Liquefaction Resistance of Silty Sands and Dynamic Properties of Cohesive Soils from Düzce,Turkey

This article presents results from a laboratory investigation into the cyclic and dynamic properties of soils from Düzce, Turkey, conducted after the destructive November 12, 1999 earthquake. The investigation was mainly conducted by means of monotonic and cyclic triaxial, as well as resonant-column tests. The triaxial tests allowed the determination of the liquefaction resistance of silty sands, as well as their critical state behavior, whereas the resonant-column tests allowed the determination of shear modulus and damping ratio of cohesive soils. The results are presented and then discussed together with their pertinence to soil behavior when subjected to earthquake loading.     

SHEAR MODULUS AND DAMPING OF CLAYEY SANDS

A series of cyclic triaxial tests on clayey sands was carried out and attempts were made to evaluate the strain dependency of shear modulus and damping. Although the strain dependency of the shear modulus of clayey sands was similar to that of clay in the low strain range, G/G ₀ became close to that of clean sand in the high strain range. The damping ratio was less for the clayey sand containing more fines, especially in the medium to large strain range. It was shown that the change in the effective confining stress with loading cycles in the undrained shear test needed to be considered, particularly in the large strain range. The consideration can be made by normalising G with G ₀' instead of using G ₀. When an initial shear stress was applied to the specimen, the amount of residual strain, which was induced with cycles, had to be added to the normal strain amplitude in order to determine the shear modulus. If the effects of excess pore pressure and the amount of residual strain were taken into account properly, it was suggested that the shear modulus of clayey sand in irregular loading could be evaluated reasonably well by using the average G/G ₀—γcurve obtained by cyclic loading tests on isotropically-consolidated specimens.     

SOIL DAMPING FORMULATION IN NONLINEAR TIME DOMAIN SITE RESPONSE ANALYSIS

Nonlinear time domain site response analysis is used to capture the soil hysteretic response and nonlinearity due to medium and large ground motions. Soil damping is captured primarily through the hysteretic energy dissipating response. Viscous damp-ing, using the Rayleigh damping formulation, is often added to represent damping at very small strains where many soil models are primarily linear. The Rayleigh damping formulation results in frequency dependent damping, in contrast to experiments that show that the damping of soil is mostly frequency independent. Artificially high damp-ing is introduced outside a limited frequency range that filters high frequency ground motion. The extended Rayleigh damping formulation is introduced to reduce the over-damping at high frequencies. The formulation reduces the filtering of high frequency motion content when examining the motion Fourier spectrum. With appropriate choice of frequency range, both formulations provide a similar response when represented by the 5% damped elastic response spectrum.

The proposed formulations used in non-linear site response analysis show that the equivalent linear frequency domain solution commonly used to approximate non-linear site response underestimates surface ground motion within a period range relevant to engineering applications. A new guideline is provided for the use of the proposed formulations in non-linear site response analysis.     

Characterization of pozzolanic lime mortars used as filling material in shaped grooves for restoring member connections in ancient monuments

This article presents a set of mechanical characteristics for a pozzolanic mortar consisting of hydrated lime and metakaolin, which is used as filler between metallic connectors and marble blocks during restoration activities of ancient monuments in Greece. Mechanical properties include the uniaxial and triaxial compressive strength, the indirect tensile strength, the bending strength, the elastic modulus and Poisson’s ratio, and a shear failure criterion. Cored cylindrical specimens are used for determining the uniaxial and triaxial compressive behavior and the indirect tensile strength of the mortar, thus ensuring the repeatability of the experimental results. The triaxial tests indicated a plastic behavior of the mortars under study, enabling them to perform well in conditions of high shear forces. Calcite and hydraulic components formed in the setting procedure contributed to the plasticity of the final mortar. The mechanical properties that were developed can be utilized to model material behavior and failure under conditions of confined or triaxial loading. The development of a shear failure criterion for a pozzolanic mortar is a substantial accomplishment that has not been previously reported in the international literature.     

