The availability of methods for the identification of nonlinear hysteretic systems is crucial for the assessment of the health and the repair of civil infrastructures during and after severe earthquakes. However, most methods used to identify hysteretic systems suffer from two problems: (1) the structural responses at all dynamic degrees of freedom(DOFs) must be measured, which is obviously impractical for real applications; and (2) the nonlinear model of a system is assumed to be known, and only the model parameters are to be identified, meaning that the nonlinear characteristics of the underlying structures may not be captured accurately. To overcome these two problems, this paper proposes a novel method that does not assume a nonlinear model and that does not require measurements at all DOFs. The new approach alternately uses the extended Kalman filter (EKF) and wavelet (W) multiresolution analysis. Within each time step, the identification can then be divided into two stages. In stage one, based on limited-state observations and the structural model at previous step, the structural responses at all DOFs are estimated using the EKF method. In stage two, based on the estimated full states, wavelet multiresolution analysis is used to identify the tangent stiffness matrix and the hysteresis-restoring force curves of the structure (i.e., the structural model is updated using the estimated full states). Two model structures with various nonlinearities at different locations, and with various state-observation schemes, are employed to conduct the numerical study. The numerical results verify the efficiency and accuracy of the proposed method. The best location for state observation is also discussed in the numerical study. DOI: 10.1061/(ASCE)EM.1943-7889.0000510. (C) 2013 American Society of Civil Engineers.
The screening effects of trenches on train-induced ground vibration are investigated by an analytical method. The embankment and soil banks beside the trench are modeled as elastic layers with appropriate widths. A fully saturated poroelastic half-space is first utilized in the investigation of the isolation of train-induced ground vibration. Biot's theory is applied to characterize the saturated poroelastic half-space, considering the coupling between the soil skeleton and the fluid. Based on Fourier transforms and Fourier series, the governing equations are solved. The various performances of the trenches on the single-phase elastic half-space and saturated poroelastic half-space as screening barriers to mitigate the ground vibration are specially investigated. It is found for the vibration generated by the axle train load with supercritical speed that the discrepancy between the screening efficiency of the trenches on the poroelastic half-space and elastic half-space is significant; the single-phase elastic half-space model may significantly underestimate the screening effect of the trench. In addition, an excellent screening effect can be achieved by an open trench to the ground vibrations generated by the dynamic wheel-rail load. DOI: 10.1061/(ASCE)EM.1943-7889.0000460. (C) 2013 American Society of Civil Engineers.
Reinforced concrete (RC) structures when subjected to sudden destruction of a column by blast pressure, experience dynamic effects in its response. Hence, reliability assessment of the damaged ductile frame against progressive collapse under dynamic loading conditions is conducted in this research. This paper aims at establishing three performance functions, two of which will consider structural collapse due to lack of strength and deformation capacity respectively while the third will incorporate the shear response of structural components for the weakest collapse mechanism. Since any of the performance functions may lead to structural collapse, a global performance function of the damaged structure is developed by considering the minimum of the functions. The results are then used in conjunction with Monte Carlo Simulation (MCS) to estimate the reliability of the damaged structure. A numerical example of a four storey RC frame is presented to address the applicability of the proposed approach and the effects of different structural parameters on its reliability against progressive collapse after the sudden column loss are investigated thereafter.
In the first part of this paper, an exact analytical solution in closed form for linear long-wave reflection by a submerged idealized breakwater or trench with various curvilinear slopes is given. The solution obtained finds almost all previous long-wave analytical solutions for wave reflection by idealized bathymetries to be its special cases, including the wave reflection by an infinite step, a continental shelf with a parabolic slope, a continental shelf with a linear slope, a rectangular obstacle, an obstacle of general trapezoidal shape with linear slopes, and a trench of general trapezoidal shape with linear slopes. In the second part, an exact analytical solution in the form of a Taylor series for linear long-wave reflection by a submerged quasi-idealized breakwater or trench is also constructed. It is shown by convergence analysis that the series solution converges in the entire physical domain. Based on the present analytical solutions, the reflection coefficients for long waves reflected by various breakwaters are calculated and the influence of the breakwater dimensions in the reflection effect is investigated. It is always found that the total reflection defined by the area under the reflection coefficient curve increaseswhen the front and back slopes become steep. It is also found that the phenomenon of zero reflection for a symmetrical rectangular breakwater still remains for a general breakwater with curvilinear slopes as long as the bathymetry is symmetrical about the breakwater. DOI: 10.1061/(ASCE)EM.1943-7889.0000483. (C) 2013 American Society of Civil Engineers.
