SOIL STRUCTURE INTERACTION AND GROUND RESPONSE ANALYSISOF TYPICAL BRIDGE PYLON STRUCTURE FOUNDATION ABSTRACT

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1 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India SOIL STRUCTURE INTERACTION AND GROUND RESPONSE ANALYSISOF TYPICAL BRIDGE PYLON STRUCTURE FOUNDATION P.J. Shukla 1, J.C.Shukla 2, C.D.Modhera 3 ABSTRACT The Gujarat region considered to be seismically active region in the peninsular India and has experienced few devastating earthquakes in past. Since the cable stayed bridges are not conventional in India, the seismic design of such type of bridge is a challenging task.in the present study the typical cable stayed bridge model is used for seismic analysis. Typical cable stayed bridge resebles to the bridge over the river Tapi at Surat City, Gujarat is selected as bridge geometry having concrete deck section of width 23 meters and a depth 2 meters. The length of cable stayed portion is considered to be 3 m and height of concrete pylon is assumed as 35 m above the deck. The proposed bridge is analysed for three different time histories: time history of, compatible time history of IRC-6-21 generated by matching the response spectra of IRC 6:21with the time history of Bhuj using computer software SeismoMatch and time history generated using ground response analysis.the time history analysis is carried out using computer application SAP2. The micro level seismic hazard assessment in the Gujarat is yet to be established and presently under research. In present study, an attempt is made to develop site specific ground motions for the proposed cable styed bridge site based on ground response analysis. Considering the response spectra suggested by IRC 6, spectrum compatible time history was generated using Bhuj earthquake data. Obtained time history was then applied at the base of the soil profile to obtain the ground response analysis output. The geotechnical investigation details made available in form of 1 bore logs are then used 1 Soil structure interaction and ground response analysis of typical bridge pylon structure foundation_ P J Shukla,Applied Mechanics Department, Shri K.J Polytechnic, Bharuch, Gujarat,India.palak.shukla8@gmail.com J.C.Shukla, Assistant General Manager, L&T-Sargent & Lundy, Vadodara. Jaykumar.Shukla@lntsnl.com C.D. Modhera, Professor & Head Applied Mechanics Department, SVNIT, Surat. cdmodhera@gmail.com

2 P.J.Shukla/J.C.Shukla/C.DModhera to estimate the geotechnical profile and associated properties. The shear wave velocities of the representative profile are estimated based on well-known correlations based on the SPT N-values observed at the site. The ground amplification is investigated considering the synthetic ground motions consistent with the developed uniform hazard spectra. Typical Pseudo-absolute Acceleration, g Layer 5 (Bed Rock) Layer 1 (Top Layer) Layer 2 Layer 3 Layer Periods (sec) Figure 1: Spectral response of various layers to theinput ground motions The displacement and key structural forces at top of pylon are monitored corresponding to three different time histories. The observations of displacement to pylon top reveals that the maximum displacement occurs due to time history generated using ground response analysis.on referring response spectra for three different time histories, compatible IRC 6 : 21 time history and time history obtained using ground response analysis, it indicates that though the response spectra suggested by IRC 6 : 21 include three types of soil condition, the specific codal provision is required to consider local site effect of bridge site. It is observed from the study that maximum structural response in form of shear force and bending moment at top of pylon occurs due to time history generated using ground response analysis. Keywords: Probabilistic seismic hazard; uniform hazard spectra; site specific ground motion; ground amplification; SPT N-value; Gujarat

