Journal of Engineering Mechanics

Actuator tracking and compensation are important in the general field of experimental structural dynamics to effectively conduct vibration testing. Real-time hybrid substructuring (RTHS) is a method of vibration testing utilized to effectively characterize the system-level performance by physically testing a component of interest while numerically simulating the remaining support structure in real-time. The physical and numerical coupling is referred to as a transfer system, and actuators typically act as this system in RTHS. The inherent dynamics of actuator systems is a main cause of RTHS instability and inaccuracy. This work presents the methodology to achieve control of a six-degrees-of-freedom shake table. The corresponding system identification and model-based linear time-varying (LTV) compensation are robust enough to facilitate stable and accurate RTHS testing of mechanical systems at small- and large-amplitude excitations. When compared with a minimum-phase inverse compensation technique, the LTV technique was superior in linearizing actuator dynamics at varying excitation amplitudes. The LTV technique was also able to accurately command the three-dimensional (3D) displacements of the 2020 magnitude 6.4 Puerto Rican earthquake.

Ultrahigh performance concrete (UHPC) is a special class of cementitious composites with superior strength and durability characteristics. Typically, UHPC contains very fine aggregate (size less than 1 mm) and short fiber reinforcement (less than 15 mm long). This sets it apart from the family of regular fiber-reinforced concrete (FRC) that has aggregate and fiber of similar lengths since the aggregate is typically coarse (10–20 mm) and that makes fiber effects on heterogeneity and crack bridging in UHPC more dominant compared to FRC. Additionally, since fiber content is relatively high (around 2% by volume), it is expected that a significant interplay exists between fiber content and traditional continuous reinforcement (rebars). Such interplay could affect the modes of failure, load-carrying capacity, and ductility of reinforced-UHPC (R-UHPC) members. Given such differences, prediction of failure mechanisms for R-UHPC members via extrapolation of design guides available for normal concrete (NC) and FRC could be debatable. Therefore, this two-part study discusses a coupled experimental and computational investigation analyzing the effect of fiber-rebar interplay on the modes of failure, strength, and ductility of R-UHPC prismatic members. The experiments included an initial campaign of testing 12 R-UHPC beams with different fiber contents and reinforcement ratios. Beams were tested with an eccentric three-point bending setup along with material level companion tests on specimens cast from same batches. Interestingly, all beams with fiber failed in bending without noticeable shear failures. The data from this initial campaign were used to calibrate and validate a computational modeling framework. This validated model was further used to predict the needed dimensions to achieve shear failure. Given these dimensions, a second experimental campaign was performed, and all tested beams failed in shear as predicted. These experimental programs are reported in the present Part I. The computational counterpart is described in the companion paper as Part II.

The first experimental campaign presented in the preceding Part I of this study is used to calibrate and validate a comprehensive computational framework called the lattice discrete particle model for fiber-reinforced concrete (LDPM-F). The model is then used to design the second experimental campaign that was also presented in the preceding Part I so that all beams fail in shear. Finally, the model is used to investigate and explain the observed failure modes, validate the fiber/reinforcement interplay effects postulated in Part I, and to analyze comprehensively the load-transfer mechanisms in the reinforced ultra-high performance concrete (R-UHPC) beams in both shear and flexural failure. This two-part study proves the effectiveness of coupling experimental analysis with comprehensive computational modeling to understand the behavior of structural members made from complex materials. Using this coupled understanding, detailed explanations of load-transfer mechanisms in shallow and deep beam shear failure as well as flexural failure are discussed and compared to simplified sectional analysis models showing the places of needed improvement in such models. These detailed discussions show the ability of the presented coupled approach to accurately predict these failure mechanisms and their dependence on fiber/reinforcement contents and their interplay. The presented accurate probing of different load-transfer mechanisms within the structural elements and how they vary during failure progression paves the road towards developing rigorous design formulations based on fundamental understanding of the complex mechanical behavior of these structural members.

The earthquake-induced pressures on arrays of cylinders have received less concern, although pile groups have been widely adopted in real practice. This paper develops an analytical method to investigate the interaction of water with arrays of rigid circular cylinders subjected to horizontal ground motion. According to seabed and surface boundary conditions and using the variable separation method, the three-dimensional governing equation of water is first transformed to a two-dimensional Helmholtz equation in the $xy$-plane and analytical vertical modes in $z$-direction. Then, the analytical solution of the hydrodynamic pressures on arrays of cylinders is deduced. This analytical solution includes the radiated wave pressures generated by each cylinder due to the ground motion, first-order scattered wave pressures due to the radiated waves, and high-order scattered wave pressure due to diffracted waves. The finite-element method is employed to validate the present analytical solution. The results show that the analytical solution agrees well with the numerical solution. The proposed method is finally used to investigate the earthquake-induced hydrodynamic forces on linear arrays of circular cylinders.

