Metro train-induced vibrations on historic buildings in Chengdu, China.pdf

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Journal of Zhejiang University-SCIENCE A (Applied Physics & Engineering) ISSN 1673-565X (Print); ISSN 1862-1775 (Online) /jzus; E-mail: jzus@

Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2011 12(10):782-793

Metro train-induced vibrations on historic buildings in Chengdu, China

Abstract: In this paper, the vibration influence on a monument caused by Chengdu Subway Line 2 is analyzed. Due to its elaborate and unique design, both structural and architectural damages should be avoided. First, the allowable root mean square (RMS) velocity at the foundation of the monument is derived and a site measurement is performed to obtain the background vibrations induced by road traffic. In addition, a train -track coupled model and 3D tunnel-soil-structure coupled finite element models are built to predict the dynamic response of the monument. Prediction models are checked by site measurement in Beijing Subway Line 5. Different kinds of fasteners and train speeds are compared and discussed as well. Results show that: (1) At a train speed of 72 km/h, all the traffic vibrations exceed the low limit no matter what kind of fastener is used, which is mainly due to the contribution of road traffic. Slowing down train speeds can cause effective vibration attenuation; (2) Vibrations drop dramatically with the train speed from 65 to 58 km/h. When the train speed is lower than 58 km/h, vibrations are lower than allowable value even if the contribution of road traffic is considered.

*

1

Meng MA

2

1, 2

, Valéri MARKINE, Wei-ning LIU, Yang YUAN, Feng ZHANG

211

(School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China)

(Section of Road and Railway Engineering, Department of Design and Construction, Delft University of Technology,

Delft 2600GA, the Netherlands)

E-mail: mameng_02231250@

Received Apr. 3, 2011; Revision accepted Aug. 18, 2011; Crosschecked Sept. 8, 2011

1

Key words: Traffic vibrations, Historic buildings, Vibration prediction, Numerical simulation doi:10.1631/jzus.A1100088 Document code: A CLC number: U231; TB533.2

1 Introduction

usually low, but lasting, which could lead to potential damage, like building material fatigue and founda-tion settlement to historic buildings. For structures that have suffered from weathering, desquamation, or have cracks, even low velocities could give rise to fatigue damage with frequent occurrences.

With the rapid development of the urban rail

transit system and the construction of buildings, ground-borne vibrations induced by subway trains and their influence on building structures have be-come major environmental concerns in urban areas. Generally, it is extremely rare for vibrations from train operations to cause any sort of building damage, even minor cosmetic damage (Heckl et al., 1996). However, there is sometimes a concern regarding long- term vibration effects on historic buildings lo-cated near the subway lines. Traffic vibrations are

*

Project supported by the National Natural Science Foundation of China (No. 51008017), and the Fundamental Research Funds for the Central Universities (Nos. 2009JBM074 and 2009JBM075), China © Zhejiang University and Springer-Verlag Berlin Heidelberg 2011

In Europe, damage to some historic buildings caused by road traffic has been reported by Bata (1971) and Clemente and Rinaldis (1998). To protect heritage buildings against traffic-induced vibrations, many studies have been performed in Great Britain, Italy, and Spain (Ellis, 1987; Bazaco et al., 1995; Chiostrini et al., 1995; Sanò et al., 1998; Crispino and D’Apuzzo, 2001; Pau and Vestroni, 2008). In China, there has been doubt that the rapid develop-ment of desquamation and cracks of frescoes and sculpture are related to the traffic vibrations in the Longmen Grottoes and Dunhuang Grottoes. Some

Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2011 12(10):782-793

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studies have indicated that, by the vibration of rail-way and highway, the disasters to the Longmen Grottoes from vibration in the last 30 years has ex-ceeded the total experienced in the prior 1000–1500 years (Zhang, 2002; Lei et al., 2009). To understand further how to protect historic buildings against subway train -induced vibrations, the route design and isolation methods have been studied for Beijing Lines 4, 5, 6, 8, and 9 and the Straight Railway Line and Xi’an Lines 2 and 6 (Liu et al., 2007; Jia et al., 2008; Ma

et al., 2009). Note that there are two kinds of building dam-age: structural and architectural. The former will destroy some structure elements and endanger the safety of these buildings, while the latter will just cause cosmetic damage or cracks which, however, are also not allowed for those with superior artistic value or historic value. In this paper, a case study on the effects of vibration from Chengdu Subway Line 2 is presented. Both site measurements and numerical simulations are performed to analyze the effect of vibration from road traffic and subway trains.

