site-specific seismic response analysis of metro …...under the seismic excitation has been...

\http://www.iaeme.com/IJCIET/index.asp 505 [email protected]

International Journal of Civil Engineering and Technology (IJCIET)

Volume 10, Issue 12, December 2019, pp. 505-530, Article ID: IJCIET_10_12_051

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=12

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication

SITE-SPECIFIC SEISMIC RESPONSE

ANALYSIS OF METRO RAIL BRIDGE

Basavana Gowda G.M

Assistant Professor, Ramaiah Institute of Technology, Bangalore - 560054

Harsha G.M

Research Scholar, Ramaiah Institute of Technology, Bangalore - 560054

Govindaraju L

Professor, Dept. of Civil Engineering, UVCE, Bangalore - 560056

ABSTRACT

The effects of ground motion modification on the characteristics and time histories

of the original seed motion have been the topic of recent studies. But, the effects of these

modifications on the results of subsequent seismic analyses have not been

studied. These papers discuss the site specific seismic response analysis of Metro Rail

Bridge situated in Bengaluru, India. For this purpose, it was necessary to consider the

local site effects with Soil-Structure Interaction (SSI) for the analysis of structure

during seismic excitation. Site Response analysis was done using RSPMatch2005 and

SHAKE2000. The wavelet based spectrum compatibility approach is used to generate

synthetic earthquake motion for study area. The effect of soil deposits on the

propagation of seismic wave motion is investigated based on the equivalent linear

approach. Static and Dynamic analysis was done using STAAD(V8i) in order to find

the variation in Natural Period, Bending Moment and Deflection of the structure, by

incorporating soil-flexibility as compared to structures with conventional fixed-base.

The study concludes that the site was amplified by 4.85times. But, structural response

for the SSI with local site effect was within the range of acceptable values. The results

will be useful for design engineer to use as a guiding tool for carrying out more

advanced dynamic analysis at the study site.

Keywords: Soil Structure Interaction, Site Specific Response Analysis, Equivalent

Linear approach

Cite this Article: Basavana Gowda G.M, Harsha G.M and Govindaraju L, Site-

Specific Seismic Response Analysis of Metro Rail Bridge, International Journal of

Civil Engineering and Technology, 10(12), 2019, pp.505-530

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=12

Basavana Gowda G.M, Harsha G.M and Govindaraju L

http://www.iaeme.com/IJCIET/index.asp 506 [email protected]

1. INTRODUCTION

1.1. General

Bengaluru is a city in India currently experiencing a hasty development towards social

suburbanisation; new structures are often built beside or above the legacy structures as cities

grow to accommodate the influx of residents and commerce. In this regards, there was a need

of report being prepared for the future construction of Elevated Metro Bridge near

Chinnaswamy Cricket stadium. In fact, Bengaluru has experienced several minor earthquakes

in the 20th century. Recent studies by Ganesha Raj and Nijagunappa (2004), Sitharam et. al.

(2006) and Sitharam and Anbazhagan (2007) have proposed that Bengaluru need to be

upgraded from the current seismic zone II (BIS, 2002) to zone III based on the local seismo

tectonic details and hazard analysis[1],. It has also been highlighted that, the presence of

potentially active geological structures in vicinity of Bengaluru [2]. The Rock level PGA map

for Bengaluru varies from 0.09g to 0.16g [1] and the amplification factor ranged from 1 to 5.8,

Sitharam et. al. [2]. Hence sub soil classification for the Bengaluru region is very much required

to evaluate seismic local site effects for an earthquake. The assessment of local site effects on

seismic ground motions is of great importance in earthquake engineering practice. Soil-

Structure Interaction (SSI) is one of the design issues, which is often disregarded and even in

some cases ignored. Therefore, it is necessary to consider the local site effects with SSI for the

analysis of structure during seismic excitation. The dynamic behaviour of soil is quite complex

phenomenon and requires models which can distinguish the important aspects of cyclic

behaviour and need to be simple. Ground response effects generally are evaluated using one

dimensional models which assumes that seismic waves will propagate in vertical direction

through the horizontal layers of the soil. Hypothetical modelling of 1Dimension site response

can be generally accomplished using Equivalent-linear (EL) or nonlinear (NL) analysis. The

advantages of equivalent-linear modelling include small computational effort and few input

parameters.

This paper discusses the estimation of ground motion parameters for the performance of

Metro Rail Bridge. In the first part of the paper, Site-Specific response analysis is carried out

based on the Seismic Down-Hole Test parameter (Shear wave Velocity). One of the problems

in carrying out site-specific study for Bengaluru is the lack of previously recorded strong

motion data in the city. Hence, the current study highlights a methodology to predict the local

soil condition effects at site where no past earthquake recorded data are available. The

methodology adopts wavelet-based spectrum compatibility approach to generate synthetic

earthquake motions for the study area. Based on the equivalent linear approach, The effect of

soil deposits in the region on propagation of seismic wave motion parameters to the ground

surface is investigated. In the second part of the paper, the performance of a Metro Rail Bridge

under the seismic excitation has been analyzed for the pier supported on three approaches,

namely, pier with fixed support, pier with support accounting for soil-flexibility with pile

foundation and pier with support on underground station box. Individually, each of these

structures has a unique seismic response.

