Journal of Engineering Mechanics

Single-bolted angle joints are widely used on secondary bracings of lattice steel towers due to their low cost and ease of construction. It has been observed that the hysteretic behavior of these joints has a significant impact on the dynamic performance of the tower structure and the nonstructural components supported by the tower. However, the various phases of this hysteretic cycle have never been investigated in detail. Therefore, there are no accurate numerical models for these types of joints in the popular software packages for structural analysis. This in turn hinders advanced studies on the performance of these critical infrastructure components (utility towers) under strong wind or seismic loads. This paper first explains the mechanics and the various stages of the hysteretic behavior of the joint, including friction, slippage, bolt bearing, and plasticity. Finite-element models are built and validated for the analytical modeling of the joint under monotonic loading. The model for hysteretic behavior is then presented, considering cyclic joint slippage and bolt-hole elongation (damage accumulation). A novel algorithm is developed to efficiently incorporate the analytical model into a computer program. In particular, the analytical model is the basis for a new zero-length finite element that can be used in the OpenSees framework. The methodology is applied to single-bolted angle joints with 10 typical configurations. The proposed analytical zero-length element is shown to agree well with brick element simulation results under both monotonic and cyclic loading. Therefore, the hysteretic behavior of the single-bolted angle connections can be incorporated into the dynamic analysis of lattice steel towers by inputting easily obtained physical properties, for example, plate thickness and width, bolt and bolt hole diameter, and material strength. This proposed element will enable engineers and researchers to efficiently study the cyclic performance of lattice tower structures capturing well the joint behavior.

The frequency domain approach (FDA) based on the power spectral description of input and output has been widely applied in the random vibration analysis of structures because of its high efficiency and distinct relationship between the input and output. However, for large structures with closely spaced natural frequencies and nonproportional damping, the efficiency of this method for nonstationary response analysis is impacted by the high demand posed by the time-history analysis. In this study, a fast convolution integration method based on Duhamel integration is introduced to enhance the efficiency of the FDA for the nonstationary response analysis of nonproportionally damped structures. First, an efficient impulse excitation approach is proposed to identify the discrete-time impulse response of linear structures. Accordingly, the nonstationary response statistics can be directly evaluated by the convolution with respect to the discrete-time impulse response. Subsequent application of the fast Fourier transform (FFT)–based algorithm accelerates the evaluation of the convolution. Therefore, the proposed scheme is efficient as it eliminates a direct time-history analysis and benefits from the implementation of the FFT. The proposed method is also illustrated to enhance the analysis of nonlinear structures under nonstationary random excitations by invoking an equivalent statistical linearization scheme. Finally, numerical examples are presented to demonstrate the accuracy and efficacy of the proposed method.

This paper addresses the identification problem of multiple open cracks in beams based on damage-induced variations in the static deflection of the beam. A two-step non-model-based damage localization and quantification methodology is proposed. First, the damage locations can be identified from the slope discontinuities in the deflection variation using a specific linear-trend filtering function. Then, the crack depths can be assessed from characteristic expressions of rotational spring models. An experimental case study of a simply supported beam with multiple cracks is used to exam the performance of the method. The deflections of the beam were measured with a digital photogrammetric system using partial measurements. The difference between the idealized lumped spring model for the crack and the actual effect of a real crack in the damage identification process and the performance of the method for statically indeterminate beams are illustrated through numerical examples. The paper shows that the proposed method can accurately locate and quantify multiple closely spaced cracks in beams in practical applications.

This paper presents the results of a comprehensive micromechanical study to improve the understanding of the coupled hydro-thermo-mechanical (HTM) processes during injection of pressurized cold fluid in permeable hot rock. At the microscopic level, it is expected that fluid flow–induced convective temperature changes in the voids will dominate the conductive heat flow from grain to grain. However, the coupled interactions between fluid flow, thermal convection, and conduction, and the resulting changes in local permeability due to thermo-poroelastic stresses at the grain scale remain poorly understood. Moreover, almost all previous studies at a large scale have focused only on thermal conduction. This study used the discrete-element method (DEM) modified to account for convective heat transport to model the particulate interactions between rock grains and between rock deformation, fluid pressure, and temperature. The results indicate that wellbore pressure, provided it is less than the fracturing threshold, has a less significant role in rock cooling around the wellbore, which is governed predominately by rock permeability, as previously thought. Different applied flow rates into the wellbore resulted in low and high wellbore pressures at short times in the absence of rock fracturing. As expected, convective heat transport dominated over conduction, resulting in a cooled ring around the wellbore without a significant thermal gradient zone due to conductive heat flow. As the fluid infiltrates and cools the rock, preferential fluid flow paths occur as fingering instabilities that are oriented toward the minimum principal horizontal stress even in the absence of initial local anisotropic porosity variations.

