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The reliable performance of composite tubes conveying nanofluid is an essential issue for robust design under dynamic loads. Further, the frequency failure mode of these structures is a considerable performance function to provide a balance between mass and stiffness of the structure. The ability of the first-order reliability method (FORM)-based finite-step adaptive length (FAL) is discussed for reliability analysis of composite tubes under frequency failure mode. The performances for efficiency and robustness to evaluate the failure probability FORM formulas are discussed. Therefore, the capability of FAL using the steepest descent sensitivity vector is compared with three FORM formulations that are extended by the steepest descent search direction for nanocomposite tubes conveying nanofluid. In this current work, the Navier scheme is employed for calculating the frequency failure mode. Furthermore, the robustness and efficiency related to the steepest descent FORM formulas of HL-RF, directional stability transformation method (DSTM), and finite-step length (FSL) are compared with FAL. Based on this discussion, FAL contains efficient formula in comparison with other FORM formulas. Moreover, it is found that FSL formulas as well as FAL formulas are more robust than HL-RF and DSTM. This structure's failure probability rises as the fluid velocity and pipe diameter increase.

Concrete-filled steel tubular (CFST) truss is a type of composite structure with CFST chords and hollow tubular braces. CFST trusses have been increasingly used in large-scale structures such as towers, bridge girders, piers and arch ribs. The compression and flexural behaviour of CFST trusses are greatly improved compared to hollow tubular trusses due to the concrete infill in chords. For such complex composite structural systems, traditional structural analysis approaches are readily applied for the safety checks of individual members and connections, whilst system-level inelastic analysis and reliability calibration are very limited. In the past, relevant studies on the reliability of CFST structures mostly focused on structural components such as individual columns and beams, and there is no mature system-based design regulations nor reliability evaluations on this complex composite system. This thesis aims to address this gap by adopting stochastic finite element analysis (FEA) modellings of CFST truss systems through numerical approach considering both the structural nonlinearities and random uncertainties. With the complex configuration, nonlinear material interaction and sophisticated construction process, initial imperfections may largely affect the strength and stability of a CFST truss structure. The deterministic and probabilistic studies of CFST trusses with random imperfections based on the statistics of imperfections obtained through experiments and on-site measurements are carried out first, which lays a foundation for further study on the comprehensive reliability analysis of composite truss structures with uncertainties in material properties, structural configurations, initial imperfections and model uncertainties. The verified FEA models of CFST truss systems (two-chord, three-chord and four-chord) are then further developed to implement random uncertainties, including the elastic modulus of steel (Es), the yield strength of steel (fy), the thickness of steel (ts), the cylinder strength of concrete (fc’), the initial steel imperfection (χs) and the initial concrete imperfection (χc1, χc2), to facilitate a comprehensive reliability analysis. To consider the random variation of these uncertainties based on statistics, the Monte Carlo (MC) simulation and the Latin Hypercube Sampling (LHS) technique are incorporated on the basis of the above-described Abaqus modelling. A large number of stochastic FEA models of CFST truss samples are randomly analysed using Abaqus-Python technique with random variables considered to estimate the statistics of the flexural strengths of CFST trusses. Using the obtained statistics of system resistance, reliability analysis is then undertaken to calculate the system reliability indices (β) of the three typical CFST truss systems. Random dead loads and live loads are implemented to the FEA models. The structure reliability of the CFST trusses is evaluated in MATLAB using the First-Order Reliability Method (FORM). The relationship between reliability indices (β) and system resistance factors (ϕs) under various load cases is obtained. Finally, a comprehensive reliability-based design guideline of CFST truss systems by advanced analysis is suggested. The proposed novel computational approach and the reliability analysis contribute to both the practical design and the standard drafting for CFST truss systems.

The estimation of the failure probability for complex systems is a crucial issue for sustainability. Reliability analysis methods are needed to be developed to provide accurate estimations of the safety levels for the complex systems and structures of today. In this paper, a novel hybrid framework for the reliability analysis of engineering systems and structures is extended to reduce the computational burden. The proposed hybrid framework is named as SVR–CFORM and consists of coupling two parts: the first is an enhanced first-order reliability method (FORM) using nonlinear conjugate map (CFORM); the second is an artificial intelligence technique called support vector regression (SVR). The conjugate FORM (CFORM) is adaptively formulated to improve the robustness of the original iterative FORM algorithm, whereas the SVR technique is used to enhance the efficiency of the reliability analysis by reducing the computational burden. The performance of the proposed SVR–CFORM formulation is compared in terms of efficiency and robustness with several FORM formulas (i.e. HL–RF, directional stability transformation method, conjugate HL–RF and finite step length) through different numerical/structural reliability examples. Results indicate that the proposed SVR–CFORM formulation is more accurate and efficient than other reliability methods. Based on the comparative analysis results, the SVR technique can highly reduce the computational costs and accurately model the response of complex performance functions, while the iterative CFORM formulation found to provide stable and robust reliability index results compared to the others reliability methods.

In the European Union since 2010, the design of any type of structures must comply with EN-1997 Geotechnical Design (CEN 2004) (EC7) referring to engineering projects in the rock mechanics field. However, the design of debris flow countermeasures in compliance with EC7 requirements is not feasible: EC7 uses partial safety factors for design calculations, but safety factors are not provided for phenomena such as debris flows and rock falls. Consequently, how EC7 can be applied to the design of debris flow barriers is not clear, although the basic philosophy of reliability-based design (RBD), as defined in EN1990 (CEN 2002) and applicable to geotechnical applications, may be a suitable approach. However, there is insufficient understanding of interactions between debris flows and structures to support RBD application to debris flow barrier design, as full-scale experimental data are very limited and difficult to obtain. Laboratory data are available but they are governed by scale effects that limit their usefulness for full-scale problems. The article describes an analysis, using the first-order reliability method (FORM), of two different datasets, one obtained through laboratory experiments and the other reflecting historical debris flow events in the Jiangjia Ravine (China). Statistical analysis of laboratory data enabled a definition of the statistical distributions of the parameters that primarily influence debris flow and barrier interactions. These statistical distributions were then compared to the field data to explore the links between flume experiments and full-scale problems. This paper reports a first attempt to apply RBD to debris flow countermeasures, showing how the choice of the target probability of failure influences the barrier design resistance value. An analysis of the factors governing debris flows highlights the applicability and limitations of EN1990 and EN1997 in the design of these rock engineering structures.