Soils being natural geological material, its properties are often difficult to assess. Due to

near surface geology with inhomogeneous nature, each earthquake produces a spatial variation

of ground motion and, consequently, spatial variability in the damage pattern. Therefore it has

been customary to assign large factors of safety to soil stresses when compared to that of

superstructure. In this situation, the soil stresses generally do not cross the elastic range except

in the small overstressed zones. Hence, in the current analysis the soil system is assumed to be

in elastic state. The process in which the response of the soil influences the motion of the

Site-Specific Seismic Response Analysis of Metro Rail Bridge


structure and the motion of the structure influences the response of the soil is termed as “Soil-

Structure Interaction” (SSI).

Pile Foundations are widely used for supporting important structures. Currently, Bengaluru

is undergoing huge infrastructure development and therefore it is important to adopt the best

engineering practices in such projects. Pile foundation is known as one of the popular method

of construction to overcome the difficulties of foundation in medium and soft soils. But, till

nineteenth century the design was entirely based on experience (Poulos and Davis, 1980)[3]. It

is for the convenient of an engineer to divide the design of major structures into two

components: the design of the structure and the design of foundation. But in reality, the loads

on foundation determines their movement, and this movement affects the loads imposed by the

structure; inevitably interaction between structure, foundation and soil or rock forming the

founding material together comprise one interacting structural system (Poulos and Davis, 1980)

)[3].The performance of superstructures during earthquake completely depends on foundation

performance which is interlinked with ground conditions at the site and the ground motion

experienced by the foundation. The response of structures is essentially a function of the

seismicity of the region, the nature of source mechanism, local site conditions and geology of

the site. Therefore, identifying the soil deposits which are susceptible to ground-motion

amplification is an important for accurate assessment of the structural analysis.

1.2. Study Area

Bengaluru city spreads over an area of 220 square kilometers and it is situated at an altitude of

around 910m above mean sea level (MSL). It is the administrative, commercial, industrial,

educational and cultural capital of Karnataka state, in the South India. The population of

Bengaluru region is over 6 million. It is situated on a latitude of 12° 58' North and longitude of

77° 37' East.

Figure: 1 a. Study Site (Source - Google Map)



1.3. Seismicity and Geology of study area

As per Seismic zonation map IS: 1893 (Part I): 2002 [4], Bengaluru falls under Zone II. Recent

earthquake that occurred to Bengaluru was in the range of 2.0 to 5.5 on Richter scale. On

29thJanuary, 2001, an earthquake with a magnitude of 4.3 on the Richer scale hit in the Mandya

district, Karnataka state, India, with its epicentre distance about 35km south of

Bengaluru.Geology wise,the Mandya district shows dominant presence of 'Archean' crystalline

formation comprising peninsular neissic complex with small patch of hornblende schist in the

northern part and intrusive closepet granites all along the western part of the district. The

overburden soil generally is dominated by Silty sand /residual silt and shows the presence of

clay as a nominal interstitial binder.

1.4. Geotechnical site characterizations

In order to perform a detailed site-specific seismic response analysis of the study site, sub soil

information is one of the most essential data. The characteristics of geological layer up to the

depth of first 30m are very important in amplification of earthquake shaking (Nandy 2007) [5].

In the current study, extensive geophysical site investigation data were collected from CEC-

SOMA-CICI JV (UG-2, BMRCL Project) [6] and analyzed. The data collected includes

geotechnical bore hole data and laboratory test data providing information such as standard

penetration test (SPT) values, thickness of subsoil strata, index and the engineering properties

of deposits of Study area. Table 1 presents classification of site with soil deposits along with

standard penetration test ‘N’ values.

Table 1. Sub Soil Details for Bore hole AB14

(Near Chinnaswamy Cricket Stadium location)

Depth (m) Symbolic Engineering Classification SPT Values RQD (%)

0

Loose, red, brow at

places, Clayey Silt

4 7

6 38

7

Highly weathered, light grey, weak to very weak, granitic

genesis fragments

9 > 100

12 > 100

15 > 100

17.5

18.5

Extremely strong, light grey, hard massive, granitic gneiss

86

19.5 54

20.5 54

21.8 93

1.5. Geophysical Site Characterizations

Characterization of the site was carried out using shear wave velocity (Vs) for the Bore Hole

AB-14 using Seismic down Hole Test (Geophysical method). Shear wave velocity at 30m

depth (Vs30) is accepted for site classification as per NEHRP (National Earthquake Hazard

Research Programme) (Dobry et. al. 2000) [7] (Kanli et. al, 2006) [8]. Usually, for amplification

and site response study, the average 30m depth Vs is considered. However, if the rock is found

within a depth of about 30m, near surface shear wave velocity of soil has to be considered.

Otherwise, Vs30 obtained will be higher due to the velocity of rock mass [1]. Table 2 presents

the Shear wave velocity (Vs) along with Dynamic module of soil deposit. Using 1-dimensional

shear wave velocity, the average shear wave velocity of soil has been calculated for depths of



3m, 6m, 12m, 18m and 21m depths, which is been shown in Table 3. Based on the average

shear wave velocity of 21m depth of study site, sub soil classification has been carried out for

local site effect evaluation using NEHRP classification. The Study site falls into “Site class C”

type of soil (Very Dense soil).

