Search

A hysteretic model for concrete structure subjected to cyclic tension and tension–compression reversals is presented. The proposed model was intended to predict the complex hysteretic behavior of concrete under cyclic loading in a simple and practical way. Based on the analysis of the characteristic hysteretic behavior of concrete, the residual deformation in tension was considered principally due to the incomplete closure of the opening cracks. The mechanism for the hysteretic behavior of concrete under tension–compression reversals was suggested as the crack closing and opening. Considering the application within different numerical approaches, dimensionless stress-deformation coordinates was adopted to perform the hysteretic model. The unloading and reloading paths have been derived from the crack closing and opening mechanism and were represented as straight lines in the model. Partial unloading and reloading were considered in both cyclic tension and tension–compression reversals. The proposed model has been validated by comparison with available experimental results and the seismic response of a SDOF system with the hysteretic model has been analysed.

Low strain dynamic testing is an important nondestructive testing method in the engineering. However, the pile foundation material is usually assumed as having a uniform elastic modulus in low strain simulations. In this paper, we consider the elastic modulus of concrete as having an inhomogeneous elastic modulus that is described by the Weibull distribution model. An explicit algorithm was adopted in order to solve the model. The finite element method (FEM) was used to simulate the low strain dynamic test of a 3D pile. The response velocity characteristics of different shape parameters were obtained using this method, and the Daubechies wavelet transform was used to analyze the characteristics of the wavelet modulus. The result shows that simulation response velocity has a correlation with the different homogeneity of the elastic modulus.

This paper investigates the free vibration of a homogeneous Euler–Bernoulli beam with multiple transverse cracks under non-symmetric boundary conditions. The differential equation is formulated by introducing Dirac’s delta function into the uniform flexural stiffness, and the close-form solution of mode shapes is then derived by applying the Laplace transform technique. The proposed method is validated against existing experimental method for damaged cantilever beams. With the validated method, a parametric study is performed to study the effect of crack numbers, damage parameters and crack locations on the natural frequencies and mode shapes for three non-symmetric boundary conditions (pinned–clamped, clamped–free shear and pinned–free shear).

The fracture parameters (the fracture energy and the effective length of the fracture process zone for an infinitely large specimen) in the size effect model of concrete exhibit a large scatter as measured in most of the experimental studies. This phenomenon is ubiquitous and has presented a great challenge to characterize the structural failure over the last decade. In order to remove the perplexing issue, this paper develops two models to predict the two fracture parameters using the artificial neural network (ANN) methodology. The proposed models are verified by using 77 experimental data collected from the literature. The results demonstrate that the two ANN-based size effect model models (ANN-I and ANN-II) are viable for predicting the fracture parameters and yield more accurate results than those obtained from the conventional regression formulations. Additionally, a parametric study is employed to evaluate the impact of each independent material parameter on the fracture parameters.

Structural failure process and failure mode are two important issues that a lot of attentions has been paid in structural failure analysis. It has been revealed that the structure failure is mostly started from local material level, and then developed to global destruction. The failure process is inherent with the multiscale effects, and there could be varies failure modes in the process. A characteristic failure curve could reflect the failure process and the singular point of the curve indicates the change of system status. To obtain the local information of the characteristic failure curve, the wavelet multiscale method is adopted for analyzing. With the local extreme value of wavelet coefficients in each scale, the singularity point of the curve can be figured out and correspondingly the structural critical load is determined. Furthermore, by defining an index called the Failure Extent Index (FEI) F m , the incremental ratio of plastic energy and elastic energy, two mainly failure modes, namely the progressive and catastrophic, are studied. Analysis results show that the proposed method, with strict mathematic theory basis, could capture the local feature of failure curve, and could obtain the critical load accurately. Besides, by analyzing the variation tendency of the index F m , the failure mode can be determined. The proposed method for determining the critical load and failure mode for solid structure can provide some valuable information for the structure and the obtained results are available for further analysis and practical operation.

Damage in the structures alters the structural dynamic behavior. Damage generally occurs locally in the components of structures, and its effect often exhibits in the changes of both local and global characteristics of structures. Better understanding of this relationship helps to monitor and assess the condition of structures and develop dynamics-based structural health monitoring techniques. In this study, the relationship between the local damage and structural dynamic behavior is investigated. To validate the concept, an experiment of a three-dimensional (3-D) steel frame structure with different magnitudes of local damage is illustrated. The experimental results indicate that the change of global resonant response between the intact and damage structure is not obvious. The change of local resonant responses measured near the location of saw-cut crack damage is quite significant. The experimental study conducted indicates that the local resonant responses at the low order resonance can be used as an effective damage identification method to detect and quantify the local damage in the 3D frame structures.

It is a common method to strengthen the damaged RC structures with bonded steel plates. At present the ultimate bearing capacity of RC structures strengthened with bonded steel plates is calculated mostly using the theory based on the test. Four beams, including one reference beam, two strengthened concrete beams in primary force and secondary force respectively, and one strengthened concrete beam which was not anchored enough, were tested under four-point bending (4PB) in order to get the data of strain of longitudinal bars, bonded bottom steel plate in tension and deflection of beams in the middle span. The experimental program was supported by a three-dimensioned finite analysis using ABAQUS. At the end of experiments and finite analysis, it is concluded that the investing strengthening technique can significantly improve the load-carrying capacity and the phenomenon of stress concentration at the end of interface, as well as the damage at interface, can be well simulated with cohesive element provided by ABAQUS.

