Unidirectional Loading Research Articles

Consideration of Unidirectional Cyclic Loading on Bond in Reinforced Lightweight Concrete in Standards

The present research deals with the cyclic and standard pull-out resistance of deformed steel bars embedded in lightweight and normal concrete. This paper is a continuation of a previous paper, where the experimental results are detailed. In the present paper, the experimental results are set against the formulas and the diagrams provided by the Eurocode standard and the Model Code 1990, and then a comparative discussion is performed. In the case of cyclic loading, the damage defined by the Palmgren–Miner hypothesis, as well based on the recommendations of various national annexes of Eurocode and the Model Code, is calculated. A multiplier corresponding to the maximum load is calculated, which indicates by how much the applied load should be multiplied to obtain a damage value equal to 1.

Accounting for Resilience in the Selection of R Factors for a RC Unsymmetrical Building

Several design codes consider the non-linear response of a building by using one of the most important seismic parameters, called the response reduction factor (R). The lack of a detailed description of the R factor selection creates the need for a deeper study. This paper emphasises a methodology for the selection of a proper R factor based on resilience aspects. Unsymmetrical/irregular buildings have become the most common in recent times due to aesthetic purposes. However, because of the complexity due to the torsional effect, the selection of the R factor is even more difficult for this type of building. Therefore, a high-rise G+10-storey L-shaped building is herein considered. The building has re-entrant corners based on the structural/plan arrangement. Different R factors were used in the building design, considering buildings subjected to both unidirectional and bidirectional seismic loading scenarios. The building response with respect to various R factors (R equal to 3, 4, 5 and 6) in terms of its performance level, functionality, damage ratio and resilience was assessed at two design levels, i.e., design basic earthquake (DBE) and maximum considered earthquake (MCE). The study concludes that, considering the above criteria along with the resilience aspect, a maximum R factor up to 4 can be recommended for unidirectional loading, whereas for bidirectional loading, the maximum recommended R factor is 3.

Response analysis of single pile embedded in saturated sand under bidirectional cyclic loading

In this study, the dynamic responses of a single pile in liquefiable sand under bidirectional loading are investigated using fully coupled three-dimensional nonlinear finite element analyses. A multi-surface plasticity model is adopted to simulate the liquefiable sand behaviour. The analysis procedures elaborate the bidirectional shaking patterns of 2-D linear, circular, and oval shape simultaneously. Results reveal that compared to the unidirectional loading, the bidirectional loading reduces the soil liquefaction resistance and increases the depth of liquefied soil layer. The results also showed that the phase difference between orthogonal components of base excitation and the frequency content of the input motion have a significant impact on the dynamic responses of soil and pile. In addition, an increase in the phase difference reduces the excess pore water pressure and the thickness of liquefied layer, and delays the liquefaction occurrence. Moreover, the maximum pile bending moment and its location correspond to the dominance of either inertia or kinematic interaction forces, which are primarily influenced by the frequency content of input motion.

Full-scale experimental tests on unbonded fiber reinforced elastomeric isolators under bidirectional excitation

Unbonded fiber reinforced elastomeric isolators (UFREIs) represent a lower-cost alternative to conventional steel reinforced elastomeric isolators (SREIs), in which the internal steel shims are replaced with fiber reinforcement and the installation is accomplished without anchorage bolts or dowels, but only relying on frictional mechanisms and interfacial roughness. In the literature, experimental tests on UFREIs were generally presented under unidirectional seismic excitation and considering small-scale devices. In this contribution, the hysteretic behavior of UFREIs is investigated experimentally by testing full-scale devices (diameter 620 mm) not only under unidirectional loading, but also including bidirectional orbits, thus quantifying the degree of biaxial coupling. A simple phenomenological model is then set up to numerically simulate the experimental outcomes. This model consists of a set of nonlinear springs arranged in a circular configuration. The spring parameters are calibrated by either curve-fitting 1D test results or considering 2D tests, thus selectively incorporating the lateral coupling. This model makes it possible to investigate the influence of bidirectional interaction of UFREIs on the seismic performance of base-isolated structures. Nonlinear time-history analysis on a regular three-dimensional reinforced concrete building with UFREIs modeled through the proposed approach demonstrates that the lateral coupling of the UFREIs leads to lower seismic response, of around 10-20%, in comparison to a simpler, uncoupled model that ignores the bidirectional interaction of the isolation devices.

