• Computers and Concrete
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• v.8 no.2
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• pp.193-206
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• 2011

The use of Carbon Fiber Reinforced Polymers (CFRP) to strengthen reinforced concrete beams under bending and shear has gained rapid growth in recent years. The performance of shear strengthened beams with externally bonded CFRP laminate or fabric strips is raising many concerns when the beam is loaded under cyclic loading. Such concerns warrant experimental, analytical and numerical investigation of such beams under cyclic loading. To date, limited investigations have been carried out to address this concern. This paper presents a numerical investigation by developing a nonlinear finite element (FE) model to study the response of a cantilever reinforced concrete T-beam strengthened in shear with side bonded CFRP fabric strips and subjected to cyclic loading. A detailed 3D nonlinear finite element model that takes into account the orthotropic nature of the polymer's fibers is developed. In order to simulate the bond between the CFRP sheets and concrete, a layer having the material properties of the adhesive epoxy resin is introduced in the model as an interface between the CFRP sheets and concrete surface. Appropriate numerical modeling strategies were used and the response envelope and the load-displacement hysteresis loops of the FE model were compared with the experimental response at all stages of the cyclic loading. It is observed that the responses of the FE beam model are in good agreement with those of the experimental test. A parametric study was conducted using the validated FE model to investigate the effect of spacing between CFRP sheets, number of CFRP layers, and fiber orientation on the overall performance of the T-beam. It is concluded that successful FE modeling provides a practical and economical tool to investigate the behavior of such strengthened beams when subjected to cyclic loading.

• Tunnel and Underground Space
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• v.9 no.4
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• pp.350-363
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• 1999

The precision cyclic shear test system was established to investigate the mechanical characteristics of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for saw-cut joints and artificial rough rock joints using Hwangdeung granite and Yeosan marble. Surface roughness and aperture characteristics of specimens were examined by measuring surface topography using the laser profilometer. Peak shear strength, phase difference during loading and unloading, and anisotropic shear behavior were investigated throughout the cyclic shear test results. These features and their subsequent variations in each loading cycle are significantly dependent upon the second order asperities and the strength of intact rock. It was observed that degradation of asperities for rough rock joints under cyclic shear loading followed the exponential degradation laws of asperity angle and that the mechanism for asperity degradation would be different depending upon the normal stress level, roughness of joint surface and the loading stage.

• Structural Engineering and Mechanics
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• v.58 no.6
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• pp.1021-1044
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• 2016

Different versions of a damage index (DI) along with a formulation to find the number of cycles at failure due to fatigue, applicable to reinforced concrete (RC) structures are presented. These are based on an energetic analysis method and applicable to both global and local levels. The required data can be found either from the numerical simulation of structures or from the experimental tests. A computer program has been developed to simulate numerically the nonlinear behavior of RC columns under cyclic loading. The proposed DI gives a regular distribution of structural damages up to failure and is validated by the results of the tests carried out on RC columns subjected to cyclic loading. In general, the local and global damage indices give approximately similar results, while each of them has its own advantages. The advantage of the implicit version of DI is that, it allows the comparison of the results with those of the monotonic loading case, while the explicit version makes it possible to estimate the number of loading cycles at failure due to fatigue, and the advantage of the simplified version is that; the monotonic loading data is not needed for the cyclic loading case.

• Journal of the Korea Concrete Institute
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• v.13 no.3
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• pp.244-250
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• 2001

One of the reasons for brittle failure in reinforced concrete structures subjected to severe earthquake is due to large slip between reinforcing steel and concrete. This study aims to evaluate effects of deformation patterns of ribbed reinforcing bars on bond under cyclic loading. Bond test specimens were constructed with machined bars to test the newly developed reinforcing bars with high relative rib areas. The degree of confinement is also another key parameter in this bond test. From the test results under monotonic and cyclic loading, bond strength and stiffness were evaluated. Bond strength and bond stiffness increase as relative rib areas under cyclic loading for specimens highly confined by transverse reinforcement. The increase rates of the bond performance under cyclic loading are larger than those of specimens under monotonic loading. The developed bars with high relative rib areas will contribute for better bond performance for reinforced concrete structures subjected to severe seismic loadings.

• The Journal of Advanced Prosthodontics
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• v.10 no.6
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• pp.415-421
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• 2018

PURPOSE. This study investigated the effects of abutment screw lengths on screw loosening and removal torque in external connection implants after oblique cyclic loading. MATERIALS AND METHODS. External connection implants were secured with abutment screws to straight abutments. The abutment-implant assemblies were classified into seven groups based on the abutment screw length, with each group consisting of five assemblies. A cyclic load of 300 N was applied at a $30^{\circ}$ angle to the loading axis until one million cycles were achieved. Removal torque values (RTVs) before and after loading, and RTV differences were evaluated. The measured values were analyzed using repeated measures of analysis of variance with the Student-Newman-Keuls multiple comparisons. RESULTS. All assemblies survived the oblique cyclic loading test without screw loosening. There was a significant decrease in the RTVs throughout the observed abutment screw lengths when the abutment-implant assemblies were loaded repeatedly (P<.001). However, the abutment screw length did not show significant difference on the RTVs before and after the experiment when the abutment screw length ranged from 1.4 to 3.8 mm (P=.647). CONCLUSION. Within the limit of this experiment, our results indicate that the abutment screw length did not significantly affect RTV differences after oblique cyclic loading when a minimum length of 1.4 mm (3.5 threads) was engaged. These findings suggest that short abutment screws may yield stable clinical outcomes comparable to long screws in terms of load resistance.

