Journal of Engineering Mechanics

An innovative articulated steel frame system consisting of continuous beams, columns, and end plates with gap bolts is proposed in this study. The gap bolts are untightened with initial gaps and used to connect beam flanges with column end plates, enabling the frame to rock freely at the initial state and avoid overturning of the whole frame when the initial gaps of bolts are exceeded. The proposed system can switch between a free-rocking column system and a rigid-jointed frame when subjected to strong excitations. Recentering of the structure is provided only by gravity loads without the addition of post-tensioning elements. This system with additional damping elements is expected to be a highly efficient seismic control system in which a high-mode effect can be mitigated through multiple-rocking interfaces along the structural height direction. As a fundamental study, this paper focuses on a single-story articulated steel frame without additional damping devices. An analytical model is established, and small shaking table tests are conducted. The effects of structural parameters and excitations on the seismic performance of the proposed system are investigated. Responses of the corresponding rigid steel frame are compared with those of the proposed system in both time and frequency domains. A method based on the seismic energy spectrum is proposed to estimate free-rocking uplifts within the initial gaps. A finite element model is also established to simulate dynamic responses. The analytical and numerical results are compared with the experimental results with acceptable accuracy. This study is of significant importance for similar rocking column systems and further study on corresponding controlled multiple-rocking systems.

Stability of real-time hybrid simulation (RTHS) has attracted considerable attention given that actuator delay might destabilize the real-time test, especially when the compensation is not sufficient. Previous research by the authors explored the stability of RTHS with time-varying delay, but the derived stability criteria are relatively conservative due to the application of the Lyapunov-Krasovskii (L-K) theory. For overcoming such defect and pursuing a more accurate stability analysis, this study introduces a delay decomposition approach to reduce the conservatism of matrix inequality with convexity property. For both constant- and time-varying delay systems, the delay decomposition approach performed remarkably in stability analysis. Moreover, with the increase in number of decomposition, this approach can further improve the accuracy of analysis results and reduce its conservatism; however, the computational efforts will rise rapidly. Computational simulation verified the effectiveness of the delay decomposition approach especially for the physical substructure involving a small stiffness ratio.

After sketching the history of the size effect models for the shear strength of reinforced concrete (RC) in design codes, the energy-based size effect law (SEL), recently incorporated into the American Concrete Institute (ACI) design code articles for beam shear and slab punching, is briefly discussed. A general derivation of the SEL based only on the first principles involving energy conservation and dimensional analysis (or laws of similitude) is presented. Attention is then focused on recent articles that present a severe critique of the SEL and various arguments in support of the Muttoni et al.’s critical shear crack theory (CSCT)—an update of the Collins et al.’s modified compression field theory (MCFT)—that some researchers propose to be introduced into the fib Model Code and the Eurocode as an alternative to the SEL. In a point-by-point analysis, it is shown that this critique and these arguments are incorrect and baseless. It is hoped that the present clarification would lead to progress in design codes, enhancing the safety and efficiency of RC structures.

Multiple crack propagation and coalescence in two-dimensional linear elastic media were modeled using the finite-element method (FEM) in conjunction with the subregion generalized variational principle. The proposed approach computes the stress intensity factor (SIF) accurately, as proved in a previous work. The approach results in regular geometric meshes. A multiple crack propagation scheme was developed in this study. The scheme follows the crack propagation by moving just the complementary energy subregion on the basis of the regular mesh. Consequently, it requires only minimal local remeshing without affecting most of the existing mesh. The scheme can handle crack coalescence easily. These advantages simplify the modeling and thus improve the efficiency. Four numerical examples, including a structure containing 10 cracks, were investigated to demonstrate the accuracy and efficiency of the proposed approach.

Fluid-structure interaction (FSI) of earthquake shaking is a design consideration in the civil, nuclear, chemical, and mechanical industries. Preliminary sizing of structures and components in these industries will often use analytical solutions because there is insufficient information to warrant the analysis of numerical models. This study extends prior analytical work on seismic FSI of base-supported tanks in the 1980s to accommodate head-supported tanks with geometries similar to those proposed for new-build liquid (fluid)-filled advanced reactors. Analytical solutions are derived for flexible head-supported cylindrical tanks subjected to small-amplitude unidirectional horizontal seismic motion and address frequencies of lateral motions of the tank, hydrodynamic pressures, frequencies of waves, wave heights, and reactions at the head support. Solutions are presented for a range of tank dimensions and could be used for (1) preliminary design of head-supported cylindrical tanks and their supporting structures with idealized seismic input, and (2) verification of numerical models to be used for final seismic analysis, design, and probabilistic risk assessment.

