Michael P. Enright

Probabilistic analysis of resistance degradation of reinforced concrete bridge beams under corrosion

Bridges subjected to environmental attack can experience changes in resistance which are time-variant. In this study, flexural strength loss in concrete bridge beams due to corrosion of steel reinforcement is considered. The sensitivity of the corrosion initiation time of steel reinforcement to changes in the main descriptors of diffusion related random variables is illustrated. Both the mean and standard deviation of the corrosion initiation time increase with an increase in the coefficient of variation of each of the diffusion related random variables for the range of values considered in this study. An example problem is given which illustrates the effect of various variables including corrosion rate and corrosion initiation time on the time-variant area of steel reinforcement and flexural strength of an existing reinforced concrete bridge beam. For the range of parameters and the damage scenarios considered, the mean value of the resistance loss function appears to increase about linearly with time. The results can be used to develop optimal lifetime reliability-based maintenance strategies for reinforced concrete bridges under environmental attack.

Reliability-based condition assessment of deteriorating concrete bridges considering load redistribution

A wide variety of models have been proposed for estimating the reliability of highway bridges. For reinforced concrete bridges subjected to environmental attack, time-variant reliability methods have to be used. In this study, the condition of reinforced concrete girder bridges is assessed using a time-variant system reliability approach in which both load and resistance are time-variant quantities. Several system models are considered, including failure of any girder (series system) and failure of a specified number of adjacent girders (series-parallel system). Adaptive importance sampling is used to determine the cumulative-time system failure probability. An existing reinforced concrete T-beam bridge located near Pueblo, Colorado, is investigated. The influence of resistance degradation and post-failure load redistribution is included. A comparison of reliability estimates for several system models is given, including the influence of correlation among initial girder strengths. The results can be used as a guide for the selection of system models for bridge reliability analysis, identification of critical girders in a bridge system, and for the development of optimal reliability-based maintenance strategies for reinforced concrete highway bridges.

A system level partitioning approach for analyzing the origins of variability in life prediction of tungsten filaments for incandescent lamps

Abstract The assessment of the origins of variability in the life expectancy has important ramifications in optimizing the cost–benefit ratio for the design and manufacture of a ‘system’. In this paper we study a model example, the tungsten filament, to develop a methodology for this process. The lifetime of tungsten filaments, in service, can vary for several reasons. The microstructure of tungsten may vary, the geometrical parameters of the filament coil or the radius of the tungsten wire may change, or the operating voltage for the filament may fluctuate. Additional uncertainty may arise from the fundamental material parameters, such as the activation energies for self-diffusion. Using a system level partitioning of system variables’ approach permits a quantitative examination of each of these parameters on the total lifetime of the filament. Important and surprising results emerge when the partitioning approach is utilized, notably, the uncertainty arising from fluctuation in temperature, which is related to fluctuation in the operating voltage, produces a variability that is comparable to the variability expected from the distributed nature of the microstructure. Furthermore, we find that a relatively narrow normal (Gaussian) distribution in the operating voltage transforms into a broad and log-normal distribution for the lifetime because of the thermally activated mechanisms that control the rate of deformation in tungsten. The results presented here may have a significant influence on cost analysis; they suggests that gains in reliability emanating from better microstructural control must be weighed against variability that is inherent in the system’s operating environment.