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Scenario based seismic re-qualification of caisson supported major bridges – A case study of Saraighat Bridge
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
Contents lists available at ScienceDirect
Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn
Scenario based seismic re-qualification of caisson supported major bridges – A case study of Saraighat Bridge
MARK
⁎
Pradeep Kumar Dammalaa,b, Subhamoy Bhattacharyab, , Adapa Murali Krishnaa, Shiv Shankar Kumara, Kaustubh Dasguptaa a b
Indian Institute of Technology Guwahati, India Chair in Geomechanics and Director-SAGE Laboratory, University of Surrey, Guildford GU2 7XH, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Keywords: Liquefaction Well foundations Ground response Natural period
Many major river bridges were constructed in highly active seismic areas of India much before the seismic code development. Bridges are lifelines infrastructure and as a result, it is necessary to requalify/reasses these structures in the light of the new and improved understanding of seismic resistant design philosophies. The aim of the paper is to develop a simplified methodology to carry out scenario based seismic requalification of major river bridges supported on caisson foundations (also known as Well Foundation). An example problem of Saraighat Bridge located in highly active Himalayan seismic zone is considered to demonstrate the application of the methodology. Field investigation and advanced laboratory tests on soil samples from the bridge site were carried out. The test results reveal that the soil is susceptible to liquefaction and as a result, soil structure interaction analyses are carried out. It is shown that good performance of these type of bridges depend on the displacement response of the pier head so as not to cause unseating of the decks. It is concluded, owing to the large stiffness of the foundations, bridges supported on caisson foundations may not be adversely affected by liquefaction induced effects.
1. Introduction Caisson foundations (also known as Well foundations), see Fig. 1, are preferred type of foundations for supporting major bridges in India. Typical lateral dimensions of such foundations range between 10 to 20 m and the depth can range between 30 and 60 m. Many of these bridges supported on well foundations were constructed much before the development of first Indian seismic code [10] and the design was mostly guided by the river and scour protection works. Fig. 1 show the schematic views of Saraighat Bridge supported on double D shaped well foundation. Historically, these foundations performed well during the limited earthquakes they experienced. However, there are currently two concerns: (a) As these were constructed before the seismic codes development, there is a need to reassess these lifeline structures for the worst possible seismic event, in the light of the latest scientific understanding; (b) It is often argued that these large dimension foundations are uneconomic and piles may be used instead. Therefore, the aim and scope of the paper are as follows: (A) Develop a methodology for scenario based seismic requalification of such river bridges supported on well foundations.
⁎
Corresponding author. E-mail address: [email protected] (S. Bhattacharya).
http://dx.doi.org/10.1016/j.soildyn.2017.06.005 Received 17 March 2017; Received in revised form 7 June 2017; Accepted 8 June 2017 0267-7261/ © 2017 Elsevier Ltd. All rights reserved.
(B) Demonstrate the application of the methodology by taking the example of Saraighat Bridge constructed in 1962 over the 1300 m wide Brahmaputra River. 2. Proposed methodology for seismic requalification of bridges While state of the art understanding of seismic behaviour is often incorporated in new projects, there are several important structures that were built much earlier and did not benefit from better understanding. In this context, seismic requalification/re-assessment is defined as the process of verifying the expected performance of a structure under estimated seismic scenario conditions. Due to the sheer size of the well foundations (see Fig. 1), bending, buckling and shear type of failure can be ruled out and the performance criteria will be governed by the pier head displacement to arrest the possible deck unseating failures. Reconnaissance survey following the recent earthquakes showed that midspan collapse of bridges often occur due to liquefaction induced effects and possibly due to elongation in natural period of bridges, see for example the Showa bridge analysis carried out in Bhattacharya et al. [2]. Fig. 2 shows a simple model to describe the dynamic soil-structure interaction following the work of Lombardi and
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
Fig. 1. (a) Schematic view of the Saraighat Bridge (b) Central portion of the bridge (c) Cross section along the foundation (Z-Z) (d) Cross section along the pier (X-X).
unsupported length (from Lfix-pre before liquefaction to Lfix-post after liquefaction). This results in an increased displacement demand of the pier head often leading to unseating/failure of deck slabs, especially when the Tpost falls in the displacement sensitive zone. This is an issue associated mostly with simply supported bridges. From the point of view of bridge requalification, predicting displacement response of the pier head/top for possible deck unseating failure is critical. Further details of the methodology can be found in Krishna et al. [13] and the essential ingredients/input required are: Expected hazard mapping (scenario earthquake information), ground conditions under the foundation, and details of the structure. The methodology involves various stages and can be carried out in few steps: Step 1: Quantification of seismic hazard: In this step, it is necessary to quantify the seismic hazard. Effectively, this would mean obtaining ground motion parameters [for example Peak Ground Acceleration (PGA), Moment Magnitude (Mw)] and time histories of expected ground motion at the site either using deterministic approach or by probabilistic approach-see for example the work of Boore [3], Galasso et al. [7], Alexander et al. [1]. Step 2: Site response analysis: In this step, information such as the ground type and stratification, slope and topography of the ground, location of the ground water level are obtained. Subsequently, ground response analyses together with liquefaction analyses are carried for the earthquake scenario predicted in Step 1. If necessary, additional site investigations (borehole or geophysical methods) and advanced soil testing may be needed. Step 3: Seismic analysis: This stage involves seismic analysis of the structure using the results obtained from the earlier stages with the help of seismic design philosophies and compare with the required performance. The loading obtained in Step 1 & 2 together with the details of the superstructure and foundations are used to predict the various performance levels. Step 4: Seismic strengthening and retrofitting methodologies are to
Fig. 2. Effects of liquefaction on acceleration response spectrum.
