Abstract in English:Abstract The present study was focused on evaluating the strength and failure properties of hybrid micro-fibre concretes containing micro brass and steel fibre additions. Experimental tests were conducted on different hybrid fibre concretes containing aligned fibres in the direction of beam axis and the relative mechanical performance was compared to randomly distributed fibre additions in concrete. Statistical analysis on the test results were used to compare the distinctive performance of fibre combinations at different dosage levels. The effect of binder to aggregate ratio and the varying proportions of fibre combinations on the concrete properties were assessed in terms of flexural strength, fracture toughness and ductility. The statistical analysis of the experimental results indicated the dual action of hybrid fibre combinations in improving the composite properties and the crack arresting mechanism of fibres at varying stages of loading as observed from the load-CMOD curves. Among the various concretes tested, the hybrid fibre concretes containing steel-brass fibres dosages at 1.0% and 0.03%Vf performed consistently in overall mechanical properties.
Abstract in English:Abstract Because soluble salt in saline soil dissolves with water, utility tunnels built in saline soil foundation are more likely to damage due to by groundwater and earthquake. In this study, new cementitious composite materials were developed by using slag, building gypsum, quicklime, and magnesia. The unconfined compressive strengths of the saline soil solidified by new cementitious composite materials and cement are investigated, and the optimum proportion of the different components of the new cementitious composite materials is determined. We found from microscopic characteristics of the saline soil solidified by the new cementitious composite materials and cement that the new materials could better absorb chloride ions. Finally, the new cementitious composite materials were applied to saline soil foundation reinforcement of a utility tunnel. By using the finite element method (FEM) and shaking table tests, it can be seen that the displacement, acceleration and extent of damage of the utility tunnel after saline soil foundation reinforcement using new cementitious composite materials significantly decreased; therefore, the new cementitious composite materials can improve the seismic behaviour of the utility tunnel and shows potential future engineering application value.
Abstract in English:Abstract Traditional probability-based structural reliability analysis method can only consider random uncertainties described by random distribution functions, which required sufficient experimental samples. On comparison, interval uncertainties can be more appropriate when lacking information. In realistic situations, several structural reliability models considering mixed probabilistic and interval variables are proposed. Besides choosing reliability model according to the information available of the uncertainties, this paper presents a perspective in structural design that for a specific design, the controllable structural uncertain parameters should be better described as interval variables while other uncontrollable uncertain parameters such as external loads described as random variables if sufficient information is available. The corresponding reliability analysis model is proposed. Further, for simple truss structures, this paper demonstrates that the extreme hybrid reliability index can be attained when the interval variables reach their upper or lower bounds. To solve the hybrid reliability index, a sequential single-loop strategy combined with an intermediate-variable based response surface and the method of moving asymptotes optimization solver is proposed. Finally, numerical examples are given to demonstrate the applicability of the proposed method and concluding remarks are made.
Abstract in English:Abstract In this study, a mechanical model for tunnels excavated in a non-uniform stress field is developed. A new strain-softening model simultaneously considers the weakening of cohesion and internal friction angle is proposed. Then, an analytical solution for the post-peak region radii, stresses, and displacements is deduced. Taking a tunnel in Taoyuan coal mine as an engineering example, the post-peak region radii, surface displacement, and stresses distribution are determined. The effects of the horizontal-to-vertical stress ratio, intermediate principal stress, residual cohesion, and residual internal friction angle on tunnel deformations are discussed. The results show that the post-peak region radii and stresses distribution around the tunnel varies with direction due to the non-uniform stress field. The post-peak region radii and surface displacement are larger with consideration of intermediate principal stress. Tunnels surrounded by rock masses with a higher residual cohesion and internal friction angle experience lower post-peak region radii and surface displacement.
Abstract in English:Abstract The ultimate limit state of stability by equilibrium bifurcation, the limit states for stress and strain resulting from this condition were evaluated for an extremely slender real structure of reinforced concrete, with geometry varying along its length. The aspects related to nonlinearities of the material were considered through the recommendations on NBR 6118:2014, from the Brazilian Association of Technical Standards (ABNT). In the analytical solution, developed for stability analysis, all elements of the structural dynamics present in the system were taken into account, including the column self-weight. The critical buckling load was then dynamically defined to different instants of time. Reductions of 70% for the modulus of elasticity and 59% for the critical buckling force were found in analyses performed from zero and five thousand days. It was also possible to obtain the induced stresses on the homogenized cross-sections and those transferred to reinforcement steel bars.