• Structural Engineering and Mechanics
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• v.61 no.5
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• pp.625-635
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• 2017

Sets of nonlinear formulations together with an energy-based damage index (DI) are proposed to model the behavior and quantify the damage of the confined and unconfined concretes under monotonic and cyclic loading. The proposed formulations and DI can be employed in numerical simulations to determine the stresses and the damages to the fibers or the layers within the sections of reinforced concrete (RC) components. To verify the proposed formulations, an adaptive finite element computer program was generated to simulate the RC structures subjected to monotonic and cyclic loading. By comparing the simulated and the experimental test results, on both the full-scale structural members and concrete cylindrical samples, the proposed uniaxial behavior modeling formulations for confined and unconfined concretes under monotonic and cyclic loading, based on an iterative process, were accordingly adjusted, and then validated. The proposed formulations have strong mathematical structures and can readily be adapted to achieve a higher degree of precision by improving the relevant coefficients based on more precise tests. To apply the proposed DI, the stress-strain data of concrete elements is required. It can easily be calculated by using the proposed nonlinear constitutive laws for confined and unconfined concretes in this paper.

• Geotechnical Engineering
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• v.10 no.1
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• pp.7-18
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• 1994

A new model for describing the behavior of clay under monotonic and cyclic loading is proposed. This model uses the hyperbolic representation for the stress -strain relationship in overconsolidated state and it describes undrained effective stress path on the basis of the critical state theory. The developed constitutive model by using an energy dissipation equation can describe the behavior of clay in heavily overconsolidated state as u.ell as lightly overconsolidated state under monotonic loading. In order to extend the model for the behavior of clay under cyclic loading, a shift function of undrained stress spacing ratio is introduced in the constitutive model developed for monotonlc loading. A single additional parameter is required to represent the cyclic effect and it can be reasonably deter mined from the test results. The measured behavior in undrained cyclic triaxial tests has been easily and precisely predicted by the newly developed constitutive model.

• Geotechnical Engineering
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• v.11 no.1
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• pp.23-40
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• 1995

In order to investigate the characteristics of the lateral earth pressure at rest under hysteretic Ko -loading l unloading conditions. Seven types of multicyclic models have been studied experimentally using dry sand. For this study a new type of Ko -oedometer appal attn is developed, and horizontal pressure is accurately measured. The multi cyclic models consist of largely 3 cases : (i) Ko-test under the same loading/unloading condition, (ii) multi-cyclic loading /unloading Ko -test exceeding the maximum prevertical stress, and (iii) multi-cyclic loading l unloading Ko -test within the mazimium prevertical stress. As a result, the multi -cyclic model showed that single-cyclic model could be extended as well, in which the exponents for unloading condition(a and a') and the reloading coefficients(m, and m*) were mainily dependent upon type of stress model, number of cycles and relati ve density.

• Steel and Composite Structures
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• v.28 no.1
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• pp.83-98
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• 2018

This research was conducted to study the behavior of exterior concrete beam-column joints with reinforced shape memory alloy (SMA) bars tested under cyclic loading. These bars benefit from superelastic behavior and can stand high loads without residual strains. The experimental part of the study, 8 specimens of exterior concrete beam-column joints were made and tested. Two different types of concrete with 30 and 45 MPa were used. Four specimens contained SMA bars and 4 specimens contained steel bars in beam-column joints. Furthermore, different transverse reinforcements were used in beams investigate the effects of concrete confinement. Specimens were tested under cyclic loading. Results show that SMA bars are capable of recentering to their original shape after standing large displacements. Due to the superelastic behavior of SMA bars, cracks at the joint core vanish under cyclic loading. As the cyclic loading increased, bending failure occurred in the beam outside the joint core. In the analytical parts of the study, specimens were simulated using the SeismoStruct software. Experimental and analytical results showed a satisfactory correlation. Plastic hinge length at the beam joint for specimens with SMA and steel bars was calculated by empirical equations, experimental and analytical results. It was shown that Paulay's and Priestley's equations are appropriate for concrete beam-column joints in both types of bars.

• Geomechanics and Engineering
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• v.14 no.2
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• pp.115-129
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• 2018

This paper presents a set of results of plate load tests that imposed incremental cyclic loading to a sandy soil bed containing multiple layers of granulated rubber-soil mixture (RSM) at large model scale. Loading and unloading cycles were applied with amplitudes incrementally increasing from 140 to 700 kPa in five steps. A thickness of the RSM layer of approximately 0.4 times the footing diameter was found to deliver the minimum total and residual settlements, irrespective of the level of applied cyclic load. Both the total and residual settlements decrease with increase in the number of RSM layers, regardless of the level of applied cyclic load, but the rate of reduction in both settlements reduces with increase in the number of RSM layers. When the thickness of the RSM layer is smaller, or larger, settlements increase and, at large thicknesses may even exceed those of untreated soil. Layers of the RSM reduced the vertical stress transferred through the foundation depth by distributing the load over a wider area. With the inclusion of RSM layers, the coefficient of elastic uniform compression decreases by a factor of around 3-4. A softer response was obtained when more RSM layers were included beneath the footing damping capacity improves appreciably when the sand bed incorporates RSM layers. Numerical modeling using "FLAC-3D" confirms that multiple RSM layers will improve the performance of a foundation under heavy loading.