Bhattacharya [14] where it may be noted that as soil liquefies there is an increase in natural period (from Tpre before liquefaction to Tpost after liquefaction) of the bridge/structure due to the increase in the
271
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
Fig. 3. (a) Saraighat Bridge (b) Location of the bridge in Assam, India (c) Soil profile near the centre of the bridge.
of double D shaped well (Fig. 1c). The reinforced deck of the bridge rest on bearings (simply supported) which are supported on 1.35 m pier cap. The pier cap is supported on twin circular piers of 10.68 m high and 4.5 m diameter each (Fig. 1d). The bottom parts of the piers are connected to the well cap of 1.95 m thickness. The mass of each main span is 2100 t [20].
be planned to meet the requirements in order to make the structure seismically safe. These requirements would be different for different structures and for different performance levels. 3. Case study – Saraighat Bridge Saraighat Bridge (Fig. 3a) was designed in 1958, construction finished in 1962 and is the first bridge constructed over the mighty Brahmaputra River (the widest river in Asia). The bridge is located in Guwahati (26°11′ N and 91°38′ E) city in Assam, one of the seven North Eastern (NE) states of India (Fig. 3b). It is of interest to note that the first Indian seismic code was developed in 1964 and liquefaction studies were initiated globally following the 1964 Niigata earthquake. Therefore it is required to carry out seismic requalification of this bridge and the essential calculation is the revised seismic displacement due to liquefaction.
3.3. Local soil conditions Fig. 3c presents the typical soil profile near the centre of the bridge (pier-P6). Soil samples near the vicinity of the bridge were collected for geotechnical characterization and further details about the tests conducted and results can be found in Dammala et al. [5] and Rouholamin et al. [17]. 3.3.1. Ground Response Analysis (GRA) A free field one dimensional GRA based on equivalent linear approach was performed on the considered soil profile to determine the induced stresses and strains. The input modulus degradation (ratio of shear modulus to initial shear modulus, G/Gmax) and damping (D) curves were developed for the Assam sand by performing Resonant Column (RC) tests. The four ground motions discussed in the earlier section have been considered as the input bedrock motions. Details about the RC testing methodology, test results and the free field GRA can be found in Dammala et al. [5].
3.1. Seismic Hazard Assessment (SHA) The entire NE region of India, including Guwahati is regarded as one the most seismic active regions of the world and Indian standards for earthquake resistant design of structures [10] classified this region as one of the highest seismic zone in the country with an expected PGA of 0.36 g. Since 1869, this region has witnessed two great earthquakes (Mw ≥ 8.0) and 20 large earthquakes (8.0 ≤ Mw ≥ 7.0), [11]. Severe seismic events were not observed in the region for the last 67 years and seismologists are of the opinion that this temporal area called the “Assam Gap”, is a potential source for an impending large earthquake with an expected moment magnitude of 8.5 [12]. In view of these, researchers [15] have performed SHA of Guwahati city. In order to consider the worst possible seismic scenario for the present case study, the closest potential seismic source to the site (Oldham fault) and the past largest seismic event recorded at the source (1897 Great Assam earthquake, Mw = 8.1) are considered. Kanth et al. [11] simulated bedrock ground motions for Guwahati city for the considered event and the PGA values obtained from the simulations were found to match well with the corelated PGA values from the witnessed intensities in terms of MSK (Medvedev-Sponheuer-Karnik) intensity scale. Four artificial ground motions for Guwahati city are presented with bedrock PGA ranging from 0.146 g to 0.185 g.
3.3.2. Liquefaction assessment Footprints of severe ground fissures representing extensive liquefaction were observed in Guwahati city during the past severe seismic events. Kanth et al. [11] concluded that most parts of the city are prone to severe liquefaction for large earthquakes. The depth of liquefaction (Dliq) was estimated based on the traditional semi-empirical procedure by Idriss & Boulanger [8] using the available Standard Penetration Test (SPT) data and it predicted the Dliq ranging between 6 and 11 m from surface depending on the magnitude (6.0–8.0) and PGA (0.15–0.36 g) parameters. 3.4. Modelling the dynamic soil-structure interaction The bridge decks are simply supported on the piers and as a result the analysis of one pier will be sufficient. The central pier of Saraighat Bridge is modelled as this has the highest free standing length in water and therefore unsupported and most flexible. Finite Element Analysis was carried out using SAP2000 [4] where the pier and the well foundation (here in referred as SWP system i.e. Soil-Well-Pier system) are modelled as rigid beam elements with their corresponding material properties, see Fig. 4. The well-soil interaction is modelled using discrete springs and dashpots obtained following the work of Novak et al. [16] and Veletsos & Wei [18]. It is assumed that the SWP system is stable vertically and the base is allowed to rotate and translate. For the
3.2. Bridge design details Saraighat Bridge is a typical double warren steel truss bridge of a rhomboidal shape, the lower deck to accommodate a two metre gauge railway track and the upper deck hosting a two lane highway. Fig. 1(a–d) present the schematic views of the geometrical details of the bridge and its support system. The super structure of the bridge is made of steel trusses while the substructure (piers and foundation) is of reinforced concrete. Foundations for the main span piers (P1 to P11) are 272
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
Fig. 4. (a) Actual central pier system of the bridge and idealized 1D SWP system (b) before and (c) after liquefaction.
calculated locations on the middle span of the bridge and reported in Debnath et al. [6]. Based on operational modal analysis using both the monitored acceleration response and finite element modelling, the fundamental frequency of Saraighat Bridge is reported to range between 0.9031 and 0.9119 Hz and the damping ratio approximated as 3.02% [6]. Our numerical model is in a good agreement (5% error) with the measured data and it is reasonable to assume that the model is verified and validated. To predict the performance of the bridge at full liquefaction, modal analyses were performed for different depths of liquefaction (Dliq). It is assumed that springs in the liquefied zone had zero stiffness (Fig. 4c). Fig. 5(a) presents the variation of natural period and percentage change in the period for different depths of liquefaction (Dliq). As expected, natural period of the bridge before liquefaction (Tpre) 1.18 s increased to 1.37 s (Tpost) for the maximum predicted Dliq of 11 m. Spectral displacements (Sd) are predicted for all the considered depths of liquefaction based on Indian Standard for earthquake resistant design of buildings [10].
sake of completeness, the formulations are given in Eqs. (1) and (2).