2 Problem outline

2.1 Description of historic buildings

The historic monument is located in the Peo-ple’s Park, center of Chengdu City, China (Fig. 1). It was built to remember the martyrs struggling for defending railway sovereignty in the Autumn of Xinhai Year (1911). This brick-masonry structure, with a height of about 30 m, was built in 1913, and was listed as a State Protected Historic Site. There are fine handwritings and reliefs in the four faces. Due to its elaborate and unique art design, both structural and architectural damages should be avoided. Re-cently, Chengdu Subway Line 2 was built close to it. The distance between the monument and new sub-way station is about 50 m.

2.2 Vibration criteria for historic buildings As vibration velocity can reflect the intensity of potential damage to structures, and the peak value has a direct relationship to the fatigue damage to buildings, peak particle velocity (PPV) is often used to evaluate the effects of vibration on building structures. For modern steel structures or reinforced

structures, it is believed that a PPV lower than 10 mm/s will rarely cause any sort of building dam-age. For historic buildings and memorials, the criteria are usually stricter, from 3 to 10 mm/s PPV, de-pending on local codes, which are summarized by Ma et al. (2009). Numerous measurements show that typical ground vibration levels from underground tunnels lie in the range of 0.05–1.00 mm/s, lower than the values in the codes mentioned above, but they remains the possibility of damages to historic buildings. That is, the PPVs between 3 and 10 mm/s do not generally consider architectural damage, but rather structural damage.

Fig. 1 Monument to the martyrs in the autumn of Xin-hai Year (1911), in the struggle for defending railway sovereignty

The Chinese National Code (GB/T 50452-2008) gives a criterion of 0.10–0.75 mm/s, which is now one of the strictest allowable values in the world and could be used to evaluate architectural damage. In this code, the fatigue limit is used to fix the allowable velocities. When the cyclic stress is lower than the limit, the materials and structures are unlikely to be destroyed by fatigue. In this case study, for the brick-masonry structure listed as a State Protected Historic Site, the allowable horizontal PPV at the top of the monument is 0.15–0.25 mm/s, according to GB/T 50452-2008. Nevertheless, the vibration re-sponses on the top of the monument are not easy to measure. In order to obtain a predictable response with different kinds of traffic, the allowable root-mean-square (RMS) velocity at the bottom of the

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building is more useful.

First, the relation between the PPV at the top and bottom can be expressed as

n Vmax Vr D Vr

( j j ) 2

,

(1)

j 1

where Vmax is the horizontal PPV at the top of the building,

Vr is the horizontal PPV at the foundation of the building, γj and βj are the participating factor and dynamic amplifying factor of the j th vibration mode, respectively, j=1,2…,n , and D is a dynamic factor of the ratio of Vmax and Vr. By numerical modeling, which will be discussed in Section 3.2, the factor D is about 4.5–5.5. Therefore, the allowable PPV at the foundation Vr that can be calculated by Eq. (1) is 27.27

–55.56 μm/s. Second, the crest factor provides the possibility to change PPV into RMS velocity. The traffic-induced vibrations can be seen as a stationary nar-row-band Gaussian process, whose mean value μ approaches 0 and the variance is σ2

. Therefore, the RMS velocity of a group of sample data of vibration signal is

RMS n

x

2

n

E(xi2

)

i

2

D(x ) E(x ) (2)

i

i

2

2

,

where n is the number of signal sample x, and E and D are the functions of expected value and variance, respectively. That is, RMS is just the standard de-viation σ of this signal. For the Gaussian distribution, the probabilities of sample data within 3nσ, 4nσ, and 5nσ can be obtained by

p p

(499.993666%,

(3 ) 99.730020%,

(3)

p(5 ) 99.999943%.

This means, if the vibration responses obey the

Gaussian distribution absolutely, a crest factor larger than 3 yields a probability over 99%. Suppose sub-way trains operate 10 h every day and the sampling frequency is 1280 Hz, the values of crest factor data

larger than 4 and 5 are about 4600 and 41, respec-tively. For important historic buildings, the former

value is still too large, but the latter is acceptable. In order to define a conservative allowable RMS ve-locity, the crest factor in this study is defined as 5. Therefore, the maximum RMS velocity in the hori-zontal direction at the ground of the building (Point 1 in Fig. 2) should be 5.45–11.11 μm/s, which is re-garded as the vibration criteria in this case study.