Understanding of cracking behavior of cover concrete is important for assessment of the remaining service life of corrosion-affected concrete structures. Predicting initiation of concrete cracking is commonly based on the evolution of stresses caused by the volume expansion of corrosion products on reinforcing bars. This paper presents an analytical method that is capable of providing the solution to two-dimensional (2D) elastic stress field caused by nonuniform volume expansion around the corroding reinforcing bars. The developed method features the formulation of a displacement model, which is applied as a boundary condition to simulate the effect of nonuniform corrosion on the deformation of the surrounding concrete. A closed-form solution to the stress field is obtained using the complex variable method of Muskhelishvili. Validation of the solution is supported by a comparison with finite-element-based simulation results. The solution reveals the evolution of stresses surrounding two typical locations...

The global stability of high arch dam is one of the key problems in the safety study of arch dams, but no feasible method with theoretical basis is available. In this paper, based on the stability theory of mechanical system, it is demonstrated that the global failure of high arch dams belongs to a physical instability starting from local strength failure, which is the extreme point instability according to the characteristics of load-displacement curve obtained from the failure process of dam-foundation system. So the global failure of dam-foundation system should be studied with the stability theory of mechanical system. It is also pointed out that the current stability analysis methods used in engineering are consistent with the stability theory, but not established according to the mechanical system stability theory directly. A perfect method can be obtained through the study of physical disturbance equations.

A general initial water penetration (seepage) fracture criterion for concrete is proposed to predict whether or not harmful water penetration (hydraulic fracturing), other than microcracking, will occur in concrete structures in a severe high water pressure environment. The final regression, of the different macroscopic failure types in concrete to microscopic Mode I cracking, allows the use of only one universal criterion to indicate the damage. Thus, a general initial water penetration fracture criterion is approximately defined as a strain magnitude of 1000×10−6, based on the concept of tensile strain derived from experimental results in the relevant literature. Then, the locations of harmful water penetration fracture (hydraulic fracture) in the high arch dam mass of the Jinping first class hydropower project are analyzed using the nonlinear finite element method (FEM) according to the proposed criterion. The proposed criterion also holds promise for other concrete structures in high water pressure environments.

In this paper, the problem of modeling crack in 2D viscoelastic media is studied using the extended finite element method. The paper focuses on the definition of enrichment functions suitable for cracks assessment in viscoelastic media and the generalized domain integrals used in the determination of crack tip parameters. The opening mode and mixed mode solutions of crack tip fracture problems in viscoelastic media are also undertaken. The results obtained by the proposed method show good agreement with the analytical methods and provide reasonable background information to enhance the modeling of crack growth in viscoelastic media.

To get the actual ultimate bearing capacity of concrete dam, the effect of geometric nonlinearity and strain softening on it, which appears in the failure process of concrete dam, is studied. Overload method is adopted to obtain the bearing capacity of a concrete dam by taking into consideration strain softening in the material constitutive law, geometric nonlinearity in geometric equation and equilibrium differential equation. Arc-length method is used to find the extreme point and descending branch of the load-displacement curve of the dam. The results present that the effect cannot be ignored. And geometric nonlinearity of structure and strain softening of materials should be considered for numerical analysis of ultimate bearing capacity of a concrete dam.

Gravity dam is a typical structure that has been frequently used in the fields of water conservancy engineering, and the safety of the structure has received widespread attention recently. Due to earthquakes or other reasons, gravity dams normally have damage such as cracks in practical service. Damage in the structures can alter the structural dynamic behavior and seriously affect structural performance. Maintaining safety and integrity of the gravity dam structures requires a better understanding of dynamic response of structure with damage and associated damage detection method. In order to study thoroughly the dynamic behavior of gravity dam with damage, the sweep vibration responses of the gravity dam with and without damage are investigated. The experimental results show that the peak-peak acceleration responses all increase for the structure is with crack. At the same time, a structural damage detection method, i.e., the local damage factor (LDF) method, is considered in the study of gravity dam damage detection when the dam is subjected to the base excitation. It is shown that the LDF method can be used as a damage index and is capable of evaluating both the presence and relative severity of structural damage, and it can be used as a viable condition assessment and damage identification technique to detect and quantify the damage in the gravity dam.

The paper is devoted to proposing a constitutive model based on micromechanics. The joints in rock masses are treated as penny-shaped inclusion in solid but not through structural planes by considering joint density, closure effect, joint geometry. The mechanical behavior of the joints is represented by an elasto-plastic constitutive law. Mori-Tanaka method is used to derive the relationship between the joint deformations and macroscopic strains. The incremental stress-strain relationship of rock masses is formulated by taking the volume average of the representative volume element. Meanwhile, the behavior of joints is obtained. By using implicit integration algorithms, the consistent tangent moduli are proposed and the method of updating stresses and joint displacements is presented. Some examples are calculated by ABAQUS user defined material subroutine based on this model.

