SEISMIC RESPONSE AND DAMAGE MECHANISM OF

UNDERGROUND STRUCTURE OF SUBWAY TRANSFER STATIONS

Zhong-Yang YU1, Hong-Ru ZHANG

2, Chao-Qun HUANG

3, Chun-Sheng QIAO

4

ABSTRACT

Urban rail transit has been considered a significant solution for efficient mass transport in the major cities of

China. On mainland China 57 cities have approval to develop urban rail transit systems, which had totalled 3832

km in operation by the end of 2016. It is predicted that the total planned distance in operation by the year 2020

will be greater than 10,000 km, most of which will be underground. Most urban areas in China are located in

earthquake zones. Therefore, the seismic behaviour of underground structures in the rail transit system is of great

concern, following the collapse of the Daikai station in Kobe, Japan during the 1995 earthquake. To understand

the seismic performance and damage mechanism of the complex underground structures of subway transfer

stations, numerical models have been developed, which include dynamic soil-structure interactions, soil property

variations, structural connection conditions and damage mechanisms. A simplified model for the design and

performance criteria of underground subway transfer stations is proposed.

Keywords: Underground Structure; Seismic Response; Damage Mechanism; Subway Station

1. INTRODUCTION

Underground structures had been considered to have better seismic performance due to the constraints

of the surrounding soil. For this reason seismic research was rarely focused on underground structures,

with the exception of lifeline pipes. However, the destruction of the structure of Dai Kai underground

station during the Hanshin earthquake in 1995, has provided a new understanding of subway station

structure (Tiwatate Y et al. 2000). Metro engineering withstanding seismic events is important in

urban disaster prevention and mitigation.

Existing studies are primarily focused on simple underground structures. Zhuang Haiyang (2006, 2007

and 2008) and Chen Lei (2012) have conducted seismic numerical simulations for symmetrical

underground subway stations, such as double story double span structures, in which the model

boundary effect, soil layer and underground structure acceleration response, earthquake stress and

strain are studied. Liu Jingbo (2006, 2007, 2008 and 2014) analysed problems with the underground

seismic structural design and proposed the static elastic-plastic analysis method, pushover analysis

method and integral response displacement method for underground structures, and the approximate

boundary conditions and earthquake parameters in numerical analyses are presented. Based on the

Beijing and Nanjing subway typical underground station structures, shake table model tests were

carried out by Tao Lianjin (2006) and Chen Guoxing (2007) to measure the dynamic response

characteristics of underground structures. Actual earthquake damage case study, model tests and

numerical simulations in the above literatures have shown that the destruction of the underground

structure is mainly influenced by the deformation of soil layer, while the inertia effect is not

significant. For an underground structure containing an interior column, the central pillar is the weak

1PhD student, School of Civil Engineering, Beijing Jiaotong University, Beijing, China, 15115294@bjtu.edu.cn

2Professor, PhD, School of Civil Engineering, Beijing Jiaotong University, Beijing, China, hrzhang@163.com

3MSc student, School of Civil Engineering, Beijing Jiaotong University, Beijing, China, 15121021@bjtu.edu.cn

4Professor, PhD, School of Civil Engineering, Beijing Jiaotong University, Beijing, China, cs-qiao@163.com

2

point within the main structure, and the damage caused by an earthquake starts with the failure of the

central column.

However, for complex structures, such as cross-transfer subway stations and other large underground

station complexes, there are few studies reported in the literature due to the complex dynamic response

characteristics, which are difficult to test. Dai Kai station, as a simple, single layer, double span

structure, is the only example of underground station earthquake destruction. Its damage can not

represent the seismic behaviour of more complex structures. For complex, large space structures, there

is not rich seismic experience. Therefore, thorough research is urgently needed. The cross structure of

a cross-transfer station can lead to an uneven deformation of the station structures during a seismic

event. Shear deformation caused by seismic waves in one direction, leads to complex forces, such as

tension or torsion in other directions. The dynamic interaction effect of the connected structures is

significant. Therefore, it is difficult to simulate a 3D structure system using a 2D model.

Based on a typical cross-transfer station used in practical engineering, which is the cross structure of

the three-story-three-span structure and two-story-three-span structure, detailed dynamic finite element

simulation analysis were carried out in this study. The results were compared with the 2D analysis to

determine the area most vulnerable to forces in the cross structure. In addition, the pushover method

was used to analyse the failure characteristics of the three-story-three-span and two-story-three-span

structures, and the damage mechanism of the underground structure was discussed.