3 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India SOIL-STRUCTURE INTERACTION AND GROUND RESPONSE ANALYSIS OF TYPICAL BRIDGE PYLON STRUCTURE FOUNDATION P.J. Shukla, Lecture in Applied Mechanics Dept, Shri K. J. Polytechnic, Bharuch.palak.shukla8@gmail.com J.C.Shukla, Assistant General Manager, L & T Sargent and Lundy, Vadodara Jaykumar.Shukla@lntsnl.com C.D. Modhera, Professor & Head Applied Mechanics Department, SVNIT, Surat, cdmodhera@gmail.com ABSTRACT:The Gujarat region considered to be seismically active region in the peninsular India and has experienced few devastating earthquakes in past. A 9m long twin pylon supported cable stayed bridge is planned across the river Tapi in Surat city of Gujarat and significant cost is invested in the project. Since the cable stayed bridges are not conventional in India, the seismic design of such type of bridge is a challenging task. The micro level seismic hazard assessment in the Gujarat is yet to be established and presently under research. In present study, an attempt is made to develop site specific ground motions for the proposed cable styed bridge site based on ground response analysis. Considering the response spectra suggested by IRC 6, spectrum compatible time history was generated using Bhuj earthquake data. Obtained time history was then applied at the base of the soil profile to obtain the ground response analysis output. Obtained the surface level earthquake data was then applied to pylon structure to observe the time history response of pylon during strong earthquake shaking. The geotechnical investigation details made available in form of 1 bore logs are then used to estimate the geotechnical profile and associated properties. The shear wave velocities of the representative profile are estimated based on well-known correlations based on the SPT N-values observed at the site. The ground amplification is investigated considering the synthetic ground motions consistent with the developed uniform hazard spectra. Present paper describes the geotechnical data, ground response analysis sand other important highlight of the findings. Results of the ground amplification study are presented in form of the amplified spectra at the end of the paper. INTRODUCTION Cable supported bridges have been widely used as major structures to connect areas separated by a body of water such as a river, a bay or a sea or by a valley. Bridges are lifeline facilities that must remain functional even after major earthquake shaking; their damage and collapse may not only cause loss of life and property, but also hamper post-earthquake relief and restoration activities. Due to importance of these structures the prevention of progressive collapse of a cable supported bridge under any condition is one of the major and important objectives in design of these bridges. The Gujarat region considered to be seismically active region in the peninsular India and has experienced few devastating earthquakes in past. The time history analysis is used to get more accurate response of structure when it is subjected to earthquake loading. The time history generated using ground response analysis is more accurate for the local seismic and site conditions. Jia&Ou(28)[5] evaluated the seismic performance of long span cable-stayed bridge by time history analysis method. The seismic behavior of the bridge to nearfault with and without pulse effect and common used ground motion were investigated and compared. It is conclude from study that pulse- type near fault ground motions will impose more severe damage potential to engineering structures with long fundamental period. McGuire & Toro (28)[6] investigated that the sitespecific hazard calculations can be made using rock hazard calculations as a basis, if site-amplification calculations include all certainties in site characteristic and site response. For each rock amplitude at which site-amplification calculations are made it is necessary only to know mean M of the causative earthquake. Setiawan&Saidi (212)[8] studied that the soil amplification has caused major structure damages founded on thick soft soils. They conclude that site specific ground response analysis to be carried out to establish the most influential parameter i.e. peak ground acceleration and the spectral fundamental frequency. Shah et al. (21)[9] studied the effect of pylon shape on seismic response of cable stayed bridge.they consider soil-structure interaction through the soil-