AbstractHybrid simulation combines numerical simulation and experimental testing in a loop of action and reaction to capture the dynamic behavior of a structure. With an extended time scale, convergence of the desired displacements or forces can be assured in each actuator connected to the experimental component before advancing to the next time step. However, when the rate-dependent behavior of an experimental component is of interest, the hybrid simulation must be conducted in real time [i.e., real-time hybrid simulation (RTHS)]. In RTHS, the dynamic behavior of the loading system (i.e., actuators, controllers, and computers) is directly introduced into the RTHS loop. These dynamics consist of both time delays and frequency dependent time lags. At the same time, the phenomenon of control-structure interaction leads to a coupling of the dynamic behavior of the actuators and the structure. Traditional actuator control approaches for RTHS compensate for an apparent time delay or time lag rather than addres...
In this paper, an analytic solution to the modified mild-slope equation (MMSE) for wave reflection by a submerged rectangular breakwater with two scour trenches is explored. Because of the use of the MMSE with effects of the bottom curvature and the slope-squared terms, the solution is not only valid in the whole wave range from shallow water to deep water, but also valid for topographies not restricted to vary moderately. The present analytic solution includes an existing analytic long-wave solution as its special case, and the computing results show good agreement between two solutions, except for a slight difference when waves approach intermediate-wave range. It is found that this slight difference used to lead to an incorrect conclusion that the reflection coefficient for wave reflection by a rectangular breakwater or trench is a periodic function to the ratio of the breakwater length to the wavelength. This analysis shows that the reflection coefficient is a periodic oscillation function with a variable oscillation amplitude rather than a periodic function with a constant oscillation amplitude. It is also found that the discrepancy between the two solutions, respectively based on the MSE and the MMSE, mainly occurs for intermediate waves. Based on the present MMSE-based solution, the influence of trench dimensions on the reflection effect is investigated. It is shown that in the whole wave range, the phenomenon of zero reflection occurs more frequently for symmetrical bathymetry. DOI: 10.1061/(ASCE)EM.1943-7889.0000481. (C) 2013 American Society of Civil Engineers.
The fiber-reinforced polymer (FRP) plate or sheet debonding or cover delamination (concrete cover separation) failure mode in externally strengthened reinforced concrete beams has attracted a lot of attention. In this paper, a closed-form analytical solution is developed to determine the nonlinear shear stress distribution along the laminate interface and cover area for any load stage assuming a perfect bond. Trilinear moment-curvature and moment-extreme compression fiber strain is assumed to realize the analytical results. By differentiating the FRP axial tension force with respect to position along the beam, closed-form derivatives in terms of curvature and extreme compressive fiber strain are obtained. The results show three distinct regions of constant or stepwise linear shear distribution in each. These correspond to the uncracked, postcracked, and postyielded zones of the shear span. The results are shown to yield an exact match to those numerically obtained by dividing the shear spans into a large number of small segments and applying nonlinear sectional analysis in the middle of each segment. The analytical solution also compares well with the finite-element results using ABAQUS. The analysis of a number of strengthened beams at experimental debonding or cover delamination failure load show that the interface shear stress distribution varies from cases having no cracking at the plate tip (three regions) to those encountering two regions only (postcracked and postyielded) when the FRP plates or sheets extend close to the supports. It also shows that this distribution, at failure, consists of two regions in most of the cases and may only have a postcracked region in beams with relatively shorter plates or sheets. DOI: 10.1061/(ASCE)EM.1943-7889.0000341. (C) 2013 American Society of Civil Engineers.
