Bio: Q Cai is an academic researcher from University of Pretoria. The author has contributed to research in topics: Gravity dam & Fracture mechanics. The author has an hindex of 1, co-authored 1 publications receiving 8 citations.
TL;DR: In this article, a smeared crack model based on nonlinear fracture mechanics was developed which allows for either linear or bilinear softening and assumes shear retention dependent on the strain normal to a crack.
Abstract: A smeared crack model, based on non-linear fracture mechanics, was developed which allows for either linear or bilinear softening and assumes shear retention dependent on the strain normal to a crack. A mesh objectivity verification study proves that the proposed crack modelling method is mesh objective. The crack model and its computational procedure is verified for a benchmark concrete gravity dam model and an existing concrete gravity dam by comparing the results with those of numerical investigations obtained by other researchers. Furthermore, an existing concrete gravity dam in South Africa is analysed and evaluated with regard to dam safety in terms of the maximum overflow level. A higher imminent failure flood is predicted in the analysis than that obtained by classical strength-based methods. The study proves the usefulness and applicability of the proposed crack model and implementation procedure in predicting crack response and evaluating the safety of concrete gravity dams. A sensitivity study on the material fracture properties and fracture parameters is included for the purpose of investigating the uncertainties often encountered in this type of analysis. The influence of the fracture properties and parameters on the cracking response and the overall structural behaviour is discussed.
TL;DR: In this paper, a nonlinear extended scaled boundary finite element method (X-SBFEM) was developed incorporating the cohesive fracture behavior of concrete, which consists of an iterative procedure to accurately model the traction distribution within the fracture process zone (FPZ) accounting for the cohesive interactions between crack surfaces.
Abstract: Fracture mechanics is one of the most important approaches to structural safety analysis. Modeling the fracture process zone (FPZ) is critical to understand the nonlinear cracking behavior of heterogeneous quasi-brittle materials such as concrete. In this work, a nonlinear extended scaled boundary finite element method (X-SBFEM) was developed incorporating the cohesive fracture behavior of concrete. This newly developed model consists of an iterative procedure to accurately model the traction distribution within the FPZ accounting for the cohesive interactions between crack surfaces. Numerical validations were conducted on both of the concrete beam and dam structures with various loading conditions. The results show that the proposed nonlinear X-SBFEM is capable of modeling the nonlinear fracture propagation process considering the effect of cohesive interactions, thereby yielding higher precisions than the linear X-SBFEM approach.
TL;DR: In this paper, a 3D finite element method (3-D FEM) is developed to simulate the temperature and thermal stress distribution in the concrete overflow dam during the construction period.
TL;DR: In this article, the Drucker-Prager non-linear finite element method (DP NL FEM) yield model is presented as a method to overcome the problem of the stress peaks at singularity points, and to produce more realistic stresses at the base of the dam wall.
Abstract: For many decades the 'classical' method has been used to design gravity dams. This method is based on the Bernoulli shallow beam theory. The finite element method (FEM) has become a powerful tool for the dam design engineer. The FEM can deal with material properties, temperatures and dynamic load conditions, which the classical method cannot analyse. The FEM facilitates the design and optimisation of new dams and the back analysis of existing dams. However, the linear elastic FEM has a limitation in that computed stresses are sensitive to mesh density at 'singularity points'. Various methods have been proposed to deal with this problem. In this paper the Drucker-Prager non-linear finite element method (DP NL FEM) yield model is presented as a method to overcome the problem of the stress peaks at singularity points, and to produce more realistic stresses at the base of the dam wall. The fundamentals of the DP NL FEM are presented. Benchmark studies of this method demonstrate the method's viability to deal with zones in a structure with stresses beyond the elastic limit where yielding of the material occurs. A case study of a completed gravity dam is analysed, comparing several analysis techniques. The service and extreme load cases are investigated. Different material properties for the concrete and rock, including weathered material along the base of the wall, are considered. The application and merits of the DP NL FEM are presented. The calculation of the critical factor of safety against sliding is done with a more realistic determination of the conditions along the base of the wall.
TL;DR: In this paper, a two-step approach for discrete crack analysis of concrete gravity dams under earthquake force is presented, based on the intuitive conjecture that the effect of cracks on structural acceleration in gravity dams is small, thus allowing the actual inertia force (the product of mass and acceleration) to be approximately obtained by linear response analysis.
TL;DR: Fiber Bragg Grating (FBG) strain sensor is presented to obtain the strain of small-scale model during testing and shows advantages of ease for installation, high sensitivity, and reliability compared with traditional resistance strain gauge.
Abstract: A 203-m-high gravity dam being built in earthquake-prone areas needs to be investigated very carefully to determine its dynamic responses, damage mechanism, and safety evaluation. The dynamic characteristics, seismic responses, failure mode, and safety evaluation of the above structure are presented through dynamic fracture test for small-scale model on shaking table. Because the strength of the model material is very low, the traditional strain gauge is also not easy to be glued to the surface of model. It is difficult to measure the accurate strain data of small-scale model during testing. Therefore, Fiber Bragg Grating (FBG) strain sensor is presented to obtain the strain of small-scale model during testing, due to its high sensitivity. The dynamic strain and residual strain are obtained with the FBG sensors embedded in model. The FBG sensor is adhered to model material completely and shows advantages of ease for installation, high sensitivity, and reliability compared with traditional resistance strain gauge. The model during testing is submitted with earthquake wave from the Chinese Code. In the experiment, the peak ground acceleration (PGA) of the first crack in the model indicates the safety level of the gravity dam. The crack locations and forms determine the damageable part of gravity dam under intense earthquake. After the final analysis, the safety evaluation result of the gravity dam under strong earthquake is given in order to guide the implementation of the project.