4 P.J.Shukla/J.C.Shukla/C.DModhera spring at the base and observed that due to soil-structure interaction as the soil stiffness decreases the maximum acceleration in the tower increases. METHODOLOGY OF PRESENT STUDY For seismic analysis on cable stayed bridge the typical cable stayed bridge model having a concrete deck section of width 23 meters and a depth 2 meters is used. The length of cable stayed portion is 3 m and height of concrete pylon above deck is 35m. The proposed bridge is analysed for three different time histories: time history of (Figure 1), compatible time history of IRC-6-21 generated by matching the response spectra of IRC 6:21with the time history of Bhuj using computer software SeismoMatch (Figure 2), and time history generated using ground response analysis (Figure 3). For ground response analysis the typical borelog is obtained from the Unique Engineering and Testing Services, Surat and from the available geotechnical investigation details BH 1 (Location AP 15) has been selected as representative geotechnical profile. To carry out ground response analysis, the shear wave velocity of the soil layer is very important parameter and should be estimated precisely however, there are no shear wave velocity mapping study available for the bridge site. Alternatively, in the present analysis the shear wave velocity of the soil layers are estimated through the correlations established by Thaker and Rao (211). Thaker and Rao (211) has carried out very extensive shear wave velocity mapping for the Surat city and established a correlation between SPT N-value and shear wave velocity (Vs = N.42 ) The typical soil parameters used in the ground response analysis is outlined in the Table 1. The ground response analysis has been carried out using computer code shake 91and longitudinal component of the Bhuj Time history has been used as input ground motion to estimate the free field ground motion at the top of the geotechnical profile. The obtained results are present in Figure 3 in the form of the response spectra. It is important to note that the Figure 3 clearly indicates that the ground motions are amplified by the soil layering available at the site and the observed amplification ratio is in the order of 1.6 to 1.7. Various three time history functions as describe earlier were applied to pylon and analysis is carried out using computer code SAP2. RESULT AND DISCUSSION Comparison of Displacement Time Histories of Pylon Top Fig. 4 shows comparison of displacement time histories of pylon top for three different time history functions. The Maximum displacement of pylon top of typical cable stayed bridge occurred due to time history using ground response analysis is 2.5 mm which is nearly same as displacement due to time history of Bhuj Earthquake 21, while the IRC 6 : 21 compatible time history gives minimum value of displacement 44.2mm. The pseudo acceleration spectrum of pylon top is also similar for and time history using ground response analysis. Layer No. Thk (m) Black Silty Clay 2 12 Medium to Fine Sand 3 3 Table 1. Table 1: Representative Geotechnical Profile Type of Soil Unit Wt. N-Value Vs (m/s) G curve (kn/m 3 ) (G/G max ) Medium Sand with gravels High plastic clay with Sand Sun et al. (1988) Seed and Idriss (197) Average Seed and Idriss (197) Average Sun et al. (1988) Damping Curve Sun et al. (1988) Seed and Idriss (197) Average Seed and Idriss (197) Average Sun et al. (1988) 5 9 Very Dense Sand Seed and Idriss (197) Average Seed and Idriss (197) Average

5 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India IRC 6 : 21 compatible time history Displacement (mm) Time (sec) Figure 1 : Time History of Bhuj Earthquake Time (s) Figure 4 : Comparison of Displacement Time Histories of Pylon Top Comparison of Response Spectra of Pylon Top Figure 5 represents Pseudo Acceleration response spectrum for three different time history functions. The maximum response of structure for 5% damping is obtained for time history generated using ground response analysis which considers the local site effects followed by. The structural response for 5% damping is quite less for IRC 6-21 compatible time history Time (sec) Figure 2: Compatible IRC 6 : 21 Time History Pseudo-absolute Acceleration, g Layer 5 (Bed Rock) Layer 1 (Top Layer) Layer 2 Layer 3 Layer Periods (sec) Figure 3: Response Spectra of The Layers to The Input Ground Motion for Geotechnical Profile Shear Force and Bending Moment at Base of Pylon Figure 6 indicates the comparison of shear force at base of pylon for the different time history. The maximum shear force is 4.46 MN for time history obtained by the ground response analysis while the minimum shear MN is obtained for IRC 6:21 compatible time history. The shear force obtained due to time history of (longitudinal component) is MN.Figure 7 shows the graph of bending moment obtained due to different time history functions. The positive bending moment due to time history obtained by the ground response analysis is maximum MN-m. The positive bending moment obtained due to time history of Bhuj Earthquake 21 is MN-m. The IRC 6:21 compatible time history gives minimum value of positive bending moment ( MN-m).

6 P.J.Shukla/J.C.Shukla/C.DModhera Pseudo Spectral Acceleration IRC 6 : 21 compatible time history Maximum Bending Moment (-ve) (MN-m) IRC 6 : 21 compatible time history.1 1 Period(sec) Figure 5 : Comparison of Response Spectra for Pylon Top Shear Force (MN) IRC 6 : 21 compatible time history Figure 6 : Comparison of Shear Force at Base of Pylon Maximum Bending Moment (+ve) (MN-m) IRC 6 : 21 compatible time history Figure 7 : Comparison of Bending Moment (+ve) at Base of Pylon Figure 8 : Comparison of Bending Moment(-ve) at Base of Pylon The comparison of maximum bending moment (-ve) at base of pylon due to different time history function is indicated in figure 8. The negative bending moment MN-m is occurred for time history obtained by the ground response analysis, MN-m is occurred for and 8.73 MN-m is obtained for the IRC 6:21 compatible time history. It is observed that maximum negative bending moment is occurred due to ground response analysis while IRC 6 :21 compatible time history gives minimum value. In all above results the maximum response of typical cable stayed pylon is occurred due to the time history obtained using ground response analysis. When seismic waves reach at a site the ground motions they produce are affected by the geometry and properties of the geologic materials at the site. The greatest degree of amplification occurs at frequencies corresponding to thecharacteristic site period, T = 4H v. Because the characteristic site period is proportional to shear wave velocity and inversely proportional to thickness, it is clear that the response of given soil deposit will be influenced by stiffness and thickness of deposit. CONCLUSIONS A typical bridge model which reveals the proposed cable stayed bridge across the river Tapi in Surat city of Gujarat is considered for seismic analysis. The proposed bridge has concrete deck section of width 23 meters and a depth 2 meters. The length of cable stayed portion is 3 m and height of concrete pylon above deck is 35m. The bridge is anlysed for three different