AbstractA suitable and simple two-step model able to predict the nonlinear response of fiber-reinforced polymer (FRP)-strengthened three-dimensional masonry structures is presented. In the first step, nonstrengthened masonry is substituted by a macroscopically equivalent homogeneous material through a kinematic model based on finite elements and working on a heterogeneous assemblage of blocks. Nonlinearity is concentrated exclusively on joints reduced to interfaces exhibiting a frictional behavior with limited tensile and compressive strength with softening. The homogenized stress-strain behavior evaluated at the mesoscale is then implemented at a structural level in a finite-element nonlinear code, relying on an assemblage of rigid infinitely resistant six-noded wedge elements and nonlinear interfaces, exhibiting deterioration of the mechanical properties. The FRP-reinforcing strips are modeled through rigid triangles and nonlinear interfaces between adjoining triangles. Delamination from the support is ...
Rounding of the corners of a polygonal bar (or a polygonal tube conveying fluid) is unavoidable in practice. This paper studies the effect of rounding on the torsional rigidity by introducing a family of homotopy cross sections and an improved Ritz method. Rounding decreases both rigidity and cross-sectional area. For a given mass, there exists an optimal shape for maximized rigidity. DOI: 10.1061/(ASCE)EM.1943-7889.0000447. (C) 2013 American Society of Civil Engineers.
In the current study, a semianalytical closed-form solution is presented for the first time for buckling analysis of two-directional, functionally graded (FG) circular plates with variable thickness supported by both constrained edges and two-parameter elastic foundations. It is assumed that the material properties of the functionally graded material (FGM) vary in the transverse and radial directions, simultaneously. While variations of the elasticity modulus in the transverse direction is described by a power-law, variations of the material properties and the thickness in the radial direction are assumed to obey exponential laws. Mindlin's shear deformation plate theory and the differential transform technique are employed to develop the governing equations. A sensitivity analysis including evaluation of effects of various edge conditions, geometric parameters, coefficients of the elastic foundation, and material heterogeneity is performed. Results reveal that the strength degradation caused by the radial thickness reduction may be compensated by an appropriate increasing of the elasticity modulus in the radial direction. Furthermore, the elastic foundation may significantly affect the buckling load in some circumstances. DOI: 10.1061/(ASCE)EM.1943-7889.0000522. (C) 2013 American Society of Civil Engineers.
With this special issue, the ASCE Engineering Mechanics Institute wishes to recognize the contributions and accomplishments of the late Professor AhmedM. Abdel-Ghaffar to the field of structural and earthquake engineering. It is a tribute to his memory by his students, colleagues, and friends who were fortunate to cross paths with him during his outstanding career. We all share with him the passion and love for long-span bridges and dams and we feel that the best way to commemorate him is through our work. With an undergraduate degree from Cairo University and a Ph.D. from Caltech, Ahmed did seminal work on dynamics and identification of long-span bridges and dams during his academic career at the University of Illinois, Princeton University, and finally at the University of Southern California. He was a pioneer for his time: he was among the first engineers to recognize the importance of monitoring structural vibrations and to use them to learn more about the structural conditions, planting the seeds forwhat would become, in modern times, structural health monitoring. He was among the first to understand the importance of nonlinear dynamics andmechanics when dealing with long, slender structures or with complex materials such as soil. For his students, he was always an example to emulate: his enthusiasm in his work, in both research and teaching, instilled in many of us a passion for academia that has changed our lives. For his colleagues, Ahmed was an unlimited source of ideas, never afraid to try new directions and always ready for new challenges. Ahmed was a scholar in the true sense of the word.
Verification of vibration-based damage detection through a full-scale actual structural testing is an important learning opportunity. From such a test, the evolution of dynamic characteristics can be observed, damage detection methods can be validated, and baseline criteria for typical structural damage can be formulated. This paper describes a case study on a full-scale destructive testing of an overpass reinforced concrete bridge. Damage is introduced by cutting one of the bridge piers at the footing level allowing vertical settlement. This type of damage is expected to simulate the condition in which a bridge suffers from nonuniform pier settlement or hidden damage inside piles of buried foundations. By applying time and frequency domain vibration analysis, as well as a system identification technique, changes in dynamic characteristics caused by the damage are evaluated. The results clearly indicate the changes in frequencies as an indicator of damage presence, while the change in mode shapes can be used to locate the damage. The paper also discusses the application of the damage detection method based on outlier analysis of the autospectra function using the bridge ambient acceleration responses. The results indicate that the presence of damage at an early stage can be detected by observing the outliers in multivariate data, and the detection accuracy improved when damage has significantly changed the dynamic characteristics of the structure. DOI: 10.1061/(ASCE)EM.1943-7889.0000280. (C) 2013 American Society of Civil Engineers.