Table 2. Down Hole Test for Bore hole AB14


Depth (m) Vp (m/s) Vs (m/s) Density

(kN/m3) μ

Young’s

Modulus

(MPa)

Shear

Modulus

(MPa)

Bulk Modulus

(MPa)

1.5 353.4 144.6 17 0.40 99.5 35.53 164.95

3.0 671.2 229.3 17 0.43 256.4 89.40 646.65

4.5 1445.7 264.7 17 0.48 353.2 119.11 3394.18

6.0 1856.9 791.5 17 0.39 2958.2 1064.88 4441.71

7.5 2345.1 1264.4 19 0.30 7867.3 3037.34 6399.05

9.0 2370.8 1274.6 19 0.30 8005.7 3086.91 6563.45

10.5 2399.6 1303.4 19 0.29 8332.8 3227.89 6636.87

12.0 2443.9 1347.9 19 0.28 8846.2 3451.71 6745.32

13.5 2540.9 1388.7 19 0.29 9431.4 3664.01 7381.07

15.0 2625.4 1444.4 19 0.28 10171.7 3964.13 7811.01

16.5 2643.1 1438.7 19 0.29 10142.5 3932.74 8030.00

18.0 3072.3 1719.1 21 0.27 15789.6 6206.17 11546.61

19.5 3183.7 1772.6 21 0.28 16830.8 6598.40 12487.53

21.0 3220.6 1789.2 21 0.28 17167.2 6722.81 12818.15

Table 3. Typical Average Shear Wave Velocity Calculations


Depth

(m)

Shear wave

velocity

"Vs" (m/s)

Soil

thickness

"di" (m)

di/Vs

Average "Vs" of the soil depths

3m

(m/s)

6m

(m/s)

12m

(m/s)

18m

(m/s)

21m

(m/s)

1.5 144.58 1.50 0.0104

177.35 245.12 412.31 543.18 603.05

3.0 229.33 1.50 0.0065

4.5 264.70 1.50 0.0057

6.0 791.45 1.50 0.0019

7.5 1264.36 1.50 0.0012

9.0 1274.63 1.50 0.0012

10.5 1303.41 1.50 0.0012

12.0 1347.85 1.50 0.0011

13.5 1388.68 1.50 0.0011

15.0 1444.43 1.50 0.0010

16.5 1438.70 1.50 0.0010

18.0 1719.10 1.50 0.0009

19.5 1772.60 1.50 0.0008

21.0 1789.23 1.50 0.0008



1.6. Quantification of the Ground motion

To estimate the maximum horizontal forces that can be expected at the study site, Peak ground

acceleration value is used. Determining the spatial variation of peak ground acceleration is not

adequate, because peak acceleration often correspond to high frequencies, which are out of

range of the natural frequencies of most structures. The largest amplification of the soil will

occur at the lowest natural frequency. (Equation 1) [1]. the period of vibration corresponding to

the fundamental frequency is called the “Characteristic Site Period” (Equation 2) [1]

wo = πVs

2H (1)

Ts = 2π

wo (2)

The characteristic site period, depends on the shear wave velocity of the soil and soil

thickness. When the natural frequency of the ground motion coincides with the natural

frequency of the structures, Structures have a high probability to achieve resonance. Resonance

will cause increase in swing of the structure and given sufficient duration, amplification of

ground motion can result in total destruction. Hence, it was essential to evaluate the spectral

acceleration, i.e. the amplification of the seismic signal for the Bore Hole AB-14, which is been

shown in Table 4.

2. SOIL STRUCTURE INTERACTION

SSI has been recognized as an important factor that may considerably affect the relative

structure response, the motion of base and motion of surrounding soil (Todorovska and

Trifunac, 1990) [9]. Structures subjected to earthquake motion are generally analyzed using free

field ground motions. This assumption, which simplifies the analysis, it is although valid for

structure on rock or stiff soils, might not be appropriate for a structure on soft soils. For the

structures founded on rock or stiff soils, The fixed base assumption imply that high stiffness

of elastic soil media constrain the motion at the foundation of structure close to the free field

motion. However, there is always some soil-structure interaction during earthquake shaking

and the interaction is particularly major for structures founded on soft soils in which elastic

media confining the foundation acts as flexible column and due to inertial and kinematic

effects, overall response of structure gets modified. SSI results in increase of fundamental time

period of structure thereby reduces base shear, lateral forces, and overturning moments but

might increase the Displacement Demands. There are two kinds of analysis methods that are

used in contemporary software packages: (1) Direct method and (2) Sub-structure method.

