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The gamma process is a stochastic cumulative process with gamma distributed increments. Professor van Noortwijk's research work played a key role in modelling uncertain and time-dependent degradation of structures as a gamma process, making it popular in the engineering community (van Noortwijk, J.M. and van Gelder, P.H.A.J.M., 1996. Optimal maintenance decisions for berm breakwaters. Structural Safety, 18 (4), 293–309). The maintenance optimisation models mostly use the renewal theorem to evaluate the asymptotic cost rate and use it to optimise the maintenance policy. However, many engineering structures have a relatively short and finite time horizon in which the application of the asymptotic formula becomes questionable. This article presents a finite time model for computing the expected cost of a condition-based maintenance program considering degradation as the gamma process. The article investigates the suitability of the asymptotic cost rate formula and provides a rich discussion about the qualitative nature of the results of the cost optimisation. [ABSTRACT FROM PUBLISHER]

Flow accelerated corrosion (FAG) is a serious form of degradation in primary heat transport piping system (PHTS) of the nuclear reactor~ Pipes transporting hot coolant from the reactor to steam generators are particularly vulnerable to FAG degradation, such as tight radius pipe bends with high flow velocity. FAG is a life limiting factor, as excessive degradation can result in the loss of structural integrity of the pipe. To prevent this, engineering codes and regulations have specified minimum wall thickness require- ments to ensure fitness for service of the piping system. Nuclear utilities have imple- mented periodic wall thickness inspection programs and carried out replacement of pipes prior to reaching an unsafe state. To optimize the life-cycle management of PHTS, accu- rate prediction of time of replacement or ~`end of life" of pipe sections is important. Since FAG is a time-dependent process of uncertain nature, this paper presents two probabilis- tic models for predicting the end of life. This paper illustrates that the modeling assump- tions have a significant impact on the predicted number of replacements and life-cycle management of the nuclear piping system. A practical case study is presented using wall thickness inspection data collected from Canadian nuclear plants. [DOl: 1O.1115/1.4000897J [ABSTRACT FROM AUTHOR]

Electrical transmission lines in Canada and elsewhere widely utilize wood structures to support electrical conductors. The Gulfport structure is one of the largest wooden structures used in Canada’s electricity transmission networks. Continuous exposure of crossarms to wide-ranging temperature, moisture, and load conditions makes them vulnerable to internal decay and failure under adverse weather conditions, such as windstorms and snowstorms. These failures can result in forced outages and customer disruptions with significant economic losses, and therefore the electrical utility industry has taken a keen interest in developing condition-assessment methods to support cost-effective replacement of aged wood crossarms. At present, there is no dataset available regarding the remaining strength of wood pole crossarms exposed to the aboveground environment for 30-40 years of service life in Canada. This paper presents a comprehensive experimental study focussed on evaluating the remaining strength distribution in a sample of crossarms removed from service. The experimental data presented in the paper are essential to assess remaining life and develop guidelines for condition assessment of crossarms. Les lignes de transport d’électricité au Canada et ailleurs utilisent beaucoup les structures de bois pour soutenir les conducteurs électriques. La structure Gulf Port est l’une des plus grandes structures de bois utilisées dans les réseaux de transport d’électricité au Canada. L’exposition perpétuelle des traverses de poteaux à une large gamme de température, d’humidité et de charge, les rendant vulnérables à la pourriture interne et à la défaillance sous diverses conditions climatiques, tels que des tempêtes de vent et de neige. Ces défaillances peuvent engendrer des indisponibilités fortuites et des pannes de courant chez les usagers, impliquant de fortes pertes financières. L’industrie des services électriques a donc montré un intérêt à développer des méthodes d’évaluation des conditions afin de soutenir le remplacement rentable des vielles traverses de poteaux en bois. Présentement, aucun ensemble de données n’est disponible concernant la résistance résiduelle des traverses en bois des poteaux exposés aux conditions environnementales au-dessus du sol pendant 30 à 40 ans au Canada. Le présent article porte sur une étude expérimentale complète portant sur l’évaluation de la distribution de la résistance résiduelle dans un échantillon de traverses de poteaux retirées du service. Les données expérimentales présentées dans cet article sont primordiales pour évaluer la durée de vie restante et mettre sur pied des directives pour l’évaluation de l’état des traverses de poteaux. [ABSTRACT FROM AUTHOR]

