Simplified discrete systems for dynamic analysis of structures on footings and piles

A simplified discrete system in the form of a simple oscillator is developed to simulate the dynamic behavior of a structure founded through footings or piles on compliant ground, under harmonic excitation. Exact analytical expressions for the fundamental natural period and the corresponding damping coefficients of the above system are derived, as function of geometry and the frequency-dependent foundation impedances. In an effort to quantify the coupling between swaying and rocking oscillations in embedded foundations such as piles, the reference system is translated from the footing–soil interface to the depth where the resultant soil reaction is applied, to ensure a diagonal impedance matrix. The resulting eccentricity is a measure of the coupling effect between the two oscillation modes. The amounts of radiation damping generated from a single pile and a surface footing are evaluated. In order to compare the damping of a structure on a surface footing and a pile, the notion of static and geometric equivalence is introduced. It is shown that a pile may generate significantly higher radiation damping than an equivalent footing, thus acting as an elementary protective system against seismic action.     

Full-scale experimental modal analysis of an arch dam: The first experience in Iran

Forced vibration field tests and finite-element studies were conducted on the Shahid Rajaee concrete arch dam in Northern Iran to determine the dynamic properties of the dam–reservoir–foundation system. The first forced vibration tests on the dam were performed with two different types of exciter units, with a limited maximum force, bolted on the dam crest for alternative in-phase and out-of-phase sequencing. Because of an insufficient number of recording sensors, two arrangements of sensors were used to cover sufficient points on the dam crest and one gallery during tests. Two kinds of vibration tests, on–off and frequency sweeping, were carried out on the dam. The primary natural frequencies of the coupled system for both symmetric and anti-symmetric vibration modes were approximated during on–off tests in two types of sequencing of exciters, in phase and out-of-phase, with a maximum frequency of 14 Hz. The principal forced vibration tests were performed at precise resonant frequencies based on the results of the on–off tests in which sweeping around the approximated frequencies at 0.1 Hz increments was performed. Baseline correction and suitable bandpass filtering were applied to the test records and then signal processing was carried out to compute the auto power, cross power and coherence spectra. Nine middle modes of vibration of the coupled system and corresponding damping ratios were estimated. The empirical results are compared against the results from calibrated finite-element modeling of the system using former ambient vibration tests, considering the dam–reservoir–foundation interaction effects. Good agreement is obtained between experimental and numerical results for eight middle modes of the dam–reservoir–foundation system.     

Damping coefficients for soil–structure systems and evaluation of FEMA 440 subjected to pulse-like near-fault earthquakes

In this study, attempts are made to investigate the effects of inertial soil–structure interaction (SSI) on damping coefficients subjected to pulse-like near-fault ground motions. To this end, a suit of 91 pulse-like near-fault ground motions is adopted. The soil and superstructure are idealized employing cone model and single-degree-of-freedom (SDOF) oscillator, respectively. The results demonstrate that soil flexibility reduces and amplifies the damping coefficients for structural viscous damping levels higher and lower than 5%, respectively. The coefficients reach one for both acceleration and displacement responses in cases of dominant SSI effects. The effect of structure dimensions on damping confidents are found insignificant. Moreover, damping coefficients of displacement responses are higher than those of acceleration responses for both fixed-base and flexible-base systems. Evaluation of damping correction factor introduced by FEMA 440 shows its inefficiency to predict acceleration response of soil–structure systems under pulse-like near-fault ground motions. Soil flexibility makes the damping correction factor of moderate earthquakes more pronounced and a distinctive peak value is reported for cases with dominant SSI effects.     

Application of perfectly matched layers in the transient analysis of dam–reservoir systems

The transient analysis of dam–reservoir systems by employing perfectly matched layers has been investigated. In previous studies, boundary conditions of the PML region in the reservoir have been neglected. In this paper, they are incorporated completely in the formulation. Moreover, a technique is introduced to involve the effect of incident waves caused by vertical ground motions at the reservoir bottom in the analysis. Performing several numerical experiments indicates that applying boundary conditions of the PML domain and utilizing the proposed method for vertical excitation cases reduce the computational cost significantly and make the PML method a very efficient approach for the transient analysis of dam–reservoir systems.     

Site-specific and spatially-distributed ground-motion intensity estimation in the 2010–2011 Canterbury earthquakes

This paper presents site-specific and spatially-distributed ground-motion intensity estimates which have been utilized in the aftermath of the 2010–2011 Canterbury, New Zealand earthquakes. The methodology underpinning the ground motion intensity estimation makes use of both prediction models for ground motion intensity and its within-event spatial correlation. A key benefit of the methodology is that the estimated ground motion intensity at a given location is not a single value but a distribution of values. The distribution is comprised of both a mean and standard deviation, with the standard deviation being a function of the distance to nearby observations at strong motion stations.The methodology is illustrated for two applications. Firstly, maps of conditional peak ground acceleration (PGA) have been developed for the major events in the Canterbury earthquake sequence, which among other things, have been utilized for assessing liquefaction triggering susceptibility of land in residential areas. Secondly, the conditional distribution of response spectral ordinates is obtained at the location of the Canterbury Television building (CTV), which catastrophically collapsed in the 22 February 2011 earthquake. The conditional response spectra provide insight for the selection of ground motion records for use in forensic seismic response analyses of important structures at locations where direct recordings are absent.     

