No abstract is provided for this article.
No abstract is provided for this article.
A large proportion of structural failures are due to human error in the design stage of a structural engineering project, and many of these failures could have been averted if there had been adequate design checking. Results are reported herein from surveys investigating the effectiveness of three typical design‐checking processes: self‐checking, independent detailed design checking, and overview checking. Following a review of current work in this area, appropriate mathematical models, which examine the effects of error magnitude, time, and experience, are proposed for each design checking process. These are compared to the limited data obtained from the surveys. Although preliminary, the results have interesting implications for practitioners.
This paper attempts to reconcile information from a number of different sources about the effect of small changes in carbon content on the immersion corrosion of specimens of normal commercial mild and low alloy steels. It does so through interpreting the data reported in the literature in terms of the recently proposed theoretically based phenomenological model for marine immersion corrosion. This model postulates different corrosion phases as corrosion progresses. When the experimental results are interpreted in terms of the model it is found that carbon content has minimal effect on the kinetically controlled corrosion phase. The next phase, when corrosion rate is controlled by oxygen diffusion, is also unaffected, in agreement with theoretical predictions. However, carbon content does affect the two anaerobic phases, with increased corrosion as the carbon content and the water temperature increase. The model allows apparently conflicting observations to be reconciled and shows that carbon content may be influential for longer-term corrosion and for corrosion in tropical waters.
A common practice in the offshore oil and gas (O&G) industry is to leave abandoned decommissioned pipelines and other steel infrastructure on the seafloor. Decisions about long-term environmental and other impacts and about the possibility of recovery require estimates of the likely long-term rate of deterioration, including corrosion loss and pit depth. These are considered as functions of time and environmental conditions including seawater temperature, dissolved oxygen concentration, salinity, seawater velocity, water pollution, microbiological activity, water depth, calcareous deposition and the effect of burial, all interpreted using established physico-chemical behaviour relevant for long-term exposures. Data for exposures up to some 600 years in seawaters are reviewed. Remaining research gaps and future perspectives of marine corrosion control are briefly discussed. Specific attention is given, by way of an example, to the influence on long-term durability of protective coatings and remnant cathodic protection, both areas in which further research is required.
Much concern exists about the apparently short durability of modern reinforced concrete structures exposed in marine atmospheric and tidal environments. Modern practice specifies high quality low permeability concretes in an effort to reduce the inward diffusion of chlorides, considered critical in the initiation of reinforcement corrosion in marine environments. However, for such environments a review of more than 300 cases reported in the literature found many reinforced concrete structures survived for long periods of time, despite poor quality concrete, low cover and very high chloride levels next to the reinforcement. Mostly these structures showed little or no external evidence of reinforcement corrosion. The available data shows a degree of correlation between long-term immunity from reinforcement corrosion and the composition of the concrete and its aggregates. It is shown that alkaline materials such as calcium carbonates and non-reactive dolomite add alkalinity to the concrete and that this is related to long term durability. While calcium carbonate usually is associated in the reinforcement corrosion literature with 'carbonation' which reduces pH levels and thus allows initiation of reinforcement corrosion, this not supported by corrosion science principles and by careful field observations. It is noted also that some reinforced concrete structures can have very aggressive localized corrosion without obvious exterior signs. This has serious practical implications.
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This article discusses the role of structural reliability theory in the wider context of structural safety and its role as part of safety in the community generally. It commences by noting that the rules for the design of structures have advanced from 'factor of safety' to more rational procedures, including the use of structural reliability theory. It is common practice, for this theory can be considered somewhat in isolation from the wider perspective of societal expectations of structural engineering. In this context, there remain a number of issues for clarification, including how the structural probability measures relate to observations about failures of actual structures. Some of these concerns can be addressed through the adoption of a more clearly defined decision-theoretic framework adapted to structural engineering safety issues. The components for such a framework are outlined herein. It is argued also that structural engineering safety and reliability measures must be part of the wider perspective of societal risk and safety assessment, and therefore must be compatible with procedures adopted for other potentially hazardous industries.