Guidelines for rail quality selection to increase service life based on wear and fatigue behavior of various wheel and rail materials

The Austrian rail network is very diverse: The lines in the alpine areas have very tight radii while the lines in the flat areas (east and north of Austria) are mainly straight high-speed lines. This leads to different levels of wheel and rail wear. Accordingly, the objective of this study was to experimentally model a laboratory-scale cyclic wheel-rail contact to investigate wear behavior and rolling contact fatigue at different radii (RCF).A novel test system was set up at AC2T research GmbH to reproduce unidirectional loading on a real track. Tribotests were performed with rail (grades R260, R350HT & R400HT) and wheel (grades ER7 & ER9) parts from the field. This newly designed principle is an effective method to characterize the wheel-rail contact. Wear characterization was done after defined testing cycles to detect (i) area loss and (ii) change in surface topography.

Finite-time stability of any polyhedral sweeping process with uni-directional loading and application to elastoplastic models

A significant progress in the study of the dynamics of elastoplastic lattice spring models has been recently due to the development of a sweeping process framework where the dynamics of the stresses of springs is linked to a solution of the associated sweeping process. By using the sweeping process framework, a recent result by Gudoshnikov et al. (2022) ensures finite-time convergence of a lattice spring model under assumption that the vector g′(t) of the applied displacement controlled-loading lies strictly inside a suitable normal cone of the sweeping process. In this paper we drop the latter assumption by proving that g′(t) always belongs to a suitable normal cone when g′(t)=const (i.e. the loading is uni-directional) and when certain genericity condition holds. By using this result we establish that every elastoplastic lattice spring model with a generic associated polyhedron reaches a unique distribution of plastic deformations in finite time.

Assessment of Web Panel Zone in Built-up Box Columns Subjected to Bidirectional Cyclic Loads

The behavior of the web panel zone has a direct effect on the cyclic performance of steel moment connections. While the mechanisms of web panel zone failure are known under cyclic load, little is known about the behavior of the web panel zone under bidirectional loads in bolted connections. Using experimental tests and calibrated numerical models, this research evaluated the web panel zone behavior under unidirectional and bidirectional cyclic loads. The results showed that bidirectional load can modify the stress and strain distribution in the web panel zone. Moreover, the increasing of the width-to-thickness ratio of the column influences the failure mechanism of the joint configuration and increases the plastic incursion in the column. These data demonstrate that bidirectional effects improve the web panel zone performance under cyclic loads.

Seismic Response of an Isolated Curved Bridge with Lead Rubber Bearing by Considering Design Aspect

Studies on the application of the lead rubber bearing (LRB) isolation system to various structures carried out as one of the measures to improve seismic performance. Curved bridges play noteworthy role in the modern transportation system. Due to complex interchange and the desire to conform to existing geometric confinements have made this type of structures more alluring. Horizontally curved bridges make up of a noteworthy portion of the bridge population in various countries. In the present study, Lead rubber bearings have been used in an Isolated Curved bridges. Seismic loadings have been considered for three recorded ground motions from PEER strong motion database (PEER 2014) without scaling viz. (1) Imperial Valley (1940) with two orthogonal horizontal ground motion components (PGA0.313g and PGA0.215g); (2) Kobe (1995) with two orthogonal horizontal ground motion components (PGA 0.83g and PGA 0.63g) and (3) Turkey (1992) with two orthogonal horizontal ground motion components (PGA 0.5g and PGA 0.39g). These three ground motions have been considered to represent conditions close to far-fault, near-fault and forward directivity of earthquakes, respectively. Unidirectional and Bi-directional loading have been considered in the present study. Design aspects of the Lead rubber bearings has been considered by deploying the interaction diagram of bearing displacement between the unidirectional and bi-directional responses. It has been found that the bidirectional loading should be considered while designing the isolators.

EVALUATION METHOD OF SHEAR CAPACITY OF REINFORCED CONCRETE WALLS IN CONSIDERATION OF TRI-DIRECTIONAL LOADING

Tri-directional loading tests were conducted with out-of-plane deformation and axial force conditions as parameters. The in-plane maximum load capacity was reduced by 8.8% due to out-of-plane deformation and varying axial load. The results were discussed along with previous experiments by the authors. As a result, it was found that the out-of-plane deformation reduced the maximum load capacity by 19% compared to that under uni-directional loading, while the axial force variation reduced that by 2% compared to that under constant axial load. An evaluation method considering tri-directional loading proposed by the authors could accurately evaluate the in-plane shear capacity.