There has been ever-increasing interest over the past decade in improving understanding of the mechanisms responsible for the progressive collapse of structures. Existing design recommendations and analyses are largely limited to instantaneous (time-independent) collapse. However, recent experiments revealed that reinforced concrete (RC) structures may be susceptible to delayed collapse, prompting the consideration of time-dependent material behavior as part of progressive collapse analysis and design. A reduced-order computational model for delayed collapse behavior of RC structures is introduced here in which the potential damage zones that evolve within a structure are treated as cohesive elements. The constitutive model of the cohesive element accounts for viscoelastic deformation and time-dependent damage accumulation of the concrete and hardening plasticity of the steel reinforcement. The remaining part of the structure is treated as a viscoelastic continuum. The model is first applied to simulate a pushdown experiment on a RC frame subassemblage under displacement-controlled loading. The deformation and failure mechanisms are in good agreement with experimental observations. The model is then used to investigate the behavior of the subassemblage in a “static fatigue scenario” in which the load is monotonically increased to a prescribed level and is then held constant until ultimate structural failure resulting from the assumed time-dependent response of the concrete material. The corresponding timescale of the delayed failure is on the order of hours, a result that has important implications not only for the analysis and design of RC structures against progressive collapse but also for the safety of first responders who enter structures that may collapse within that period of time.

Apparent weakening is the softening of a structure’s apparent stiffness by adding negative stiffness to the overall system, which has the benefit of reducing the peak accelerations and base shears induced in the structure due to a seismic event. Apparent weakening is an elastic effect that does not reduce the main structural strength. To attain this behavior, negative stiffness can be introduced through the integration of negative stiffness devices. This study presents a novel smooth negative stiffness device (SNSD) consisting of cables, pulleys, and tension springs. A nonlinear mathematical model of the force-deflection behavior of the SNSD was developed and used to determine the optimal geometry for such a device. A prototype device was designed and fabricated for installation in a bench-scale experimental structure, which was characterized through static and dynamic tests. This paper also presents a numerical study on two other device configurations designed to achieve different force-deflection relations for use in an inelastic model building subject to a suite of historic and synthetic ground motions. In both the experimental prototype and the numerical study, the SNSDs successfully produced apparent weakening, effectively reducing accelerations and base shears of the structures.

A multifield model of the early-age behavior of massive concrete structures is presented. The hydration process and the thermal evolution are both described through a coupled chemo-thermal model that predicts the degree of hydration and temperature fields that are consistent with experimental measurements. The mechanical model is of the damage–plasticity type and relies on the assumption of additive strains. The classical evolution functions for damage and instantaneous plastic strain are introduced in effective stress space. The extended microprestress-solidification (MPS) theory is implemented to account for the effects of stress, temperature, and degree of hydration on the creep strain. To account for nonlinear creep at relatively high stress, a damage-dependent nonlinear creep function is introduced to couple damage and creep. The autogenous shrinkage and thermal strains are characterized by linear functions of degree of hydration and temperature, respectively. Moreover, the early-age evolutions of strengths and peak strain are also considered to be functions of hydration degree. The model is calibrated and validated through numerical simulations of simple creep tests and a three-dimensional finite element analysis of a massive concrete wall. The results suggest that the proposed model offers promise for the analysis of early-age cracking within massive concrete structures.

This study investigates the dynamic and nonlinear behavior of an eddy current (EC) damping mechanism by using shaking table tests and finite-element modeling. A noncontact and friction-free EC damper is firstly designed and tested, illustrating the existing frequency-dependent characteristic. Following a primary convergence study on the mesh resolution, the three-dimensional (3D) transient analysis with the finite-element model is presented to reproduce the experimental observations, and the favorable agreement between simulation and experiment is utilized to validate the accuracy and reliability of modeling. The observations on instantaneous distribution and the phase lag of the EC are discovered and discussed. Then, a detailed parametric analysis on EC damping is further performed by evaluating the equivalent damping and stiffness coefficients generated from linear regression. The sensitivity of the two evaluation parameters on the excitation frequency and amplitude is presented with valuable insights obtained. Following that, the regressed functions of the excitation frequency for calculating the equivalent stiffness and damping coefficient are presented, and an illustrative case study is adopted to discuss the potential influence of the frequency dependence of the EC damping. Finally, the EC damper with real-size dimensions is modeled and investigated in ANSYS.