Inelastic Demand Spectra of Bi-Linear Models for Capacity Spectrum Method and Response of Simple Structures under Near-Source Records

A very useful tool for the preliminary design of structures is the elastic demand spectrum that can be used in the capacity spectrum method. A pseudo-acceleration relationship has to be assumed when constructing a demand spectrum. This assumption results in large errors for long period structures with large damping ratios and the conventional demand spectra require a substitute elastic structure. In the present study, the conventional demand spectra are extended to bi-linear models. Pseudo-acceleration is still assumed but results in acceptably small errors, when a constant viscous damping coefficient for a single-degree-of-freedom (SDF) structure is calculated from the tangent stiffness and the damping ratio is set at 5% in both elastic and yield phases. For nonlinear structures, tangent stiffness dependency of damping force could be acceptable because energy absorption is primarily the result of structural nonlinear deformation. To extend the conventional demand spectra to a bi-linear model, effective period calculated from the secant stiffness has to be used. The use of effective period introduces no approximation because the peak displacement of the SDF structure is computed from nonlinear analysis in the time domain. The method presented in this study is also valid if damping coefficient proportional to initial elastic spectra is used. In this case, the pseudo-acceleration is defined as the base shear coefficient that is required to produce the peak displacement of the SDF structure in a static manner. We present demand spectra of bi-linear models for a number of near-source records from large earthquakes, and spectral ratios of two horizontal components. The effects of different types of ground motion on the response reduction factor due to inelastic deformation are investigated.     

Assessment of Multi-Wythe Stone Masonry Subjected to Seismic Hazards

Heritage stone buildings are typically vulnerable to seismic events. Results of an experimental program were analyzed to evaluate the shear strength and intrinsic damping of stone masonry representative of the West Block of the Canadian Parliamentary Precinct in Ottawa. Eight representative wall specimens with different rehabilitation schemes had been tested under different static and dynamic loadings. Each wall had wythes of sandstone and limestone connected by a rubble core. The results show the shear strength of the walls could be predicted accurately and that no strengthening scheme was particularly beneficial. The effective viscous damping ratios varied between 7 and 9%.     

SYSTEM IDENTIFICATION OF LANDFILL SEISMIC RESPONSE

Assessment of landfill seismic response necessitates the availability of reliable dynamic material properties. During the past decade, geophysical surveys and computational studies have been conducted to investigate the seismic response of the Operating Industries, Inc. (OII) landfill in Southern California. In this paper, a survey and summary of available research results is presented. In addition, a set of Oil input-output seismic records during six earthquakes is thoroughly analysed. Spectral analyses are conducted to shed light on the landfill dynamic response characteristics. A simple shear beam model is found to be useful in modelling the landfill resonant behaviour. System identification techniques are employed to estimate the landfill stiffness and damping properties. These properties are defined by minimising the difference between computed and recorded acceleration response spectra at the landfill top. The identified stiffness properties are found to be near the lower bound of those documented through geophysical measure-ments. Identified damping of about 5% (at resonance) is within the range of earlier investigations. Comparisons of the computed and recorded accelerations show: (I) effectiveness of a linear viscous shear beam model in simulating the landfill dynamic behaviour, for the recorded small to moderate levels of dynamic excitation (up to 0.26 g peak lateral acceleration), and (ii) potential of the employed system identification procedure for analysis of input-output seismic motions.     

Seismic Performance Assessment of Slender T-Shaped Reinforced Concrete Walls

T-shaped slender reinforced concrete (RC) structural walls are commonly used in medium-rise and high-rise buildings as part of lateral force resisting system. Compared to its popularity, experimental results on seismic performance of these walls are relatively sparse, especially for data regarding these walls in the non-principal bending directions. This article aims at providing additional experimental evidence on seismic performance of T-shaped RC structural walls. Experimental results of six T-shaped RC walls were presented. These walls resemble the structural walls found in existing buildings in Singapore and possess slightly inferior details compared to the requirements of modern design codes. The test variables were the loading direction and the axial load ratio. The experimental results were discussed in terms of the failure mechanisms, cracking patterns, hysteretic responses, curvature distributions, displacement components, and strain profiles. In addition, the experimental results were compared with methods commonly adopted in current design practice including the nonlinear section analyses, shear strength models and effective width of the tension flange. The experimental data illustrate that the shear lag effect not only was not accurately accounted for by the effective width method but also significantly affected the strength and stiffness of the tested specimens.     

