Journal of Engineering Mechanics

This paper attempts to develop a reduction-based model updating technique for jacket offshore platform structure. A reduced model is used instead of the direct finite-element model of the real structure in order to circumvent such difficulties as huge degrees of freedom and incomplete experimental data that are usually civil engineers’ trouble during the model updating. The whole process consists of three steps: reduction of FE model, the first model updating to minimize the reduction error, and the second model updating to minimize the modeling error of the reduced model and the real structure. According to the performance of jacket platforms, a local-rigidity assumption is employed to obtain the reduced model. The technique is applied in a downscale model of a four-legged offshore platform where its effectiveness is well proven. Furthermore, a comparison between the real structure and its numerical models in the following model validation shows that the updated models have good approximation to the real structure. Besides, some difficulties in the field of model updating are also discussed.

In this paper, three-dimensional turbulent flow field around a complex bridge pier placed on a rough fixed bed is experimentally investigated. The complex pier foundation consists of a column, a pile cap, and a $2\times 4$ pile group. All of the elements are exposed to the approaching flow. An acoustic-Doppler velocimeter was used to measure instantaneously the three components of the velocities at different horizontal and vertical planes. Profiles and contours of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy, and Reynolds stresses, as well as velocity vectors are presented and discussed at different vertical and horizontal planes. The approaching boundary layer at the upstream of the pile cap separated in two vertical directions and induced an upward flow toward the column and a contracted downward flow below the pile cap and toward the piles. The contracted upward flow on the pile cap interacts with downflow in the front of the column and deflects toward the side of the pier, which in return produces a strong downflow along the side of the pile cap. The flow at the rear of the pile cap is very complex. The strong downward flow at the downstream and near the top of the pile cap in interaction with the reverse flow behind the column and upward flow near the bed produce two vortices close to the upper and lower corners of the pile cap with opposite direction of rotation. On the other hand, the back-flow from the wake of the pile cap is forced into the top region resulting in a secondary recirculation at the wake of the column. The contracted flow below the pile cap and toward the piles, a strong downflow along the sides of the pile cap at the upstream region, and a vortex flow behind the pile cap and an amplification of turbulence intensity along the sides of the pile cap at the downstream region are the main features of the flow responsible for the entrainment of bed sediments.

A nonhydrostatic model with a higher-order top-layer pressure treatment is developed. Accuracy with respect to linear wave dispersion and wave nonlinearity is carefully examined. The model is thereafter applied to simulate nonlinear deep-water wave groups. For slowly modulated wave groups, the model well predicts the characteristics of bichromatic waves better than those obtained by the fourth-order nonlinear Schrödinger equation and the multilayer Boussinesq model. For fast evolution of focusing wave groups, the model accurately captures the limiting extreme wave conditions. Particularly the predicted local wave steepness of a narrow-banded wave group is higher than that of a broad-banded wave group, supporting the importance of spectral bandwidth in determining the limiting wave condition in the previous study. Overall, the agreement between the model’s results and experimental data are excellent, demonstrating the capability of the model on resolving wave-wave interactions in the nonlinear deep-water wave groups.

In order to study the dynamic response of an asphalt road, a dynamic model of the road under a moving load is proposed, in which the viscoelastic characteristics of the base and pavement are all considered, the pavement is regarded as an infinite beam on a Kelvin viscoelastic base. By using Green’s functions, Laplace transforms, and Fourier transforms, the analytical solution in transient is deduced. As the viscosity of the pavement is included in the model, the analytical solution can be used to investigate more of the factors that affect the dynamic response, such as vehicle speed, temperature, and road material properties. Using this analytical solution, some numerical calculations are given to illustrate the effects of vehicles’ speeds and different damping on the deflection with the displacement.

Compressive strength of porous materials, especially of cement gels, has been estimated by means of fractal dimension of fracture surfaces. The relationship between mechanical strength and fractal characteristics of porous gels has been derived and tested experimentally using samples of cement gels. The dimensions of fracture surfaces have been found to be general parameters independent on the fractal dimensions of inner material components.

In this paper, the basic methodology for the fatigue reliability assessment of randomly vibrating multidegree-of-freedom systems is presented within the coupled response-degradation model. The fatigue process in the system components is quantified by the fatigue crack growth equations which—via the stress range—are coupled with the system response. Simultaneously, the system dynamics is affected by fatigue process via its stiffness degradation so that it provides the actual stress values to the fatigue growth equation. In addition to the general coupled response-degradation analysis, its special case of noncoupled fatigue crack growth is treated as well for the wide-band stationary applied stress by the use of its first four spectral moments and the approximate, empirically motivated, Dirlik’s probability distribution for the stress range. Both, the general analysis and the illustrating exemplary problems elaborated in the paper provide the route to the fatigue reliability estimation in complex–hierarchical vibratory systems under random loading.

