Unidirectional Cyclic Loading Research Articles

Spatial rotational behaviour of intact and damaged column foot joints: numerical modelling

Columns in traditional Chinese timber structures barely rest on stone bases, and the column foot joints are capable of resisting moment around any direction. This study numerically investigated the spatial rotational behaviour of intact and damaged columns based on experiments. Unidirectional cyclic loading tests were carried out on two intact and three damaged column foot joint specimens. A fibre element-based numerical model for the column foot joints was developed and validated by use of the unidirectional loading test results. Numerical analyses were performed based on the fibre element-based model to obtain the rotational behaviour of the intact and damaged column foot joints under spatial loading. The analyses indicated that the moment–rotation curve of an intact column foot joint under spatial loading was on an umbrella-shaped surface. The damage at the column foot not only unidirectionally decreased the moment-resisting capacity of a column foot joint but also resulted in a rotational performance degradation valley on the moment–rotation surface of the joint. The rotational behaviour of both the intact and damaged column foot joints under spatial loading was highly non-linear. This developed fibre element-based model can be further utilised to analyse the structural performance of traditional timber structures under three-dimensional excitations.

Experimental investigation on flexural performance of concrete beams strengthened with SMA-GFRP-ECC smart composite materials

The structural performance improvement of concrete members is by far a crucial theoretical issue for engineers, and the development of modern smart and composite materials makes it possible to gradually enhance the durability design of the concrete structure. In this study, six beams of the same size and reinforcement ratio, the proposed composite beam (SMA-GFRP-ECC) and five comparative beams (RC, R-ECC, SS-ECC, GFRP-ECC, SMA-ECC), were designed and tested under low-cycle unidirectional cyclic loading and unloading conditions. The energy dissipation capacity, displacement ductility, residual deformation, and self-repairing performance of each concrete beam were evaluated. Afterward, a concise calculation model for the studied composite beam is deduced and developed based on the existing relevant constitutive models and concrete assumptions. The test results indicate that compared with RC beams, the composite reinforced ECC beams show obvious multi-cracking and smaller crack width during the loading process and have good bending ductility. The innovative SMA-GFRP-ECC beam is capable of a high bearing capacity, ductility, and damage self-repairing. The new proposed beam has more than 80% of the maximum crack width recovery capacity during unloading. Hence, the proposed SMA-GFRP-ECC beam is a rather good first attempt of strengthened beams, combining the advantages of SMA, GFRP, and ECC.

Efficient three-dimensional shear-flexure interaction model for reinforced concrete walls

This paper describes the formulation of a computationally stable and efficient shear-flexure interaction model for reinforced concrete walls. The model is derived by combining two previously available models, a two-dimensional E-SFI model, and a three-dimensional SFI-MVLEM-3D model. The major enhancement in the model formulation compared to its parent SFI-MVLEM-3D comes from the implementation of a closed-form solution for the calculation of horizontal axial strains at fibers of the wall element. This significantly reduced the number of element degrees of freedom, which resulted in analysis run-time that is reduced to approximately 25% and convergence rate that is increased roughly two times. Validation of the developed analytical model is performed using test results obtained from a slender, flexure-controlled, C-shaped RC wall specimen tested using multidirectional loading protocol, as well as two squat, shear-controlled, box-shaped wall specimens subjected to unidirectional cyclic loading. The comparison of analytical and experimental results revealed that the proposed model captures with good accuracy the stiffness and lateral load capacity of all examined wall specimens under cyclic loading. Model accuracy when predicting contributions of shear deformations was also good, while vertical strains are predicted with reduced accuracy mostly due to plane-sections assumption implemented in model formulation. The proposed analytical model is implemented in computational platform OpenSees.

Cyclic p-y curve model of unidirectional loaded pile in sand considering the characteristic of friction in the strain wedge model

Pile foundations are always subjected to cyclic loads due to the action of wind and waves. A cyclic p-y curve model considering the characteristic of friction force is proposed to study the lateral response of pile foundations under unidirectional cyclic loads in sand. The soil resistance calculated by the improved nonlinear strain wedge model (SWM) is divided into friction resistance and compression resistance according to the different characteristics of soil resistance. The friction resistance depends on the relative displacement between pile and soil, whose direction and magnitude will constantly change in the process of cyclic load, while the compression resistance is analyzed by combining the extended Masing rule and soil degradation model. The validity of the model proposed in saturated sand is verified by centrifuge test results. Furthermore, the influence of cyclic loads on pile deflection, bending moment and soil resistance is analyzed through the results calculated by the cyclic p-y curve model.

