• Steel and Composite Structures
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• v.34 no.4
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• pp.499-509
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• 2020

The purpose of this paper is to investigate the degradation law of stiffness of steel-concrete composite beams after certain fatigue loads. First, six test beams with stud connectors were designed and fabricated for static and fatigue tests. The resultant failure modes under different fatigue loading cycles were compared. And an analysis was performed for the variations in the load-deflection curves, residual deflections and relative slips of the composite beams during fatigue loading. Then, the correlations among the stiffness degradation of each test beam, the residual deflection and relative slip growth during the fatigue test were investigated, in order to clarify the primary reasons for the stiffness degradation of the composite beams. Finally, based on the stiffness degradation function under fatigue loading, a calculation model for the residual stiffness of composite beams in response to fatigue loading cycles was established by parameter fitting. The results show that the stiffness of composite beams undergoes irreversible degradation under fatigue loading. And stiffness degradation is associated with the macrobehavior of material fatigue damage and shear connection degradation. In addition, the stiffness degradation of the composite beams exhibit S-shaped monotonic decreasing trends with fatigue cycles. The general agreement between the calculation model and experiment shows good applicability of the proposed model for specific beam size and fatigue load parameters. Moreover, the research results provide a method for establishing a stiffness degradation model for composite beams after fatigue loading.

• Advances in aircraft and spacecraft science
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• v.11 no.1
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• pp.1-21
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• 2024

The stiffness reduction of cross-ply composite laminates featuring a transverse cracking and delamination within the mid-layer is predicted through utilization of a modified shear-lag model, incorporating a stress perturbation function. Good agreement is obtained by comparing the prediction models and experimental data. The material characteristics of the composite are affected by fluctuations in temperature and transient moisture concentration distribution in desorption case, based on a micro-mechanical model of laminates. The transient and non-uniform moisture concentration distribution induces a stiffness reduction. The obtained results demonstrate the stiffness degradation dependence on factors such as cracks density, thickness ratio and environmental conditions. The present study underscores the significance of comprehending the degradation of material properties in the failure progression of laminates, particularly in instances of extensive delamination growth.

• Steel and Composite Structures
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• v.51 no.6
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• pp.697-712
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• 2024

In this paper, the dynamic flexural stiffness of concrete-filled steel tubular (CFST) members is investigated based on vibration modal testing and a Bayesian model updating procedure. To reflect the actual service states of CFST members, a 3-stage modal testing procedure is developed for 6 circular CFST beam-columns, in which the modal parameters of the specimens under varying axial load levels are extracted. In the model updating procedure, a Timoshenko beam element model is first established, in which the influence of shear deformation and rotational inertia are incorporated. Subsequently, a 2-round Bayesian model updating strategy is proposed to calculate the dynamic flexural stiffness of the specimens, which could effectively consider the influence of physical constraints in the updating process and achieve reasonably well results. Analysis of the updating results shows that with the increase of the axial load level, degradation of the flexural stiffness is significantly influenced by the load eccentricity. It shows that the cracking of the core concrete is the primary reason for the flexural stiffness degradation of CFST beam-columns. Finally, based on comparison with equations proposed by several design standards, the calculation methods for the dynamic flexural stiffness of CFST members is recommended.

• Structural Engineering and Mechanics
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• v.45 no.5
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• pp.655-676
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• 2013

In this study, strength reduction factors and inelastic displacement ratios are investigated for SDOF systems with period range of 0.1-3.0 s considering soil structure interaction for earthquake motions recorded on soft soil. The effect of stiffness degradation on strength reduction factors and inelastic displacement ratios is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastoplastic model is used to represent non-degrading structures. The effect of negative strain - hardening on the inelastic displacement and strength of structures is also investigated. Soil structure interacting systems are modeled and analyzed with effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. New equations are proposed for strength reduction factor and inelastic displacement ratio of interacting system as a function of structural period($\tilde{T}$, T) ductility (${\mu}$) and period lengthening ratio ($\tilde{T}$/T).