Kx = G [Su1 (ao , v, D) + iSu2 ((ao , v, D))]
(1)
K θ = G ϒ o2 [Sψ1(ao × Ds ) + iSψ2(ao × Ds )]
(2)
where Kx and K θ are the dynamic impedance of soil for distributed translational and rotational vibrations respectively; v and G are the Poisson's ratio and shear modulus of the soil, respectively; D and ϒo are the equivalent diameter and radius of the arbitrarily shaped well foundation, respectively; ao (=roω/Vs) is the dimensionless frequency; ω (=2Πf) is the circular natural frequency; f is the predominant frequency of the ground motion (≈5 Hz for the considered ground motions); Ds is the material damping. The real (k x and kθ ) and the imaginary parts (cx and cθ ) of the above complex notations represent the stiffness and material damping coefficients (combined material damping of soil and radiation damping), respectively. Su1, Sψ1 are the stiffness coefficients to evaluate k x and kθ respectively, while Su2 , Sψ2 are the damping coefficients to evaluate cx and cθ , respectively. These coefficients are frequency (ω) and Poisson's ratio (v ) dependant and Novak et al. [16] proposed design charts to evaluate the coefficients at different values of ω and v . Veletsos & Wei [18] and Velestos & Verbic [19] proposed dynamic impedance to calculate the base springs (translational and rotational) and dashpot and is used in this study. The formulations are given by Eqs. (3) and (4).
kbx =
8Gro 2−v
(3)
cbx =
8Gro ⎛ 0. 6 × ao ⎞ ×⎜ ⎟ 2−v ω ⎠ ⎝
(4)
Sd =
Sa ω2
(5)
where Sa is the spectral acceleration; ω is a circular natural frequency ( = 2 × Π × f ) , with f being the fundamental frequency ( = 1/ T ) . It may be observed with the increase in liquefaction depth, there is an increase in spectral displacements in turn the pier head displacement (Fig. 5b). An increase in Sd of approximately 150 mm is observed with a Dliq of 11 m. In this respect, it is of interest to review the 10th edition of Indian Bridge code [9] which provides the minimum support/seating length (Lsup) given by Eq. (6) to avoid unseating failures. Following Eq. (6), for, Saraighat Bridge the minimum seating length required is 730 mm and the actual provided is more than 1000 mm. The analysis presented shows that the predicted maximum displacement of the pier head for a probable Dliq of 11 m corresponding to magnitude 8.0 earthquake is about 610 mm indicating that the bridge is most likely to be safe against deck unseating.
Equivalent diameter of 11.63 m for a circular well foundation for the actual double D shape is obtained by maintaining the same moment of inertia. The Poisson's ratio for the soil is assumed as 0.40 and the shear modulus is obtained from the equivalent linear soil properties and free field GRA presented in Dammala et al. [5].
Lsup = 305 + (2. 5 × span) + (10 × average column or or pier height ) (6)
3.5. Analysis results and validation Modal analyses were performed on the developed SWP model and the fundamental natural period of the bridge at no liquefaction condition is 1.17 s (0.8547 Hz). Measurements of ambient vibration data (in form of acceleration response) were carried out using thirteen uniaxial accelerometer (modal ES-U2) and four numbers of tri-axial accelerometer (model FBAES-T) by placing them at 28 different pre-
4. Discussion and conclusion It is of interest to note the difference of behaviour between a bridge supported on a pile foundation and a caisson (Well) foundation as soil progressively liquefies. In this context, the well documented pile-supported Showa Bridge is considered. It has been reported in 273
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
800
50
50
Time period % change in time period
1.3
30
1.2
20
1.1
10
1.0 0
3
6
Spectral displacement (mm)
40
% change it time period
Time period (sec)
1.4
Spectral Displacement % change in spectral displacement
(b)
(a)
40
700
margin for unseating failure for the maximum possible event
30
650 D for M 7.0_0.186g
600
D for M 8.1_0.186g
20
D for M 6.0_0.186g
10
550
500
0 12
9
Cut off for unseating=750 mm
750
0
3
Depth of liquefaction (m)
6
% change in spectral displacement
1.5
0 12
9
Depth of liquefaction (m)
150
200
(b)
(a) 125
Saraighat Showa
50
br id ge
80
40
ge d brid porte n sup
25
120
su pp or te d
75
Saraighat Showa
Pi le
100
% change in Sd
160
Pi le su pp or ted br idg e
Percentage change in time period
Fig. 5. Variation of (a) natural period and percentage increase in natural period and (b) Sd and percentage change in Sd with Dliq.
idge pported br Caisson su
o Caiss
0
0
5
10
15
20
25
0
3
6
9
12
15
Depth of Liq (m)
Depth of Liq (m)
Fig. 6. Comparison of percentage change in (a) Time period and (b) Sd with liquefaction for Showa and Saraighat Bridges.
in natural period due to liquefaction is insignificant and will result in a very low spectral displacement of the pier head. Though they consume enormous concrete, Caisson/Well foundations may be preferable type of foundations for major bridges in liquefiable soils. The proposed simplistic methodology assumes single scenario based hazard assessment (deterministic), conventional equivalent linear ground response analysis, traditional semi-empirical liquefaction evaluation and linear modal analysis of the structure. However, this can be a very useful tool for a quick reassessment of any caisson supported bridge in seismic regions. If the performance of the structure is found not to be in the allowable limits, advanced performance based assessment can be conducted considering the uncertainties in hazard assessment (probabilistic approach). Furthermore, advanced nonlinear ground response studies, probabilistic liquefaction assessment and advanced continuum nonlinear structural analysis can also be carried out.