Influence of static–cyclic load misalignment on the drained tilting response of offshore monopiles in sand

Offshore monopile foundations are exposed to misaligned wind and wave loadings, which are respectively dominated by (nearly) static and cyclic load components. While the response of these systems to unidirectional cyclic loading has been extensively investigated, only a few studies have been devoted to the realistic case of misaligned static and cyclic loads, and particularly to the effects of such misalignment on the accumulation of pile rotation under prolonged cycling. This paper presents a 3D finite-element (FE) modelling study on the relationship between load misalignment and cyclic monopile tilt under drained conditions, based on the use of the SANISAND-MS model to enable accurate simulation of cyclic sand ratcheting. After qualitatively identifying the relationship between relevant loading parameters and cyclic stress/densification mechanisms in the soil, specific parametric studies are performed to explore the impact on pile tilt accumulation. The results show that, in comparison to unidirectional loading, misaligned static–cyclic loading gives rise to lesser-known pile–soil interaction mechanisms: when the direction of cycling deviates from that of the static load, “cyclic compression” and “direct cyclic shearing” mechanisms begin to co-exist. This is quantitatively captured by a newly proposed empirical equation for monopile tilt calibrated against the 3D FE simulation results obtained in this work.

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.

Influence of Unidirectional Cyclic Loading on Bond between Steel Bars Embedded in Lightweight Aggregate Concrete

The topic of present research is the experimental investigation of pull-out resistance of B500B-type ribbed reinforcing steel bars embedded in lightweight aggregate concrete (LWAC) under unidirectional cyclic loading. Only a limited amount of standardized data on bond strengths are available in the case of cyclic loading, especially for lightweight aggregate concrete. This paper deals with the experimental comparison of bond behavior of steel bars embedded in LWAC with expanded clay (Liapor) aggregate and normal weight aggregate concrete (NWAC) specimens by means of standard (non-cyclic) and cyclic pull-out tests. The LWAC and NWAC specimen’s mixes were identical except for the type of the coarse aggregate. It was found that the bond strength determined by non-cyclic pull-out tests showed a higher standard deviation compared to that for compression and splitting tensile strength tests. The shape of the characteristic bond strength–the slip curve was similar in the case of LWAC and NWAC and the failure mode was the same for both types of aggregate. Moreover, the maximum value of bond stress was identical for LWAC and NWAC and also no significant difference was found in the low cycle number fatigue. Both mixes were able to resist the maximum pull-out force multiple times in the case of cyclic loading because there was no time for the development of cracks.

Mechanical characteristics of circular hollow section damper under bidirectional cyclic loading

During earthquake events, hysteretic dampers installed on the elevations composed of the columns and beams of structural frames can resist forces in any direction other than the in-plane direction because of the three-dimensional behavior of buildings and the interrelationship between the dampers and the adjacent members. Circular hollow section dampers (CHSDs), which form a type of steel hysteretic dampers, can be employed as seismic devices because they provide equal structural performance in any horizontal direction according to the geometrical characteristics of the cross-sectional shape. However, the cumulative hysteretic characteristics of CHSDs for multidirectional behavior remain unelucidated; therefore, an empirical review is needed. This study experimentally analyzed the cumulative hysteresis behavior of a CHSD repeatedly loaded sequentially in two horizontal directions (bidirectional cyclic loading). The results obtained were compared with previous experimental results under unidirectional cyclic loading conditions. The comparative analysis using empirical data revealed that the hysteretic curve of the CHSD under bidirectional cyclic loading maintained a relatively constant strength increase ratio and effective stiffness along with a stable spindle shape until the damper reached the ultimate state despite the continuous change in the loading direction. Further, compared with the unidirectional cyclic loading condition, the CHSD under bidirectional cyclic loading exhibited similar structural trends considering the damper aspect ratio and material influence. Moreover, the total accumulated plastic deformation in each loading direction was approximately the same as the cumulative plastic deformation caused by unidirectional cyclic loading.

