T.D. Righiniotis

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.

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.

A simplified crack model for weld fracture in steel moment connections

This paper presents a simplified two-dimensional crack model for assessing the fracture of bottom flange welds in steel beam-to-column connections. The formulation of the model includes the determination of approximate expressions for stress intensity factors related to the cracked geometry, accounting for typical stress conditions and taking due consideration of the presence of the backing bar. Idealized residual stress distributions are also incorporated in the model to examine their influence on the behaviour. Particular attention is given to typical connection configurations, which have suffered considerable damage in the Northridge earthquake. Comparisons are made between the results obtained from the proposed model and those available from a number of experimental investigations as well as two-dimensional finite element analyses. Within the range of results examined in this study, the proposed model is shown to provide good, and generally conservative, predictions in terms of both the fracture moments and reduction in stiffness.

A Simplified Fragility Methodology for Regular Steel MRFs

During the last decade, significant progress has been made toward the development of probabilistic methods for the seismic assessment of structures. However, the use of existing analytical fragility methodologies for practical applications related to structures of moderate to low importance remains prohibitive, in part due to substantial computational demands. This paper explores, through a case study, the robustness of a simplified methodology for fragility assessment of regular steel Moment Resisting Frames (MRFs). Through comparisons with a more accurate but computationally demanding methodology, it was confirmed that the “2000 SAC/FEMA” procedure gives robust fragility estimates. Moreover, it was found that a closed-form fragility assessment, where the structural response is evaluated by means of an Equivalent Single Degree Of Freedom (ESDOF) oscillator, can yield sufficiently accurate results for regular steel MRF structures, providing acceptable construction quality has been achieved in the connections.

STIFFNESS AND FRACTURE CHARACTERISTICS OF THE NORTHRIDGE STEEL MOMENT RESISTING CONNECTIONS

The Northridge fractures demonstrated the inability of the welded beam-to-column connection, typically used in California prior to the 1994 earthquake, to fully develop the beams plastic moment and, in some cases, to even reach yield. The unanticipated character of these failures implies that loads at which fracture occurs under Northridge-type conditions as well as the effects the latter has on a buildings stiffness need to be quantified. This paper addresses these issues in a quantitive manner. To simplify matters, the behaviour of a simple single degree-of-freedom (SDOF) frame, with connection characteristics modelled in a way that reflects the Northridge conditions, is examined by taking into account the random nature of the connections resistance to fracture. The fracture event is found to result in a modest reduction in the frames lateral stiffness, with the post-fracture behaviour being governed by the behaviour of the remaining connecting elements.