• Structural Engineering and Mechanics
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• v.50 no.2
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• pp.231-240
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• 2014

In the present research, fatigue behavior of chopped strand mat/epoxy composites has been studied with two different techniques. First, the normalized stiffness degradation approach as a well-known model for unidirectional and laminated composites was utilized to predict the fatigue behavior of chopped strand mat/epoxy composites. Then, the capability of the fatigue damage accumulation model for chopped strand mat/epoxy composites was investigated. A series of tests has been performed at different stress levels to evaluate both models with the obtained results. The results of evaluation indicate a better correlation of the normalized stiffness degradation technique with experimental results in comparison with the fatigue damage accumulation model.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2000.04a
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• pp.357-366
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• 2000

Longitudinal dynamic behaviors of a bridge system under seismic excitations are examined with various magnitudes of peak ground accelerations. The stiffness degradation due to abutment-soil interaction is considered in the bridge model which may play the major role upon the global dynamic characteristics. The idealized mechanical model for the whole ridge system is proposed by adopting the multiple-degree-of-freedom system which can consider components such as pounding phenomena friction at the movable supports rotational and translational motions of foundations and the nonlinear pier motions. The abutment-soil interaction is simulated by utilizing the one degree-of-freedom system with nonlinear spring. The stiffness degradation of the abutment-soil system is found to increase the relative displacement under moderate seismic excitations.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2001.04a
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• pp.353-360
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• 2001

An analysis procedure of 2-dimensional bridge dynamics has been developed by using force-deformation model, which simulates the pier motion under biaxial bending due to the bi-directional input seismic excitations. A three-dimensional mechanical model is utilized, which can consider the other major phenomena such as pounding, rotation of the superstructure, abutment stiffness degradation, and motions of the foundation motions. The bi-directional dynamic behaviors of the bridge are then examined by investigating the relative displacements of each oscillator to the ground. It is found that the nonlinearity of the pier due to biaxial bending affects the pier motions, but the global bridge behaviors are greatly governed by the pounding phenomena and stiffness degradation of the abutment-backfill system. Especially, the relative displacement of the abutment system (A2) with movable supports to the ground is increased about 30% due to the abutment stiffness degradation.

• Structural Engineering and Mechanics
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• v.45 no.6
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• pp.741-758
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• 2013

In this study, inelastic displacement ratios are investigated for existing systems with known lateral strength considering soil structure interaction. For this purpose, SDOF systems for period range of 0.1-3.0 s with different hysteretic behaviors are considered for a number of 18 earthquake motions recorded on soft soil. The effect of stiffness degradation on inelastic displacement ratios is investigated. The Modified Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastoplastic model is used to represent non-degrading structures. Soil structure interaction analyses are conducted by means of equivalent fixed base model effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. A new equation is proposed for inelastic displacement ratio of system with SSI with elastoplastic or degrading behavior as a function of structural period ($\tilde{T}$), strength reduction factor (R) and period lengthening ratio ($\tilde{T}$/T). The proposed equation for $\tilde{C}_R$ which takes the soil-structure interaction into account should be useful in estimating the inelastic deformation of existing structures with known lateral strength.

• Structural Engineering and Mechanics
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• v.64 no.3
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• pp.287-292
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• 2017

In the present research, an available flexural stiffness degradation model was modified and a new comprehensive model called "X-NFSD" was developed. The X-NFSD model is capable of predicting the flexural stiffness degradation of composite specimen at different states of stresses and at room temperature. The model was verified by means of different experimental data for chopped strand mat/epoxy composites under displacement controlled bending loading condition at different displacements and states of stresses. The obtained results provided by the present model are impressively in very good agreement with the experimental data and the mean value of error of 5.4% was achieved.

• Journal of the Korean Society for Railway
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• v.16 no.6
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• pp.466-472
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• 2013

The use of a concrete track is gradually growing in urban and high-speed railways in many part of the world. The resilient pad, which is essentially when concrete tracks are used, plays the important role of relieving the impact caused by train loads. The simple fatigue test[1] to estimate the variable stiffness of resilient pads is usually performed, but it differs depending on the practical conditions of different railways. In this study, the static stiffness levels of used resilient pads according to passing tonnages levels were measured in laboratory tests. Also, the simple fatigue test and the multi-stress accelerated degradation test for new resilient pads were performed in a laboratory. The static stiffness of the used pad was compared with the results of tests of usage times and cycles. The results of the comparison showed that the variable static stiffness levels of the used pad were similar to results of the multi-stress accelerated degradation test considering the fatigue and heat load. With a T-NT equation related to the degree of the multi-stress accelerated degradation, a model of multi-stress accelerated degradation for a resilient pad was devised. It was found through this effort that the total acceleration factor was approximately 2.62. Finally, this study proposes an equation for a multi-stress accelerated degradation model for polyurethane resilient pads.