To improve the effectiveness and the robustness of a tuned liquid column damper (TLCD) in mitigating structural vibrations, an upgraded type of tuned liquid column damper (UTLCD) is proposed in this paper. A UTLCD is composed of a single TLCD fixed on an undamped tuned mass damper (TMD). The effect of parameters of UTLCD on its performance was investigated through a numerical example. It was found that the mass ratio between TMD and TLCD in a UTLCD has a significant influence on the effectiveness of UTLCD. In addition, the performance and the robustness of a UTLCD with optimal parameters were compared with those of the optimum TLCD, which has the same weight as the UTLCD. This study confirms that an optimized UTLCD is more effective than a TLCD. Meanwhile, the robustness of the most effective UTLCD is also better, but not by much, than that of an optimized TLCD. However, a UTLCD can be much more robust than an optimal single TLCD if they are considered on the same level of efficiency.

This paper presents numerical modeling of nanoparticles injected into cementitious material using the electromigration method. In order to remediate leaking wells in ${\mathrm{CO}}_2$ storage formations, beneficial nanoparticles can be injected into the cement. This process can not only repair the leakage and cracks, but also remove harmful ions from the cementitious material. A general framework for numerical modeling is developed to describe the proposed nanoparticle injection technology. This is done with a two-step approach: A model for injecting nanoparticles is developed first, in which an externally applied current on the system is characterized, then simultaneous chloride ion removal is used to calibrate the developed model. The ionic concentration problem in this paper is solved using the Nernst-Planck equation coupled with Poisson’s equation. The general framework uses the finite element method. The results are further compared with an experimental study conducted to prove the effectiveness of this model.

This paper extends the idea of using nonuniform rational B-splines (NURBS) based shape functions in the scaled boundary finite-element method (SBFEM) to plate bending formulations, which inherits the advantages of the isogeometric analysis (IGA) as well as the scaled boundary finite-element method. The NURBS are introduced to reconstruct an exact boundary for analysis domain with arbitrary complicated geometry, $h$-, $p$-, and $k$- refinement strategies, which can maintain the same exact geometry as the computer-aided design (CAD) model at all levels. The NURBS basis functions are also used for the approximation of physical quantities inspired by the sense of isoparametric concept, and the high-order continuity of the NURBS basis functions contributes to the better accuracy, convergence, and efficiency of the present isogeometric scaled boundary finite-element method (IGSBFEM). The proposed technique is derived based on the exact three-dimensional elastic theory, which contributes to its high-accuracy property, whereas only discretization of the midplane is required for the present model due to the characteristics of dimensionality reduction and analytical property along the radial direction from the conventional SBFEM, and the solutions along the thickness direction are described as analytical expressions. Five numerical examples involving complicated geometries and multiconnected domains are carried out to examine the applicability of the present approach. Available solutions computed by several other methods (such as the analytic method, FEM, conventional SBFEM, and IGA) are used for comparison. Higher accuracy and efficiency compared with the traditional approaches are achieved.

In this investigation, an approximate analytical solution to the axisymmetric consolidation problem of unsaturated soils improved with vertical drain is presented under the equal-strain condition. The smear effect of soil, the well resistance of vertical drain, and the application of time-dependent loading are considered in the solution. Matrix analysis is used to solve the solution under constant loading for comparison against the available solution. The influence of soil parameters, smear parameters, and well resistance parameters on the consolidation behavior is investigated under ramp loading. Moreover, two types of loading which are noncyclic forms (i.e., instantaneous, multistage ramp, and exponential loading) and cyclic forms (i.e., triangular, sinusoidal, and cosine cyclic loading) are incorporated in the analyses. It is found that a higher hydraulic conductivity ratio in the undisturbed zone and a larger radius of the influence zone can speed up the consolidation process. The consolidation process is slowed down by increasing the radius of the disturbed zone and reducing its hydraulic conductivity, and the influence of hydraulic conductivity is more significant. The effect of radius and hydraulic conductivity of vertical drain on the consolidation behavior is similar. The consolidation behavior generally follows the pattern of the applied loading apparently.

The transient wind field during a nonsynoptic wind event (e.g., thunderstorm downburst) presents time-varying mean and nonstationary fluctuating components, and hence is not easy to be reproduced in a conventional boundary-layer wind tunnel with various passive devices (e.g., spires, roughness elements, and barriers). As a promising alternative, the actively controlled multiple-fan wind tunnel has emerged to effectively generate the laboratory-scale, spatiotemporally varying wind flows. The tracking accuracy of target wind speed histories at selected locations in the multiple-fan wind tunnel depends on the control signals input to individual fans. Conventional hand-design linear control schemes cannot ensure good performance due to the complicated fluid dynamics and nonlinear interactions inside the wind tunnel. In addition, the determination of the control parameters involves a time-consuming manual tuning process. In this paper, an accurate and efficient control scheme based on deep reinforcement learning (RL) is developed to realize the prescribed spatiotemporally varying wind field in a multiple-fan wind tunnel. Specifically, the fully connected deep neural network (DNN) is trained using RL methodology to perform active flow control in the multiple-fan wind tunnel. Accordingly, the optimal parameters (network weights) of the DNN-based nonlinear controller are obtained based on an automated trial-and-error process. The controller complexity needed for active simulation of transient winds can be well captured by a DNN due to its powerful function approximation ability, and the “model-free” and “automation” features of RL paradigm eliminate the need of expensive modeling of fluid dynamics and costly hand tuning of control parameters. Numerical results of the transient winds during a moving downburst event (including nose-shape vertical profiles, time-varying mean wind speeds, and nonstationary fluctuations) present good performance of the proposed deep RL-based control strategy in a simulation environment of the multiple-fan wind tunnel at the University at Buffalo.