Offshore oil spills can greatly harm marine ecological environments. The containment boom is a simple and effective tool that can effectively prevent the spread of oil spills, reduce the oil-spreading region, and work with other recovery measures. In the study described, a two-dimensional multiphase model was developed to reproduce the oil containment process and to comprehensively investigate the water–oil boom interaction. Good agreement was achieved between the modeled and measured results in an application of the oil containment process with respect to variation in oil thickness. The verified model was then employed to investigate oil containment subject to waves and currents. In addition, the containment performance of floating booms with different skirt lengths, buoyancy/weight ratios, and boom shapes was investigated. The numerical results showed that boom motion can significantly change the flow field around the boom and thereby the boom’s containment effectiveness. Booms with arc-shaped or polyline-shaped skirts consistently exhibit satisfactory oil-controlling performance.

The present work aims to understand the stress–strain response of an igneous rock, granite, under high loading rate through detailed experimental and numerical analyses. The high-strain-rate characterization of granite rock was performed for two different diameters and five different slenderness ratios of the rock specimens using a 76-mm-diameter split Hopkinson pressure bar (SHPB) device in an effort to determine the appropriate specimen dimension for granite to be used in an SHPB test. The stress–strain response of the rock specimens was studied by systematically varying the length of the striker bars and the gas gun pressure values of the SHPB device. The petrological and static characterization of the granite rock was also carried out to assess the response of the rock specimens. Finally, a methodology was proposed to characterize granite rock specimens under high loading rate. Further, a numerical simulation of the SHPB test on granite rock was performed using the strain rate–dependent Johnson-Holmquist 2 (JH-2) model available in the finite-element software LS-DYNA. The simulation results were compared with the experimental data; thus, the parameters of the JH-2 model for granite rock were determined. The determined parameters were then used in blast analysis of a tunnel subjected to 20-kg TNT explosive.

The multi-time-scale responses of a two-degree-of-freedom pendulum coupled with a nonlinear absorber were studied. The absorber was positioned in an arbitrary direction with respect to those of the pendulum oscillations. The phase-dependent slow invariant manifold of the system and its stable zones were traced at a fast time scale while system responses were studied at a slow time scale around the slow, invariant manifold, leading to detection of equilibrium and singular points. Moreover, the amplitude–frequency curves of the system were detected, showing the possibility of the existence of isolated branches, which could correspond to high energy levels for pendulum oscillations. All analytic developments were confronted with numerical results collected from direct numerical integration of system equations. Depending on the characteristics of external excitations, the system can face periodic or modulated regimes.

The pore water dissipation of sand–clay mixtures is significantly affected by the sand fraction due to nonuniform stress distribution. On the basis of the elastic visco-plastic modeling concepts of Yin and Graham, a new elastic visco-plastic (EVP) model based on Lagrangian formulation was proposed to consider the effects of sand fraction in a sand–clay mixture on the time-dependent stress–strain behavior at finite strain. In hydraulic dredging and marine deposit improvement projects, the initial water content of mixtures is relatively high, leading to a high compressibility. Therefore, the soil skeleton of the mixtures was fixed to Lagrangian coordinates to facilitate the definition of soil boundary. The governing equation was formulated by combining an equivalent time concept with the mixture theory. A finite difference method was adopted for the benchmark analysis of boundary–initial value problems. The proposed model contained eight parameters. Seven of them pertained to the clay matrix that can be calibrated from the reference time line, instant time line, and consolidation curves of the pure clay in the mixture. The structure parameter represented the intergranular structure and can be calibrated based on the compressibility of a sand–clay mixture. Two multistage oedometer tests (including unloading stages) can be performed to calibrate the model parameters, one on the pure clay and the other on the sand–clay mixture with a predefined sand fraction. A benchmark analysis of the proposed model revealed a significant difference in excess pore pressure dissipation between Eulerian and Lagrangian coordinates. The calibrated model based on Lagrangian coordinates was found to reproduce the effect of sand fraction on the overall responses of sand–clay mixture well when compared with the experimental data of sand–bentonite mixtures and sand–marine clay mixtures from the literature.