Table 3.Quantification of the Ground Motion

Soil Classification based

on bore hole (AB-14)

Soil

depth

(m)

Shear Wave

velocity "Vs"

(m/s)

Fundamental

frequency "wo"

(rad/sec)

Fundamental

frequency "fo"

(Hz)

Characteristic

Site period

"Ts" (sec)

Loose, Red, Brown at

places, CLAYEY SILT

1.5 144.58 151 24 0.041

3.0 229.33 120 19 0.052

4.5 264.70 92 15 0.068

6.0 791.45 207 33 0.030

Highly weathered, light

grey, weak to very weak,

granitic gneiss fragments

7.5 1264.36 265 42 0.024

9.0 1274.63 222 35 0.028

10.5 1303.41 195 31 0.032

12.0 1347.85 176 28 0.036



13.5 1388.68 162 26 0.039

15.0 1444.43 151 24 0.042

16.5 1438.70 137 22 0.046

Extremely strong, light

grey, white at places,

hard, massive, granitic

gneiss

18.0 1719.10 150 24 0.042

19.5 1772.60 143 23 0.044

21.0 1789.23 134 21 0.047

2.1. Sub Structure Method or Multilevel Method

Substructure method divides the analysis in three simple steps and uses the principle of

superposition to obtain the final response. It is solved in frequency domain and soil-structure

behavior is assumed to be linear.

The three steps followed in sub-structure method are

Evaluation of foundation input motion

Performing Dynamic analysis of soil media to obtain stiffness and damping

characteristic of foundation-soil interaction.

Analyzing the structure supported on springs represented by step 2 and subjected to

base excitation obtained in step 1.

In the present study, SHAKE2000 was used for seismic site response analysis to provide

the input ground motion parameters required for SSI analysis in STAAD V8i.

3. METHODOLOGY

The following figure shows the methodology adopted in the present study.(Figure b)

Figure 2. Flow Chart Showing the Methodology Adopted



4. SITE SPECIFIC RESPONSE ANALYSIS

It is generally believed that Dynamic analysis provides the most realistic predictions of

structural response induced by earthquake ground motions. The forces on structures depend on

time period and peak ground acceleration of the structure. Subsoil condition is responsible for

the amplification of ground motion and amplification may increase or decrease the value of

spectral acceleration coefficient. The phenomena where in the local soils act as a filter and

modify the ground motion characteristics is referred as Site amplification. The seismic waves

travel from the bedrock to the surface with certain change in its characteristics (i.e. amplitude

and frequency of seismic waves) as they pass through the different soil deposits. During this

process, large accelerations may transfer to the structure and thus causing large deflection. Site

specific response analysis aims to determine the effects of local soil conditions on amplification

of seismic waves and hence estimating the ground response spectra for the design purposes. In

this study, an attempt has been made to study site specific ground response by equivalent linear,

1D analysis using SHAKE2000 for the study site. The term ‘one-dimensional’ means the soil

profile extends to infinity in all horizontal directions and the bottom layer is considered a half

space.

4.1. Selection of Peak Ground Acceleration

As per seismic zoning map of India (IS 1893–part I: 2002) [4], Bengaluru region, lies in low

seismic zone (Zone II) with a zone factor of 0.1 and expected earthquake of magnitude 5 to 6.

It is quite interesting to note that Sitharam and Anbazhagan (2006) [2] have identified Mandya-

Channapatna-Bengaluru lineament as the most vulnerable source for Bengaluru and have

reported that the MCE for this region has a moment magnitude of 5.1 thus developing a Peak

Ground Acceleration (PGA) of 0.146g at rock level. Hence, in the present study the maximum

PGA of 0.15g has been selected for the study site from the Rock level PGA Map (Figure 1) to

study the effect of local soil condition on the Metro Rail Bridge due to seismic excitation.

Figure3: Rock Level PGA Map for Bengaluru [1]

4.2. Development of Synthetic Earthquake Motion

As cited earlier, there are no records of strong ground motion available in the study site and

therefore there was a need for the development of synthetic earthquake for the study site. A

wavelet-based method was used for the generation of spectrum-compatible time-history of

earthquake. RSPMatch2005 is one such wavelet-based method developed by Hancock et al.



(2006) [10] [11] and same is used in the present study for developing synthetic earthquakes. As a

basic step, the methodology requires the use of strong motion records available from historical

earthquake. Several important factors are considered while selecting a suitable strong motion.

These include peak acceleration close to the target value, similar magnitude and similar site

conditions (Kramer 1996) [12]. Based on these factors, earthquake recorded at Boston of

magnitude (M) 5.9 and PGA 0.2g was selected from the earthquake data base. The selected

earthquakes represent nearly the similar magnitude of 5 to 6 for Bengaluru site condition.

Figure 2 shows the time history of selected strong motion for Boston earthquake having peak

acceleration of 0.2 g.

Based on the anticipated PGA (0.15g) and target response spectra at 5% damping for rock

or hard soil as per IS : 1893 (part I): 2002 [4], the program RSPMatch2005 was used to scale

down the PGA from 0.2g to 0.15g and generate the spectrum compatible time-history of the

earthquake. Figure 3 shows the time history of selected strong motion after scaling down the

PGA to 0.15 g. Figure 4 shows the response spectrum of selected earthquake motion before

and after matching the target spectra for the Boston earthquake.