In this paper, it is aimed to perform clustering analysis on a large set of near-fault ground motions based on the intensity of velocity pulses. For this purpose, the velocity pulses of these ground motions are extracted based on the asymmetric Gaussian chirplet model (AGCM), the adapted dictionary-free orthogonal matching pursuit (ADOMP) algorithm, and the Newton method. The location of first AGCM atom is considered as the location of real velocity pulse, and its combination with adjacent atoms reconstructs the real velocity pulse of near-fault ground motions. In addition, hierarchical clustering and $k$-means clustering are used to cluster near-fault ground motions into two, three, and five groups based on the features of their velocity pulses. The results showed that the clustering results for two and three groups are more suitable than the results of five groups according to the silhouette and Davies-Bouldin measures. Also, the clustering results of five groups are shown to be appropriate for the design of structures with different importance factors. According to the results, agglomerative clustering with complete linkage provides more appropriate clustering results for moderate and strong pulselike ground motions. The $k$-means and agglomerative clustering with complete linkage provide more uniform clustering results for three and five groups, respectively. A sensitivity analysis showed that the $k$-means provides more stable clustering results when the records for the Chi-Chi earthquake are removed from the data. Also, the clustering results were used to extract the possible relation of the intensity of velocity pulses with the seismological parameters, which were in accordance with the results of previous research.

Pipes conveying fluids are primary components in almost all industries. Internal surface attack is inevitable in pipes and can be crucial to their structural integrity and dynamic response characteristics. In this study, the dynamic response analysis of composite pipes with internal surface discontinuity is investigated in the presence of fluid flow. The equations of motion are obtained using the extended Hamilton’s principle and discretized using the wavelet-based finite-element method (WBFEM). The internal surface defect is allowed to occupy any length along the pipe span, while its cross section can vary both in radial and angular directions. The dynamic response of the defected pipe is obtained by integrating the equations of motion forward in time using MATLAB solver ODE45. The developed dynamic model has been validated using ANSYS and some benchmark results are presented to underline the influence of the internal surface defect on dynamic response of composite pipes conveying fluid.

Submarine buried pipelines interact with shallow soil layers that are often loose and prone to fluidization/liquefaction. Such occurrence is a possible consequence of pore pressure build-up induced by hydrodynamic loading, earthquakes, and/or structural vibrations. When liquefaction is triggered in sand, the soil tends to behave as a viscous solid–fluid mixture of negligible shear strength, possibly unable to constrain pipeline movements. Therefore, pipelines may experience excessive displacement, for instance, in the form of vertical flotation or sinking. To date, there are no well-established methods to predict pipe displacement in the event of liquefaction. To fill such a gap, this work proposes a computational fluid dynamics (CFD) framework enriched with soil mechanics principles. It is shown that the interaction between pipe and liquefied sand can be successfully analyzed via one-phase Bingham fluid modeling of the soil. Postliquefaction enhancement of rheological properties, viscosity, and yield stress can also be accounted for by linking soil–pipe CFD simulations to a separate analysis of the pore pressure dissipation. The proposed approach is thoroughly validated against the results of small-scale pipe flotation and pipe dragging tests from the literature.

A minimum mesh reinforcement, called the shrinkage and temperature reinforcement, has been required by ACI design code for $92\mathrm{years}$ to attain ductility with no formation of large localized cracks. The required steel ratio, which is 0.18%, has been empirical. In this paper, it is shown that it can be explained theoretically and justified approximately by finite-element analysis of the size effect and crack growth based on quasibrittle fracture mechanics, in which the microplane model M7 and crack band model are used. The premise, which simplifies the analysis, is that the cracking would localize into wider cracks if and only if there is a size effect. The size effect can be completely avoided only if, for the same cross-section area, the tensile strength of yielding reinforcement is greater that the tensile strength of concrete. The effect of increasing the reinforcement ratio is also explored. The calculations indicate that fracture mechanics can, and should, be used to check ductility and size effect implications in the two-sided reinforced members, boundary beams, and more complicated designs such as in shear walls.

The dynamical behavior of a system containing a surface foundation atop a fluid-conveying pipe that is embedded in soil beneath the foundation is investigated. A Winkler–Voigt model is presented to simulate the interaction between the pipe and the foundation. The extended Hamilton’s principle is utilized to formulate the governing equation of the system. A generalized Fourier series is used to discretize the infinite-dimension model to an $n$-degree-of-freedom system. Maximum displacement of the surface foundation and pipe due to flow-induced vibrations is obtained via a modal analysis method. In addition, a numerical solution is used to verify the results. Good agreement was observed between the numerical and semianalytical results, even though the system is not classically damped. Variations of key factors such as flow velocity and geometrical properties that could affect the dynamical behavior of the system are considered. Although a relatively sharp rise of maximum amplitude was seen when changing the values of key factors, a unique change pattern was not seen in the maximum amplitude when varying different parameters.