2.3 Site measurement for background vibrations The arterial street lays on the northeast of the monument with a distance of about 80 m. Site mea-surements have been performed to obtain an under-standing on the background vibration mainly caused by road traffic. Fig. 2 shows the positions of the ac-celerometers. Point 1 is at the ground of the monu-ment, Point 2 is on the pedestal of the monument, while Point 3 and Point 4 are on the ground nearby. At each measurement point, accelerometers both on the vertical direction and horizontal direction parallel to the street, are installed.

P0

Northeast

10 m

P2 P1 P4

P 3

Fig. 2 Schematic of measurement points

P

0–P4 represent Points 0–4

The measurement configuration consists of eight LC0130 piezoelectric accelerometers, and an INV3020A high-precision portable data acquisition system. The data acquisition hardware was con-nected to a PC via a USB interface. All these in-struments were calibrated by Measure Center of China Aerospace Science and Technology and Na-tional Institute of Measurement & Testing Technol-ogy. Velocities can be calculated by integral from acceleration responses. The final average results were based on at least 10 records, which are listed in Table 1.

Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2011 12(10):782-793 785

Table 1 Background vibration velocities by measurement

Direction

PPV (mm/s)

RMS velocity (μm/s)

(Point)

Horizontal Vertical Horizontal Vertical

1 0.0201 0.0454 4.1266 10.0833 2 0.0199 0.0405 4.8068 9.0926 3 0.0380 0.0431 7.1167 9.2366

4

0.0516

0.0444 9.4248

10.2490

2.4 Characteristics of tunnel and soil

According to the geologic survey report of Chengdu Subway Line 2, the strata are simplified to four layers from top to bottom in this area. The dy-namic soil characteristics are listed in Table 2. Ed and d are dynamic elastic modulus and dynamic Pois-son’s ratio, respectively, and H and ρ are the thickness and density of each soil layer, respectively.

Table 2 Dynamic soil characteristics

E d Layer Soil type (m) (MPa) d (kg/m3

) 1 Miscellaneous fill 5.5 150 0.38 1900 2 Loose gravel 5.7 425 0.28 1900 3 Dense gravel 16.4 893 0.27 2200 4 Weathered mud stone ∞ 1000 0.35 2350

The Jiangjun Yamen Station near the monument is 138 m long and the embedded depth of the track is about 13 m. It is a double-decked station with island platform. The reinforced concrete lining of the sub-way has a Young’s modulus of 3.5×104

MPa, a Pois-son’s ratio of 0.25, a density of 2500 kg/m3

, and a hysteretic material damping ratio of 0.02.

3 Numerical models

In this section, to study the vibration responses caused by subway trains, a coupled train-track-tunnel-soil-structure model for vibration analysis is built. For the convenience of calculation, it is a simple method wherein the 3D system is simplified into a 2D train-track model and a 3D tunnel-soil-structure model. On one hand, the dynamic effects of moving train loads are mainly on the vertical direction, much larger than those on the horizontal and longitudinal directions. Therefore, in the train-track model, the structure is loaded by vertical loads only and in the horizontal plane the structure properties are symmet-

ric with respect to the track axis. On the other hand, it is not a good idea to simplify a building as a plane-strain problem. The moving effect is important; thus, a 3D tunnel-soil-structure model is employed. As an output of the first 2D model, two groups of moving train forces will be applied on the second 3D model.

3.1 Dynamic train-track models

To obtain the dynamic forces acting in the tunnel-soil -structure models, numerical models are created in the computer software DARTS_NL de-veloped at Delft University of Technology, the Neth-erlands. DARTS_NL is specialized finite element software for analysis of the dynamic railway vehicle-track interaction. Various track structures such as ballasted track, slab track, and viaduct, can be mod-elled in DARTS_ NL. The software has been suc-cessfully used for various railway applications such as optimization of a slab track (Markine et al., 2000; 2001), identification of dynamic properties of track components (Markine et al., 2003), assessment of various high-speed track structures (Esveld and Markine, 2006; Markine and Esveld, 2007), and analysis of the dynamic forces due to bad welds (Steenbergen, 2008). The main parts of the software and the developed models of subway train-track are described below.