Discrete-Element Simulation of Dense Sand under Uni- and Bidirectional Cyclic Simple Shear Considering Initial Static Shear Effect

Huge and critical structures, such as fuel storage tanks and nuclear power plants, are often built on dense sandy grounds, which may cause excessive plastic deformations under multidirectional cyclic shearing conditions caused by earthquakes. The lack of investigation of sand behavior under multidirectional loading may lead to unsafe designs, and thus significantly increase the maintenance cost for infrastructures during their long operational time, which might be longer than 100 years. This study presents a numerical investigation on the liquefaction behavior of dense sand under bidirectional cyclic simple shear using the discrete element method. The bidirectional cyclic simple shear tests with varying stress trajectories on the deviatoric stress plane, including figure-8, circular, and straight lines, were conducted. Additionally, a series of unidirectional loading tests was conducted in the simulation scheme for comparison purposes. The effect of the initial static shear on the cyclic behavior and liquefaction resistance of granular materials was examined. Four loading categories, including full reversal, partial reversal, intermediate reversal, and no reversal, were implemented in the numerical simulations. Although the presence of the initial static shear may enhance the cyclic resistance of granular samples under unidirectional loading, it mitigates the liquefaction resistance and promotes the failure of specimens under bidirectional shearing conditions. The microscopic analysis was performed to reveal the relationship between the fabric evolution and external loading, which can provide profound insights into the underlying mechanism of the cyclic behavior and liquefaction susceptibility of granular material when both the bidirectional shearing and initial static shear effects are considered.

Seismic safety evaluation method based on fiber-element strain for square-section steel piers

Inconsistency and incompatibility exist between the seismic capacity values obtained from shell-element models and the seismic demand results obtained from beam-based models in engineering. In this paper, the improved fiber model which can consider local instability of steel plates is used uniformly to conduct seismic capacity and seismic demand analyses under bidirectional earthquake actions. First, the restoring-force interaction curves of square-section steel piers are obtained under horizontal unidirectional and oblique loading; however, the evaluation method based on the restoring-force interaction curves may produce incorrect evaluation results according to the elastoplastic stability theory. Therefore, a new seismic safety evaluation method, based on the fiber-element strain within the effective damaged length (Led), is further proposed through the quasistatic analysis under the square spiral loading path. Dynamic seismic response analysis is conducted to verify the effectiveness of the proposed strain-based evaluation method. The results show that the ultimate state according to the compressive strain limit obtained via the square spiral loading is consistent with that based on the elastoplastic stability theory under bidirectional earthquake actions. The proposed method can not only coordinate the demand and capacity results, but also avoid misjudgment cases and facilitate engineering application.

Seismic analysis of a base-isolated reinforced concrete frame using high damping rubber bearings considering hardening characteristics and bidirectional coupling effect

High damping rubber bearings (HDRBs) are commonly used in base isolation systems, which can protect the structures from heavy earthquake damage by elongating natural vibration periods of the structures and improving the energy dissipation capacity of the systems. For the HDRBs, the stiffness hardening effect is exhibited at large shear strain, which is favorable to control the displacement of the bearings. Meanwhile, the horizontal hysteretic behavior in the two orthogonal directions is coupled and related to the path history. Here, the influence of the stiffness hardening and bidirectional coupling effects of the bearings on the seismic responses of the isolation system is studied. The full-scale quasistatic cyclic tests of two HDRB specimens with different dimensions were carried out under unidirectional and bidirectional loads. In the numerical analysis, the DHI and the classical Bouc-Wen model are used to simulate the hysteretic behavior of the HDRBs, in which the DHI model can capture the increased damping and stiffness, and bidirectional coupling properties. The parameters of both bearing models are determined according to the test results. Time-history analyses of a six-story reinforced concrete (RC) frame building with an HDRB isolation system are carried out for a suite of earthquake ground motions. The influence on seismic response of superstructure and isolation layer is analyzed with or without considering the hardening effect and bidirectional coupling effect of HDRBs. The results demonstrate that with consideration of the stiffness hardening and coupling effects, the peak responses of the superstructure significantly increase under high-intensity earthquakes. The analytical results suggest that the stiffness hardening and the effect of bidirectional coupling of the HDRBs should be considered in the design for HDRB isolation systems.

Cyclic Behavior of the HPFRCC Column With Short Lap Splices Under Unidirectional Loading

Cyclic Behavior of the HPFRCC Column With Short Lap Splices Under Unidirectional Loading

Numerical Study on Seismic Response of Offshore Wind Turbine Monopile in Multi-Layered Soil Profile of Arabian Sea

The seismic performance of monopiled offshore wind turbine (OWT) structures was evaluated numerically. The aim was to analyze offshore wind farm sites on complicated layered seabed with high seismicity. Following pile soil analysis (PISA) model, three-dimensional (3D) numerical evaluations were undertaken for two separate locations under two independent seismic events. The effects of pile diameter, depth, site impact owing to transverse soil layering, uni-directional and multi-directional seismic loading and seismic acceleration magnitude are presented. Dynamic impedance depth variation and site response analysis via lateral displacement, lateral soil response and Fourier response amplitude are explored in frequency and time domain. In a seismic zone with stratified soil, pile diameter has a greater influence. Variations in soil profile affect wind turbine performance and seismic sensitivity. This research will give a strong platform for later studies to recommend a safe wind farm site based on simulation results.