Influence of Groundwater Depth on Pile–Soil Mechanical Properties and Fractal Characteristics under Cyclic Loading

The analysis of the behavior of soil and foundations when the piles in offshore areas are subjected to long-term lateral loading (wind) is one of the major problems associated with the smooth operation of superstructure. The strength of the pile-soil system is influenced by variations in the water content of the soil. At present, there are no studies carried out analyzing the mechanical and deformational behavior of both the material of the laterally loaded piles and soil with groundwater level as a variable. In this paper, a series of 1-g model tests were conducted to explore the lateral behavior of both soil and monopile under unidirectional cyclic loading, based on the foundation of an offshore wind turbine near the island. The influence of underground water level and cyclic load magnitude on the performance of the pile–soil system was analyzed. To visualize the movements of soil particles during the experimental process, particle image velocimetry (PIV) was used to record the soil displacement field under various cyclic loading conditions. The relationship curves between pile top displacement and cyclic steps, as well as the relationship curves between cyclic stiffness and cyclic steps, were displayed. Combined with fractal theory, the fractal dimension of each curve was calculated to evaluate the sensitivity of the pile–soil interaction system. The results showed that cyclic loading conditions and groundwater depth are the main factors affecting the pile–soil interaction. The cyclic stiffness of the soil increased in all test groups as loading progressed; however, an increase in the cyclic load magnitude decreased the initial and cyclic stiffness. The initial and cyclic stiffness of dry soil was higher than that of saturated soil, but less than that of unsaturated soil. The ability of the unsaturated soil to limit the lateral displacement of the pile decreased as the depth of the groundwater level dropped. The greater the fluctuation of the pile top displacement, the larger the fractal dimension of each relationship curve, with a variation interval of roughly 1.24–1.38. The average increment of the cumulative pile top displacement between each cycle step following the cyclic loading was positively correlated with fractal dimension. Based on the PIV results, the changes in the pile–soil system were predominantly focused in the early stages of the experiment, and the short-term effects of lateral cyclic loading are greater than the long-term effects. In addition, this research was limited to a single soil layer. The pile–soil interaction under layered soil is investigated, and the results will be used in more complex ground conditions in the future.

Bond behavior of early-age concrete and deformed steel bars under unidirectional cyclic loading: experimental study and bond stress–slip model

The bond behavior between early-age concrete and deformed steel bars under unidirectional cyclic loading is of great significance for evaluating the bearing capacity of reinforced concrete structures during construction. In this study, pull-out tests were conducted on specimens of four concrete ages and three strength grades under unidirectional cyclic loading to study the bond failure mechanism. The results showed that, regardless of whether the load level was high or low, unidirectional cyclic loading affected the bond performance between the steel bar and early-age concrete, resulting in irreversible residual deformation. The bond energy increased with the strength grade and concrete age. When the concrete strength grades were the same, the peak and residual strengths increased with increasing age, and the slip at the slip point and peak strength decreased with increasing age. Taking C50 concrete as an example, when the age was increased from 1 to 28 days, the peak and residual strengths increased by 167.87% and 199.76%, respectively, whereas the slip at the slip point and peak strength decreased by 36.84% and 17.48%, respectively. For concrete with the same curing age, as the concrete strength grade increased, the peak bond strength and residual strength gradually increased, and the slip at the slip point and peak strength decreased progressively. For example, when the concrete age was 28 d, if the strength grade increased from C30 to C50, the peak bond strength and residual strength increased by 64.79% and 151.75%, respectively, and the slip at the slip point and the peak strength decreased by 14.29% and 13.24%, respectively. Based on the characteristics of the curve under unidirectional cyclic loading, a bond stress–slip model was established and compared with the test results. The results showed that the proposed models are acceptable.

Lateral and Overturning Resistance of Wind Turbine Foundations Reinforced with Piles on Bedrock by Modelling Experiments

This study evaluated the lateral and overturning resistance of wind turbine foundations reinforced with piles on bedrock through model experiments. In particular, changes in lateral and overturning resistance of wind turbine foundations were analyzed according to cross-sectional size and the presence of piles of wind turbine foundations. As a result, by reducing the cross-section, the lateral resistance of the pile-reinforced wind turbine foundation was compared to the existing wind turbine foundation with large cross-sections and was shown to be 1.68 times greater. In the case of vertical displacements affecting overturning, the safety of overturning was also greater, as the vertical displacement of the pile-reinforced wind turbine foundation was 36% smaller than the existing wind turbine foundation. As a result of the unidirectional cyclic load on a pile-reinforced wind turbine foundation, lateral resistance value was similar to that of the static load in target displacement value, and it showed that the elastic resilience was very large due to pile reinforcement. According to the bending moment measurement of piles embedded in wind turbine foundations and bedrock, bending moments were large in the order of the front row, the right-hand row, and the back row, while the maximum bending moment generation was found on the boundary surface of the wind turbine foundation and the rubble mound layer for the front row, as well as on the boundary surface of the rubble mound layer and bedrock for the right-hand row and back row.