The dynamic analysis of track-vehicle interac-tion that includes track flexibility can be computa-tionally expensive. In order to reduce the computa-tional effort, the modelling in DARTS_NL is re-stricted to two dimensions (the vertical and longitu-dinal directions) while using linear material property elements. A track structure is modelled using a series of alternating hard and soft layers. Each hard layer consists of Timoshenko beam elements, while the elastic layers are represented by distributed spring and damper combinations (Kelvin elements).

Depending on the properties of the finite ele-ments, these layers can be used for modelling various track components, such as rails, rail pads, sleepers, ballast, concrete slabs, subgrade, and foundation piles, etc. By defining the appropriate topology and im-posing the necessary boundary conditions, various railway track structures can be modelled.

The dynamic train- track model is schematically shown in Fig. 3. In this model, each vehicle is mod-eled by a mass-spring system that consists of four

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wheels, two bogies, and a car body, all of which are rigid bodies and connected to each other by the pri-mary and secondary suspensions. The track model consists of three hard and three soft layers. The hard layers are modeled as Timoshenko beams, with both bending stiffness and shear stiffness, while the elastic layers are represented by spring-damper combina-tions. The contact wheel/rail forces are modeled by means of a non-linear Hertzian contact spring (Gras-sie, 1984):

2

K

E P R R

3

6w r

,

(4)

2

H 4(1 )

2

where P is the static wheel load, Rw and Rr are the radiuses of the wheel and rail profile, E is the Young’s modulus of the wheel and rail material, and is Poisson’s ratio.

Fig. 3 Schematic of dynamic train-track model

Proper representation of the track geometry in the numerical model is important for realistic simu-lation of the vehicle-track interaction. Containing both long and short wave irregularities, the rail ge-ometry represents the main source of the dynamic excitations in the vehicle-track system. In DARTS_ NL the vertical rail geometry can be defined either as a periodic function or as a numeric data profile obtained from measurements. As there is no existing power spectral density (PSD) of track ir-regularity for subways in China, in this study, a rail surface data file NSTO_new is employed, which was from the measurement results of a railway line with new rails and with a total length of 1 km (Fig. 4a). Compared with the measured acceleration on the rail in Beijing Subway, here the data from 400 to 750 m of the total 1 km database in the surface file NSTO_new are used.

10 (a)

8

)6 mm4

(2

edu0 tilp-2 mA-4

-6

-8

-100

200

400 600 800 1000

Distance (m)

10100 (b)

)z101 H/ . 1

210

m

2

(m10.01

D1E0-3

SP1E0-

4

5

1E0-50

1

2 3 4 5

Frequency (Hz)

Fig. 4 Surface file NSTO_new of rail geometry profile (a) and average PSD of vertical rail geometry (b)

The size of the integration step is determined by

x

t

min

,

(5)

v

where xmin is the smallest element size, which is 0.1 m; and ν is the train speed, here the largest train speed of Chengdu Subway is employed, 20 m/s. Therefore, the integration step t≤0.005 s. Finally, the step of 0.0032 s is chosen, and then the spectrum of dynamic load are between 0 and 156.25 Hz, which contains main frequencies of ground vibrations caused by train loads.

The dynamic analysis is performed in the time domain following the concept of the displacement method (Zienkiewicz and Taylor, 1988; Kok, 1995). The direct integration process has been used. The main steps in the numerical procedure are:

(1) Assembling the mass M, damping C and stiffness K matrixes and the vector of the external forces

f; (2) Generation of the equations of motion:

Mu Cu Ku f ;

(6)

(3) Solution of the equations of motion yielding displacement vector u and acceleration ü;

(4) Filtering of the obtained displacements

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vector u and acceleration ü by cutting off the high frequency contributions (optional);

(5) Calculation of responses based on the (fil-tered) displacements u. The responses include: stresses, forces, and bending moments in the rail and rail supports; accelerations of the vehicle; dynamic wheel forces; and contact forces.

Details of the numerical method implemented in the DARTS_NL software can be found in Kok (1995). 3.2 Coupled tunnel-soil-structure finite element

models

To predict the vibration response of the monu-ment, the finite element method software MIDAS-GTS was used to build 3D finite element models of tunnel-soil-structure, and subjected to train loads of different kinds of fasteners and different train speeds. According to the shear wave velocity of different soil layers and the excited frequency range, the model takes a dimension of 120 m×60 m×60 m, and the size of elements ranges from 0.5 to 5 m.