Seismic application of steel rocking column bases with replaceable cover plates in steel MRFs

This paper presented unidirectional and bidirectional cyclic loading test results of a novel steel rocking column base. Feasibility of the column base in steel moment-resisting frames was verified through nonlinear static and time history analysis. It was found that the column base was capable of dissipating seismic energy through plastic bending deformation of the replaceable cover plates and flag-shaped hysteretic curves can be observed. Nevertheless, it is not expected to provide much energy dissipation for the whole structural system, since the plastic bending deformation of each cover plate is only in one direction. A three-dimensional simplified numerical model of the column base was developed and validated against the test results. Following that, nonlinear static and time-history analysis were conducted on prototype steel moment-resisting frames with and without the rocking column bases. In general, replacing some column bases with the rocking ones (i.e., 4/24 of all in this study) did not cause much reduction in lateral resisting stiffness and capacity. Moreover, this reduction was even slighter when the loading direction was perpendicular to the weak axis of the rocking column section. On the other hand, replacing all fixed column base with the proposed rocking column base may cause drift concentration in the ground storey, which is too radical for the prototype building. The seismic responses of modified frames were generally very close to that of the original frame with all fixed column bases. Even though the maximum drifts in the ground storey of these frames were larger than that of the original frames with fixed bases, the maximum drifts of the whole structures were not expected to be amplified. • Bidirectional cyclic loading tests on a novel steel rocking column base were conducted. • The column base showed flag-shaped hysteretic cures and dissipated seismic energy through plastic deformation of the replaceable cover plates. • Feasibility of the column base in steel moment-resisting frames was verified through nonlinear static and time history analysis. • Replacing some column bases with the rocking ones did not cause much reduction in lateral stiffness and capacity.

Extrusion-Based 3D Printing of Stretchable Electronic Coating for Condition Monitoring of Suction Cups.

Suction cups (SCs) are used extensively by the industrial sector, particularly for a wide variety of automated material-handling applications. To enhance productivity and reduce maintenance costs, an online supervision system is essential to check the status of SCs. This paper thus proposes an innovative method for condition monitoring of SCs coated with printed electronics whose electrical resistance is supposed to be correlated with the mechanical strain. A simulation model is first examined to observe the deformation of SCs under vacuum compression, which is needed for the development of sensor coating thanks to the 3D printing process. The proposed design involves three circle-shaped sensors, two for the top and bottom bellows (whose mechanical strains are revealed to be the most significant), and one for the lip (small strain, but important stress that might provoke wear and tear in the long term). For the sake of simplicity, practical measurement is performed on 2D samples coated with two different conductive inks subjected to unidirectional tensile loading. Graphical representations together with analytical models of both linear and nonlinear piezoresistive responses allows for the characterization of the inks’ behavior under several conditions of displacement and speed inputs. After a comparison of the two inks, the most appropriate is selected as a consequence of its excellent adhesion and stretchability, which are essential criteria to meet the target field. Room temperature extrusion-based 3D printing is then investigated using a motorized 3D Hyrel printer with a syringe-extrusion modular system. Design optimization is finally carried out to enhance the surface detection of sensitive elements while minimizing the effect of electrodes. Although several issues still need to be further considered to match specifications imposed by our industrial partner, the achievement of this work is meaningful and could pave the way for a new generation of SCs integrated with smart sensing devices. The 3D printing of conductive ink directly on the cup’s curving surface is a true challenge, which has been demonstrated, for the first time, to be technically feasible throughout the additive manufacturing (AM) process.

