to be high adequacy to provide with such a high lateral force. As per earthquake
engineering, responses of any structure to dynamic loading is an activity of its three
basics properties i.e. mass, stiffness & damping. By develop these properties we can
deal with the responses of any structure. Among these properties are mass, stiffness,
damping and other mathematical based properties. Twin towers presents a great
challenge on structure seismic design. Different types of bearings are always used to
connect the sky corridor and towers. The connection system between sky corridors and
towers should be correctly design When the neigh-bouring tall towers coupled by a
sky bridge are subjected to earthquake or wind excitations. Twin towers are greatly in
demand due to its architectural or structural design, particularly planning along with

2. more spaces with same foundation supports. Base isolation system is one of the most
well-known system for protecting a structure against earthquake forces. Which the
superstructure is separated from the base structure by recommended a suspension
system between the base structure and main structure. The isolated bearing are
connected between sky corridor and towers. The base isolation system has been
adopted to reduce the storey acceleration responses of building mainly during large
earthquake. There are many types of base isolation devices like high damping rubber
bearing, friction pendulum bearing & rocker roller bearing, low damping rubber
bearing, etc. It mitigates the motion of structure due to seismic activity.
The concept of STMD twin tower linked by sky corridor using flexible joint.
These systems are derived by 3 DOF. The effect of frequency ratio, mass ratio, tuning
frequency ratio of corridor & damping ratio of passive control devices on structural
seismic response is investigated. Optimum parametric analysis is performed to
minimize the displacement of both towers. It investigated seismic performance of
adjacent structure connected using isolated corridor. (Qing Lye, Wensheng Lu,
Weiqiang Wang, Yue Chen). The coupling control effect of sky bridge for adjacent
tall building has been investigated. Two building structures 42 & 49 stories connected
by sky bridge and constructed in Seoul, Korea. LRB & LMB were used for connectors
between sky-bridge & examples of building. The displacement & acceleration
responses of coupled building, & reaction of bearings & member forces of sky-bridge
were estimated in comparison with uncoupled building. The coupling control effect
are investigated in this study using wind and earthquake excitation. (Dong- Guen Lee1,
Hyun-Su Kim*, † and Hyun Ko1). The enclave tower Sky Club is a super high-rise
RC apartment block constructed in Fukuoka City, Japan. The building consists of three
towers connected by three aerial gardens. To achieve a high level of safety against
earthquake & residential comfort during strong wind, various techniques were
adopted. These techniques are core-wall, hybrid base isolation system &oil and zinc
aluminum alloy dampers in aerial garden. Seismic response analyses were conducted
to confirm the performance of the structural design. To estimate the structural control
performance in detail, the time history of absorbed energy was calculated. (Akira

3. Nishimura, Hiroshi Yamamoto, Yasuhiko Kimura, Hideki Kimura, Masashi
Yamamoto, Atsumichi Kushibe).
In previous research twin towers connected at top by an isolated corridor high
damping rubber bearing (HDPE) are used. Its need to carried out friction pendulum
bearing (FPB), lead rubber bearing (LRB), linear motion bearing (LMB) is not
founded.
There is an increasing need for more efficient, effective and innovative
displacement control strategies. Seismic isolation bearing, lead rubber bearing (LRBs),
high damping rubber bearing (HDRB) are used.
The present study aims to study different base isolation system for tall
buildings with similar natural frequencies and different natural frequencies under wide
range ground motion. To study the effectiveness of seismically isolated bridge
connecting two towers structure at the top. To study the coupling effects of a sky bridge
for adjacent tall buildings with similar natural frequencies & different natural
frequencies. To study the effectiveness of the system by using different types of rubber
and friction type base isolation devices.
To study seismic behavior of tall building connected by an isolated corridor
under unidirectional earthquake ground motion. The isolation system considered for
the study are high damping rubber bearing (HDRB), lead rubber bearing (NZ system)
and friction pendulum system (FPS). The height of connected buildings (number of
stories) are equal. However, their natural frequencies are same or different. In this
study only seismic type lateral forces are considered for analysis. i.e., effect of wind is
not considered.
Application of equations for MDOF structures is achieved by reducing two
MDOF connected structures to equivalent MDOF ones using the time history method.
Numerical analysis is conducted to verify control effectiveness of connected MDOF
system subjected to the different earthquake ground motion.
2.TWIN TOWER WITH ISOLATED CORRIDOR:
In fig 1 shows an example of a typical twin tower connecting a building system
and related simplified multi- degree- of- freedom model. The isolation bearings are
installed between the sky corridor and top of the towers. The adjacent structure can be