The Rayleigh damping can be calculated by

C M K ,

(7)

where C is defined as a linear superposition of M and K with the coefficients α and β. The two coefficients can be only defined by the damping ratio of soils and the analysis frequencies. The damping ratios of soil are usually about 0.01–0.3, while those of sand and pebble are much smaller, about 0.004–0.12. Accord-ing to the lab test for sand pebble in Chengdu area by Wang (2006) and geologic survey report, a constant damping ratio 0.006 is employed here. Therefore, the coefficients 5

can be calculated as α=0.3589 and β= 1.82×

10. Soils are divided into four layers. The material properties are shown in Table 2. Special viscous-elastic artificial boundary is used to eliminate the reflection of propagation waves. The 3D finite ele-ment model is shown in Fig. 5.

3.3 Model checking

In order to check whether the model could pre-dict the vibration response effectively, measurements have been performed in Beijing Subway Line 5, both at the vibration source and on the ground above this metro line.

Fig. 5 3D finite element model of subway station, soil and monument

Fig. 6 shows the acceleration sensor on the rail. The dynamic responses and dynamic train loads can be calculated by the train-track model in Section 3.1. Comparing the accelerations between measurement and simulated results in time and frequency domain, we find that the amplitude and the frequency contents are similar, and especially the dominant frequency matches well (Fig. 7).

Fig. 6 An acceleration sensor on the rail

In addition, another ground vibration measure-ment was performed as well. The measurement point is just above the center of the two tunnel lines (Fig. 8). Another tunnel-soil 3D finite element model was set up for the section of running tunnel. The meshing,

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Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng)

6

(a)

)2

4

s/m2

( noit0 arele c-2 cA-4

-60

2

4

6

8

10

Time (s)

))

z0.10 ·H

0.08 (b)

2

(/m

(s0.06

no0.04 itare0.02

leccA0.000 20 40

60 80 100 120 140

Frequency (Hz)

6

(c)

)4

2/m (s 2

noitar0 elecc-2

A-4

-60 2 4 6 8

10

Time (s)

))z0.10 H·

(d)

2(0.08 /m (s 0.06

noita0.04 relec0.02

cA

0.000

20

40 60 80 100 120 140

Frequency (Hz)

Fig. 7 Time history (a) and frequency spectrum (b) of mea-sured acceleration on the rail; Time history (c) and fre-quency spectrum (d) of simulated acceleration on the rail

damping definition, train load simulation, and boundary conditions are the same as mentioned in Sections 3.1 and 3.2. Moreover, measured and simu-lated ground vibration velocities are shown in Fig. 9. It can be seen that both peak value and energy-averaged vibration level are very similar, which means the modeling in Sections 3.1 and 3.2 is rea-sonable and the dynamic model for the monument can be used to predict the vibration responses.

2011 12(10):782-793

Fig. 8 The soil conditions and site parameters of Beijing Subway Line 5 (unit: m)

0.4

(a)

0.3

s/m0.2

m

()0.1

ticol0.0

eV y-0.1

-0.2

-0.3

-0.40

2

4 6 8

10

Time (s)

0.4

0.3 (b)

)s/0.2 mm0.1

( y0.0

tico-0.1

leV-0.2

-0.3

-0.4

2

4 6 8

10

Time (s)

Fig. 9 Measured (a) and simulated (b) ground vibration velocity for Beijing Subway Line 5

4 Numerical results

In this section, some numerical results will be discussed. As the vibration responses caused by two trains coming across the station at the same time are larger than that caused by just one train operating at one time, all the results shown below are induced by two trains, taking into account the worst operation condition, in terms of vibration effects.

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4.1 Mode analysis and response of building

The first four natural frequencies and the related mode shapes numerically obtained from a modal analysis are shown in Fig. 10. It can be seen that all the first four natural frequencies are very low (<5 Hz), and the mode shapes show that the body is more flexible than the foundation of the monument.

