Soil-Structure System Identification with Ambient Vibration Tests (A Case-Study on a Surface Pier of Kermanshah Urban Railroad)
Majid
Lameie
Civil Eng. Dept., School of Eng., Razi Univ., Kermanshah, Iran
author
Iman
Ashayeri
Civil Eng. Dept., School of Eng., Razi Univ., Kermanshah, Iran
author
Mahnoosh
Biglari
Civil Eng. Dept., School of Eng., Razi Univ., Kermanshah, Iran
author
Mohammad Amin
Kadivar
slamic Azad Univ., Science and Research Branch, Isfahan, Iran
author
text
article
2015
per
During an earthquake, the dynamic response of a structure located on a soil deposit could be very complex compared with the analysis of the same structure on bedrock due to the interactions between the soil and the structure. This phenomenon is technically termed as Soil-Structure Interaction (SSI) effects in literature. Most of what is currently known about soil-structure interaction (SSI) is based on theoretical and mathematical models. Therefore, it is necessary to investigate the structure treatments when they response to the ground strong motions transferred by SSI. In this regard, in an experimental field, the present study investigated the SSI effects on structure, evaluating the natural frequencies and damping of a pile-group-supported pier of Kermanshah’s LRT. The frequencies and damping evaluations were performed through ambient vibration test results and system identification procedures. The purpose of system identification is to evaluate unknown properties of a system, using known inputs and outputs. There are two principal system identification procedures to build mathematical models of dynamical systems from measured data: (a) non-parametric and (b) parametric procedures. Non-parametric procedures evaluate complex-valued transmissibility functions from the input and output recordings without fitting an underlying model. Accordingly, Fourier Transform (FT), response square of transfer function, peak picking and four spectra are considered as non-parametric procedures. On the other hand, parametric procedures develop numerical models of transfer functions. More precisely, in these procedures, a mathematical model with several parameters is defined first. The considered parameters are featured with specific values determined by experimental results. Then, the system’s input-output function is obtained using this described model. The studied pier was fully instrumented with two SARA and a CEM seismometers. The seismometers recorded signals of two horizontal and a vertical components that were digitally recorded at 200 Hz sampling rate. In general, measure of SSI effects was then obtained by comparing the flexible-base and fixed-base parameters to calculate the two most important effects of SSI, period lengthening and foundation damping. SAP 2000 was used to create a finite element model of the whole structure and the accuracy of the model was tested using recorded data from ambient vibration at the structure site. In summary, the current study indicates that all the utilized system identification methods are appropriate in determining the dynamic characteristics of the structure in fixed condition. In addition, it was demonstrated that peak picking and four spectral methods did not have appropriate function in investigating the interaction. However, these two procedures have appropriate function in determining the dynamic characteristics of the structure in fixed condition. As Figure (1) shows, the diagram of the period lengthening obtained from parametric method with the ratio of the structure-to-soil stiffness for the pier is approximately consistent with system identification analyses performed for the 57 sites in Stewart et al. [1]. Accordingly, inertial interaction effects were generally observed to be small for 1/σ < 0.1 and for practical purposes could be neglected in such cases. Reference1.Stewart, J.P. and Fenves, G.L., and Seed, R.B. (1999) Seismic soil-structure interaction in buildings. I: Analytical aspects. Journal of Geotechnical and Geoenv. Engineering, ASCE, 125(1), 26-37.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
1
13
http://www.bese.ir/article_240271_ef127ea37f5b06a8d2c481198c032c2b.pdf
Tehran Subway Tunnel Effect on the Seismic Response of the Ground Surface with Linear Soil Behavior: An Experimental and Numerical Study
Mohammad Hassan
Baziar
School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran
author
Abbas
Ghalandarzadeh
Department of Civil Engineering, University of Tehran, Tehran, Iran
author
Masoud
Rabeti Moghadam
School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran
author
text
article
2015
per