Bhattacharya et al. [2] that the period of Showa Bridge increased from 2 s to 6 s as soil liquefied to a depth of about 10 m resulting in increased spectral displacement. Fig. 6(a & b) compare the percentage change in time period and predicted spectral displacement for both the bridges and the marked difference in predicted behaviours can be attributed to the differential increase in time period. For the same depth of liquefaction of 10 m, for the case of Showa Bridge, the increase in time period is 125% and on the other hand, the corresponding increase in Saraighat Bridge is about 18%. It is important to note that the Showa Bridge collapsed due to falling of decks. Based on the study, it may be inferred that the predicted good performance of Saraighat Bridge can be attributed to the high rigidity of well foundations which needs relatively low stiffness contribution from the surrounding soil. Many major bridges in high seismicity areas of India are supported on well foundations and were designed before the first seismic code implementation in the country. In this paper, a simplified scenario based requalification methodology is proposed to assess the seismic safety of caisson supported bridges together with a worked out example of Saraighat Bridge in high seismic zone. The critical failure mechanism for these bridges are deck unseating due to liquefaction induced hazards. Advanced soil tests (resonant column) were performed on the soil samples collected from the bridge site which were used for numerical analyses. The numerical analyses of the Soil-Well-Pier system based on one dimensional spring dashpot system were validated with limited dynamic measurements on the bridge. The results show that the change
Acknowledgements The financial assistance provided by UK India Education and Research Initiative (UKIERI) to the project titled Seismic Requalification of Geotechnical Structures (ref no: UKUTP 201100296) is gratefully acknowledged. The first author would like to thank the Commonwealth Scholarship Commission (CSC), UK for providing splitsite fellowship (CSC ref no: INCN-2016-214) in order to carry out the research at the University of Surrey (UK). The authors acknowledge 274
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[7] Galasso C, Zareian F, Iervolino I, Graves W. Validation of ground‐motion simulations for historical events using SDoF systems. Bull Seismol Soc Am 2012;102(6):2727–40. [8] Idriss IM, Boulanger RW. Semi-empirical procedures for evaluating liquefaction potential during earthquakes. Soil Dyn Earthq Eng 2006;26(2):115–30. [9] IRC 6 Standard specifications and code of practise for Road bridges. Section II, 2014, New Delhi, India. [10] IS 1893. Criteria for earthquake resistant design of structures. New Delhi: Bureau of Indian Standards, Govt of India; 2002. [11] Kanth SR, Sreelatha S, Dash SK. Ground motion estimation at Guwahati city for an Mw 8.1 earthquake in the Shillong Plateau. Tectonophysics 2008;448(1):98–114. [12] Khattri K. Probabilities of occurrence of great earthquakes in Himalaya. Earth Planet Sci Proc Indian Acad Sci 1999;108(2):87–92. [13] Krishna AM, Bhattacharya S, Choudhury D. Seismic requalification of geotechnical structures Indian Geotechnical Journal; 2014: 44(2). p. 113–118. [14] Lombardi D, Bhattacharya S. Evaluation of seismic performance of pile‐supported models in liquefiable soils. Earthq Eng Struct Dyn 2016:1–20. [15] Nath SK, Raj A, Thingbaijam KKS, Kumar A. Ground motion synthesis and seismic scenario in Guwahati city—a stochastic approach. Seismol Res Lett 2009;80(2):233–42. [16] Novak M, Nogami T, Aboul-Ella F. Dynamic soil reactions for plane strain case. J Eng Mech Div ASCE 1978;104:953–9. [17] Rouholamin M, Bhattacharya S, Orense RP. Effect of initial relative density on the post-liquefaction behaviour of sand. Soil Earthq Eng 2017;97:25–36. [18] Veletsos AS, Wei YT. Lateral and rocking vibration of footings. J Soil Mech Found Div ASCE 1971;97:1227–48. [19] Veletsos AS, Verbic B. Basic response functions for elastic foundations. J Eng Mech Div ASCE 1974;100:189–202. [20] Wadhwa Y. Seismic analysis of bridges – A case study of Saraighat bridge [Master's thesis]. India: Indian Institute of Technology Guwahati; 2007.
Gammon India and Northeast Frontier Railway (NEF), India for providing required soil bore log data near the bridge. The authors are also thankful to Dr. Raghukanth, IIT Chennai for providing the simulated bedrock ground motions for the ground response analysis. The contributions of Mr. George Nikitas, Dr. Mehdi Rouholamin and Mr. Praveen Dammala for their cooperation during the laboratory testing is fully acknowledged. Constructive and insightful comments and suggestions by the reviewers were very helpful in improving the paper. References [1] Alexander NA, Chanerly AA, Crewe AJ, Bhattacharya S. Obtaining spectrum matching time series using a Reweighted Volterra Series Algorithm (RVSA). Bull Seismol Soc Am 2014;104(4):1663–73. [2] Bhattacharya S, Tokimatsu K, Goda K, Sarkar R, Shadlou M, Rouholamin M. Collapse of Showa Bridge during 1964 Niigata earthquake: a quantitative reappraisal on the failure mechanisms. Soil Dyn Earthq Eng 2014;65:55–71. [3] Boore DM. Stochastic simulation of high-frequency ground motions based on seismological models of the radiated spectra. Bull Seismol Soc Am 1983;73(6A):1865–94. [4] CSI. SAP2000:V11. 0 Integrated software for structural analysis and design. Berkeley, CA, USA: Computer and Structures Inc. (CSI); 2004. [5] Dammala PK, Krishna AM, Bhattacharya S, Nikitas G, Rouholamin M, Dynamic soil properties of Northeast Indian soils for ground response analysis, (Accepted for publication). [6] Debnath N, Deb SK, Dutta A. Multi-modal vibration control of truss bridges with tuned mass dampers under general loading. J Vib Control 2016;22(20):4121–40.