Figure 4: Time-History of Selected Motion Record (PGA 0.2g)

0 2 4 6 8 10 12 14 16 18 20

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Accele

rati

on

(g

)

Time (sec)

Boston Saguenay, Baie-St-Paul,amax

=0.2g



Figure 5: Time-History of Selected Motion Record (PGA 0.15g)

Figure 6: Comparison of Target and Response Spectrum of Boston Earthquake Time History

4.3. Site Response Analysis

In the present study, equivalent linear approach using the computer program SHAKE2000

developed by Idriss and Sun (2004) [13] [14] was used for the analysis. The shear wave velocity

of each soil layer obtained from Seismic down hole test is used as the input parameter for the

model. Also to account for behavior of soil under irregular cyclic loading, the dynamic

properties of soils such as modulus reduction and damping versus shear strain curves proposed

by Seed and Idriss, Average formula for Sand soil and Schnabel formula for Rock were

considered in the SHAKE analysis. Synthetic earthquake motions are then specified at the base

0 2 4 6 8 10 12 14 16 18 20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Accele

rati

on

(g

)

Time (Sec)

Boston(saguenay,Baie-St-Paul)

amax

=0.15g

0 1 2 3 4

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Sp

ectr

al A

ccele

rati

on

(g

)

Period (Sec)

Target Spectra

(IS1893:2002 Rock or Hard Soil)

Spectra of original motion

(Boston,Saguenay)

Spectra of Modified motion



of soil profile to carry out dynamic analysis. The output from the site response analysis includes

time history of ground motion at any depth, amplification of motion at the ground surface,

fundamental frequency of the ground and spectral response at the ground surface. Figure 5

shows the acceleration time history at the ground surface.

Figure 7: Acceleration Time-History at ground surface

4.4. Response Spectra

The frequency content of an earthquake motion will strongly influence the effects of that

motion and hence only the PGA value cannot characterize the ground surface motion. A

response spectrum is used extensively in earthquake engineering practice to indicate the

frequency content of an earthquake motion. A Response Spectrum describes the maximum

response of a single-degree-of-freedom (SDOF) system to a particular input motion as a

function of the natural frequency/period and damping ratio of the SDOF system. Figure 6

shows variation of spectral accelerations (Sa) with period corresponding to 5% damping for the

study site. Also shown is the BIS 1893 Part (1): 2002 [4] code-specified response spectra for

medium soil condition at 5% damping for comparison. It may be observed that the spectral

values exceed the code specified values at some period. The maximum spectral acceleration

is 1.6117 for the earthquake input motion.

0 5 10 15 20

-0.4

-0.2

0.0

0.2

0.4

0.6

Accele

rati

on

(g

)

Period (Sec)

amax

=0.485g



Figure 7: Response Spectra at Ground Surface (5% damping)

4.5. Peak Ground Acceleration

The Peak Horizontal Acceleration (PHA) values at bedrock level are amplified based on the

soil profile at AB14 location. The acceleration-time histories at different depths were obtained

as output from SHAKE analysis. Figure 7 shows the variation of peak acceleration with depth.

The PGA value for the study site was 0.485g.

Figure 7: Variation of Peak Acceleration with Depth

4.6. Amplification Factor

The term “Amplification Factor” is used to refer to the ratio of the peak horizontal acceleration

at the ground surface to the peak horizontal acceleration at the bedrock. This factor was

evaluated for AB-14 using the PHA at bedrock obtained from the synthetic acceleration time

history for the borehole and the peak ground surface acceleration obtained as a result of ground

response analysis using SHAKE2000. The amplification factor thus calculated for the study

0 1 2 3 4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Accele

rati

on

(g

)

Period (Sec)

Target Spectra

(IS 1893-2002, Medium Soil)

Spectral Acceleration at Ground Surface Motion



site was 3.17. It can be observed that the amplification factor for most of Bengaluru region is

in the range of 2-3. This is in agreement with Sitharam et. al. (2005) [2] who have also concluded

that most of Bangalore region has a moderate amplification potential.

5. ANALYSIS OF BRIDGE STRUCTURE

Generally structures which are subjected to earthquake ground motion are analyzed using free

field ground motions. This assumption, which simplifies the analysis, is although valid for

structure supported on rock or stiff soils, might not be appropriate for a structure resting on

other soils. According to Gazetas and Mylonakis (1998) [15] SSI effect on the response of the

bridge was to increase the natural period of vibration and thus increase the ductility demand on

the pier by two times due to the increased base shear forces. Gazetas and Mylonakis (1998) [15]

stated that ignoring SSI has lead to oversimplification in the design leading to an unsafe design

of foundation and superstructure. Soil-structure interaction can be divided into two phenomena

inertia interaction and kinematic interaction. Earthquake ground motion causes soil

displacement known as free-filed motion. However the foundation embedded in to the soil will

not follow the free field motion. This inability of the foundation to match the free field motion

causes the kinematic interaction. On the other hand, the mass of the super structure transmits

the inertia force to the soil causing further deformation in the soil which is termed as inertia

interaction.