The long-range monitoring of civil infrastructure systems monitored with dense sensor arrays that are capable of generating voluminous amounts of data from continuous online monitoring requires the implementation of a proper data processing and archiving scheme to maximize the benefits of structural health monitoring operations. This paper focuses on the areas of data management, data quality control, and feature extraction of meaningful parameters to describe the response of large-scale infrastructure systems to ambient excitation in the context of structural health monitoring (SHM). Recordings from the monitoring system installed on the Vincent Thomas Bridge (VTB) in San Pedro, California form the database of the proposed data-management and archiving methodology. The data processing methodology for the VTB is based on the calculation of the sensor array acceleration covariance matrices for every hour of available data and the subsequent orthogonal decomposition of the covariance matrices. The dominant proper orthogonal modes of the bridge are determined, and their statistical variations over an extended observation period covering several months of continuous data are quantified and analyzed. The empirical probability density functions for the mean daily bridge accelerations are computed and used to compare the statistical variations in different periods of operation of the bridge (working days, weekends, holidays). It is shown that the computed statistical distributions of the bridge response can provide a quantitative baseline through which to facilitate the early detection of any anomalies indicative of a possible structural deterioration resulting from fatigue (service loads) or extreme loading events, i.e., earthquakes, artificial hazards, or other natural hazards.
The detection of changes in vibrational behavior has long been applied as a potential means of damage detection. However, existing damage diagnosis techniques are primarily limited to linear models in which the stiffness of some of the elements is reduced to represent damage. Although these methods appear to be adequate for locating and quantifying the present damage in the structure, they are not sufficient for determining future performance of the structure, which can only be determined using nonlinear models. This paper focuses on the challenging task of updating nonlinear models for reinforced concrete civil engineering structures. A nonlinear hysteretic material model is applied to accurately portray the fundamental hysteretic behavior of concrete structures. A systematic methodology to perform damage detection and, more importantly, update the nonlinear model for prediction is proposed. This new method is designed to use low-level ambient vibration data to detect changes in the modal parameters. With the acquired modal information, the damage parameters that control the nonlinear material model are updated. The final updated nonlinear model may be utilized not only to evaluate the structure's current damage state but also to predict its future behavior. A concrete shear wall is analyzed numerically to demonstrate the proposed method. DOI: 10.1061/(ASCE)EM.1943-7889.0000519. (C) 2013 American Society of Civil Engineers.
Laboratory tests, including splitting tension, three-point bending, and direct shear tests, were conducted on polymer cement mortar (PCM)-concrete composite specimens with various degrees of interface roughness (R-a). The PCM-concrete bond strength, fracture energy, and fracture surface were investigated qualitatively. The failure mode and the condition of the fracture surface significantly affected the interface bond strength and fracture energy. The bond strength and fracture energy were found to increase with increasing interface roughness until the fracture location shifted from the PCM-concrete interface to within either the PCM or the concrete. The interface roughness has a greater influence on the interface fracture energy and the flexural strength than on the tensile and shear strengths. Considering the single mode (Mode I or Mode II) bond and fracture properties, as well as retrofitting costs, a roughness index of approximately 1mm(0.9 <= R-a <= 1.1 mm) is proposed as the optimum value of interface roughness, and the corresponding recommended treatment is water-jetting (WJ) to a depth of 2-2.5 mm. Finally, an interfacial tension softening model that takes into account the effects of fracture energy, tensile strength, and failure mode is presented. DOI: 10.1061/(ASCE)EM.1943-7889.0000486. (C) 2013 American Society of Civil Engineers.