275
Contents lists available at ScienceDirect
Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn
Scenario based seismic re-qualification of caisson supported major bridges – A case study of Saraighat Bridge
MARK
⁎
Pradeep Kumar Dammalaa,b, Subhamoy Bhattacharyab, , Adapa Murali Krishnaa, Shiv Shankar Kumara, Kaustubh Dasguptaa a b
Indian Institute of Technology Guwahati, India Chair in Geomechanics and Director-SAGE Laboratory, University of Surrey, Guildford GU2 7XH, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Keywords: Liquefaction Well foundations Ground response Natural period
Many major river bridges were constructed in highly active seismic areas of India much before the seismic code development. Bridges are lifelines infrastructure and as a result, it is necessary to requalify/reasses these structures in the light of the new and improved understanding of seismic resistant design philosophies. The aim of the paper is to develop a simplified methodology to carry out scenario based seismic requalification of major river bridges supported on caisson foundations (also known as Well Foundation). An example problem of Saraighat Bridge located in highly active Himalayan seismic zone is considered to demonstrate the application of the methodology. Field investigation and advanced laboratory tests on soil samples from the bridge site were carried out. The test results reveal that the soil is susceptible to liquefaction and as a result, soil structure interaction analyses are carried out. It is shown that good performance of these type of bridges depend on the displacement response of the pier head so as not to cause unseating of the decks. It is concluded, owing to the large stiffness of the foundations, bridges supported on caisson foundations may not be adversely affected by liquefaction induced effects.
1. Introduction Caisson foundations (also known as Well foundations), see Fig. 1, are preferred type of foundations for supporting major bridges in India. Typical lateral dimensions of such foundations range between 10 to 20 m and the depth can range between 30 and 60 m. Many of these bridges supported on well foundations were constructed much before the development of first Indian seismic code [10] and the design was mostly guided by the river and scour protection works. Fig. 1 show the schematic views of Saraighat Bridge supported on double D shaped well foundation. Historically, these foundations performed well during the limited earthquakes they experienced. However, there are currently two concerns: (a) As these were constructed before the seismic codes development, there is a need to reassess these lifeline structures for the worst possible seismic event, in the light of the latest scientific understanding; (b) It is often argued that these large dimension foundations are uneconomic and piles may be used instead. Therefore, the aim and scope of the paper are as follows: (A) Develop a methodology for scenario based seismic requalification of such river bridges supported on well foundations.
⁎
Corresponding author. E-mail address: [email protected] (S. Bhattacharya).
http://dx.doi.org/10.1016/j.soildyn.2017.06.005 Received 17 March 2017; Received in revised form 7 June 2017; Accepted 8 June 2017 0267-7261/ © 2017 Elsevier Ltd. All rights reserved.
(B) Demonstrate the application of the methodology by taking the example of Saraighat Bridge constructed in 1962 over the 1300 m wide Brahmaputra River. 2. Proposed methodology for seismic requalification of bridges While state of the art understanding of seismic behaviour is often incorporated in new projects, there are several important structures that were built much earlier and did not benefit from better understanding. In this context, seismic requalification/re-assessment is defined as the process of verifying the expected performance of a structure under estimated seismic scenario conditions. Due to the sheer size of the well foundations (see Fig. 1), bending, buckling and shear type of failure can be ruled out and the performance criteria will be governed by the pier head displacement to arrest the possible deck unseating failures. Reconnaissance survey following the recent earthquakes showed that midspan collapse of bridges often occur due to liquefaction induced effects and possibly due to elongation in natural period of bridges, see for example the Showa bridge analysis carried out in Bhattacharya et al. [2]. Fig. 2 shows a simple model to describe the dynamic soil-structure interaction following the work of Lombardi and
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
Fig. 1. (a) Schematic view of the Saraighat Bridge (b) Central portion of the bridge (c) Cross section along the foundation (Z-Z) (d) Cross section along the pier (X-X).
unsupported length (from Lfix-pre before liquefaction to Lfix-post after liquefaction). This results in an increased displacement demand of the pier head often leading to unseating/failure of deck slabs, especially when the Tpost falls in the displacement sensitive zone. This is an issue associated mostly with simply supported bridges. From the point of view of bridge requalification, predicting displacement response of the pier head/top for possible deck unseating failure is critical. Further details of the methodology can be found in Krishna et al. [13] and the essential ingredients/input required are: Expected hazard mapping (scenario earthquake information), ground conditions under the foundation, and details of the structure. The methodology involves various stages and can be carried out in few steps: Step 1: Quantification of seismic hazard: In this step, it is necessary to quantify the seismic hazard. Effectively, this would mean obtaining ground motion parameters [for example Peak Ground Acceleration (PGA), Moment Magnitude (Mw)] and time histories of expected ground motion at the site either using deterministic approach or by probabilistic approach-see for example the work of Boore [3], Galasso et al. [7], Alexander et al. [1]. Step 2: Site response analysis: In this step, information such as the ground type and stratification, slope and topography of the ground, location of the ground water level are obtained. Subsequently, ground response analyses together with liquefaction analyses are carried for the earthquake scenario predicted in Step 1. If necessary, additional site investigations (borehole or geophysical methods) and advanced soil testing may be needed. Step 3: Seismic analysis: This stage involves seismic analysis of the structure using the results obtained from the earlier stages with the help of seismic design philosophies and compare with the required performance. The loading obtained in Step 1 & 2 together with the details of the superstructure and foundations are used to predict the various performance levels. Step 4: Seismic strengthening and retrofitting methodologies are to
Fig. 2. Effects of liquefaction on acceleration response spectrum.