The extended finite element method (XFEM) is a new numerical method for modeling discontinuity. Research about numerical modeling for concrete hydraulic fracturing by XFEM is explored. By building the virtual work principle of the fracture problem considering water pressure on the crack surface, the governing equations of XFEM for hydraulic fracture modeling are derived. Implementation of the XFEM for hydraulic fracturing is presented. Finally, the method is verified by two examples and the advantages of the XFEM for hydraulic fracturing analysis are displayed.

Damage in a structure alters its dynamic characteristics. Significant research has been conducted in damage detection and structural health monitoring using dynamics-based techniques. But simultaneously determining the presence, severity, and location of damage using the existing damage detection methods can still prove challenging. In this study, a new practical method of structural damage detection called Local Damage Factor (LDF) is presented, which is capable of determining the presence, severity, and location of structural damage at the same time. By including the dynamic characteristics of the intact local structure in the LDF method, the influence of structural nonlinearity, imperfections, and system noise is considered, so that the accuracy of damage detection is improved. Furthermore, a modified LDF (MLDF) method is proposed, which can detect damage without requiring benchmark data for the intact local structure. As a demonstration, the proposed LDF and MLDF methods are applied to damage detection in a 3-D steel frame structure, and the experimental results indicate that both methods can effectively determine the presence, severity, and location of a crack cut into one of the pillars in the frame with a saw. The LDF method is effective in a way that can eliminate both the nonlinear severity effect of structure itself and the ambient noise inherent in the intact structure, whereas the MLDF method is advantageous in that it does not require information about the intact local structure, which is often unavailable for damage detection. The proposed LDF method of in situ damage detection is illustrated using the concrete columns in a wharf structure. The successful detection of damage in the 3-D steel frame, as well as the wharf, demonstrates that the proposed local damage factor technique can be effectively and efficiently used in damage detection and structural health monitoring of structures.

The numerical evaluation of high-order modes of a uniform Euler–Bernoulli beam has been studied by reformatting the classical expression of mode shapes. That method, however, is inapplicable to a stepped beam due to the nonuniform expressions of the mode shape for each beam segment. Given that concern, this study develops an alternative method for the numerical evaluation of high-order modes for stepped beams. This method effectively expands the space of high-order modal solutions by introducing local coordinate systems to replace the conventional global coordinate system. This set of local coordinate systems can significantly simplify the frequency determinant of vibration equations of a stepped beam, in turn, largely eliminating numerical round-off errors and conducive to high-order mode evaluation. The efficacy of the proposed scheme is validated using various models of Euler–Bernoulli stepped beams. The principle of the method has the potential for extension to other types of Euler–Bernoulli beams with discontinuities in material and geometry. (The Matlab code for the numerical evaluation of high-order modes for stepped beams can be provided by the corresponding author upon request.)

As it is not enough to assess the integral reliability of the complex 3D high arch dam-foundation system just from local damage in the deterministic analysis, the improved response surface method is firstly introduced to get the failure possibility of individual uncertain factor, and then PNET method to fix the integral reliability considering several uncertain factors. Then the main works are presented as following: (1) find out the main individual failure modes; (2) establish the limit state functions of the main individual failure modes respectively; (3) fix the relevance among the main failure mode and calculate the integral reliability. Finally the results are used to assess the failure possibility and integral reliability of Jinping high arch dam-foundation system.

Since the anisotropic characteristic of deformation and strength of layered rock masses, giving the reasonable constitutive model is of great significance for numerical analysis of the rock engineering concerned of layered rock masses. Based on the McLamore & Gray strength criterion, the isotropic Drucker-Prager strength criterion which is widely used in rock engineering is extended to be an anisotropic elastoplastic constitutive model for layered rock masses by making use of the conversion relationship between the Mohr-Coulomb strength criterion and the Drucker-Prager strength criterion. Then the presented anisotropic constitutive model is embedded in ABAQUS for its numerical implementation based on the derivation of the implicit integration algorithm and consistent tangent modulus. Numerical example is given for demonstration of the validity of the proposed constitutive model and its numerical implementation.

Currently, in order to adequately exploit the abundant hydropower energy resources in China, a number of high concrete dams are under construction. It is quite prevalent that dams' cracking caused by inappropriate handling of temperature during the construction process of concrete dams endangers the dams' safety. Therefore, it is absolutely significant to study the variations of the thermal stress field during the construction process of high concrete dams. Based on Finite Element Method (FEM) software ANSYS 10.0, a newly developed program is edited and applied to study and calculate the non-steady temperature field and the thermal stresses through ANSYS Parametric Design Language (APDL), and verified to be reliable. The thermal stress field during the construction process of Longtan Roller Compacted Concrete (RCC) gravity dam is numerically simulated by the above developed program. The distributions of the thermal stress field and the water-thermal coupling stress field under reservoir normal storage are simulated and analyzed. Some meaningful conclusions are obtained from numerical simulations.