5.1. Bridge Structure Description

To study the dynamic behavior while accounting the effect of soil-structure interaction, Metro

Bridge pier was modeled with three approaches, namely, pier with fixed support (Figure 8),

pier with supports accounting for soil-flexibility with pile foundation (Figure 9) and pier

supporting on underground station box (Figure 10). The cross section of the superstructure of

the box girder bridge consists of single cell box (Trapezoidal Shape). The bridge is simply

supported and having a span of 25mC/C. The structural parameters that used in this study were

collated from CEC-SOMA-CICI-JV (BMRCL Project) [16]



Figure 8: Typical Metro Rail Bridge with Conventional Fixed Base Support

(NOTE: Dimensions are in meter)

5.2. Modelling and Analysis using STAAD (V8i)

The method adopted in software (STAAD V8i) [17] for modeling and analysis was Finite

Element Analysis (FEM). The flexibility of soil was modeled by inserting springs between the

foundation member and soil medium. For the accuracy of the results the Bridge pier was

divided into small segments (0.75m each segment). To study the seismic behavior of the

structure, it was idealized as 3D space frame. The geometry and material properties of Bridge

structure used for the study are tabulated in table 4 and 5. Equivalent Static and Response

Spectrum given in IS: 1893 (Part I): 2002 [4] was used for the analysis and Time History based

on local site effect analysis was being performed and compared.

Table 4: Sectional Properties of the Under Ground Station Box Members Considered for Analysis.

Members Section(b x d x l)in mm

Roof slab * 20700 x 1200 x 24000

Concourse slab * 20700 x 750 x 8700

Base slab * 6530 x 1335 x 24000

1820 x 800 x 24000

Side Wall * 1200 x 13200 x 24000

Base slab and Roof Slab at Pier location * 6000 x 2000 x 9000

*Grade of Concrete=M35



Table 5: Sectional Properties of the Metro Rail Bridge Considered for Analysis

Members Section(b x d x l)in mm

Depth box girder *** 2100

Width and Thickness of top flange *** 8800 x 250 to 500

Width and Thickness of bottom flange *** 3600 x 300 to 500

Web Thickness *** 375 to 500

Pier Cap ** 3600 x 3100 x 1500

Pier ** (R= 800)2600 x 1600 x 9000

Pile Cap * 5200 x 5200 x 2000

Pile * 4 #, 1200 Dia. 17m length

*** Grade of Concrete = M55 ** Grade of Concrete = M45 * Grade of Concrete = M35

NOTE: Dimensions are along the alignment of Bridge.

HARD ROAD

GROUND LEVEL

Figure 9: Typical Metro Rail Bridge with Pile Support


5.3. Modelling of the Foundation System

The foundation system of the bridge comprises of three alternate approaches, namely, fixed

support, pile support and underground station box support. The interaction between foundation

system and the soil is represented by sets of linear springs and is summarized below. The



estimation of superstructure load coming on to the foundation system and the numerical model

used for the study are described in the following section. In the present analysis, the foundation

systems (Piles and Underground station box) are treated as linear elastic concrete elements as

nonlinear behavior of the material is not expected.

5.4. Load Details

As per IITK-RDSO guidelines [18], the inertia forces due to each component or portion of the

bridge as obtained shall be applied @ centre of mass of the corresponding component or portion

of the bridge for the seismic force calculation (Figure 11)

In Longitudinal Direction of Bridge = 100% Total DL of Super structure + 0 % Live load

In Transverse Direction of Bridge = 100% Total DL of Super structure + 50 % Live load

Table 6: Summary of Nodal loads for Bridge components

Nodal Load Details Longitudinal

Direction

Transverse

Direction

p1 = Super Structure Load 5422 kN 6262 kN

p2 = Pedestal Load 9.8 kN 9.8 kN

p3 = Pier Cap Load 332.35 kN 332.35 kN

p4 = Pier Load 779.90 kN 779.90 kN

Horizontal Design Seismic Force [P] has been calculated:

P1 = p1 x Ah

P2 = p2 x Ah

P3 = p3 x Ah

P4 = p4 x Ah

Base Shear (VB) = P1 + P2 + P3 + P4

Horizontal Design Seismic Moment [M] has been calculate:

M1 = P1 x H1

M2 = P2 x H2 M3 = P3 x H3

M4 = P4 x H4

Bending Moment (M) = M1 + M2 + M3 + M4



GROUND LEVELGROUND LEVEL

BASE SLAB

CONCOURSE SLAB

ROOF SLAB

LO

WE

R S

IDE

WA

LL

UP

PE

R S

IDE

WA

LL

Figure 10: Typical Metro Rail Bridge with Underground Station Box Support




HORIZONTAL LOAD APPLICATIONTYPICAL CROSS SECTION OF METRO BRIDGE

P 1 "SUPER STRUCTURE LOAD"

P 2 "PEDESTAL LOAD"

P 3 "PIER CAP LOAD"

P 4 "PIER LOAD"

Figure 11: Typical Metro Rail Bridge with Seismic Force application (IITK-RDSO GUIDELINES) [18]


5.5. Modelling of Piles

The Bored cast in situ concrete piles are modeled as column with specified lateral and axial

stiffness. The Pile Diameter 1200mm was recommended by the Contractors availability of Rig

Machine. The Pile length was calculated based on the recommended Vertical Capacity of

Single pile was 5000kN. The pile was considered has “End Bearing Pile” socketing in to rock.

The ultimate pile load capacity was estimated by means of static formula based IRC 78:2000 [19], IS 2911:2010 [20] and IS 14593:2003 [21]. The Frictional resistance in soil was ignored and

length of pile was calculated based on “End Bearing” in Rock and “Frictional Resistance” in

weathered rock alone. In the present study, the length of pile was estimated to be 17m.

