It has been generally accepted that the tensile strength and elastic modulus of concrete is proportional to the square-root of its compressive strength. This relationship, however, may not be applicable for high-performance concrete. The study presents data on strength and stiffness of concretes containing a laboratory produced metakaolin and commercial silica fume as cement replacement materials, with water-to-cementitious materials ratio of 0.27 to 0.33. Approximately 750 specimens were tested and compressive strength of up to 110 MPa at 90 days was reported. Analysis of the best-fit relationships for tensile-compressive strength and stiffness-compressive strength found that the square-root function recommended by most codes of practice is inadequate when applied to concretes of higher strength, particularly in the case for tensile strength prediction.      

The contribution of diagonal stiffeners in the shear capacity of composite plate girders has not been studied so far. In this study, the behavior of diagonally stiffened simply supported composite plate girders is investigated and a theoretical formula is proposed to estimate the ultimate shear strength of such girders. The proposed analytical method considers the tension field action within the plate girder web panel and the shear failure of the concrete slab. Several validated finite element models of the composite plate girders, having different configurations of diagonal stiffeners, are also generated to verify the proposed method by comparing the analytical and numerical outcomes. These three-dimensional finite element models are developed to account for the geometric and material nonlinear behavior of composite girders. The models are first verified by experimental values obtained for girders with no diagonal stiffeners tested by other researchers and afterwards different configurations of diagonal stiffeners are added to the models. A total of four tested specimens are selected from previous studies. Based on the results of finite element analyses, a design method is proposed which incorporates the effects of the concrete slab, the composite action, and the web shear buckling of composite girders. The proposed method is approximate, simple and does not require any complex mathematical operations and can be applied to composite plate girders at the preliminary stages of design. The calculated ultimate shear strengths using the proposed method are in good agreement with both numerical results and experimental values, which indicates that the proposed analytical equations can be applied to predict the ultimate shear strength of girders for design office use. In comparison with un-stiffened girders, it is also observed that diagonal stiffeners are able to reduce the buckling effects of the web steel plate, increase elastic shear buckling strength and therefore the ultimate shear capacity of the girders.

In this study, a three-dimensional finite element (FE) model for armchair, zigzag and chiral single-walled carbon nanotubes (SWCNTs) is proposed. To create the FE models, nodes are placed at the locations of carbon atoms and the bonds between them are modeled using three-dimensional elastic beam elements. The FE model is used to investigate the influence of chirality and Stone-Wales defects on the ultimate strength (Ultimate stress and ultimate strain) of SWCNTs. Results indicate that Stone-Wales defect significantly reduces the ultimate stress and strain of armchair CNTs. But this defect has a negligible effect on the ultimate strength of zigzag nanotubes. Based on the results, the crack growth path in zigzag and armchair nanotubes have 90 and 45 degree angle to the long axis of the nanotube, respectively.

Assessing the ultimate strength of the stiffened plates forming the ship structure is the first step in assessing its ultimate strength. Over time and increase the life of the structure, failures such as cracks reduce the load-bearing capacity of the structure. The main purpose of this paper is to present a machine learning method based on XGBoost algorithm to calculate the ultimate compressive strength of stiffened plates with crack failure using the results of multiple finite element analyzes. To achieve the best possible results from the XGBoost algorithm, some of the hyperparameters in this algorithm have been optimized using the Bayesian optimization method. The results of this method show that the accuracy of using the optimized XGBoost algorithm is much higher than conventional methods based on linear regression.