Experimental and Numerical Seismic Response of a 65 kW Wind Turbine

A full-scale shake table test is conducted to assess the seismic response characteristics of a 23 m high wind turbine. Details of the experimental setup and the recorded dynamic response are presented. Based on the test results, two calibrated beam-column finite element models are developed and their characteristics compared. The first model consists of a vertical column of elements with a lumped mass at the top that accounts for the nacelle and the rotor. Additional beam-column elements are included in the second model to explicitly represent the geometric configuration of the nacelle and the rotor. For the tested turbine, the experimental and numerical results show that the beam-column models provide useful insights. Using this approach, the effect of first-mode viscous damping on seismic response is studied, with observed experimental values in the range of 0.5–1.0% and widely varying literature counterparts of 0.5–5.0%. Depending on the employed base seismic excitation, damping may have a significant influence, reinforcing the importance of more accurate assessments of this parameter in future studies. The experimental and modeling results also support earlier observations related to the significance of higher modes, particularly for the current generation of taller turbines. Finally, based on the outcomes of this study, a number of additional experimental research directions are discussed.     

Non-Iterative Equivalent Linearization Based on Secant Period for Estimating Maximum Deformations of Existing Structures

The capacity spectrum method of ATC-40 uses the secant period as the equivalent period of equivalent linear systems. Therefore, it results in a direct graphical comparison. The maximum inelastic displacement and acceleration demands of structures can be simultaneously obtained from the intersection of the demand and capacity diagrams. However, for evaluation of existing structures, the demands need to be determined through iterations since the equivalent period and damping of the equivalent linear systems currently available are both a function of the (displacement) ductility ratio, which is unknown and is the target of evaluation. In addition, the equivalent damping used in the capacity spectrum method is independent of periods of vibration. It may lead to poor estimations of maximum responses especially for short-period systems. This article proposes two equivalent linear systems based on the secant period to estimate the maximum displacement and acceleration responses of existing structures. Both the recommended equivalent period and damping are defined by the strength ratio (elastic lateral strength/yield lateral strength), rather than the ductility ratio. Because the strength ratio of existing structures is a known parameter, the maximum displacement and acceleration responses of these structures can be determined without iterations. Besides, effects of periods of vibration on the equivalent linear systems are also included in this study. The equivalent damping is derived from statistical analyses for bilinear single-degree-of-freedom (SDOF) systems with different periods of vibration, strength ratios and post-yield stiffness based on 72 earthquake ground motions recorded on firm sites. Procedures and examples for applications of the proposed equivalent linear systems on nonlinear static analysis procedures are also provided.     

Completeness Verification of Complex Response Spectrum Method for Underdamped and Overdamped Multiple-Support Systems Regarding the Decoupled Damping as Mathematical Parameter without Physical Meaning

The completeness of the complex response spectrum method for both formally underdamped and overdamped modes is theoretically proved, and the physical meanings of the decoupled modes as well as involved parameters are recognized and clarified in this paper. For the system with relatively large non-classical damping, the eigenvalue pairs generated by the complex mode decomposition method are real and the so-called modal damping ratios are larger than unity. In this paper, we firstly clarified that the decoupled modes are virtual and the so-called modal frequency and damping ratio are mathematical parameters that have no physical meaning. Then, the completeness of the complex response spectrum method for both formally underdamped and overdamped modes is rigorously proved by allowing the “damping frequency” to be an imaginary number. For the virtually overdamped modes, Duhamel integral involved in the calculation for formally underdamped modes automatically convert to hyperbolic Duhamel integral. A numerical example taken from the published literature is given to verify this method. Structural responses for the system with coupled damping under multi-support seismic excitations are further analyzed and numerical results indicate the accuracy of complex response spectrum method.     