In the current study, a series of 1 g shaking table tests was performed to study the Tehran subway tunnel effect on the ground surface acceleration response. Two reduced-scale 1 g shaking table models, designated as FF and SF, were constructed in 1/32 scale. The FF was constructed to study the seismic response of the soil layer in free field condition, while the SF model includes a subway tunnel to study its effect on the acceleration response of nearby ground. In prototype scale, the subway tunnel with 8 m diameter and 0.35 m thickness was embedded in a soil layer with 32 m thickness. The soil was dense sand with 70% of relative density. The models were constructed in a rigid box made from Plexi-glass with dimensions of 178*80*120 cm (L.H.W.). Lateral boundaries of the models were covered with conventional foam in order to reduce the lateral boundary effect on the seismic response of the soil layer. The constructed SF model is depicted in Figure (1a). The accelerometers and LVDT transducers installed in the models to record the acceleration in the soil and settlement at model surface are illustrated in Figure (1b). The experimental study revealed that the tunnel does not affect the incident waves with dimensionless period (λ/D) larger than 10. Previous numerical studies [1-2] also demonstrated that an underground tunnel does not affect the free field response at λ/D greater than 10. Up to now, this finding has not been demonstrated by any experimental research. However, the physical modeling performed here is suffered from some limitations regarding the applied frequencies. Therefore, a numerical model was developed based on the results of the shaking table tests, and the effect of the tunnel on the excitations with higher frequency ranges was investigated. Besides, the effect of different parameters such as shear wave velocity of the soil, flexibility ratio and depth of the tunnel on the acceleration at the ground surface was numerically determined. Figure (2) shows the numerical model of the SF model in prototype scale. Six real earthquake motions that were matched to the response spectrum of ground type I in Standard 2800 were used in the parametric analyses. PGA of the motions was scaled to 0.35 g. The push of the amplification during the analyses were considered as the maximum response and depicted as amplification pattern at the ground surface. Amplification pattern at the ground surface for a tunnel at h/a=1.5 in soils with different shear wave velocities (VS) is depicted in Figure (3). As presented, the maximum amplification occurred at X/a = 1.5 for all Vs. Moreover, as the shear wave velocity increases, the amplification ratio decreases. The study revealed that the amount of the amplification on the ground surface depends on the tunnel depth and shear wave velocity of the soil. The maximum amplifications at the ground surface was equal to 5, 8 and 10 percent for the tunnel depth rations of 1.5, 2 and 3, respectively, in a soil medium with 175 m/s of the shear wave velocity. The effect of tunnel depth on the amplification pattern was investigated in the parametric study. It was concluded that as the tunnel depth increases, the amplification ratio decreases. The tunnel depth affects the location of the maximum amplifications. As the tunnel depth increases, the location of the maximum amplification gets away from the tunnel center and occurs at longer distance from the tunnel center. The effect of the Tehran subway tunnel on the response spectrum at the ground surface in different soil for different ratios of the tunnel depth was investigated. It was concluded that the subway tunnel in soils with different shear wave velocity affects the different ranges of the periods. A subway tunnel with 8 m diameter influences the seismic response of the buildings with the period lower than 0.4 sec or the buildings smaller than 10 m. It means that the tunnel has an adverse effect on the short buildings. References 1. Yiouta-Mitra, P., Kouretzis, G., Bouckovalas, G., and Sofianos, A. (2007) Effect of underground structures in earthquake resistant design of surface structures. Dynamic Response and Soil Properties, Geo-Denver: New Peaksin Geotechnics. 2. Alielahi, H., Kamalian, M., and Adampira, M (2015) Seismic ground amplification by unlined tunnels subjected to vertically propagating SV and P waves using BEM. Soil Dynamics and Earthquake Engineering, 71, 63-79.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
15
36
http://www.bese.ir/article_240272_bcd030265befee63fa9db929cbb4b16f.pdf
Seismic Bearing Capacity Factor of Unit Weight Under Inclined Load Using the Kötter Equation
Morteza
Jiryaei Sharahi
Department of Civil Engineering, Qom University of Technology, Qom, Iran
author
Mojtaba
Mousavi Ourimi
Department of Civil Engineering, Qom University of Technology, Qom, Iran
author
text
article
2015
per
Experimental and theoretical investigations indicate that the seismic bearing capacity of foundations is affected by earthquake excitation. In the present study, an analytical procedure is presented to obtain the seismic bearing capacity factor of shallow strip footing NγE for a foundation under inclined load on cohesionless soils. The limit equilibrium method with numerical iteration technique is utilized to calculate the seismic bearing capacity factor NγE. In the proposed analysis the Kötter’s equation and a failure surface consisting log-spiral and planar surface are employed. The results indicate that the seismic bearing capacity is reduced due to an increase in horizontal coefficient of earthquake acceleration. Besides, the results are in good agreement with solutions available in the literature.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
37
49
http://www.bese.ir/article_240273_b736019b3d7a5456a7552311f5f80edd.pdf
