Journal of Engineering Mechanics

An analytical model is developed to describe the out-of-plane response of one-way spanning unreinforced masonry (URM) walls by investigating the effects of various parameters. Horizontal crack height, masonry compressive strength, and diaphragm support stiffness properties are assumed as variables, and sensitivity analyses are performed to study the influence of these parameters on the cracked wall characteristic behavior. The parametric studies show that crack height significantly influences wall stability by affecting both the instability displacement and the wall lateral resistance. The reduction in cracked wall lateral resistance and in the instability displacement caused by finite masonry compressive strength is shown to be significantly amplified by the applied overburden. A study using the typical configuration of flexible diaphragms and URM walls indicates that the wall top support flexibility does not significantly influence cracked wall out-of-plane response. An existing simplified wall behavioral model is improved, and a procedure is proposed for calculation of the wall out-of-plane response envelope.

Classical shear beam theory does not consider the rotation of cross sections. This paper studies the free vibration of axially loaded shear beams carrying lumped masses at elastically supported ends where rotational motion of the cross section is taken into account. By using asymptotic analysis of the Timoshenko beam theory, a unified analytical approach for dealing with free-vibration problems of nonclassical shear beams subjected to axial compressive or tensile force according to Engesser’s model is presented. A simple characteristic equation is derived for axially loaded shear beams with translational and rotational springs and with attached lumped end masses. The resulting frequency equation is compared with the classical one. A condition causing the nonclassical shear beams to collapse to the classical ones is found. Natural frequencies are evaluated and mode shapes are given explicitly. The influences of the spring coefficients, axial loads, and rotational inertia on the natural frequencies are expounded. The frequency equations of shear beams with typical ends such as free-free and free-pinned ends can be recovered from the present study as special cases.

A numerical simulation of three-dimensional flow around rectangular prisms (two dimensional) of different aspect ratios, ranging from 0.3 to 7.0, is conducted at a Reynolds number of 10⁵, using large eddy simulation. This study identifies the influence of aspect ratio on the flow field around prisms and the attendant aerodynamics. Results show salient features in the flow field and associated pressure/aerodynamic forces caused by changes in the afterbody, which demonstrate good agreement with observations from wind tunnel experiments. A time-frequency analysis is introduced to identify the transient nature of fluctuations in the drag and lift coefficients, highlighting temporal variations in the frequency contents of these fluctuations and their resulting influence on force coefficients.

Strong form collocation using radial basis functions approximation, which is called the radial basis collocation method, is an efficient method for solving partial differential equations or integral equations because it has implementation simplicity and exponential convergence. In this work, the radial basis collocation method is introduced in conjunction with a least-squares scheme to suppress the volumetric locking in nearly incompressible elastic problems. Radial basis approximation, which possesses infinite continuity, is rich enough to ensure the divergence-free condition. Modal analysis proves that all the nonphysical locking modes can be removed, which eliminates the locking effect. Numerical studies validate the good performance of the proposed method. Exponential convergence is obtained as expected, and no pressure oscillation is observed.

A parametric approach is proposed for identifying the inelastic response of individual elements, which provide resistance against lateral loads in torsionally coupled buildings. Depending on the severity of the ground shaking in each lateral direction and the amount of eccentricity, different lateral load resisting elements (LLREs) will experience different levels of permanent deformation and energy dissipation based on their location and orientation. The proposed method utilizes the well-known Bouc-Wen model for representing the hysteretic response of each LLRE. An unscented Kalman filtering approach is implemented for identifying the Bouc-Wen model parameters, and for direct estimation of the force-displacement loops experienced by each LLRE during damaging seismic events. The simulated response of a single-story building to ground motion is used for demonstrating the utility of the proposed method, which may also be applied to multistory frames in a straightforward fashion, if certain instrumentation requirements are met.

Filters are commonly used in the postprocessing of noise-contaminated measurement signals for achieving noise removal or data fusion. Quantifying variance of noise embedded in the signal is an important task for the calibration and operation of filters. In this study, an adaptive subspace technique is proposed to identify and track noise variances with unknown and potentially time-varying characteristics from noise-contaminated signals. The technique is developed on the premise that the covariance of the clean signals is rank-deficient, i.e., the number of excited vibration modes is less than that of measured channels. After separating the vector space of noisy signals into a signal subspace and a noise subspace, noise variances are estimated using the energy of the noise subspace. The projection approximation subspace tracking technique is then employed to adaptively update the signal subspace and track the changes of noise variances from the measurement data. A numerical example of a 10-story building model and a field test on a pedestrian bridge are performed to validate the proposed technique. Results show that the proposed technique can identify and track time-sensitive noise variances accurately and efficiently from noisy measurement data. It is also illustrated that the proposed technique can be integrated into the Kalman filter for fusing acceleration and displacement data and provides a better displacement estimate when the data contain time-varying noises.