Bhattacharya [14] where it may be noted that as soil liquefies there is an increase in natural period (from Tpre before liquefaction to Tpost after liquefaction) of the bridge/structure due to the increase in the
271
Soil Dynamics and Earthquake Engineering 100 (2017) 270–275
P.K. Dammala et al.
Fig. 3. (a) Saraighat Bridge (b) Location of the bridge in Assam, India (c) Soil profile near the centre of the bridge.
of double D shaped well (Fig. 1c). The reinforced deck of the bridge rest on bearings (simply supported) which are supported on 1.35 m pier cap. The pier cap is supported on twin circular piers of 10.68 m high and 4.5 m diameter each (Fig. 1d). The bottom parts of the piers are connected to the well cap of 1.95 m thickness. The mass of each main span is 2100 t [20].
be planned to meet the requirements in order to make the structure seismically safe. These requirements would be different for different structures and for different performance levels. 3. Case study – Saraighat Bridge Saraighat Bridge (Fig. 3a) was designed in 1958, construction finished in 1962 and is the first bridge constructed over the mighty Brahmaputra River (the widest river in Asia). The bridge is located in Guwahati (26°11′ N and 91°38′ E) city in Assam, one of the seven North Eastern (NE) states of India (Fig. 3b). It is of interest to note that the first Indian seismic code was developed in 1964 and liquefaction studies were initiated globally following the 1964 Niigata earthquake. Therefore it is required to carry out seismic requalification of this bridge and the essential calculation is the revised seismic displacement due to liquefaction.
3.3. Local soil conditions Fig. 3c presents the typical soil profile near the centre of the bridge (pier-P6). Soil samples near the vicinity of the bridge were collected for geotechnical characterization and further details about the tests conducted and results can be found in Dammala et al. [5] and Rouholamin et al. [17]. 3.3.1. Ground Response Analysis (GRA) A free field one dimensional GRA based on equivalent linear approach was performed on the considered soil profile to determine the induced stresses and strains. The input modulus degradation (ratio of shear modulus to initial shear modulus, G/Gmax) and damping (D) curves were developed for the Assam sand by performing Resonant Column (RC) tests. The four ground motions discussed in the earlier section have been considered as the input bedrock motions. Details about the RC testing methodology, test results and the free field GRA can be found in Dammala et al. [5].
3.1. Seismic Hazard Assessment (SHA) The entire NE region of India, including Guwahati is regarded as one the most seismic active regions of the world and Indian standards for earthquake resistant design of structures [10] classified this region as one of the highest seismic zone in the country with an expected PGA of 0.36 g. Since 1869, this region has witnessed two great earthquakes (Mw ≥ 8.0) and 20 large earthquakes (8.0 ≤ Mw ≥ 7.0), [11]. Severe seismic events were not observed in the region for the last 67 years and seismologists are of the opinion that this temporal area called the “Assam Gap”, is a potential source for an impending large earthquake with an expected moment magnitude of 8.5 [12]. In view of these, researchers [15] have performed SHA of Guwahati city. In order to consider the worst possible seismic scenario for the present case study, the closest potential seismic source to the site (Oldham fault) and the past largest seismic event recorded at the source (1897 Great Assam earthquake, Mw = 8.1) are considered. Kanth et al. [11] simulated bedrock ground motions for Guwahati city for the considered event and the PGA values obtained from the simulations were found to match well with the corelated PGA values from the witnessed intensities in terms of MSK (Medvedev-Sponheuer-Karnik) intensity scale. Four artificial ground motions for Guwahati city are presented with bedrock PGA ranging from 0.146 g to 0.185 g.
3.3.2. Liquefaction assessment Footprints of severe ground fissures representing extensive liquefaction were observed in Guwahati city during the past severe seismic events. Kanth et al. [11] concluded that most parts of the city are prone to severe liquefaction for large earthquakes. The depth of liquefaction (Dliq) was estimated based on the traditional semi-empirical procedure by Idriss & Boulanger [8] using the available Standard Penetration Test (SPT) data and it predicted the Dliq ranging between 6 and 11 m from surface depending on the magnitude (6.0–8.0) and PGA (0.15–0.36 g) parameters. 3.4. Modelling the dynamic soil-structure interaction The bridge decks are simply supported on the piers and as a result the analysis of one pier will be sufficient. The central pier of Saraighat Bridge is modelled as this has the highest free standing length in water and therefore unsupported and most flexible. Finite Element Analysis was carried out using SAP2000 [4] where the pier and the well foundation (here in referred as SWP system i.e. Soil-Well-Pier system) are modelled as rigid beam elements with their corresponding material properties, see Fig. 4. The well-soil interaction is modelled using discrete springs and dashpots obtained following the work of Novak et al. [16] and Veletsos & Wei [18]. It is assumed that the SWP system is stable vertically and the base is allowed to rotate and translate. For the
3.2. Bridge design details Saraighat Bridge is a typical double warren steel truss bridge of a rhomboidal shape, the lower deck to accommodate a two metre gauge railway track and the upper deck hosting a two lane highway. Fig. 1(a–d) present the schematic views of the geometrical details of the bridge and its support system. The super structure of the bridge is made of steel trusses while the substructure (piers and foundation) is of reinforced concrete. Foundations for the main span piers (P1 to P11) are 272
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Fig. 4. (a) Actual central pier system of the bridge and idealized 1D SWP system (b) before and (c) after liquefaction.