4. simplified as an MDOF structure, which is characterized by mass m, stiffness k, and
damping 𝑐𝑖. The natural frequency and viscous damping ratio of towers are
ω1=√k1/m1 & ζ1=c1/2√k1m1 respectively.
It consists of mass(md), stiffness(kd), damping (Cd). Let, ωd=√kd/md &
ζd=cd/2√kdmd.
FIGURE 1: sketch of a connected building system and simplified multi-degree-
of- freedom system
2.1 Governing equation of motion:
The equations that describe behaviour of the mass shown in Figure 1 are as follows:
𝑚1𝑢̈1 + 𝑐1𝑢̇1 + 𝑘1𝑢1 = −𝑐𝑑1(𝑢̇1-𝑢̇𝑑)-𝑘𝑑1(𝑢1-𝑢𝑑)-𝑚1𝑥̈𝑔 (1)
𝑚2𝑢̈2 + 𝑐2𝑢̇2 + 𝑘2𝑢2 = −𝑐𝑑2(𝑢̇2-𝑢̇𝑑)-𝑘𝑑2(𝑢2-𝑢𝑑)-𝑚2𝑥̈𝑔 (2)
𝑚𝑑𝑢̈𝑑 + (𝑐𝑑1 + 𝑐𝑑2)𝑢̇𝑑 + (𝑘𝑑1 + 𝑘𝑑2)𝑢𝑑 = 𝑐𝑑1𝑢̇1 + 𝑐𝑑2𝑢̇2 + 𝑘𝑑1𝑢1𝑘𝑑2𝑢2𝑚𝑑𝑥̈𝑔(3)

5. Where u,𝑢̇, 𝑢̈
̇ are the displacement, velocity, and acceleration of the masses and 𝑥̈𝑔 is
ground acceleration. The dynamic equation can be expressed in a matrix form as
M𝑥̈(t)+C𝑥̇ (t)+Kx(t)=M𝑙𝑛𝑥̈𝑔(t) (4)
Where, M=[
𝑚1 0 0
0 𝑚2 0
0 0 𝑚3
]; C= [
𝑐1 + 𝑐2 0 −𝑐𝑑1
0 𝑐2 + 𝑐𝑑2 −𝑐𝑑2
−𝑐𝑑1 −𝑐𝑑2 −𝑐𝑑1 + 𝑐𝑑2
] ;
K=[
𝑘1 + 𝑘𝑑1 0 −𝑘𝑑1
0 𝑘2 + 𝑘𝑑2 −𝑘𝑑2
−𝑘𝑑1 −𝑘𝑑2 𝑘𝑑1 + 𝑘𝑑2
]; X= {
𝑢1
𝑢2
𝑢𝑑
} (5)
TABLE 1 Building model parameters:
Tower No. of
floors
Damping
ratio (%)
Floor
mass (kg)
Floor Stiffness
(N/m)
Natural
frequency (Hz)
1 10 2 1.02× 106
1.5× 109 0.912
2 10 2 1.40× 106
2.5× 109 1.005
3 10 2 1.60× 106
4.6× 109 1.275
TABLE 2 Building model parameters:
Connected
Tower
𝒌𝐝 (N/m) 𝒄𝐝 (N.s/m)
1-2 6.91 × 106
3.31× 105
2-3 8.82× 106
7.08× 105
TABLE 3 Details of Earthquakes:
Sr.
No.
Earthquakes Magnitude Location PGA(g)
1 1940 Imperial Valley 6.95 El Centro 0.313
2 1987 Superstition Hills 6.7 El Centro Imp. Co. Center 0.512
3 1999 Chamoli
Earthquake
6.4 Gopeshwar 0.359
4 1994 Northridge 6.7 Northridge-Saticoy St 0.529
5 1989 Loma Prieta 6.9 Capitola 0.42
6 1941 Imperial Valley 6.7 Canoga Park-Topanga
Canyon
0.477