The calculation of dynamic contact between the bottom of a hammer and the surface of soil is the key technical issue in dynamic compaction. In this paper, a large deformation hypothesis is used based on the relationship between strain and displacement. Then, nonlinear governing equations of soil, based on a finite-element approach, are established and an iterative calculation form is presented. According to the contact and separation processes of the hammer and soil, the dynamic contact conditions are given in the calculation of the dynamic contact stress. Finally, according to the energy conservation law, an optimization model is used and an additional energy equation is established. Furthermore, a specific algorithm for optimizing the model is given. The calculation results show both the time-interval changing features and spatial distribution features of the stress and displacement. The numerical results and field test comparisons of the rammed pit demonstrate that this method is effective.

This paper presents the material response of a granular material under different gravity fields using the discrete element method (DEM). A bidispersive porous medium is prepared and sheared under various gravity conditions. The medium is composed of two kinds (shape and size) of ellipsoids. Each type of particle occupies 50% of the whole mass. The ellipsoids are randomly generated and deposited in lunar gravity (1/6g), Earth’s gravity (1g), and elevated gravity (10g). Various friction coefficients are used during the deposition stage, which yields samples of different initial densities. Afterward, samples are isotropically consolidated to different initial confining pressures, and the friction coefficient between particles is 0.5. A total of 25 numerical samples have been created. Very large strain is applied to the sample under drained or undrained conditions. The granular material comes into the critical state. Overall, the gravity effect on peak friction angle (shear strength of the material) is negligible. The critical states of all samples are very similar. There is one observed gravity effect; a greater level of gravity produces samples of higher coordination number.

For reasons more related to functionality than safety, it is not uncommon for heavy mechanical and electrical equipment to be placed on wheels. Examples of such devices are medical carts, mechanical equipment in hospitals, electrical transformers, and recently, even supercomputers. Although wheels facilitate the operation of these devices, they also affect the response of these objects during earthquakes, but not necessarily in a beneficial way. While a wheel rolling favors the translational displacement of the body in the horizontal direction parallel to its plane over rocking, rocking is still possible along the plane perpendicular to the plane of the wheel. Moreover, because the plane of the wheel is in most cases free to rotate with respect to the body, it is not easy to identify the directions that favor rocking or displacement at any time. The problem becomes even more complicated, if one considers that one of the wheels, which can swivel, may be locked. Thus, in the most general case, a body on wheels experiences three-dimensional displacements and rotations along with the three-dimensional dynamics of the wheels. In this work, a model for the dynamic behavior of bodies on wheels is presented and the corresponding response behavior is examined for several common cases.

A finite-element formulation is developed for the lateral torsional buckling analysis of plane frames with moment connections consisting of two pairs of welded plate stiffeners. The finite element provides a realistic representation of the partial warping restraint provided by the joint to the adjoining members. The element consists of two nodes with four generalized buckling degrees of freedom per node and is thus devised to interface with two classical beam buckling elements connected at right angles. The new finite element extends the functionality of the classical beam finite element to predict the lateral buckling load for noncollinear structures, such as portal frames. A comparison with results based on shell finite-element analysis demonstrates the ability of the new formulation to reliably predict the lateral buckling resistance of plane frames at a fraction of the computational and modeling cost of shell-based finite-element solutions.

An evolutionarily coupled finite-element (FE) and reproducing kernel (RK) formulation is implemented in the Karagozian and Case–finite element/mesh-free (KC-FEMFRE) code for modeling concrete behaviors under blast and impact loadings. Mesh-free methods, such as the RK particle method, have the capacity to overcome regularization requirements and numerical instabilities that encumber finite-element methods in large deformation problems, and they are also more naturally suited for problems involving material perforation and fragmentation. To enhance efficiency, a novel approach is developed by coupling the FE approximation with the RK approximation in a controllable and evolutionary fashion. A unique domain integration, stabilized conforming nodal integration, is applied to both FE and RK domains, and therefore the state variables are stored at nodal points directly, and thus no state variable transition is required when mesh conversion is performed. This has a wide range of utility, not only related to efficiency of RK simulations, but by providing a consistent numerical approach for element-to-particle conversion. Moreover, the Karagozian and Case concrete (KCC) model is implemented into the KC-FEMFRE framework, and the internal damage variable of the KCC model is used to evolve mesh-free particles. Quasi-static, blast, and high-speed impact problems are simulated by the coupled formulation, and realistic responses are obtained.

The focus of this work is on methods for modal identification of civil structures using output data only. An important family of time domain methods uses autoregressive time series models and exploits formulations developed in the field of system control. Another strategy consists of using methods well tested in the identification of structures on the basis of impulse response or free decay and extending them to the analysis of response signals generated by excitations of a more general nature. A third option refers to the Ho-Kalman minimal realization algorithm, which was extended by Akaike and Aoki to stochastic systems. These approaches, or their combinations, include a sizable proportion of the methods actually used in output-only identification of civil structures subjected to natural excitation, and most of them are based on stationarity assumptions. The question that prompted this study was as follows: what degrees of reliability and accuracy can such methods ensure when they are used, as is often the case in actual practice, in nonstationary conditions? An answer to this question was sought numerically by focusing on nonstationary conditions deemed typical of the actions naturally applied to civil structures.