calculated locations on the middle span of the bridge and reported in Debnath et al. [6]. Based on operational modal analysis using both the monitored acceleration response and finite element modelling, the fundamental frequency of Saraighat Bridge is reported to range between 0.9031 and 0.9119 Hz and the damping ratio approximated as 3.02% [6]. Our numerical model is in a good agreement (5% error) with the measured data and it is reasonable to assume that the model is verified and validated. To predict the performance of the bridge at full liquefaction, modal analyses were performed for different depths of liquefaction (Dliq). It is assumed that springs in the liquefied zone had zero stiffness (Fig. 4c). Fig. 5(a) presents the variation of natural period and percentage change in the period for different depths of liquefaction (Dliq). As expected, natural period of the bridge before liquefaction (Tpre) 1.18 s increased to 1.37 s (Tpost) for the maximum predicted Dliq of 11 m. Spectral displacements (Sd) are predicted for all the considered depths of liquefaction based on Indian Standard for earthquake resistant design of buildings [10].
sake of completeness, the formulations are given in Eqs. (1) and (2).
Kx = G [Su1 (ao , v, D) + iSu2 ((ao , v, D))]
(1)
K θ = G ϒ o2 [Sψ1(ao × Ds ) + iSψ2(ao × Ds )]
(2)
where Kx and K θ are the dynamic impedance of soil for distributed translational and rotational vibrations respectively; v and G are the Poisson's ratio and shear modulus of the soil, respectively; D and ϒo are the equivalent diameter and radius of the arbitrarily shaped well foundation, respectively; ao (=roω/Vs) is the dimensionless frequency; ω (=2Πf) is the circular natural frequency; f is the predominant frequency of the ground motion (≈5 Hz for the considered ground motions); Ds is the material damping. The real (k x and kθ ) and the imaginary parts (cx and cθ ) of the above complex notations represent the stiffness and material damping coefficients (combined material damping of soil and radiation damping), respectively. Su1, Sψ1 are the stiffness coefficients to evaluate k x and kθ respectively, while Su2 , Sψ2 are the damping coefficients to evaluate cx and cθ , respectively. These coefficients are frequency (ω) and Poisson's ratio (v ) dependant and Novak et al. [16] proposed design charts to evaluate the coefficients at different values of ω and v . Veletsos & Wei [18] and Velestos & Verbic [19] proposed dynamic impedance to calculate the base springs (translational and rotational) and dashpot and is used in this study. The formulations are given by Eqs. (3) and (4).
kbx =
8Gro 2−v
(3)
cbx =
8Gro ⎛ 0. 6 × ao ⎞ ×⎜ ⎟ 2−v ω ⎠ ⎝
(4)
Sd =
Sa ω2
(5)
where Sa is the spectral acceleration; ω is a circular natural frequency ( = 2 × Π × f ) , with f being the fundamental frequency ( = 1/ T ) . It may be observed with the increase in liquefaction depth, there is an increase in spectral displacements in turn the pier head displacement (Fig. 5b). An increase in Sd of approximately 150 mm is observed with a Dliq of 11 m. In this respect, it is of interest to review the 10th edition of Indian Bridge code [9] which provides the minimum support/seating length (Lsup) given by Eq. (6) to avoid unseating failures. Following Eq. (6), for, Saraighat Bridge the minimum seating length required is 730 mm and the actual provided is more than 1000 mm. The analysis presented shows that the predicted maximum displacement of the pier head for a probable Dliq of 11 m corresponding to magnitude 8.0 earthquake is about 610 mm indicating that the bridge is most likely to be safe against deck unseating.
Equivalent diameter of 11.63 m for a circular well foundation for the actual double D shape is obtained by maintaining the same moment of inertia. The Poisson's ratio for the soil is assumed as 0.40 and the shear modulus is obtained from the equivalent linear soil properties and free field GRA presented in Dammala et al. [5].
Lsup = 305 + (2. 5 × span) + (10 × average column or or pier height ) (6)
3.5. Analysis results and validation Modal analyses were performed on the developed SWP model and the fundamental natural period of the bridge at no liquefaction condition is 1.17 s (0.8547 Hz). Measurements of ambient vibration data (in form of acceleration response) were carried out using thirteen uniaxial accelerometer (modal ES-U2) and four numbers of tri-axial accelerometer (model FBAES-T) by placing them at 28 different pre-
4. Discussion and conclusion It is of interest to note the difference of behaviour between a bridge supported on a pile foundation and a caisson (Well) foundation as soil progressively liquefies. In this context, the well documented pile-supported Showa Bridge is considered. It has been reported in 273
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800
50
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3
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1.4
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margin for unseating failure for the maximum possible event
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br id ge
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Pi le su pp or ted br idg e
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Fig. 5. Variation of (a) natural period and percentage increase in natural period and (b) Sd and percentage change in Sd with Dliq.
idge pported br Caisson su
o Caiss
0
0
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0
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Fig. 6. Comparison of percentage change in (a) Time period and (b) Sd with liquefaction for Showa and Saraighat Bridges.
in natural period due to liquefaction is insignificant and will result in a very low spectral displacement of the pier head. Though they consume enormous concrete, Caisson/Well foundations may be preferable type of foundations for major bridges in liquefiable soils. The proposed simplistic methodology assumes single scenario based hazard assessment (deterministic), conventional equivalent linear ground response analysis, traditional semi-empirical liquefaction evaluation and linear modal analysis of the structure. However, this can be a very useful tool for a quick reassessment of any caisson supported bridge in seismic regions. If the performance of the structure is found not to be in the allowable limits, advanced performance based assessment can be conducted considering the uncertainties in hazard assessment (probabilistic approach). Furthermore, advanced nonlinear ground response studies, probabilistic liquefaction assessment and advanced continuum nonlinear structural analysis can also be carried out.