Evaluation of fatigue strength of high-strength grouted connections subjected to unidirectionally reversible cyclic loading

Grouted connections have been widely used in offshore construction to connect two different-sized cylindrical steel tubes; these connections include pile-to-sleeve grouted connections for mono-piles of wind turbine towers and pile-to-jacket-leg grouted connections for jacket structures. Because the grouted connections in seawater are mainly subjected to environmental loadings induced by waves and wind loading, high-strength grouted connections have been widely applied in offshore structures to enhance the structural performance and to prevent fatigue failure generated by the cyclic environmental loading.In this study, the fatigue performance of high-strength grouted connections with shear keys subjected to unidirectional reversible cyclic loadings in dry conditions was evaluated based on experimental tests, numerical evaluations, and comparative analysis based on a test database established using prior studies. It was revealed that the grout strength, shear-key density, and stress ratio have a significant influence on fatigue performance. Finally, general aspects of effects of grout strength, shear-key density, and stress ratio that influence the fatigue capacity of high-strength grouted connections were discussed, and a design equation for the unidirectional cyclic loading condition in dry conditions was presented.

Analysis of influencing factors of lateral soil resistance distribution characteristics around monopile foundation for offshore wind power

A numerical calculation model of offshore wind power monopile foundation in heterogeneous soil was established by finite element software ABAQUS. The wave, ocean current and wind load on the monopile foundation were equivalent to the bidirectional symmetrical cyclic load. The influence rules of different factors on variation characteristics along pile circumferential direction of pile inner and outer walls soil resistance under horizontal cyclic load were studied. The results show that the load frequency has a great influence on the passive earth pressure at the side of pile outer wall and the load on the inner wall of the pile in loading area increases with the increase of load frequency. As the loading height increases, the load on both sides of the inner wall of the pile increases, while the load on the middle part of the inner wall of the pile decreases. The resistance of soil in the inner wall of pile varies most along the circular direction of pile under unidirectional cyclic load. When the positive horizontal cyclic load is applied, the larger the vertical load is, the larger the horizontal load borne by the pile outer wall along the pile ring is, but the more conducive to the stability of the pile inner wall, while the negative load is just the opposite. When the wall thickness is small, the soil resistance of the inner and outer walls of the pile changes obviously along the pile circumferential direction.

Empirical approach based on centrifuge testing for cyclic deformations of laterally loaded piles in sand

A systematic study into the response of monopiles to lateral cyclic loading in medium dense and dense sand was performed in beam and drum centrifuge tests. The centrifuge tests were carried out at different cyclic load and magnitude ratios, while the cyclic load sequence was also varied. The instrumentation on the piles provides fresh insights into the ongoing development of net stresses, bending moments and deflections as cycling progresses. Parallels between the test results and corresponding cyclic triaxial tests are drawn. The paper combines the results from this study with those from previous experimental investigations to provide empirical design recommendations for monopiles subjected to unidirectional cyclic loading.

Flexural behavior of ungrouted post-tensioned concrete masonry beams with unbonded bars

Precast and post-tensioned systems increase the efficiency of masonry beams in terms of labor and time of construction. This study is aimed at evaluating the flexural behavior of ungrouted post-tensioned concrete masonry beams, with eccentric bars, and the effect of the main design parameters on their structural performance when subjected to monotonic and unidirectional cyclic loading until failure. All beams failed in flexure by crushing of the masonry in the compression region. Masonry strength was the design parameter that had the most significant effect on the load-deflection behavior of post-tensioned masonry beams. The test results showed that the measured nonlinear load-deflection behavior and the correlation between bar stress increase and midspan deflection can be estimated using a multi-linear relationship. Beams tested under monotonic and unidirectional cyclic loading exhibited comparable load-deflection envelope response.

The response of suction caissons to multidirectional lateral cyclic loading in sand over clay

Offshore wind turbines and their foundations experience loading from varying directions, yet understanding of the effect of changes in the direction of applied cyclic loading on foundation response remains limited. This study investigates the behaviour of a suction caisson foundation in sand over clay under various types of multidirectional cyclic loading. The findings are significant: although the caisson rotation (or tilt) is affected by changes in load direction, the caisson rotation is less than that in a unidirectional test with the same cyclic load magnitude and symmetry, meaning that a unidirectional test may serve as a conservative estimate of caisson rotation for multidirectional loading. This contrasts with previous findings for monopiles in sand, where prediction of the accumulated displacement based on unidirectional cyclic loading was substantially un-conservative for multidirectional loading. Changes in unloading stiffness due to changes in load direction are slight, varying by an amount that is no greater than the change measured over the course of a unidirectional test. Foundation stiffness and ultimate capacity following multidirectional cyclic loading are largely unaffected, unlike unidirectional cyclic loading, where consolidation of the clay layer improves both the stiffness and capacity.