Nonlinear Seismic Analysis of Pile Groups in Layered Soils due to Kinematic Interaction Effects
Hossein
Tahghighi
University of Kashan, Kashan, Iran
author
Majid
Shabkhan
Islamic Azad University of Kashan, Kashan, Iran
author
text
article
2015
per
During earthquakes, piles undergo stresses due both motion of the superstructure (i.e. inertial interaction) and that of the surrounding soil (i.e. kinematic interaction). In practice, structural engineers commonly take into account stresses induced by the inertial interaction, which is responsible for pile head failure, but they neglect the effects of the kinematic interaction that is responsible for failures along pile’s length in the case of layered soils with highly contrasting mechanical characteristics even in the absence of the superstructure. Thus, the evaluation of kinematic forces developing in piles during earthquakes has been receiving increased interest from the researchers. Numerical methods for the analysis of kinematic soil-pile interaction can be classified into two groups; continuum-based approaches and Winkler methods [1-3]. It has been customary in professional engineering and research practices to assume a linear behaviour for the soil and the pile foundation. However, under strong excitation, the nonlinear behaviour of soil media at the soil-pile interface has a strong influence on the response of the pile foundation. The aim of this study is to investigate the influence of soil nonlinearities on the kinematic interaction forces of pile groups embedded in layered soil deposits during seismic actions. Figure (1) shows the assumed soil-pile group case with 5 by 5 piles embedded in two layer subsoil profile. The pile has been considered as an elastic beam, while the soils have been modelled using the elastic-plastic solid element. The corresponding 3D finite element mesh has been shown in Figure (2). Based on the symmetry, only half of the model is meshed.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
51
62
http://www.bese.ir/article_240274_d3c1d619402b4f37669adf86000e35e3.pdf
Role of Seismic Load Characteristics on Permanent Settlement of Shallow Foundations on Liquefiable Sand
Mohamad Ali
Moradi
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
Yaser
Jafarian
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
text
article
2015
per
In this study, the role of input motion characteristics on the amount of residual settlement of shallow foundations relied on liquefiable soil is investigated. First, two-dimensional numerical model is created using finite difference software FLAC-2D, and the UBCSAND constitutive model. After verification of the numerical model with the results of a centrifuge test, behavior of foundation is investigated under different earthquake loadings. To evaluate the effect of the input motion characteristics on foundation settlement, three records from Irpinia 1980, Northridge 1994, and Landers 1992 earthquakes are used. The frequency contents of these records are considerably different. These records are scaled to five levels of PGA (including 0.05g, 0.1g, 0.2g, 0.3g, 0.45g) to be used in the analyses. The results of the analyses demonstrate that the amounts of foundation settlement for the same PGA in three records are considerably different. Therefore, the use of PGA alone might be insufficient to predict the amount of foundation settlements. This fact is shown in Figure (1a). Figure (1b) illustrates the amounts of foundation settlement against the mean period of records at different PGA levels. It is observed that the mean period of records which indicates their frequency content, has a significant influence on the values of foundation settlements.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
63
75
http://www.bese.ir/article_240275_6c21773fa190369c33b3d55b7b27c0f4.pdf
Influence of Masonry Infills with and without Opening on Progressive Collapse of Buildings (A Case Study: San Diego Hotel)
Majid
Mohammadi
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
Nahid
Inanloo
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
text
article
2015
per
Progressive collapse starts with local destruction of a few elements of structures, which extends into a significant part of the building. Regular buildings are designed for dead, live, wind, earthquake and other normal loads. Nevertheless, there are other possible risks and loads, including firing, vehicle collision, gas explosion, design or construction error, bomb blast, etc. These risks occur very rarely, but they may cause a catastrophic collapse; therefore, for very important structures, they should be considered in the designing phase [1]. Infills have a considerable improvement in the stiffness and the strength of the frame. Therefore, their influences on progressive collapse of buildings should be considered. Considering such elements in structural modelling is very complicated, that’s why many standards and codes ignore their local and global effects and just consider their effects in decreasing the building natural period of vibration [2]. However, they are considered in rehabilitation projects and should be considered in progressive collapse analyses [3-4]. Many methods have already been proposed