Marios K. Chryssanthopoulos

Elastic buckling and postbuckling behaviour of widely-stiffened conical shells under axial compression

In thin-walled shells of revolution with widely-spaced meridional stiffeners loaded in compression, a local panel buckling may develop. For the particular case of cylindrical panels, the buckling and postbuckling behaviour has been investigated in detail, whereas limited research is available for the more general case of conical panels. In this paper, the linear and non-linear elastic buckling response of the conical panel is studied for a wide range of shell and stiffening parameters by means of an appropriate finite element model. The classical buckling load is determined on the basis of linear analysis. The imperfection sensitivity is studied through non-linear analysis of imperfect conical panels with imperfections affine to the critical mode. Different aspects of the behaviour are quantified through suitably defined curvature parameters.

Uncertainty analysis of strength and ductility of confined reinforced concrete members

The strength and ductility of reinforced concrete beam and column cross-sections with varying geometries and levels of confinement are investigated. Material properties are modelled as random variables and their effect on section behaviour is assessed through fibre modelling and the Response Surface Methodology. The section ductility is in many cases found to be mainly dependent on the ultimate concrete strain. The uncertainty involved in the estimation of the latter using various models for confined concrete is examined with the aid of existing experimental data. Other failure criteria such as first hoop fracture and buckling of the longitudinal bars are also considered. Monte Carlo simulations show that a significant amount of variability exists in both the strength and ductility of confined concrete sections due to variations in material properties. In the case of ductility, this variability is greatly enhanced if model uncertainty is taken into account, especially for high axial loads. Furthermore, simple expressions are derived for the estimation of the strength parameters for any cross-section subject to varying material properties. Finally, the confined concrete model and curvature ductility provisions of Eurocode 8 are evaluated.

Probabilistic evaluation of behaviour factors in EC8-designed R/C frames

A methodology for the probabilistic assessment of reinforced concrete (R/C) frames which takes into account material variability, confinement model uncertainty and the uncertainty in local and global failure criteria is applied for the derivation of vulnerability curves for the serviceability and ultimate limit states of a multi-storey frame designed to Eurocode 8. By combining the uncertainties affecting structural vulnerability and seismic hazard, the seismic reliability is quantified in terms of the probability of failure for any given design life period. It is found that, while adequate safety margins exist for the ULS, the reliability against the SLS strongly depends on the structural criterion adopted for the definition of this state. The variability in the actual behaviour factor of this frame is also estimated, and the appropriateness of the EC8 specified value is assessed.

Probabilistic fatigue analysis under constant amplitude loading

Abstract Use of a fracture mechanics-based fatigue analysis for bridge details requires that the random nature of fabrication, crack growth, fatigue-fracture failure and applied loading be properly accounted for. Prior to investigating the last item, the others may be addressed by comparing with corresponding S-N curves, which have been established over many years of experimental code development. In this paper, four typical fatigue-sensitive details are examined under constant amplitude loading and the 50 and 97.7% probability of survival lines are obtained using appropriately selected stress intensity factors, a bi-linear crack growth model and a failure assessment diagram. To this end, various material and geometric parameters are treated as random. The good agreement observed between the fracture mechanics-based S-N curves and their code-specified counterparts, especially for low stress ranges, increases confidence in the model parameters used and hence in subsequent bridge reliability analyses using probabilistic fracture mechanics.

Fatigue and fracture simulation of welded bridge details through a bi-linear crack growth law

This paper deals with the application of a probabilistic fracture mechanics approach to predict the fatigue life of welded steel details in the presence of cracks under bridge spectrum loading. It is based on a recently proposed bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. The formulation leading to the expected number of cycles to failure is first outlined, followed by a simple example on a butt-welded detail. Uncertainty modelling, especially on fatigue crack growth parameters, is undertaken with the aid of recently published data in support of the bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using simulation with Latin Hypercube Sampling, and emphasis is placed on a comparison between linear and bi-linear crack growth models. The latter is found to lead to higher fatigue life estimates and significantly different crack size distributions, both of which have implications on inspection schemes for steel bridge components.