5.6. Modelling of Soil

Soil is an extremely nonlinear material when subjected to large deformation and its stiffness

also varies with depth. The pile foundation behavior when subjected to seismic loading

significantly depends on the soil-pile interaction. Several analytical and semi-empirical

methods have been adopted in practice to model the soil-pile interaction including (a)

Continuum model, (b) Numerical Finite element model, and (c) Winkler spring model



In this study, the soil interacting with the foundation of the structure was represented by

Winkler springs. Three types of soil springs (Winkler type) are used in the present analysis

(Figure 12)

Axial soil springs (t-z springs): representing soil resistance at pile surface along its

length,

Lateral soil springs (p-y springs): representing the lateral resistance of soil to the

piles, and

End bearing springs (q-z springs): representing the end bearing of soil at the bottom

of the piles

Figure 12: Schematic Showing Soil Springs along the Pile depth

The Joint springs was considered for the Pile foundation model. Spring constant k (Modules

of subgrade reaction) was obtained for the joint springs by referring to API guidelines [22].

Since, the pile is resting on the Hard Rock, the End bearing springs (q-z springs) has been

considered as “Pin Support condition”.

The Area springs was considered for the Under Ground Station model. Spring constant k

was obtained for the area springs by referring to “Modulus of Subgrade Reaction Ks” has

illustrated in “Foundation Analysis – BOWLES” [23] and the Multi linear spring was calculated

by considering 50mm settlement (Allowable settlement for Raft foundation). A 3-D linear

model was constructed in a finite element program STAAD V8i [17] to analyze the foundation

as a whole.

6. PARAMETRIC STUDY

The Metro Rail Bridge under the seismic excitation has been analyzed for the pier supported

on three approaches, namely, pier with fixed supports, pier with supports accounting for soil-

flexibility with pile foundation and pier supporting on underground station box. Studying the

site response of the soil using the RSP Matching and SHAKE software’s and Equivalent Static,

Response Spectrum and Time History analysis is done using finite element software STAAD

V8i [17] (Linear) in order to find the variation in Natural Time Period ‘Tn’, Bending Moment

(M) in pier and Deflection ‘Δ’ of super structure by incorporating soil flexibility as compared

to structures with conventional fixed base. These are discussed one by one as followed.

6.1. Natural Time Period (Tn)

The variation in Natural Time Period of structure for the fixed base support, Pile support and

Box support is tabulated in Table 7 and 8. Empirical equations given by IITK-RDSO [18]



guidelines were used for the calculation of time Period (Tn) of the structure and compared

withSTAAD (V8i) for the Fixed base support. “Rayleigh Method” has been adopted to

calculate the “Fundamental Natural Time Period” of the Structure in STAAD (V8i) [17]. Default

STAAD Calculates “Tn”as per IS 1893–part I: 2002 [4] for RC structure. Hence, the Time Period

in longitudinal and transverse direction of the Bridge Pier [Cantilever Structure] shall be

assigned manually using STAAD by Rayleigh Frequency Approach.

Table 7: Fundamental Time Period for Different Support Conditions (Longitudinal Direction)

Type of

Support

Fixed Base

Support (Manual)

Fixed Base Support

(STAAD)

Pile Support

(STAAD)

Box Support

(STAAD)

Time Period

(Tn) 0.454 0.492 0.659 0.659

Table 8: Fundamental Time Period for Different Support Conditions (Transverse Direction)

Type of

Support

Fixed Base

Support (Manual)

Fixed Base Support

(STAAD)

Pile Support

(STAAD)

Box Support

(STAAD)

Time Period

(Tn) 0.914 0.969 1.091 1.091

From the results of detailed study, it has been shown that the Fundamental period is more

for pile and box supported structure due to flexibility of the surrounding soil and time

period obtained for the Fixed Base (STAAD) was similar to the time period calculated for the

Fixed Base (Manual - As per IITK-RDSO guidelines [18]).

6.2. Seismic forces and Base Shear (VB)

The seismic forces for the Metro Rail Bridge Structure was estimated by “Seismic Coefficient

Method” (Single Mode Method), Response Spectrum Method (Multi Mode Method) and Time

History Method in following cases. Design Horizontal Seismic force (Ah) was calculated from

Equation 3.

Ah = Z

2x

I

Rx

Sa

g 3

Base Shear (VB) was computed from the Eq. 4

VB = Ah x Wi 4

Where,

(Z) Zone factor, (I) Importance factor, (R) Response reduction factor, (Sa/g) Bridge

Flexibility factor and (Wi) Seismic Weight of the structure. Base Shear evaluated for various

methods under different support conditions are presented in Table 9 and Table 10

Table 9: Base Shear for various methods with Different Support Conditions (LONGITUDINAL)

Base Shear (VB)

Method of Analysis

Manual Static Response Time

History

Type

of

Support

Fixed Support 490.0 490.0 490.0 490.0

Pile Support 404.5 404.4 404.5 404.5

Box Support 481.2 480.9 480.9 480.6



Table 10: Base Shear for various methods with Different Support Conditions (TRANSVERSE)

Base Shear (VB)

Method of Analysis

Manual Static Response Time

History

Ty

pe

of

Su

pp

ort

Fixed Support 331.8 310.4 310.4 310.4

Pile Support 275.8 275.8 275.8 275.8

Box Support 292.3 292.7 292.3 292.3

From the results of detailed study, it has been shown that the Dynamic analysis performed

either by the Time History method or by the Response Spectrum Method are matching to the

Base Shear obtained from Static Method (Calculated using a fundamental period Tn).It has

been observed that the reduction in base shear for the Box support and further reduction in base

shear for Pile support. Base shear is more in case of fixed support for all the three types of

analysis in comparison with pile and box support due to more stiffness offered at the support

level.