Bhattacharya et al. [2] that the period of Showa Bridge increased from 2 s to 6 s as soil liquefied to a depth of about 10 m resulting in increased spectral displacement. Fig. 6(a & b) compare the percentage change in time period and predicted spectral displacement for both the bridges and the marked difference in predicted behaviours can be attributed to the differential increase in time period. For the same depth of liquefaction of 10 m, for the case of Showa Bridge, the increase in time period is 125% and on the other hand, the corresponding increase in Saraighat Bridge is about 18%. It is important to note that the Showa Bridge collapsed due to falling of decks. Based on the study, it may be inferred that the predicted good performance of Saraighat Bridge can be attributed to the high rigidity of well foundations which needs relatively low stiffness contribution from the surrounding soil. Many major bridges in high seismicity areas of India are supported on well foundations and were designed before the first seismic code implementation in the country. In this paper, a simplified scenario based requalification methodology is proposed to assess the seismic safety of caisson supported bridges together with a worked out example of Saraighat Bridge in high seismic zone. The critical failure mechanism for these bridges are deck unseating due to liquefaction induced hazards. Advanced soil tests (resonant column) were performed on the soil samples collected from the bridge site which were used for numerical analyses. The numerical analyses of the Soil-Well-Pier system based on one dimensional spring dashpot system were validated with limited dynamic measurements on the bridge. The results show that the change
Acknowledgements The financial assistance provided by UK India Education and Research Initiative (UKIERI) to the project titled Seismic Requalification of Geotechnical Structures (ref no: UKUTP 201100296) is gratefully acknowledged. The first author would like to thank the Commonwealth Scholarship Commission (CSC), UK for providing splitsite fellowship (CSC ref no: INCN-2016-214) in order to carry out the research at the University of Surrey (UK). The authors acknowledge 274
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[7] Galasso C, Zareian F, Iervolino I, Graves W. Validation of ground‐motion simulations for historical events using SDoF systems. Bull Seismol Soc Am 2012;102(6):2727–40. [8] Idriss IM, Boulanger RW. Semi-empirical procedures for evaluating liquefaction potential during earthquakes. Soil Dyn Earthq Eng 2006;26(2):115–30. [9] IRC 6 Standard specifications and code of practise for Road bridges. Section II, 2014, New Delhi, India. [10] IS 1893. Criteria for earthquake resistant design of structures. New Delhi: Bureau of Indian Standards, Govt of India; 2002. [11] Kanth SR, Sreelatha S, Dash SK. Ground motion estimation at Guwahati city for an Mw 8.1 earthquake in the Shillong Plateau. Tectonophysics 2008;448(1):98–114. [12] Khattri K. Probabilities of occurrence of great earthquakes in Himalaya. Earth Planet Sci Proc Indian Acad Sci 1999;108(2):87–92. [13] Krishna AM, Bhattacharya S, Choudhury D. Seismic requalification of geotechnical structures Indian Geotechnical Journal; 2014: 44(2). p. 113–118. [14] Lombardi D, Bhattacharya S. Evaluation of seismic performance of pile‐supported models in liquefiable soils. Earthq Eng Struct Dyn 2016:1–20. [15] Nath SK, Raj A, Thingbaijam KKS, Kumar A. Ground motion synthesis and seismic scenario in Guwahati city—a stochastic approach. Seismol Res Lett 2009;80(2):233–42. [16] Novak M, Nogami T, Aboul-Ella F. Dynamic soil reactions for plane strain case. J Eng Mech Div ASCE 1978;104:953–9. [17] Rouholamin M, Bhattacharya S, Orense RP. Effect of initial relative density on the post-liquefaction behaviour of sand. Soil Earthq Eng 2017;97:25–36. [18] Veletsos AS, Wei YT. Lateral and rocking vibration of footings. J Soil Mech Found Div ASCE 1971;97:1227–48. [19] Veletsos AS, Verbic B. Basic response functions for elastic foundations. J Eng Mech Div ASCE 1974;100:189–202. [20] Wadhwa Y. Seismic analysis of bridges – A case study of Saraighat bridge [Master's thesis]. India: Indian Institute of Technology Guwahati; 2007.
Gammon India and Northeast Frontier Railway (NEF), India for providing required soil bore log data near the bridge. The authors are also thankful to Dr. Raghukanth, IIT Chennai for providing the simulated bedrock ground motions for the ground response analysis. The contributions of Mr. George Nikitas, Dr. Mehdi Rouholamin and Mr. Praveen Dammala for their cooperation during the laboratory testing is fully acknowledged. Constructive and insightful comments and suggestions by the reviewers were very helpful in improving the paper. References [1] Alexander NA, Chanerly AA, Crewe AJ, Bhattacharya S. Obtaining spectrum matching time series using a Reweighted Volterra Series Algorithm (RVSA). Bull Seismol Soc Am 2014;104(4):1663–73. [2] Bhattacharya S, Tokimatsu K, Goda K, Sarkar R, Shadlou M, Rouholamin M. Collapse of Showa Bridge during 1964 Niigata earthquake: a quantitative reappraisal on the failure mechanisms. Soil Dyn Earthq Eng 2014;65:55–71. [3] Boore DM. Stochastic simulation of high-frequency ground motions based on seismological models of the radiated spectra. Bull Seismol Soc Am 1983;73(6A):1865–94. [4] CSI. SAP2000:V11. 0 Integrated software for structural analysis and design. Berkeley, CA, USA: Computer and Structures Inc. (CSI); 2004. [5] Dammala PK, Krishna AM, Bhattacharya S, Nikitas G, Rouholamin M, Dynamic soil properties of Northeast Indian soils for ground response analysis, (Accepted for publication). [6] Debnath N, Deb SK, Dutta A. Multi-modal vibration control of truss bridges with tuned mass dampers under general loading. J Vib Control 2016;22(20):4121–40.
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