Buckling design of conical shells based on validated numerical models

In most shell buckling codes, guidance on the design of conical shells is restricted to unstiffened cones and even in this case the clauses are based on the procedures for cylindrical shells. Virtually no guidance is offered on stiffened cones and the particular characteristics of conical shells are not treated in detail. In this paper, use is made of finite element analysis to quantify critical elastic response and imperfection sensitivity through numerical models, whose adequacy has been quantified through comparisons with test data. The finite element results obtained were aimed at validating existing design recommendations for unstiffened cones and at developing a design approach for stringer-stiffened cones under compression, with a philosophy and format compatible with the European Shell Buckling Recommendations (ECCS).

Performance updating of concrete bridges using proactive health monitoring methods

Abstract Uncertainties associated with modelling of deteriorating bridges strongly affect management decisions, such as inspection, maintenance and repair actions. These uncertainties can be reduced by the effective use of health monitoring systems, through which information regarding in situ performance can be incorporated in the management of bridges. The objectives of this paper are twofold; first, an improved chloride induced deterioration model for concrete bridges is proposed that can quantify degradation in performance soon after chlorides are deposited on the bridge, rather than when initiation of corrosion at the reinforcement level takes place. As a result, the implications of introducing proactive health monitoring can be assessed using probabilistic durability criteria. Thus, the second objective of the paper is to present a methodology for performance updating of deteriorating concrete bridges fitted with a proactive health monitoring system. This methodology is illustrated via a simple example of a typical bridge element, such as a beam or a part of a slab. The results highlight the benefits from introducing ‘smart’ technology in managing bridges subject to deterioration, and quantify the reduction in uncertainties and their subsequent effect on predictions of future bridge performance.

Fragility and hazard analysis of a welded steel moment resisting frame

With the move towards performance and consequence-based design and assessment of structures under seismic loading, the engineering community is becoming increasingly convinced that design practices need to be developed and checked using probabilistic methods. In this article, a methodology for the probabilistic assessment of low-rise steel buildings is presented and applied to a welded Moment Resisting Frame (MRF). In light of recent field experience for this form of construction, emphasis is given to the modeling of connections, particularly with respect to fracture characteristics. The seismic behavior of the building is assessed by means of nonlinear dynamic time history analyses, using a set of ground motions scaled according to spectral acceleration. Randomness related both to structural properties and earthquake excitation is explicitly taken into account. Fragility curves are generated using the Monte Carlo (MC) simulation method coupled with the Latin Hypercube Sampling (LHS) technique, and the failure probabilities are presented in terms of drift angles at different performance levels. Furthermore, evaluation of the seismic risk through a hazard analysis is presented in order to compare present results with previous pertinent studies. The study reveals that structures experiencing brittle connection fractures undergo large deformations, resulting in a low reliability in terms of achieving code-related performance requirements.

Probabilistic buckling analysis of plates and shells

Abstract For many years, a significant amount of research has been directed towards experimental modelling of thin-walled plates and shells, as well as towards the development of analytical and numerical methods to improve their design against buckling. This paper presents methodologies for probabilistic buckling analysis and reliability assessment of such structural components and examines the link between probabilistic and deterministic studies. In particular, the effect of manufacturing variabilities, such as initial geometric imperfections and residual stresses, on elasto-plastic buckling response is investigated through parametric reliability studies of plate panels and cylinders under axial compression.

Probabilistic imperfection sensitivity analysis of axially compressed composite cylinders

Buckling analysis of cylinders under axial compression is sensitive to the assumptions made in the modelling of initial imperfections. Normally, imperfection modes are selected solely on the basis of buckling mode considerations and their amplitudes are determined using tolerance specifications in codes or experimentally recorded values. Whilst this approach may be used for metal cylinders with some confidence, due to the many test results available for validation purposes, it is not appropriate for the analysis and design of fibre-reinforced composite cylinders where the test results are limited and the effects of manufacturing on the imperfection characteristics have not yet been studied in detail. This paper presents a methodology for probabilistic buckling strength assessment based on the results of a statistical analysis on imperfections on two groups of composite cylinders manufactured by lay-up. The dominant features are quantified and the effect of fibre orientation on imperfections is examined. Simple models describing the random variability of imperfection modal amplitudes are presented. Using these probabilistic models, characteristic imperfection shapes are developed for fibre-reinforced cylinders and their use in buckling strength prediction and tolerance specification is demonstrated