The accurate estimation of piping failure frequency is an important task to support the probabilistic risk assessment and risk-informed in-service inspection of nuclear power plants. Although probabilistic models have been reported in the literature to analyze the piping failure frequency, this paper proposes a stochastic point process model that incorporates both a time dependent trend and plant-specific (or cohort) effects on the failure rate. A likelihood based statistical method is proposed for estimating the model parameters. A case study is presented to analyze the Class I pipe failure data given in the OPDE Database. [ABSTRACT FROM AUTHOR]

In the life-cycle management of infrastructure systems, the decisions regarding the time and frequency of inspection, maintenance and replacement are confounded by sampling and temporal uncertainties associated with deterioration of the structural resistance. To account for these uncertainties, probabilistic models of deterioration have been developed under two broad categories, namely the random variable model and the stochastic process model. This paper presents a conceptual exposition of these two models and highlights their profound implications on age-based and condition-based preventive maintenance policies. The stochastic gamma process model of deterioration proposed here is more versatile than the random rate model commonly used in structural reliability literature. [ABSTRACT FROM AUTHOR]

Closed form expressions for the accurate calculation of the resonance frequency of a hexagonal and half hexagonal microstrip antennas, based on their equivalence with rectangular microstrip antenna, have been presented. These proposed empirical formulae have been validated using simulations and experiments. Design curves, for the input impedance variation along the feed axis, have been presented for hexagonal and half hexagonal patches for two feed positions. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 448–452, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24087 [ABSTRACT FROM AUTHOR]

Three configurations of compact gap-coupled half hexagonal microstrip antennas have been presented for broad bandwidth. Parametric studies for the effect of coupling gap and feed point location on the performance of these antennas have been investigated and the results have been validated experimentally. Increase in bandwidth is more than twice for the two configurations and about four times for the third case when compared with that of the corresponding hexagonal microstrip antenna. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 271–275, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23069 [ABSTRACT FROM AUTHOR]

Formulations to determine the resonance frequencies of hexagonal and half hexagonal microstrip antennas have been proposed based on their equivalence with circular and semicircular microstrip antennas, respectively. The new configuration of half hexagonal patch formed by bisecting the full hexagon along its resonant length, has the same resonance frequency as that of the full hexagonal patch, with half the size. Simulations and experiments have been performed to validate the proposed method. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2876–2879, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22843 [ABSTRACT FROM AUTHOR]

The design wind pressures specified in the 2005 National Building Code of Canada (NBCC) have been derived from the Gumbel distribution fitted to annual maximum wind speed data collected up to early 1990s. The statistical estimates of the annual maxima method are affected by a relatively large sampling variability, since the method considers a fairly small subset of available wind speed records. Advanced statistical methods have emerged in recent years with the purpose of reducing both sampling and model uncertainties associated with extreme quantile estimates. The two most notable methods are the peaks-over-threshold (POT) and annually r largest order statistics (r-LOS), which extend the data set by including additional maxima observed in wind speed time series. The objective of the paper is to explore the use of advanced extreme value theory for updating the design wind speed estimates specified in the Canadian building design code. The paper re-examines the NBCC method for design wind speed estimation and presents the analysis of the latest Canadian wind speed data by POT, r-LOS, and annual maxima methods. The paper concludes that r-LOS method is an effective alternative for the estimation of extreme design wind speed. [ABSTRACT FROM AUTHOR]