(a)

(b)

(c)

(d)

Fig. 10 The first four mode shapes and related natural frequencies of the building

(a) f1=1.46 Hz; (b) f 2= 1.49 Hz; (c) f3=1.75 Hz; (d) f4=2.13 Hz

Fig. 11 describes the response transfer ratio of the building caused by train vibrations, defined as the velocity ratio of the top (Point 0) and bottom (Point 1) in one-third octave band. Between 8 and 80 Hz, the ratio is always larger than one in both horizontal (x-direction) and vertical (z-direction) directions. Vibrations will be enlarged from Point 1 to Point 0 in these frequencies. At frequencies below 5 Hz, the ratio is smaller than one in vertical direction; how-ever, it is very large (even up to 20) in horizontal direction, which means train-induced vibrations can easily cause resonance at low frequencies.

4.1 Dynamic response with different fasteners It is an effective isolation method that designs different kinds of vibration absorption fasteners. Three kinds of fasteners will be studies here, all of which are widely used in subway track design. The

first one, DTVI2 fastener, is typical of the high resil-ience direct fixation fasteners (Ding, 2010). It is usu-ally directly installed on short sleepers (Fig. 12a). The second kind of fastener is the shearing vibration ab-sorption fastener (Fig. 12b). Generally, this type of fastener possesses double stiffness and good vibration reduction capacity. The last one is Pandrol Vanguard fastener (Fig. 12c), which was initially developed for the British Rail. The elastic wedges are held in place of side bracket which are fastened to the track foun-dation. The low vertical stiffness gives improved attenuation of the dynamic forces generated at the wheel/rail interface, reducing the level of dynamic forces transmitted through the fastener and into the supporting structure.

10

o

itar

refsn1

arT

Horizontal direction

Vertical direction

0.11

10

100

1/3 octave frequency (Hz)

Fig. 11 Transfer ratios of the top and bottom of the building

(a)

(b)

(c)

Fig. 12 Sketches (left) and pictures (right) of three kinds of fasteners

(a) High resilience direction fixation fastener; (b) Shearing vibration absorption fastener; (c) Vanguard fastener

790 Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2011 12(10):782-793

RMSvelocity

All three fasteners have a supporting interval of 0.6 m. The type of rail is UIC-60, which has a cross

32

section area of 7.725×10 m, a mass per unit length

6

of 60.64 kg/m, and a bending stiffness of 6.434×10

2N·m. The stiffness of the rail pads of DTVI2

7

fastener is 7.8×10 N/m. The track damper fastening model III is chosen as a typical shearing vibration absorption fastener, whose equivalent stiffness is

7

about 1.0×10 N/m. The equivalent stiff-ness of

6

Vanguard fastener is 4.2×10 N/m (Jia, 2009).

DTVI2 fastener, the other two are effective in at-tenuating vibrations.

Fig. 14 describes the RMS velocities in hori-zontal direction at Point 1. Compared with the

7.5

( )µm/s

7.0 6.5

Allowable RMS velocity Subway+road traffic Subway

6.0 5.5 5.0

Velocity µ( )m/s

Train load (kN)

The time history of the dynamic train loads and 4.5 horizontal velocities at Point 1 are shown in Fig. 13. It 4.0

7 8

can be seen that, decreasing the stiffness of fastening 1010

Stiffness of fastener (N/m)

system is a good way to promote vibration absorption,

Fig. 14 Dynamic response with different kinds of fasteners especially at the vibration sources. Compared with

30 40

20 20

10 0

0 -20

2

8

10

-10 -20 -30

Train load (kN)

)m/s

-40 0

40

20 0 -20 -40 0

2

4 6

4 6

Time (s)

(a)

-400

8 10 12 14 16

Time (s)

30 20 10 0

2

4

8

10

µ(

-10 -20

Train load (kN)

-30 -400

2

4

6

6

Time (s)

(b)

Velocity

8 10 12 14 16

Time (s)

40 20 0 -20 -40 0

2

8

10

µ(

)m/

s

30 20 10 0

-10 -20

Veocty

2

4

-30

4 6

Time (s)

-400

6 8 10 12 14 16

Time (s)

Fig. 13 Dynamic train loads of different kinds of fasteners (left) and horizontal velocity responses at Point 1 (right)

(a) DTVI2 fastener; (b) Model III fastener; (c) Vanguard fastener

(c)

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allowable RMS velocity, which was 5.45–11.11 μm/s defined in Section 2.2, vibrations induced only by trains could be satisfactory when low-stiffness fas- teners are used.