to model solid infills in the structure; the most common approach is modelling by diagonal struts [4, 5]. For infills with openings, there is not a verified approach for the modelling. This study is to propose a method for modelling infills with and without opening and investigating their effects on progressive collapse of buildings. Perforated infills are modelled by the equivalent struts, considering the influence of the opening as a reduction factor for the width of the strut, verified in previous studies [6]. The proposed model is verified by the results of an experimental study on San Diego Hotel, obtained by Sasani [3]. The plan of the hotel is shown in Figure (1). The hotel had reinforced concrete structure and was destructed by explosion in two columns (A2 and A3 in Figure 1) of the first story.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
77
88
http://www.bese.ir/article_240276_836e6da2cc29d4ec8249f5226cce9954.pdf
Using Base Isolation Method to Mitigate the Seismic Response of Liquefied Natural Gas
Mohammad Ali
Goudarzi
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
Parviz
Rafati
Imam Hossein Comprehensive University, Tehran, Iran
author
Soheil
Rostam Kolaee
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
author
text
article
2015
per
During the last 20 years, part of the research work has focused on the seismic analysis of Liquefied Natural Gas (LNG) tanks, due mainly to (1) the increasing number of LNG tanks constructed in seismically active regions, resulting from the adoption of LNG as an environmentally friendly fossil fuel, and (2) the catastrophic environmental impact, associated with a potential local or total failure of such tanks, caused by the earthquake motion. Seismic isolation is a well-known method to mitigate the earthquake effects on the structures by increasing their fundamental natural periods at the expense of larger displacements in the structural system. In this study, the seismic response of isolated and fixed base vertical, cylindrical, liquid storage tanks is investigated using a numerical model, taking into account the fluid-structure interaction effects. The numerical model is validated by the comparison of its results with the experimental measurements of small-scale tank under harmonic and seismic excitations. The comparison reveals that the use of the considered model provides enough accuracy for evaluating the seismic behavior of nonlinear isolated and non-isolated tanks. Three vertical, cylindrical tanks with different ratios of height to radius (H/R=2.6, 1.0 and 0.3 as the representatives of slender, medium and broad tanks) are analyzed and the results of response-history analysis, including base shear, overturning moment and free surface displacement are reported for isolated and non-isolated tanks. The isolated tanks are equipped with lead rubber bearings isolators, and the bearings are modeled by using a non-linear spring in numerical model. Long period ground motion is the main parameter that can significantly affect the seismic response of isolated tank. It is observed that the seismic isolation of liquid storage tanks is quite effective and the response of isolated tanks is significantly influenced by the system parameters such as their fundamental frequencies and the aspect ratio of the tanks. The average reductions of base shear forces of isolated tanks are 71%, 70% and 50% for broad, medium and slender isolated tanks. It seems that the effectiveness of base isolation system to mitigate the base shear force is not significantly affected by changing of tank aspect ratio. In terms of overturning moment, the average reductions of the order of 71%, 69% and 47% for broad, medium and slender tanks is obtained due to applying of isolation system. Therefore, overturning moment is considerably mitigated by the reduction of the tank aspect ratio. The effectiveness of base isolation considerably reduces for exerted earthquake records including long period motion. Especially for slender tanks, base isolation may even increase the overturning moment. However, the base isolation does not significantly affect the surface wave height, and even it can cause adverse effects on the free surface sloshing motion. The results of free surface displacement for both isolated and non-isolated tanks have quite similar trends for considered tanks. The errors between the maximum sloshing wave height of fixed base and isolated tank are less than 8% for most of the considered cases. Even, the sloshing height is slightly amplified in some cases. Therefore, the base isolation system can cause adverse effects on the free surface sloshing motion. It can be concluded that the effectiveness of the base isolation method is very sensitive to the physical and geometrical parameters of the considered tanks. This suggests that a careful selection of isolators with a certain limit on the mechanical properties of the isolators is required for the optimal seismic isolation design of liquid storage tanks.
Bulletin of Earthquake Science and Engineering
International Institute of Earthquake Engineering and Seismology
2476-6097
2
v.
3
no.
2015
89
100
http://www.bese.ir/article_240277_380278c224e711d0beaa52bd61a8a1c7.pdf