The artificial ground freezing (AGF) method is an environmentally friendly ground improvement technique for numerous geotechnical applications. It can be used in fine-grained soils, which may not be efficiently improved via conventional cement-based ground improvement techniques. However, some of the issues hindering the application of the AGF method to fine-grained soils include inefficiency in achieving the target volume of frozen soil and degradation in mechanical properties of the soil after the freezing-thawing process. In this paper, the freezing rate and degradation in strength and stiffness of a clayey silt in South Korea were investigated using field experiments. At two different outlet temperatures ($-180°\mathrm{C}$ and $-120°\mathrm{C}$), liquid nitrogen was injected into the freezing pipe to evaluate the freezing rate. A simple equation to estimate the theoretical radial freezing rate was proposed and compared with the experimental results. In addition, a piezocone penetration test (CPTu) and pressuremeter test (PMT) were performed to assess the degradation in strength and stiffness of the soil after the freezing-thawing process. Results of the CPTu, PMT, and laboratory experiments revealed that the degradation in mechanical properties of Korean clayey silt could be attributed to the rearrangement of soil particles.

River-crossing bridges are often subject to multiple hazards, including foundation scour, seismic attacks, and environmental degradation. When river-crossing bridges are in service, they should be restored rapidly after any disruption over their lifetime. To achieve this resilient quality, it is necessary to assess the system resilience subjected to multihazard impacts and the beneficial effects of any retrofitting or hazard-countermeasure in a lifetime context. To river-crossing bridges, one important intervention is to implement scour countermeasures. This study presents a probabilistic framework to quantify the lifetime system resilience of river-crossing bridges subject to multiple hazards. Notably, the framework is designed to reveal how progressive and abrupt hazards interact and result in resilience degradation and how scour countermeasures contribute to resilience enhancement. Experimental outcomes reveal the positive and distinct effects of implementing scour countermeasures at different times. The proposed framework is expected to assist civil engineers in conducting lifecycle management of river-crossing bridges that are subject to hydraulic scour and demand timely countermeasures.

The present paper offers a micromechanical modeling approach to evaluate the impact of the porosity structure on the response of asphalt concrete as a viscoelastic multiphase material under the combined action of moisture diffusion and mechanical loading. Moisture infiltrates and degrades asphalt concrete material properties and encourages the fast propagation of cracks upon additional mechanical loading. The critical impacts of the porosity structure on the coupled moisture-mechanical responses of asphalt concrete have been highlighted by both experimental and numerical studies. It is therefore imperative to develop a clear understanding of the various impacts that the porosity has on the overall performance of this viscoelastic composite. In this work, a micromechanical modeling approach is used in combination with a methodology to randomly generate realistic pore properties within an asphalt concrete specimen. The relevant effects of the porosity structure are assessed for two sets of moisture diffusion coefficients representing low-end and high-end diffusivities for the aggregates and matrix. Finite-element models are developed using coupled nonlinear viscoelastic–moisture damage–mechanical damage constitutive relationships. The effects of varying the pore content, shape, and size distribution on the stress distribution and damage evolution within asphalt concrete specimens are assessed. A numerical approach is also presented to estimate the effective moisture diffusivity of asphalt concrete as a function of the pore shape and content. The outcome sheds light on our understanding of how the porosity structure of asphalt concrete affects its moisture-mechanical responses.

A smooth hysteretic formulation for modeling of damage is presented, emanating from damage mechanics and classical theory of plasticity. This generalized smooth, rate-independent three-dimensional (3D) Bouc–Wen type model is intended for nonlinear static and dynamic analysis of structures and is expressed in tensorial form. The hysteretic model embodies the modified von Mises yield criterion that takes into account damage evolution and nonlinear hardening laws. Based on these considerations, elastic finite elements are extended considering the plastic strains as additional hysteretic degrees of freedom together with the variables for damage and isotropic hardening as well as the back-stresses. These variables are considered at the integration points of the finite elements, and their evolution is described by the Bouc–Wen-type equations. The developed hysteretic finite element allows for geometric nonlinear analysis, assuming small strains. Numerical examples are presented, attesting to the generality of the proposed approach as well as the efficacy and accuracy.