7 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India types of time hisotries : time history of Bhuj Earthquake 21, compatible time history of IRC :6-21 and time history generated site specific ground response analysis. The study of displacement time history of pylon top reveals that the ground response analysis gives more accurate idea about ground motion than proposed by the relevant codal provision. From the study of response spectra for three different time histories it can be observed that the pseudo acceleration due to time history of Bhuj Earthquake has similar value as ground response analysis, but minimum response in terms of pseudo acceleration is due to compatible IRC 6: 21. This indicates that though the response spectra suggested by IRC 6: 21 include three types of soil condition, the specific codal provision is required to consider local site effect of bridge site. From the present study it can be observed that the structural response in term of shear force and bending moment is minimum for the compatible IRC 6 : 21 time history while the maximum shear force and bending moment at base of pylon occurs due to time history generated by use of ground response analysis, followed by shear force and bending moment due to time history of Bhuj Earthquake 21. Hence influence of soil layering on ground response can be observed in Bhuj Earthquake. REFERENCES Analysis of Structures Computers & Structures Inc., California, USA. 8. Schnabel PB, Lysmer J and Seed HB (1972)- SHAKE A Computer Program for Earthquake Response Analysis of Horizontally Layered Sites. Report No. EERC 72-12, University of California Berkeley. 9. Setiawan B. and Saidi T. (212); Preliminary results of site- specific ground response analysis of Banda Aceh, Indonesia, Biosciences conference, At Banda Aceh Indonesia, (ISSN:289-28X), pp Shah S.G., Desai J.A. and Solanki C.H. (21); Effect of Pylon Shape on Seismic Response of Cable Stayed Bridge with Soil Structure Interaction, International Journal of Civil and Structural Engineering, Vol. 1(3), pp Shukla J. C. and Choudhury D.(212); Seismic Hazard and Site-Specific Ground Motion for Typical Ports of Gujarat, Natural Hazards,(ISSN: 921-3X,IF:1.398/21) Springer, Netherlands, Vol. 6(2), pp Thaker T.P. and Rao K.S. (211); Development of Statistical Correlation between Shear Wave Velocity and Penetration Resistant using MASW Technique, Pan American Conference on Soil Mechanics, Toronto, Canada 1. IRC: 6-2 Standard Specifications and Code of Practice for Road Bridges, Section II, Loads and Stresses 2. IRC: 6-22 Interim Measures of IRC:6-2 for Seismic Provisions 3. IRC: 21-2 Standard Specifications and Code of Practice for Road Bridges, Section III, Cement Concrete (Plain and Reinforced) 4. IRC: 78-2, Standard Specifications and Code of Practice for Road Bridges, Section VII, Foundations and Substructure. 5. Jia J. F. and Ou J. P. (28); Seismic Analyses of Long Span Cable Stayed Bridges Subjected To Near Fault Pulse Type Ground Motions, The 14 th World Conference on Earthquake Engineering, October 12-17,28,Beijing, China 6. McGuire R. K. and Toro G. R. (28); Site- Specific Seismic Hazard Analysis ˮ, The 14 th World Conference on Earthquake Engineering, October 12-17,28,Beijing, China 7. SAP2 (29), Advanced14.1. Computer Software for Static and Dynamic Finite Element

Assessment of the Dynamic Response of the Soil to Strong Ground Motion at a Wind Farm

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