Wood transmission structures, such as H frames, have been extensively used to support electrical transmission lines throughout Canada. The transmission infrastructure is in general aging, and fungal decay of wood crossarms poses a significant risk of failure under adverse weather conditions. A crossarm failure in the transmission system can result in forced outages and customer disruptions that lead to significant economic losses. This paper presents a condition assessment model to prioritize the replacement of transmission crossarms that are near the end of their service life. The proposed standard involves a visual condition rating system, which is validated by results of full-scale testing of a sample of in-service crossarms. Aerial inspection of transmission lines using the proposed visual rating system is a simple, economical, fast, and effective method of assessment. The proposed approach would ensure a more consistent compliance with the condition-based replacement standard specified in the Canadian (Canadian Standards Association Standard CSA 22.3 No. 1-01) and North American (US National Electric Safety Code 2002 edition) standards. [ABSTRACT FROM AUTHOR]

A method is presented for estimating mean crossing rates for the stationary response of linear systems to non-normal excitation modeled by polynomials of Gaussian processes. Information about the non-Gaussian system response is derived in terms of statistical moments that are accurately calculated from a closed system of linear algebraic equations. This information is used to construct the most unbiased response distribution using the maximum entropy principle, a consistent method of inductive inference. Mean crossing rates are then estimated by mapping a standard Gaussian process into the nonGaussian response and by using the property that the two processes have nearly equal crossing rates. The proposed method is applied to compute mean crossing rates and probability density functions for the response of a simple oscillator to the following types of excitation processes: the square of the Ornstein-Uhlenbeck process, and third degree polynomials of Gaussian processes. [ABSTRACT FROM AUTHOR]

Probabilistic seismic risk analysis (SRA) is often used for accurately quantifying seismic risk of critical engineering structures. It combines the probabilistic model of the behavior of structural response given a ground motion parameter (GMP) (e.g., seismic fragility model) and the probabilistic seismic hazard analysis (PSHA) for the GMP in a mathematically rigorous manner. In current engineering practice, SRA is performed based on a single GMP. For structures whose responses can be better predicted using multiple GMPs, a vector-valued SRA (VSRA) gives more accurate estimates of risk. However, due to the extensive computational efforts required for the multiple-dimensional cases, VSRA has not been widely applied in engineering practice. This paper presents a simplified approach to VSRA, which can substantially improve computational efficiency without losing accuracy. A new seismic hazard deaggregation procedure is proposed to determine a set of controlling earthquakes in terms of magnitude, source-site distance, and occurrence rate for the site of interest. VSRA is then performed based on the controlling earthquakes rather than all possible earthquake occurrences. Two numerical examples are presented to validate the effectiveness and accuracy of the simplified approach. Factors affecting the approximations in the simplified approach are discussed. [ABSTRACT FROM AUTHOR]

Recently, blind source separation (BSS) methods have gained significant attention in the area of signal processing. Independent component analysis (ICA) and second-order blind identification (SOBI) are two popular BSS methods that have been applied to modal identification of mechanical and structural systems. Published results by several researchers have shown that ICA performs satisfactorily for systems with very low levels of structural damping, for example, for damping ratios of the order of 1% critical. For practical structural applications with higher levels of damping, methods based on SOBI have shown significant improvement over ICA methods. However, traditional SOBI methods suffer when nonstationary sources are present, such as those that occur during earthquakes and other transient excitations. In this paper, a new technique based on SOBI, called the modified cross-correlation method, is proposed to address these shortcomings. The conditions in which the problem of structural system identification can be posed as a BSS problem is also discussed. The results of simulation described in terms of identified natural frequencies, mode shapes, and damping ratios are presented for the cases of synthetic wind and recorded earthquake excitations. The results of identification show that the proposed method achieves better performance over traditional ICA and SOBI methods. Both experimental and large-scale structural simulation results are included to demonstrate the applicability of the newly proposed method to structural identification problems. [ABSTRACT FROM AUTHOR]