2. SESMIC RESPONSES OF UNDERGROUND SUBWAY STATION

2.1 Dynamic Finite Elements Models

The underground structure considered in this study was an underground cross-transfer station for two

subway lines, constructed using a cut and cover method. It was composed of a three-story-three-span

structure for one line crossing, and a two-story-three-span structure for the other line. The plan and

cross section of the structure is shown in Figure 1.

(b) Cross section 1 of the underground structure

(a) Plan section of the underground structure (c) Cross section 2 of the underground structure

Figure 1. Plan and cross section of the structure (Unit: mm)

3

The dimension of the three-story structure was 22.8 m in width, 21.85 m in height, 0.8 m-thick top

plates, 0.4 m-thick medium plates, 0.9 m-thick bottom plates, 0.8 m-thick side walls, 1.2 m-wide

longitudinal beams, and 0.8×1.0 m interior columns. The dimensions of the two-story structure was

22.8 m in width, 13.85 m in height, and the same dimensions for the remainder of the three-story

station structure. In addition, the longitudinal space of the interior columns was 9.0 m in both

directions and the burial depth of roof of the structure was 3.0 m.

An equivalent 2D dynamic analysis model was proposed: a standard three-story-three-span station

structure. The remainder of the parameters of the 2D model were selected to be the same as the 3D

model.

Concrete material used in the station structure was C40, with the basic properties shown in Table 1.

The main reinforcement of the structure was HRB 400 reinforced steel bars, composed of C28@150

for side walls, C25@150 for plates, and 32C28 for interior columns.

Table 1. Damage parameters of C40.

(From Code for Design of Concrete Structures GB 50010-2010)

Density (Kg/m3) Elastic Modulus (GPa) Poisson ratio Damping ratio

2500 32.5 0.25 0.05

The elastoplastic model with Mohr-Coulomb failure criteria was applied as the constructive law of the

soil. The parameters of the soils are shown in Table 2, and the depth division of the soil layer with the

cross section of the structure is shown in Figure 2.

Table 2. Soil parameters.

(From Geological Survey Report of Beijing Metro Line 7)

Soil Depth

(m)

Density

(kg/m3)

Shear Wave

Velocity(m/s)

Dynamic

Modulus Of

Shear(MPa)

Cohesion

(kPa)

Friction

Angle(º)

Poisson

Ratio

① 3 1750 170.0 50.6 5 10 0.35

② 5 1960 223.3 97.7 31 17 0.36

③ 16 1980 238.6 112.7 33 19 0.36

④ 36 2020 317.8 204.0 28 30 0.33

⑤ 20 2120 462.7 453.8 0 45 0.26

Figure 2. Soil layer distribution and structure embedment (Unit:mm)

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In order to test the possibility of using the plain strain model to analyse the 3D underground cross

structure, a 3D finite element model of the full structure and a 2D model corresponding to standard

cross section were constructed.

The size of the 3D finite element model was 239×239×80 m, which included the soil surrounding the

structure of the underground station. Figure 3 shows the total finite element model and the structure

model. The finite element model includes two typical elements: eight-node hexahedral elements for

the soil medium, and four-node shell elements for the station structure. Viscoelastic boundary

conditions were introduced to eliminate the wave reflection in the lateral boundary, by horizontal

dampening springs and vertical restraints on the four lateral boundaries of the soil. The surface of the

bedrock served as the seismic input interface. In this numerical simulation, the input had horizontal

seismic excitations, perpendicular to the two-story-three-span cross section.

(a) Full mesh model (b) Mesh of structure only

Figure 3. 3D mesh of cross transfer underground subway station

The size of the 2D finite element model was 239×80 m, which included the soil surrounding the

underground station structure. Figure 4 shows the total finite element model and the structure model.

The elements applied include two typical elements: the four-node bilinear tetragonal elements in a

plane-strain condition for the soil medium, and two-node planar linear beam elements for the station

structure. The viscoelastic boundary conditions were also introduced, modeled using vertical restraints

and horizontal dampening springs on the lateral boundaries of the soil. The surface of the bedrock

served as the seismic input interface. The input was horizontal in-plane excitations.