6.3. Pier Bending Moment (M)

The Variation in Pier Bending Moment at the top of Pile Cap was evaluated for various methods

of analysis under different support conditions are represented in Figure 13 and 14

Figure 13: Variation of Bending Moment (LONGITUDINAL)



Figure 14: Variation of Bending Moment (TRANSVERSE)

From the results of detailed study, it has been shown that the Bending Moment

In the Longitudinal direction of the Bridge, Maximum Bending moment was 5775kN-m

for the Fixed Base support (Equivalent Static Analysis) and Minimum Bending moment was

4616kN-m for the Pile Support (Equivalent Static Analysis)

Bending moment is more for fixed support condition at the pile cap level by all the three

methods of analysis (Equivalent Static, Response Spectrum and Time history method) whereas

at the top of pile, BM has reduced. Since the flexibility of soil is induced at the level of Pile

cap, it has resulted in more bending moment.

In the Transverse direction of the Bridge, Maximum Bending moment was 3714.31kN-m

for the Fixed Base support (Time History Analysis) and 3667kN-m for the Fixed Base Support

(Equivalent Static Analysis) and Minimum Bending moment was 3166kN-m for the Pile

Support (Equivalent Static Analysis)

6.4. Pier Deflection (Δ)

The Variation in Pier Deflection at the CG of Viaduct was evaluated for various methods with

different support conditions are presented in Figure 15 and 16.

In the Longitudinal direction of the Bridge, Maximum Deflection was 8.953mm for the

Fixed Base support (Time History Analysis) and Minimum Deflection was 5.135mm for the

Fixed Support (Response Spectrum Analysis).



Figure 15: Variation of Pier Deflection (LONGITUDINAL)

In the Transverse direction of the Bridge, Maximum Deflection was 10.912mm for the

Fixed Base support (Time History Analysis) and Minimum Deflection was 8.115mm for the

Fixed Support (Equivalent Static Analysis).

Figure 16: Variation of Pier Deflection (TRANSVERSE)

There is increase in pier deflection from height 9.75m to 12.0m for fixed support. SSI effect

reduces the displacement of superstructure for both Box support and Pier support condition due

to flexibility offered by the soil at the pile cap level while for the fixed support condition, it

results in more deflection.



CONCLUSIONS

1. Spectrum compatible time histories of acceleration developed from wavelet

based target spectrum matching, using computer model RSPMatch2005

for the required PGA at bed rock for the study site was identical to the Rock

level PGA Map for Bengaluru region developed by Sitharam et. al. (2005).

2. The Time period of the structure was 6 to18 times more than the Characteristic

Site Period (0.024sec to 0.068sec) obtained based on the shear wave velocity Vs

(Equation 2). Therefore the Resonance will not occur for the Bridge Structure

near the study site.

3. Based on the average shear wave velocity of 603.05m/sec measured at 3m

interval up to a depth of 21m, as per NEHRP, the study site will be classified as

“Site class C” (Very dense Soil)

4. Site Specific Response analysis for the study site shows a Peak Ground

Acceleration at ground surface was 0.485g against 0.1g for the Bengaluru region

(As per SI 1893:2002) with an amplification factor 3.17.

5. The maximum spectral acceleration at ground surface was 1.6117. Further, the

response spectrum obtained for the study site was exceeding the current I.S.

code specified values at some period.

6. The increase in time period of vibration is 18% and 31% in the Longitudinal

direction and 11% and 16% in transverse vibration of Bridge structure for Box

support and Pile support respectively compared to conventional Fixed base

support

7. Variation in Pier Bending Moment in the longitudinal direction (20%) and

transverse direction (13.6%) of the Bridge was less for the Pile support

compared to the conventional fixed base support (Equivalent Static Analysis).

8. The Deflection has increased by 74.35% in the longitudinal direction and 34.5%

in the transverse direction for the pile support (Time History Analysis)

compared to conventional Fixed support (Equivalent Static Analysis).

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest regarding the publication of this journal

paper.

ACKNOWLEDGEMENTS

First and foremost, we express our sincere gratitude to Mr. V.L. Prasad and Mr. M.M. Raju for

the continuous support extended for the research, for their patience, motivation, enthusiasm,

and immense knowledge. Their guidance helped us in all the time of research. The study was

also made possible by the support of Bangalore metro rail corporation limited (BMRCL). The

authors grateful to M/s CEC-SOMA-CICI-JV (UG-2) and Mr. Shiva Kumar-Design Expert

(M/s MOTT) consultants for their assistance.



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site-specific seismic response analysis of metro …...under the seismic excitation has been...

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