Vladimir Cervenka

Structural assessment and reliability analysis for existing engineering structures, theoretical background

The concept presented for the safety assessment of concrete structures integrates nonlinear finite element analysis with stochastic and reliability technology into an advanced engineering tool. The basic aim of the stochastic nonlinear analysis is to calculate the safety index of an existing engineering structure, which characterizes its reliability (and failure probability). The nonlinear solution enables a realistic estimation of the structural response statistics to be obtained (failure load, deflections, cracks, stresses, etc.). The possibility of randomization for such computationally intensive problems is shown. Latin hypercube sampling is used in order to keep the number of required simulations at an acceptable level. Statistical correlation, which is important for a realistic solution, is imposed by using a stochastic optimization technique called simulated annealing. The sensitivity of results to random input parameters can be evaluated using nonparametric rank-order correlation coefficients. The safety index of the analysed structure is calculated from the stochastically obtained structural resistance and expected load distribution using appropriate reliability techniques. The presented approach for the safety assessment of engineering structures supersedes the usual methods based on simplified formulas. It can lead to considerably improved results since the structure is analysed more precisely. Therefore, it supports a higher level of decision-making process in bridge administration and maintenance of transport macrostructure.

ATENA — A tool for engineering analysis of fracture in concrete

Advanced constitutive models implemented in the finite element system ATENA serve as rational tools to explain the behaviour of connection between steel and concrete. Three nonlinear material models available in ATENA are described: crack band model based on fracture energy, fracture-plastic model with non-associated plasticity and microplane material model. Nonlinear simulation using these advanced constitutive models can be efficiently used to support and extend experimental investigations and to predict behaviour of structures and structural details.

ATENA simulation of crack propagation in CONCRACK benchmark

A crack propagation in concrete and reinforced concrete structures can be simulated by numerical models based on fracture mechanics. The cracks can result from load actions or volume changes of material due to thermal and chemical processes or combinations of both. The first part of the paper describes the constitutive models implemented by the authors in the software ATENA used for the presented study. In the second part, a crack propagation in a shear wall tested within the benchmark CONCRACK was examined. The significance of appropriate modelling of the confinement effect was demonstrated. The third part of the paper describes the simulation of cracks in young concrete in the early stages after casting of a reinforced concrete element under restrained boundary conditions. This analysis considers modelling of heat development due to concrete hydration and associated crack development due to thermal and shrinkage strains. The both tests presented were performed on typical structural members with real dimensions under well-controlled testing conditions. This allowed a unique validation of the state-of-the-art theoretical constitutive models and available software implementation. Such validation is necessary for assessment of the model uncertainty in design verifications based on the non-linear finite element-based analysis.

Uncertainty of Predicting Shear Strength

The authors participated in the prediction contest for strength of 4m deep concrete slab strip set forth by University Toronto in Canada. They submitted the best prediction among 66 entries from all over the world. Their solution was achieved with a numerical analysis based on nonlinear constitutive model of concrete using fracture mechanics. The shear strength of beam was significantly affected by its large size. After the results of contest were made public the authors performed a study about mesh sensitivity and element type effects, which resulted in assessment of model uncertainty.

Probabilistic design of fibre concrete structures

Advanced computer simulation is recently well-established methodology for evaluation of resistance of concrete engineering structures. The nonlinear finite element analysis enables to realistically predict structural damage, peak load, failure, post-peak response, development of cracks in concrete, yielding of reinforcement, concrete crushing or shear failure. The nonlinear material models can cover various types of concrete and reinforced concrete: ordinary concrete, plain or reinforced, without or with prestressing, fibre concrete, (ultra) high performance concrete, lightweight concrete, etc. Advanced material models taking into account fibre concrete properties such as shape of tensile softening branch, high toughness and ductility are described in the paper. Since the variability of the fibre concrete material properties is rather high, the probabilistic analysis seems to be the most appropriate format for structural design and evaluation of structural performance, reliability and safety. The presented combination of the nonlinear analysis with advanced probabilistic methods allows evaluation of structural safety characterized by failure probability or by reliability index respectively. Authors offer a methodology and computer tools for realistic safety assessment of concrete structures; the utilized approach is based on randomization of the nonlinear finite element analysis of the structural model. Uncertainty of the material properties or their randomness obtained from material tests are accounted in the random distribution. Furthermore, degradation of the reinforced concrete materials such as carbonation of concrete, corrosion of reinforcement, etc. can be accounted in order to analyze life-cycle structural performance and to enable prediction of the structural reliability and safety in time development. The results can serve as a rational basis for design of fibre concrete engineering structures based on advanced nonlinear computer analysis. The presented methodology is illustrated on results from two probabilistic studies with different types of concrete structures related to practical applications and made from various materials (with the parameters obtained from real material tests).

Performance of Fibre Reinforced Concrete Structures - Modelling of Damage and Reliability

Steel fibre reinforced concrete (FRC) has higher ductility, it can save amount of convention reinforcement, labour and in consequence costs of the structure. However, broader use of SFRC as construction material is limited among others by lack of design codes. According to the previous study, reliability and safety of ordinary reinforced engineering can be verified using non-linear finite element analysis and several safety formats that are proposed in fib Model Code 2010. In the presented paper, safety formats are applied for fibre reinforced structures such as tunnel lining precast segment and individual approaches are compared. As tensile and shear cracks or compressive crushing can develop in the fibre reinforced concrete under severe conditions, the design combining numerical and experimental investigations together with safety formats is appropriate method how to obtain safe and reliable structure. Finite element method and advanced material models taking into account FRC properties such as shape of tensile softening branch, high toughness and ductility are described in the paper. Since the variability of FRC material properties is rather high, full probabilistic analysis seems to be the most appropriate format for evaluation of structural performance, reliability and safety.