(

kN)

40

20

(a)

load

0 -20

Train

As the square of RMS velocity stands for the vibration energy, Eq. (8) is used to calculate total environmental vibrations with a vibration superposi- tion caused by both road traffic and metro trains.

v v 2 v2

, (8)

rms,t

rms,r

rms,m

where vrms,t is the total environmental vibration ve-locity with the form of RMS, vrms,r is the RMS veloc-ity mainly induced by road traffic and vrms,m is the RMS velocity induced by metro trains with the worst

potential operation condition in terms of vibration. By site measurement the vrms,r in horizontal direction at Point 1 is 4.1266 μm/s (Table 1). Comparing the su-perposition response and allowable RMS velocity, one sees that, due to the contribution of road traffic, all the total vibrations exceed the lower limit, no matter what kind of fastener is used.

In theory, vibration responses would be reduced if the fastener with lower stiffness is used. However, there should be appropriate stiffness relationships between the wheel and track system, or wheel/rail wear or corrugation will develop.

4.2 Dynamic response with different train speeds Another way to attenuate vibrations is slowing down the train speeds. Some studies have shown that increased train speed will generate a higher dynamic load, and a doubling of the speed will increase the vibration levels by about 4–6 dB, and vice versa (Kurzweil, 1979; Remington and Kurzweil, 1987).

Here five train speeds are studied from 43 to 72

km/h. The stiffness of the fastener is 7.8×107

N/m (DTVI2 fastener). Figs. 15 and 16 show the dynamic train loads and RMS velocities in the horizontal di-rection at Point 1 with five different train speeds.

It can be seen that reducing train speeds pro-duces effective vibration attenuation, both for the train loads at source and the response of structures. The RMS velocities decrease sharply with the train speed, from 65 to 58 km/h. When the train speed is lower than 58 km/h, vibrations are lower than al-lowable value even if the contribution of road traffic is considered.

-40

-600

2

4 6 8

10

Time (s)

40 (b)

)

N20 k(

da0

ol

nia-20

rT

-40

-600

2

4 6

8

10

Time (s)

40

(c)

)20 Nk(

d0

ao l

nia-20

rT

-40

-600

2

4 6 8 10

Time (s)

40

(d)

)20

Nk(

da0

ol

nia-20

rT

-40

-600

2

4 6 8 10

Time (s)

40 (e)

)20

Nk(

da0

ol

nia-20

rT

-40

-600

2

4 6 8 10

Time (s)

Fig. 15 Train loads with different train speed (a) 72 km/h; (b) 65 km/h; (c) 58 km/h; (d) 50 km/h;

(e) 43 km/h

792 Ma et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2011 12(10):782-793

8

Allowable RMS velocity

7 Subway+road traffic

s/mSubway

) 6

µ(

yti5

cole

v4

SM3

R2

1

40 45 50 55 60 65 70 75

Train speed (km/h)

Fig. 16 Dynamic response with different train speeds

5 Conclusions

To protect buildings against long-term traffic vibration, both structural damage and architectural damage should be taken into account, especially for important historic buildings. A primary objective of this paper is to predict the vibration response on the monument in Chengdu due to road traffic and railway trains. The following conclusions have been drawn from the above analyses.

1. According to the vibration criteria listed in GB/T 50452-2008 and the analysis of mode and crest factor, the allowable RMS velocity at the foundation of the monument has been calculated as 5.45–

11.11 μm/s, which is a very strict limit for this im-portant structure.

2. A measurement at the monument shows that existing ambient vibrations are lower than the al-lowable value, but will contribute considerably to the final traffic vibration response.

3. A dynamic train-track model and 3D tunnel-soil-structure FE models have been set up to predict the vibrations at the foundation of the monument. Similar models have been built for Beijing Subway Line 5, and the site measurements at Line 5 have been performed both at the source and ground. A com-parison of the acceleration on the rail and the velocity on the ground shows that a good qualitative agree-ment has been obtained, especially in the time history.

4. Three types of fasteners were considered in the track model. Although the vibrations attenuate a lot when the track damper fastening model III and Vanguard fastener are used, the superposition RMS velocities still exceed the lower limit of allowable value.

5. Five different train speeds were considered.

Results show that reducing train speeds leads to ef-fective vibration attenuation. The RMS velocities decrease sharply with the train speed from 65 to 58 km/h. When the train speed is lower than 58 km/h, vibrations are lower than allowable value even if the contribution of road traffic is considered.

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