Abstract in English:ABSTRACT The isogeometric analysis (IGA) consists of using the same shape functions, usually employed on Computer-Aided Design (CAD) technologies, on both geometric modelling and approximation of the fields of physical models. One issue that concerns IGA is how to make the connection or apply general constraints in the connection of structures described by different curves and surfaces (multi-patch structures), particularly when the shape functions are not interpolatory at the selected point for the imposition of the constraint or the desired constraint is not related directly to degrees of freedom, which may be an issue on Kirchhoff-Love shells and Euler-Bernoulli beams, since usually no rotational degrees of freedom are employed. In this context, the present contribution presents an isogeometric 2D curved beam formulation based on Bernoulli-Euler assumptions. An approach about the implementation of multi-patch structures enforcing constraints, such as same displacement or same rotation among neighbor paths, is developed based on Penalty and Lagrange methods. The applicability of the methods is verified by examples of application.
Abstract in English:Abstract A numerical investigation is performed here using a NURBS-based finite element formulation applied to classical Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI) problems. Model capabilities related to refinement techniques are analyzed using a finite element formulation with NURBS (non uniform rational B-splines) basis functions, where B-splines and low-order Lagrangian elements can be considered as particular cases. An explicit two-step Taylor-Galerkin model is utilized for discretization of the fundamental flow equations and turbulence is considered using Large Eddy Simulation (LES) and the Smagorinsky’s sub-grid scale model. FSI is considered using an ALE kinematic formulation and a conservative partitioned coupling scheme with rigid body approach for large rotations is adopted. CFD and FSI applications are analyzed to evaluate the accuracy associated with the different refinement procedures utilized. Results show that high order basis functions with appropriate refinement and non-uniform parameterization lead to better predictions, compared with low-order Lagrangian models.
Abstract in English:Abstract This work focused on the stress response of anisotropic 30SiMn2MoVA steel gun barrel under coupled thermo-mechanical loads, namely, the contact pressure and friction of projectile, thermal load and pressure of propellant gas. The effect of temperature on the mechanical properties of gun barrel was considered. Two thermal-displacement coupled finite element models (FEM) were built by ABAQUS software. The first was the interaction model of projectile and barrel which was to calculate the stress of barrel engraved by copper jacket and steel jacket projectile. The results of barrel in the interaction model, as an initial condition, were imported to the second FEM which was applied with thermal load and pressure of propellant gas. The results show that the rifling at chamber throat has the largest stress and circumferential stress under coupled loads. It gives a request that the gun barrel should have the better circumferential mechanical properties. The plastic deformation of the rifling at the chamber throat is observed which indicates that 30SiMn2MoVA gun barrel has a low service life due to the poor mechanical properties.
Abstract in English:Abstract The present study investigates the stability conditions of reinforced concrete panels subjected to fire loading within the framework of limit analysis theory. The method relies in a first step upon the preliminary determination of the temperature dependent interaction diagrams of the structural element. Interaction diagrams derived from the static approach are shown to depend on the geometry of panel cross-section as well as on the strength properties of the constituents, which degrade continuously as fire proceeds. The second step of the method consists in determining the deformed configuration of panel from the analysis of thermo-elastic equilibrium of the structure. The stability analysis and design of the panel in its deformed geometry are then carried out by comparing the distribution of internal efforts to its reduced strength capacities expressed by means of the associated interaction diagrams evaluated in the first step. Several numerical examples are presented to assess the effect of relevant parameters on the overall fire safety of the structure, emphasizing the effectiveness of the approach for design purposes.
Abstract in English:Abstract Structural reliability theory has long been recognized as the proper tool to guide selection of partial safety factors in limit state structural design codes. Brazilian design codes, however, have never been through this calibration process. This paper addresses the reliability-based calibration of partial safety factors of Brazilian design codes for loads (NBR8681:2003), and for steel (NBR8800:2008) and concrete (NBR6118:2014) structures. The study is based on an extensive dataset of load and strength variables, addressing the Brazilian reality, as much as possible. Calibration minimizes the variations of reliability indexes of the most diverse structures designed according to the codes of interest, with regard to a target reliability index chosen by the analyst. The main result of calibration is to make reliability indexes more uniform, for different design configurations. In case of Brazilian codes, this could be achieved by increasing main variable loads, and reducing the combination values of secondary loads. This paper presents results that are not (yet) recommended for adoption in Brazilian codes, but which should be discussed with the community in order to reach minimal consensus.