Figure 4. 2D mesh of standard cross section of underground station

In this simulation, the first 30 seconds data of N-W horizontal component of El-Centro Seismic Wave

was inputted at the bottom of the model. In the analysis, additional 5 seconds zero data was added to

end to dissipate the seismic response. The peak acceleration of the record was adjusted to 0.1g for

simulating Level 7 seismic intensity. The original 30 seconds waveform is shown in Figure 5.

5

5 10 15 20 25 30

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Time(s)

Accele

rati

on

(g)

Figure 5. Original El-Centro seismic waves

(The data from https://strongmotioncenter.org/, Imperial Valley Earthquake, May 18, 1940, Station No. 117,

Comp S00E, Peak Acceleration = 341.69531 cm/sec/sec)

2.2 Dynamic Seismic Response of the Cross Transfer Underground Station Structure

3D and 2D dynamic simulations were carried out, based on the corresponding finite element models.

The internal forces of the side walls and interior columns are shown in Figure 6 for both models,

where the vertical coordinate represents the buried structural depth, and the horizontal axis presents

the internal forces of the structure. In these figures, “1CS”, “2CS”, “3CS” and “4CS”, respectively,

stand for the 3D results from the cross structure with a distance of one, two, three and four times

column space away from the cross structure, as is shown in Figure 1; while “2D” stands for the

corresponding results from the 2D model.

0 300 600 900 1200

24

21

18

15

12

9

6

3

Sidewall Moment (kN-m/m)

Str

uct

ure

Bu

ried

Dep

th (

m)

2D

1CS

2CS

3CS

4CS

0 200 400 600 800 1000

24

21

18

15

12

9

6

3

Sidewall Shear Force (kN/m)

Str

uct

ure

Bu

ried

Dep

th (

m)

2D

1CS

2CS

3CS

4CS

0 3000 6000 9000 12000

24

21

18

15

12

9

6

3

Interoir Column Axial Force (kN)

Str

uct

ure

Bu

ried

Dep

th (

m)

2D

1CS

2CS

3CS

4CS

(a) Sidewall moment (b) Sidewall shear force (c) Interior column axial force

Figure 6. The maximum internal forces of the side walls and interior columns

Some differences between the 2D and 3D internal forces were observed. The two-story structure can

be regarded as a shear wall for the three-story structure, which may explain the lateral wall bending

moment being smaller and the shear force larger in comparison with the 2D results.

This effect gradually decreases as the distance increases and at a distance greater than three times of

the column space from the cross section, the bending moment and shear forces of the side wall were

similar as those in the 2D standard structure along the depth direction. Therefore, the internal forces of

the side wall were not influenced by the cross structure at distances three times the column space.

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The axial forces of the interior column were primarily determined by the static loading and there is no

obvious distinction between the 3D and 2D standard structures.

Overall, it was demonstrated that the range of influence on the cross structure was about 3 times the

column space, and the internal forces of the structure beyond this range are not affected by the

intersection. A standard structure seismic design method could be adopted when the distance is

beyond the influence range and the enhanced design method could be used for the intersection.

3. DAMAGE MECHANISM OF UNDERGROUND SUBWAY STATION

3.1 Pushover Models

To investigate the damage mechanism, pushover simulations were performed on 2D models of a two-

story-three-span station structure and a three-story-three-span station structure, with the same size

cross structure.

C50 concrete was used in the interior columns and C35 concrete was used in all other station

structures. The reinforcement configuration with HRB 400 is shown in Table 3. When the moment of

bent dominant members reached their ultimate bending moments, hinges occurred. The failure forces

for the interior columns were the ultimate axial forces.

To simplify the analysis in the simulation, the structures were elastically linear before yield, and full

plastic after yield. The site soil was assumed to be elastic and saturated. The shear wave velocity was

160 m/s (soft soil), with a density ρg= 2.0 g/cm3, and Poisson's ratio μd = 0.47.

Table 3. Structural reinforcement and ultimate forces.

Side Wall Top Plate Medium

Plate Bottom Plate

Interior

Column

Reinforcement

(mm) C28@150 C25@150 C18@150 C28@150 32C28

Ultimate forces 919 kN·m 732 kN·m 197 kN·m 1228 kN·m 2326 kN

The total size of the 2D soil-structure system model was 200×50 m, with the soil medium represented

using four-node bilinear tetragonal elements in a plane-strain condition, and the station structure was

represented using two-node planar linear beam elements. The mesh is shown in Figures 7 and 8.

The lateral boundary was vertically constrained, while the horizontal boundary was free and the

bottom boundary fixed. Lateral push loading with an inverted triangle vertical distribution was used.