Evaluation of the natural vibration frequencies of a historical masonry building accounting for SSI

The dynamic identification of a historical masonry palace located in Benevento (Italy) has been carried out. The case study is representative of many buildings located in historic Italian centres. Since the building has been instrumented by the Department of Civil Protection with a permanent dynamic monitoring system, some of the recorded data, acquired in various operating conditions have been analysed with basic instruments of the Operational Modal Analysis in order to identify the main eingenfrequencies and vibration modes of the structure. The experimental results have been compared to the numerical outcomes provided by a detailed three-dimensional Finite Element (FE) model of the building where Soil–Structure Interaction (SSI) has been taken into account. The comparison of experimental vs. numerical frequencies and vibration modes of the palace evidenced the role exerted by the subsoil on the dynamic response of the building.     

Approximate soil–structure interaction analysis by a perturbation approach: The case of soft soils

An approximate solution of the classical eigenvalue problem governing the vibrations of a relatively stiff structure on a soft elastic soil is derived through the application of a perturbation analysis. The full solution is obtained as the sum of the solution for an unconstrained elastic structure and small perturbing terms related to the ratio of the stiffness of the soil to that of the superstructure. The procedure leads to approximate analytical expressions for the system frequencies, modal damping ratios and participation factors for all system modes that generalize those presented earlier for the case of stiff soils. The resulting approximate expressions for the system modal properties are validated by comparison with the corresponding quantities obtained by numerical solution of the eigenvalue problem for a nine-story building. The accuracy of the proposed approach and of the classical normal mode approach is assessed through comparison with the exact frequency response of the test structure.     

Full-scale model testing on a ballastless high-speed railway under simulated train moving loads

Model testing in laboratory, as an effective alternative to field measurement, provides valuable data to understand railway׳s dynamic behaviors under train moving loads. This paper presents comprehensive experimental results on track vibration and soil response of a ballastless high-speed railway from a full-scale model testing with simulated train moving loads at various speeds. A portion of a realistic ballastless railway comprising slab track, roadbed, subgrade, and subsoil was constructed in a larger steel box. A computer-controlled sequential loading system was developed to generate equivalent vertical loadings at the track structure for simulating the dynamic excitations due to train׳s movements. Comparisons with the field measurements show that the proposed model testing can accurately reproduce dynamic behaviors of the track structure and underlying soils under train moving loads. The attenuation characteristics of dynamic soil stresses in a ballastless slab track is found to have distinct differences from that in a ballasted track. The model testing results provide better understanding of the influence of dynamic soil–structure interaction and train speed on the response of track structure and soils.     

Idealisation of soil–structure system to determine inelastic seismic response of mid-rise building frames

In this study, a novel and enhanced soil–structure model is developed adopting the direct analysis method using FLAC 2D software to simulate the complex dynamic soil–structure interaction and treat the behaviour of both soil and structure with equal rigour simultaneously. To have a better judgment on the inelastic structural response, three types of mid-rise moment resisting building frames, including 5, 10, and 15 storey buildings are selected in conjunction with three soil types with the shear wave velocities less than 600 m/s, representing soil classes C_e, D_e and E_e, according to Australian Standards. The above mentioned frames have been analysed under two different boundary conditions: (i) fixed-base (no soil–structure interaction) and (ii) flexible-base (considering soil–structure interaction). The results of the analyses in terms of structural displacements and drifts for the above mentioned boundary conditions have been compared and discussed. It is concluded that considering dynamic soil–structure interaction effects in seismic design of moment resisting building frames resting on soil classes D_e and E_e is essential.     

Dynamic soil–structure interaction analysis of a telescope at the Javalambre Astrophysical Observatory

This paper presents the dynamic soil–structure analysis of the main telescope T250 of the Observatorio Astrofísico de Javalambre (OAJ, Javalambre Astrophysical Observatory) on the Pico del Buitre. Vibration control has been of prime concern in the design, since astrophysical observations may be hindered by mechanical vibration of optical equipment due to wind loading. The telescope manufacturer therefore has imposed a minimal natural frequency of 10 Hz for the supporting telescope pier. Dynamic soil–structure interaction may significantly influence the lowest natural frequency of a massive construction as a telescope pier. The structure clamped at its base has a resonance frequency of 14.3 Hz. A coupled finite element–boundary element (FE–BE) model of the telescope pier that accounts for the dynamic interaction of the piled foundation and the soil predicts a resonance frequency of 11.2 Hz, demonstrating the significant effect of dynamic soil–structure interaction. It is further investigated to what extent the coupled FE–BE model can be simplified in order to reduce computation time. The assumption of a rigid pile cap allows us to account for dynamic soil–structure interaction in a simplified way. A coupled FE–BE analysis with a rigid pile cap predicts a resonance frequency of 11.7 Hz, demonstrating a minor effect of the pile cap flexibility on the resonance frequency of the telescope pier. The use of an analytical model for the pile group results in an overestimation of the dynamic soil stiffness. This error is due to the large difference between the actual geometry and the square pile cap model for which the parameters have been tuned.