Influence of tooth root cracks on the mesh stiffness of asymmetric spur gear pair with different backup ratios

Gears are critical machine elements that transmit power and motion in diverse implementation fields. Over time, gears may produce a series of faults due to harsh operating conditions, fatigue, manufacturing errors, etc., leading to severe performance degradation. During the meshing process, the stiffness of a single tooth controls the load sharing, vibration, and noise characteristics of a geared system. An undetected fault could decrease the gear stiffness and thus may lead to a fatal breakdown, substantial economic losses, or even human casualties in safety-critical applications such as helicopters, high-speed trains, and wind turbines. Hence, the accurate quantification of the gear stiffness emerges as an important research area to obtain reliable gear designs. With this in mind, the asymmetric tooth concept offers superior bending strength, fatigue propagation life, and the ability to lessen vibration over the standard (symmetric) designs in applications where unidirectional loadings are predominant. This study investigates the influence of tooth root cracks on the single-tooth and meshing stiffness characteristics of the standard and asymmetric involute spur gears. To this end, the numerical crack propagation paths obtained in our previous works were introduced to the created 3D CAD geometries. Subsequently, the single tooth stiffness of both healthy and cracked (2 5%-50%-75%-100%) gears was calculated through the ANSYS® Workbench, and the time-varying mesh stiffness was obtained. The present study evaluated the effects of backup ratio and the tooth asymmetry on the spur gears’ meshing stiffness characteristics simultaneously and further expanded the scope of the research work. The results indicated that the single tooth stiffness and mesh stiffness could be improved by 35% and 22%, respectively, as the drive side pressure angle increased from 20° to 35°. It has been noted that the gear stiffness decreased as the crack level increased, while the increment of the backup ratio further increased the reduction in the stiffness. The findings could provide significant outputs for a better understanding of the influence of tooth asymmetry on the gear dynamics characteristics, life prediction, and early fault diagnosis.

Construction and Verification of Spherical Thin Shell Model for Revealing Walnut Shell Crack Initiation and Expansion Mechanism

Walnut shell breaking is the first step of deep walnut processing. This study aims to investigate the mechanical properties and fracture state of the Qingxiang walnut shell under unidirectional load and guide the complete separation of the walnut shell and kernel. The spherical thin shell model of the walnut (the fitting error is less than 5%) was established and verified. The process from the initiation to the expansion of walnut cracks was analyzed. The crack expansion rate was estimated in terms of the crack fracture regularity on the shell’s surface. Based on the momentless theory and finite element simulation analysis, we found that the stress on the shell surface in the concentrated force action region was gradient distributed from inside to outside and that the internal forces were equal in all directions in the peripheral force action region. The unidirectional impact shell-breaking experiments confirmed the reliability of our spherical thin shell model and verified our hypothesis of walnut shell fracture along the longitudinal grain. Our results can provide a theoretical basis for the development and structural optimization of shell-breaking machinery.

Effect of boundary conditions on values of homogenized fabrics’s elastic parameters and inelastic response

The effect of displacement, periodic and surface traction boundary conditions (BCs) on predicted elastic parameters and inelastic response of a homogenized material equivalent to either a unidirectional or a randomly distributed short fibers reinforced composite has not been extensively studied. A few such studies have focused on their elastic analyses. Here, we analyze the effect of the three types of BCs on both elastic and inelastic deformations of weaves and braids with continuum damage mechanics used in the inelastic analyses. We employ simple unit cells, Hill’s theorem, and the displacement, periodic and uniform traction BCs to deduce from the finite element stiffness matrix K the matrix C of homogenized moduli. For both elastic and inelastic problems, it is seen that predictions using the displacement and the periodic BCs are close to those reported in the literature while those from the traction BCs are not. For unidirectional loading, it is found that the nonlinearity in the axial stress - axial strain curve, the localized deformations and the resin damage occurring in regions unsupported by fibers are significantly more for the traction BCs than that for the other two BCs. Also, there is more damage for the periodic BCs than that for the affine displacement BCs.

Bearing failure mechanism of rock-socketed monopile foundation for offshore wind turbine in weathered-granite seabed

Large-scale constructions of offshore wind power are carried out in southeast coast of China, where the seabed is covered in weathered granite. However, the bearing capacity and failure mechanism of monopile foundation in weathered-granite seabed are rarely researched. In this paper, the stress-strain characteristics and strength parameters of granite with different weathering degrees were obtained through laboratory tests, and the pre-strain test method was used to study mechanical parameter variation of moderately weathered granite in the crushing process. Experiment found that the strength parameters decrease mainly after the strain reaching 50% of the ultimate strain of moderately weathered granite. In addition, the bearing capacity and failure mode of rock-socketed monopile under unidirectional horizontal loading was investigated numerically. Analysis showed that the failure mode of rock-socketed monopile under unidirectional horizontal loading is progressive and presents two obvious failure zones. More critical, the existence of vertical loading effectively improves the horizontal bearing capacity of monopile foundation in the weathered rock seabed, and the failure envelope of the monopile foundation in H-V loading space is elliptic-shaped. This provides further understanding of the mechanical properties of submarine weathered granite, and the helpful reference for stability evaluation of rock-socketed monopile foundation in weathered granite seabed.