Development and Performance Evaluation of Disk-Type Piezoelectric Transducer for Measurement of Shear and Compression Wave Velocities in Soil

A piezoelectric transducer capable of measuring both shear and compression wave velocities in soil simultaneously in triaxial testing conditions is presented. Performance evaluation of disk transducer system showed that the use of low-noise coaxial cables, proper grounding, and high resolution wave recorder can significantly enhance signal quality and eliminates crosstalk deterioration. Distortions due to near-field effects were found to diminish by increasing input frequency and by using sinusoidal input waveform, compared to square input. Disk-type piezoelectric transducers show significant future potential for laboratory determination of shear, and compression modulus of soil because of their robustness and noninvasive nature.     

Dynamic Interaction between Rigid Foundations on Multi-layered Stratum

This article concerns the cross-interaction problem among multi-foundations on a linearly viscoelastic medium at very small shear strains. In the analysis, the foundations are discretized into a number of sub square-elements. The dynamic response within each sub-element is described by the Green’s function, which is obtained by the Fourier-Bessel transform and the precise integration method. Incorporating the displacement boundary condition and the force equilibrium of the foundations, it obtains the dynamic impedance and compliance functions of the foundations. Extensive results for two rigid circular foundations placed at different separations are presented. Parametric studies are carried out on the dynamic interaction among adjacent foundations. Illustrative results for several closely spaced foundations are also illustrated.     

FRAGILITY ASSESSMENT OF SLAB-COLUMN CONNECTIONS IN EXISTING NON-DUCTILE REINFORCED CONCRETE BUILDINGS

Fragility functions that estimate the probability of exceeding different levels of damage in slab-column connections of existing non-ductile reinforced concrete buildings subjected to earthquakes are presented. The proposed fragility functions are based on experimental data from 16 investigations conducted in the last 36 years that include a total of 82 specimens. Fragility functions corresponding to four damage states are presented as functions of the level of peak interstory drift imposed on the connection. For damage states involving punching shear failure and loss of vertical carrying capacity, the fragility functions are also a function of the vertical shear in the connection produced by gravity loads normalised by the nominal vertical shear strength in the absence of unbalanced moments. Two sources of uncertainty in the estimation of damage as a function of lateral deformation are studied and discussed. The first is the specimen-to-specimen variability of the drifts associated with a damage state, and the second the epistemic uncertainty arising from using small samples of experimental data and from interpreting the experimental results. For a given peak interstorey drift ratio, the proposed fragility curves permit the estimation of the probability of experiencing different levels of damage in slab-column connections.     

REPEATABLE DYNAMIC RELEASE TESTS ON A BASE-ISOLATED BUILDING

The main activities and the relevant results of an experimental program aimed at evaluating the dynamic characteristics of a large seismically isolated building are described. The building is situated in Potenza (Italy) and is the largest of five seismically isolated blocks of the University of Basilicata. Cyclic shear tests on scaled isolators and some small amplitude free vibration tests were preliminarily carried out. The latter ones were aimed at getting a preliminary linear characterisation of the structure. Finally, many in situ release tests on the isolated structure were carried out, using a mechanical device purposely designed to statically move the building and then suddenly release it, thus making it vibrate freely. The variations of the dynamic characteristics of the system while the oscillation amplitude decreases have been evaluated by using the “Short Time Fourier Transform (STFT)”. The results were elaborated in terms of stiffness and equivalent damping and compared to the results of the laboratory tests on scaled isolators, finding an excellent agreement.     

Influence of Ground Motion Duration on Damping Reduction Factor

This article investigates the influences of the effective ground motion duration (GMD) on damping reduction factor. The GMD are associated with 25 Chi-chi earthquake ground motion records and harmonic sine wave. The study shows that damping reduction factor decreases with the increasing of the damping ratio, and decreases with the increasing of the effective duration of the ground motion and the number of cycles of harmonic excitation. A nonlinear multiple regression analysis based on the statistical mean values of the present study is employed, and a modified damping reduction factor considering the effects of GMD is suggested.     