6. 3 NUMERICAL STUDY:
To demonstrate the proposed method for connecting adjacent structures, two design
illustrations are presented. In the first illustration, the first natural frequency of two
towers (Tower-1 & 2) is close to each other, whereas in the second example, the first
natural frequencies depart from each other. Three towers are considered in total, and
the parameters of each tower are listed in Table 1. All towers are classically damped,
and the damping ratio of each tower is assumed to be 0.02. To apply the closed form
equations, we first reduce the structures to their equivalent MDOF models as shown
in figure. The parameters are used to determine the stiffness and damping properties
between the sky corridor and two towers in each example, as listed in Table 2.
3.1 Two towers with similar natural frequencies:
Tower 1 & 2 are two 10-storey structures flexibly connected by a sky corridor
at the top floor. The towers have similar natural frequencies. The time domain
responses of the displacement of the two towers are shown in figure 1. The time
domain responses of the base shear of the two towers are shown in figure 2. The time
domain responses of the acceleration of the two towers are shown in figure 3.
Figure 1 Time variation of Displacement under 1940 El centro earthquake
In fig 1 shows a decrease in overall maximum displacement responses. The largest
percentage reductions in the maximum displacement responses of tower 1 are 22.28%,
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 10 20 30
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Connected T-1
Unconnected T-1
Similar frequencies
0.1993
0.1549
Displacment
(m)
Time (sec)
0.2982
Connected T-2
Unconnected T-2
0.2030

7. which occurs at the top floor. whereas in tower 2 are 17.75% at top floor. In fig 2
shows a decrease in maximum base shear responses. The largest percentage reductions
in the maximum base shear responses of tower 1 are 19.45%, whereas in
tower 2 are 14.33%. The largest percentage reductions in the maximum acceleration
responses of tower 1 are 9.93% and tower 2 are 13.49%.
Figure 2 Time variation of base shear under 1940 El centro earthquake
Figure 3 Time variation of acceleration under 1940 El centro earthquake
-0.4
-0.2
0.0
0.2
0.4
0 10 20 30
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Connected T-1
Unconnected T-1
0.4078
Similar frequencies
0.3285
Base
shear
Time (sec)
0.5799
0.4968
Connected T-2
Unconnected T-2
-10
-5
0
5
10
0 10 20 30
-15
-10
-5
0
5
10
Connected T-1
Unconnected T-1
9.4696
7.0817
Similar frequencies
Acceleration
Time (sec)
Connected T-2
Unconnected T-2
10.999
9.9066

8. TABLE 4 Responses of earthquake ground motion of Tower 1
Similar
frequencies
Displacement (m) Base shear (W) Acceleration (g)
Sr.
No
Earthquake Connected Unconnected Connected Unconnected Connected Unconnected
1
1940 Imperial
Valley
0.140 0.189 0.322 0.4 0.712 0.952
2
1987
Superstition
hills
0.157 0.181 0.324 0.328 0.718 0.871
3 1999 Chamoli 0.164 0.169 0.299 0.362 0.890 1.103
4
1994
Northridge
0.268 0.256 0.576 0.659 1.204 1.361
5
1941 Imperial
Valley
0.157 0.181 0.394 0.411 0.88 1.172
6
1989 Loma
prieta
0.205 0.222 0.430 0.569 1.268 1.544
7 1979 El
centro
0.154 0.199 0.328 0.407 0.721 0.965
TABLE 5 Responses of earthquake ground motion Tower 2
Similar
frequencies
Displacement (m) Base shear (W) Acceleration (g)
Sr.
No
Earthquake Connected Unconnected Connected Unconnected Connected Unconnected
1
1940 Imperial
Valley
0.162 0.194 0.481 0.561 0.987 1.101
2
1987
Superstition
hills
0.177 0.214 0.265 0.314 0.723 0.965
3 1999 Chamoli 0.147 0.155 0.391 0.432 0.844 0.945
4
1994
Northridge
0.335 0.353 1.016 1.037 1.826 2.247
5
1941 Imperial
Valley
0.177 0.215 0.533 0.603 1.162 1.508
6
1989 Loma
prieta
0.139 0.168 0.551 0.688 1.392 1.508
7 1979 El
centro
0.203 0.298 0.497 0.579 1.009 1.121
3.2 Two towers with different natural frequencies:
Tower 1 and 3 are two 10-storey structures. The fundamental frequencies of
these two towers are 0.912 and 1.275 Hz, respectively. The time domain responses of
roof displacement, base shear and acceleration of these towers are shown in figures
7,8 & 9 respectively. Compared with unconnected case, the two towers flexibly
connected by a sky corridor induced a significant reduction in responses.