Figure 7. Mesh of two-story-three-span subway station model

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Figure 8. Mesh of three-story-three-span subway station model

3.2 Damage Mechanism of the Underground Station

In the seismic design for structures, the elastic limit and plastic limit are usually used as parameters to

estimate the performance of the structures in a seismic event. The elastic limit was marked by the

appearance of the first plastic hinge in structure members; while the plastic limit was presented by the

appearance of sufficient hinges to form a plastic mechanism.

3.2.1 Development of Underground Structure Damage

With the increase of seismic loading, plastic hinges in the underground structure appeared gradually

and the damage increased until the structure completely collapsed.

In the two-story underground structure, the first plastic hinge appeared at the intersection of the

interior column and the middle plate (Figure 9(a)). This was followed by the plastic region of the

structure developing rapidly. The joints of the middle plates with the side wall and the interior

columns subsequently turned into the plastic phase. The left and right joints of the side walls and the

bottom also entered into the plastic phase later (Figure 9(c)).With the load increasing, the bottom ends

of the interior columns and the top end of the side wall entered into the plastic stage, as is shown in

Figure 9(e). Finally, the plastic hinges appeared at the top ends of the interior columns (Figure 9(f)).

(a) (b) (c)

(d) (e) (f)

Figure 9. Plastic development of two-story underground structure

In the three-story underground structure, the plastic development sequence was similar to the two-

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story underground structure, with the first plastic zone appearing at the intersection of the side wall

and the middle plate (Figure 10(a)). The joints of the middle plates with the side wall and the interior

columns subsequently turned into the plastic phase. The left and right joints of the side walls and the

bottom also entered the plastic phase later (Figure 10(c)). With the load increasing, the bottom ends of

the interior columns and the top end of side wall entered into the plastic phase, as is shown in Figure

10(e). The top ends of the interior columns finally entered into the plastic stage (Figure 10(f)).

(a) (b) (c)

(d) (e) (f)

Figure 10. Plastic development of three-story underground structure

Therefore, it can be considered that the damage mechanism of cut and cover structures start with the

destruction of the middle plate.

3.2.2 Design Limit for Design Earthquake and High Level Earthquake

The performance of a structure is often determined in a design using the inter-story drift angle of the

underground structure, enabling the structure working conditions to be measured. The inter-story drift

angle is calculated by:

/R H (1)

Where: R is inter-story drift angle, Δ is the relative horizontal displacement of adjacent story and H is

the height of story.

Based on the definition of the inter-story drift angle, the elastic limit of the two-story underground

structure was 1/835 and the plastic limit was 1/225. The elastic limit of the three-story underground

structure was 1/1191 and the plastic limit was 1/296.

In the two-story structure, the inter-story drift angle exceeded 1/250, the limit of current design code

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for a high level earthquake.

In the seismic design of the ground structure, the structural working condition can be estimated based

on the inter-story drift angle. Underground structures may perform differently at the same inter-story

drift angle. Therefore, the inter-story drift angle might not be the best parameter to determine the

underground structure working conditions during a seismic event.

4. CONCLUSIONS

In order to investigate seismic behaviour of complex underground urban rail transit station precisely,

3D and 2D dynamic numerical model were used to simulate the seismic response characteristics of a

cross-transfer subway structure. The comparison between the distribution of the internal force and

displacement of both models shows that the cross structure has significant influence on the cross-

transfer subway structure. The bending moments and shear forces of the side wall are influenced by

the cross structure in a distance about 3 times of the column space, while the internal forces of the

interior columns are mainly determined by the static loading.

2D quasi-static pushover analyses for two-story-three-span and three-story-three-span underground

structures were carried out to study their failure mechanisms. The results indicate the first plastic zone

appeared in the middle plate for both structures, then the joints of side wall and the bottom entered the

plastic stage. The bottom ends of interior columns and the top end of side wall entered into the plastic

phase later. Finally, the top ends of the interior columns turned into the plastic stage and the structures

became unstable. However, there is a slight difference in the initial plastic development stage that the

location of the first plastic hinge is different for two structures. It has to be mentioned that this failure

characteristics may result from the real engineering designed structures, where the members subjecting

to vertical loads and joints had been strengthened to prevent integral structural collapse, and the soft

soil assumption.

5. ACKNOWLEDGMENTS

This project was supported by the National Natural Science Foundation of China (51678042).

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