Experimental Investigation on the Seismic Performance of Beam-Column Joints Reinforced with GFRP Bars

Glass fiber-reinforced polymer (GFRP) reinforcing bars were used recently as main reinforcement for concrete structures. The noncorrodible GFRP material exhibits linear-elastic stress-strain characteristics up to failure with relatively low modulus of elasticity compared to steel. This raises concerns on GFRP performance in structures where energy dissipation, through plastic behavior, is required. The objective of this research project is to assess the seismic behavior of concrete beam-column joints reinforced with GFRP bars and stirrups. Two full-scale exterior T-shaped beam-column joint prototypes are constructed and tested under simulated seismic load conditions. One prototype is totally reinforced with GFRP bars and stirrups, while the other one is reinforced with steel. The experimental results showed that the GFRP reinforced joint can sustain a 4.0% drift ratio and can recover its deformation without any significant residual strains. This indicates the feasibility of using GFRP bars and stirrups as reinforcement in the beam-column joints subjected to seismic-type loading.     

ACTIVE/PASSIVE SEISMIC CONTROL OF STRUCTURES

In civil engineering, structural integrity and safety are of utmost importance as the consequences of failure are devastating. Maintaining the structural integrity becomes particularly important when the structures are subjected to severe earthquakes and strong wind-loading. Various passive and active control means have been considered to avoid catastrophic failure due to seismic or wind excitations. In this paper, a new class of hybrid actuators is presented which consists of a piezoelectric stack actuator combined with a viscoelastic damper to form a passive/active brace system (PAB). The actuator is used for mitigating structural dynamic responses of a three-storey structure subjected to simulated earthquakes loading. The proposed hybrid actuator is selected because it combines the attractive attributes of active and passive systems as well as because it has high stiffness-weight ratio, high frequency bandwidth, and low power consumption. The theoretical and experimental performance characteristics of the three-storey structure with the hybrid PAB actuator are presented. Comparisons are also included when the structure is controlled with conventional viscoeiastic dampers or conventional active control braces that operate without any viscoelastic damping. These comparisons emphasize the effectiveness of the hybrid actuator in damping out simulated seismic vibrations.     

Failure Prediction and Function Determination of Archaeological Pottery by Finite Element Analysis

We present a quantitative model, based on the finite element analysis (FEA) approach, developed for predicting the failure of vessels under almost any possible loading conditions, by taking into account the shape. The vessel is divided into a large number of virtual elements, on which the strains, under any type of loading, are calculated by the parametric solution of a large number of equations. The inputs needed for the analysis are the Young's modulus and the Poisson's ratio of the material as well as the geometric characteristics of the vessel and the loading mode. The efficiency of the FEA approach is evaluated on ceramic pots manufactured for this purpose. The maximum strain experienced by the pots is calculated by FEA, under different loading modes and loads. The actual pots are subjected to mechanical loading and the load at failure was found to be within 10–20% of the calculated value. The model is also applied to archaeological pottery with a view towards predicting its mechanical behaviour.     

滚珠式文物展柜隔震装置性能试验

为有效减小馆藏浮放文物的震害,研制了一款滚珠式文物展柜水平隔震装置。制作了2个文物展柜模型,并在各模型内浮放文物仿品。分别考虑文物展柜底部安装隔震装置、展柜底部浮放地面,进行了振动台对比试验,检测了隔震装置的效果。结果表明:与非隔震条件相比,隔震体系的基频减小,阻尼比增大;地震作用下,隔震体系中的文物仿品产生的位移、加速度峰值明显减小。但是隔震条件下,文物仿品的动力放大系数仍有大于1的情况,且隔震装置的复位能力尚需提高。建议对隔震装置采取的改进措施主要适当减小基频、增大阻尼比、提高加工精度等。