9. Figure 7 Time variation of Displacement under 1940 El centro earthquake
Figure 8 Time variation of base shear under 1940 El centro earthquake
-0.2
-0.1
0.0
0.1
0.2
0 10 20 30
-0.2
-0.1
0.0
0.1
0.2
0.3
Connected T-1
Unconnected T-1
0.1296
Different frequencies
0.1211
Displacement
(m)
Time (sec)
0.2102
0.1766
Connected T-3
Unconnected T-3
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0 10 20 30
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Connected T-1
Unconnected T-1
0.5346
Different frequencies
0.5008
Base
shear
Time (sec)
Connected T-3
Unconnected T-3
0.5799
0.5126

10. Figure 9 Time variation of acceleration under 1940 El centro earthquake
TABLE 6 Responses of earthquake ground motion of tower 1
Different
frequencies
Displacement (m) Base shear (W) Acceleration (g)
Sr.
No
Earthquake Connected Unconnected Connected Unconnected Connected Unconnected
1
1940 Imperial
Valley
0.107 0.114 0.461 0.524 1.099 0.195
2
1987
Superstition
hills
0.15 0.154 0.564 0.60 1.335 1.474
3 1999 Chamoli 0.138 0.141 0.562 0.576 1.025 1.094
4
1994
Northridge
0.254 0.242 1.029 0.995 1.686 1.679
5
1941 Imperial
Valley
0.127 0.181 0.554 0.614 1.039 1.031
6
1989 Loma
prieta
0.168 0.171 0.869 0966 1.830 2.218
7 1979 El
centro
0.121 0.129 0.501 0.535 1.197 1.286
-15
-10
-5
0
5
10
0 10 20 30
-15
-10
-5
0
5
10
Connected T-1
Unconnected T-1
12.6196 Different frequencies
11.7407
Acceleration
Time (sec)
Connected T-3
Unconnected T-3
10.999
9.7244

11. TABLE 7 Responses of earthquake ground motion of tower 3
Different
frequencies
Displacement (m) Base shear (W) Acceleration (g)
Sr.
No
Earthquake Connected Unconnected Connected Unconnected Connected Unconnected
1
1940 Imperial
Valley
0.166 0.194 0.498 0.561 0.923 1.101
2
1987
Superstition
hills
0.092 0.11 0.258 0.314 0.809 0.965
3 1999 Chamoli 0.146 0.154 0.399 0.432 0.864 0.945
4
1994
Northridge
0.329 0.353 0.990 1.037 1.833 2.247
5
1941 Imperial
Valley
00.183 0.215 0.554 0.603 1.163 1.342
6
1989 Loma
prieta
0.147 0.168 0.574 0.688 1.374 1.507
7 1979 El
centro
0.206 0.210 0.513 0.579 0.991 1.121
4. CONCLUSIONS
The concept of adjacent towers connected by an isolated sky corridor is proposed in
this paper. The system consisting of sky corridor mass and flexible connecting element
has an effect of the towers. The results show that better seismic reduction effect can
be achieved if the connected towers have similar dynamic properties. The seismic
reduction effect of system depends on mass, damping, stiffness of the system.
In general, the displacement responses of the connected towers are reduced compared
with those of the unconnected towers. For the towers with similar natural frequencies.
 The maximum displacement of Tower 1 and 2 is reduced by 22.28% and
17.75%, base shear is 19.45% and 14.33%, & acceleration is 25.22% and
9.86%.
 In general, the displacement responses of the connected towers are generally
reduced compared with those of the unconnected towers. For the towers with
different natural frequencies.
 The maximum displacement of Tower 1 and 3 is reduced by 6.56% and 7.15%,
maximum base shear responses of tower 1 and 3 is 2.3% and 7.81%, &
maximum acceleration responses of tower 1 and 3 is 9.96% and 11.59%. The
design formula simplifies the design process and offers flexibility to control

12. the performance of both structures when the adjacent structures are connected
by sky corridor that uses stiffness and damping elements.
5. REFERENCES
Qing Lye, Wensheng Lu, et al. “Mechanism & optimum design of STMD for twin
tower structures connected at top by an isolated corridor : WILEY Feb-2020/21
Dong- Guen Lee, Hyun-Su Kim, et al. “Evaluation of coupling–control effect of a sky-
bridge for adjacent tall buildings” Wiley (wileyonlinelibrary.com), 11 March
2010, DOI:10.1002/tal.592
Akira Nishimura, Hiroshi Yamamoto, et al. “Base-isolated super high-rise RC building
composed of three connected towers with vibration control systems” Wiley, june
2011.
Kazuto SETO, Masaaki OOKUMA, et al, “Method of estimating equivalent mass of
MDOF system” JSME International journal, 10th March 1986.
W. S. Zhang, Y. L. Xujohn, “Dynamic characteristics and seismic response of adjacent
buildings linked by discrete dampers” Wiley & sons, Ltd 29th sept 1999.
Jie Song, K. T. Tse, et al. “Aerodynamics of closely spaced buildings with application
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