Due to the nonlinear nature of coordinate transformations, evaluation and calibration of multiaxial loading systems in global Cartesian coordinates are challenging problems. This study proposes a systematic calibration method for multiaxial loading systems in global coordinates using an external measurement system. Global coordinate measurement for a multiaxial loading system is usually obtained from a geometric transformation based on internal actuator measurements. However, any misrepresentation of initial actuator configuration (e.g., origin, pin locations, etc.) introduces errors and cross-talk in the global Cartesian coordinates. Such errors and cross-talk cannot be observed or eliminated based on internal measurements. The method proposed in this paper is based on the sensitivity of the global coordinates with respect to the initial actuator length. To validate the proposed method, calibration is performed using the state-of-the-art Load and Boundary Condition Box at University of Illinois at Urbana-Champaign as the multiaxial loading system and the Krypton Dynamic Measurement Machine as the external measurement system. Experimental results demonstrate that the proposed sensitivity-based external calibration method is effective for improving control accuracy and reducing cross-talk of multiaxial loading systems in global Cartesian coordinates.

The problem of end-plate debonding of the external reinforcement in strengthened concrete beams is analyzed in this paper. As experimentally observed, this mode of failure is highly brittle and poses severe limitations to the efficacy of the strengthening technique. A numerical analysis of the full-range behavior of strengthened beams in bending is herein proposed to study the stages of nucleation and propagation of interfacial cracks between the external reinforcement and the concrete substrate. This is achieved by modeling the nonlinear interface behavior according to a cohesive law accounting for Mode Mixity. The numerically obtained load versus midspan deflection curves for three- or four-point bending beams show that the process of end-plate debonding is the result of a snap-back instability, which is fully interpreted in the framework of the Catastrophe Theory. To capture the softening branch with positive slope, the interface crack-length control scheme is proposed in the numerical simulations. The results of a wide parametric study exploring the effect of the relative reinforcement length, the mechanical percentage of fiber-reinforced polymer sheets, the beam slenderness, and the ratio between Mode II and Mode I fracture energies are collected in useful diagrams. Finally, an experimental assessment of the proposed model completes the paper.

Corrosion-induced cracks in reinforced concrete (RC) structures degrade the stiffness of the cover concrete. The stiffness degradation is mainly caused by the softening in the stress-strain relation in the cracked concrete. Limited efforts have been made to model the cracking and the corresponding effects on the cover concrete, despite of its importance in assessing and modeling the behavior of RC structures. This paper proposes a stiffness degradation factor to model the stiffness degradation of the cover concrete subject to cracking. The proposed factor is computed in terms of the cracking strain corresponding to the maximum opening of the concrete cracks based on an energy principle applied to a fractured RC structure. The time to cracking of the cover concrete is then determined as the time from the corrosion initiation needed by the crack front to reach the outer surface of the cover concrete. The proposed stiffness degradation factor and the method to compute the time to cracking are illustrated through two numerical examples. The times to cracking of the cover concrete that are predicted using the proposed method are in agreement with the measured values from laboratory experiments.

Self-centering precast concrete walls have been found to provide excellent seismic resistance. Such systems typically exhibit low energy dissipation, requiring supplementary dissipating components to improve their seismic performance. Mild steel shear connectors can provide an economical energy dissipating element. The design and analysis of steel shear connectors for a new precast wall system has been undertaken. A series of finite-element analyses were conducted to investigate the behavior of different types of connectors. Emerged from these analyses is a oval-shaped connector (O-connector) that provided satisfactory force-displacement behavior and appeared well suited for the new wall system in high seismic regions. An extensive experimental test program was then conducted to verify the performance of the chosen O-connector, which confirmed the expected response with sufficient energy dissipation. The experimental data demonstrated good correlation with the finite-element model developed, providing satisfactory confidence in the finite-element technique used for the development of the different connectors.

This paper presents an analytical model to determine the compressive response of concrete members, having a square transverse cross section and reinforced with steel angles at the corners and steel strips connecting the steel angles. The model evaluates the effects of transverse strips and steel angles on confinement pressures, including strength and buckling phenomena. Compressive response was evaluated in cases of directly and indirectly loaded elements. The data produced correlate well with available experimental data.

Gypsum is made up of interlocked and elongated crystals. The random nature of its morphology suggests to resort to homogenization of random media to investigate its mechanical properties from the scale of the single crystals upwards. Unfortunately, the usual homogenization schemes fail to quantitatively predict the influence of the porosity on the effective Young’s modulus of gypsum. This is clearly due to the inability of such approaches to take into account the elongated nature of the crystals. A modification of the classical self-consistent scheme is proposed. It is validated against elastic characteristics computed by finite element analyses, and also against experiments on real dried gypsum samples (with empty pores). Finally, a strength model based on brittle failure is presented. The whole strength domain in the space of macroscopic principal stresses is derived. The comparison to experimental data in both simple tension and simple compression is remarkably good.

This paper experimentally investigates the feasibility of an adaptive passive control system, which consists of a magnetorheological (MR) fluid damper and an electromagnetic induction (EMI) part, for suppressing vibration of building structures subjected to ground accelerations. In the adaptive passive control system, the EMI part composed of a permanent magnet and a coil convert the kinetic energy of the relative motion between a building and a damper into the electric energy, which is used for a change in damping characteristics of the MR fluid damper. Since the EMI part can be used as a controller, which determines the command voltage input according to structural responses as well as a power source, the adaptive passive system can be much more compact, convenient, and economic than a conventional active/semiactive system that needs a power supply, a controller, and sensors. To experimentally verify the feasibility of the adaptive passive control system, a shaking table test of a small-scale building model employing the MR fluid damper with the EMI part is conducted. The performance of the adaptive passive control system is compared with that of passively operated MR fluid damper-based semiactive control systems.