This paper addresses the issue of connectivity-and cost-based optimal scheduling for maintenance of bridges at the transportation network level. Previous studies in the same field have considered the connectivity just between two points or other network performance indicators, such as the total travel time. In this paper, the maximization of the total network connectivity is chosen as the objective of the optimization, together with the minimization of the total maintenance cost. From a computational point of view, several numerical tools are combined to achieve efficiency and applicability to real cases. Random field theory and numerical models for the time-dependent structural reliability are used to handle the uncertainties involved in the problem. Latin hypercube sampling is used to keep the computational effort feasible for practical applications. Genetic algorithms are used to solve the optimization problem. Numerical applications to bridge networks illustrate the characteristics of the procedure and its applicability to realistic scenarios. DOI: 10.1061/(ASCE)EM.1943-7889.0000271. (C) 2013 American Society of Civil Engineers.
While a freestanding high-strength sheet metal subject to tension will rupture at a small strain, it is anticipated that lamination with a ductile sheet metal will retard this instability to an extent that depends on the relative thickness, the relative stiffness, and the hardening exponent of the ductile sheet. This paper presents an analytical study for the deformability of such laminate within the context of necking instability. Laminates of high-strength sheet metal and ductile low-strength sheet metal are studied assuming: (1) sheets are fully bonded; and (2) metals obey the power law material model. The effect of hardening exponent, volume fraction and relative stiffness of the ductile component has been studied. In addition, stability of both uniform and nonuniform deformations has been investigated under plane strain condition. The results have shown the retardation of the high-strength layer instability by lamination with the ductile layer. This has been achieved through controlling the aforementioned key parameters of the ductile component, while the laminate exhibits marked enhancement in strength–ductility combination that is essential for metal forming applications.
Based on a generalized variational principle of magnetoelasticity and Hamilton's principle, a dynamic theoretical model is developed for magnetoelastic vibration of a soft ferromagnetic plate with nonlinear magnetization being in a stationary magnetic field. The fundamental equations of the magnetic field and the motion of the ferromagnetic plate are derived together with the expression of equivalent magnetic force acting on the ferromagnetic plate as a result of the reciprocity between the magnetizable ferromagnetic plate and the applied field. There involve twofold nonlinearities in which one is from the magnetoelastic coupling and the other arises from the nonlinear magnetization of the ferromagnetic medium. A numerical technique that combines the finite-element method for magnetic-field distribution with the finite-difference and Newmark methods for vibration of the ferromagnetic plate is proposed. Analyses of the dynamic behaviors and stability characteristics of a ferromagnetic cantilevered beam-plate vibrating in an applied stationary magnetic field are implemented numerically, which show that the frequency of magnetoelastic vibration of the ferromagnetic beam-plate increases with the intensity and the incident angle of the applied magnetic field, and that the magnetization nonlinearity obviously influences the magnetoelastic dynamics behavior. Especially for strong magnetic fields and certain geometrical parameters of the ferromagnetic plate, the magnetoelastic dynamic behavior of the ferromagnetic plate, taking into account nonlinear magnetization, exhibits distinctive magnetoelastic characteristics compared with that of the ferromagnetic plate with linear magnetization. DOI: 10.1061/(ASCE)EM.1943-7889.0000518. (C) 2013 American Society of Civil Engineers.
Gravelly soils are used extensively for a wide range of engineering applications. One example is railway ballast, and another is the fill for rock-filled dams. These soils are usually subjected to complicated loading, including high pressure, repeated loading from trains, and earthquake loading. Depending on the natural characteristics of soil particles and the level of external loading, gravelly soils may undergo particle breakage, which modifies the strength and deformation properties of the soils. To better estimate the response of earth structures with gravelly soils, it is necessary to describe particle breakage properly, its relation with external loading, and its effect on soil properties. In this study, a generalized plasticity framework, based on critical-state soil mechanics and following the associated flow rule, was developed based on the unique responses of gravelly soils. Particle breakage and its effects were described by a translating critical-state line that was related to dissipated plastic energy through a hyperbolic function. The responses of six gravelly soils along different stress paths were simulated using the proposed model. It was shown that with 12 parameters, the constitutive model was capable of describing the responses of gravelly soils over a wide range of initial void ratios and initial confining pressures, as well as along different stress paths. The model parameters, most of which have definite physical meanings, can be calibrated through conventional triaxial compression tests. This framework will provide a basis for simulating the cyclic responses of gravelly soils. DOI: 10.1061/(ASCE)EM.1943-7889.0000513. (C) 2013 American Society of Civil Engineers.