Influence of cyclic loading and frost on the behavior of bond of three‐wire strand

This paper presents the results of an experimental research investigating the influence of static and cyclic loading and frost impact on the bond between concrete and 6.2 mm three‐wire‐strand. From the pullout tests, it was determined that a cyclic load did not negatively influence the bond strength of three‐wire strand. On the contrary, the influence of cyclic loading after 200 freezing and thawing cycles resulted in a significant decrease of bond strength of three‐wire strand. Additionally, the new parameters for bond stress–slip relationship suggested in Model Code 2010 are proposed for the 6.2 mm three‐wire strand cast with concrete having compressive strength of 82.6 MPa with standard deviation of 6.65 MPa. The results of dynamic coefficient b for 6.2 mm three‐wire strand used in relationship between a slip of reinforcement and a number of loading cycles under unidirectional cyclic loading suggested in Model Code 2010 are analyzed.

Bi-directional and uni-directional bending fatigue performance of unreinforced and carbon fiber reinforced polyamide 66 spur gears

Gears are generally subjected to uni-directional cyclic loads, however, in applications like actuators of satellite launchers, gears experience bi-directional cyclic loads due to its rotation in both clockwise and counter clockwise directions. In the present study, bidirectional and uni-directional bending fatigue performance of injection molded unreinforced and carbon fiber reinforced polyamide 66 gears were evaluated using a test rig developed in-house. During testing, torque applied and the angular displacement of the gears were continuously recorded. The net surface temperature measured by an infrared camera was higher for the gears subjected to bi-directional loading as compared to uni-directional loading. Reinforced gears exhibited about 84% lower temperature than that of unreinforced gears, due to lesser material hysteresis and better thermal conductivity. Fatigue failure of unreinforced gears exhibited almost straight root cracks and confirmed ductile failure. Fatigue failure of reinforced gears exhibited tortuous crack path due to the existence of reinforced fibers. For both the type of loads, carbon fiber reinforced gears exhibited superior bending performance than unreinforced gears (2-51 times) due to its improved mechanical and thermal properties. Significant fatigue life reduction (83-99%) of the tested gears, were observed when subjected to bi-directional loading when compared to that of uni-directional loading.

Low Reversed Cyclic Loading Test for New Precast Concrete Beam to Column Connection

On the basis of the domestic and overseas scholars’ research, it is innovative to put forward a new precast frame beam-to-column connection. One full-scale beam-to-column connection in a precast concrete（PC）frame and a beam-to-column connection in a cast-in-place concrete structures were tested under uni-directional cyclic loading that simulated earthquake-type motions. The new-type beam-to-column joint in a precast concrete frame consisted of cast-in-place columns and precast beams. Test results showed that（a）The property and action of precast connections are similar to the cast-in-place connection, it showed that both precast connections and cast-in-place connection have similar seismic performance; (b) The cracking load of precast connection are similar to the cast-in-place connection; (c) All the columns of all specimens did not destroy, it illustrate that it conform to the strong column weak beam. (d) In this experiment test, because the control factor use the bending capacity of beam, it also play the same plastic hinge mechanism as the traditional component. All the longitudinal reinforcement destruction ahead of stirrup in beams, which should be confirmed the guiding ideology of strong shear weak bending. On the basis of the test results, design considerations for the beam-to column connection were recommended.

Effects of Variation of Axial Load and Bidirectional Loading on Seismic Performance of GFRP Retrofitted Reinforced Concrete Exterior Beam-Column Joints

Most of the experimental studies available in literature on the seismic assessment and retrofit of existing, poorly detailed, reinforced concrete (RC) beam-column joints, typical of pre-1970s construction practice, have concentrated on the two-dimensional (2D) response, using unidirectional cyclic loading testing protocol and constant axial load. Even more limited information is available on the performance of exterior (corner) three-dimensional (3D) RC beam-column joints with substandard detailing subjected to bidirectional loading regime. In this study, the results of a comprehensive experimental program is presented, aiming to show the effects of varying axial and bidirectional loading on the seismic performance of deficient exterior RC beam-column joints before and after retrofit. Ten exterior beam-column joint subassemblies are tested, including four as-built specimens and six retrofitted specimens using externally bonded glass fiber-reinforced polymer (GFRP) sheets. Test results are herein presented...