The present study compares corrosion mass loss and pit depth measurements on carbon steel corrosion coupons exposed under similar operating parameters, but with different biological consortia. One set of data were obtained from standard flush disc corrosion coupons used to monitor corrosion rates in a water injection pipeline on the North Sea continental shelf. The coupons were exposed on average for 6 months over 6 years operational time. These data are compared with published corrosion data of coupons exposed in abiotic district hot water systems from several power plants situated in Europe. The exposure time for these coupons was 9 months. Both systems were anoxic and in the same temperature range and are comparable. Observations regarding relationship between MIC and bacterial consortia, bacterial numbers and type, water quality and corrosion products are also made. The corrosion rate of the water injection pipeline is approximately 10 times higher compared with the corrosion rate in the abiotic district hot water system. It is concluded that the increased corrosion on the carbon steel coupons in the early stage is caused by MIC. This is also supported by the chemical and biological information available for the pipelines. The results reported here constitute the first step of an overall study to improve the level of understanding of the bacterial contribution to the total corrosion rates of carbon steel in water injection flowlines. Such understanding is expected to improve management and operational decision-making for practical control of corrosion in the field, by providing predictions of expected life time as a function of control of biotic consortia (e.g. through pigging, and biocide treatments). Further, it will facilitate decisions concerning choice of pipeline construction materials for future design.
A finite element model is proposed for simulating the short and long term strength of Glass Fibre Reinforced Plastics. The essence of the model is a division of the composite system into a binary system comprising 'fibre elements' to represent the fibres, and 'effective medium' elements to represent the matrix material, which account for other mechanical properties such as shear and transverse stiffness and Poisson's effect. Such a representation stems from the fact that the performance of a composite material is intrinsically controlled by the microstructure of the component materials. Besides, it has the advantage of the power and versatility of the finite element technique. Typical results for short and long term strengths are compared to similar predictions obtained from shear lag theory. Good agreement is obtained for a variety of finite element discretisations.
A review of some pertinent literature is made to show that there is now sufficient evidence that classical reliability calculations are not able to correctly predict the incidence of structural failure or malfunction. The main factor which has not been included in such analyses is human error. Interest in the importance of human error, whether of the gross or variability type, has increased considerably in recent years. Several approaches have been suggested for predicting the effect of human error: herein only statistically based models are considered. An attempt is made to compare and critically review the various suggested techniques, and a further model developed in outline. For the abstracts of parts I and III see TRIS nos 394165 and 394167. (Author/TRRL)
This paper describes some aspects ofthe processes involved in the development of high-quality probabilistic models for corrosion loss and for maximum pit depth for steels exposed to marine environments. Such models increasingly are required for advanced infrastructure life-cycle management to predict likely future levels of deterioration and for the setting of rational inspection regimes. Most models currently available are empirical. They have high levels of uncertainty. The models described herein are based on appropriate levels of representation of underlying corrosion science fundamentals, including the recognition of the important role of microbiological activity and model calibration using a probabilistic approach to data interpretation. This requires careful interpretation of existing literature data and has required obtaining new data to elucidate particular aspects. A brief overview of this work is given with emphasis on the modeling of maximum pit depth using extreme value distributions. Reinterpretation of existing pit depth data was found to be necessary for consistency with new observations of pit depth development in actual in-situ tests (as distinct from laboratory observations). As a result, the new model provides maximum pit depth as a function of time based on understanding of the underlying pitting process with time. It has shown also that long-term pit depth is most appropriately represented by the Frechet extreme value distribution since this is consistent with the underlying corrosion and bacterial processes. This new interpretation of data provides estimates of pit depth uncertainty as a function of exposure time. It is shown that this can be applied even for existing data sources that were not designed to elucidate probabilistic estimates of uncertainty. The conventional application of the Gumbel extreme value distribution can lead to serious errors in long-term extreme pit depth predictions.
It is generally agreed that reinforced concrete structures exposed to harsh marine environments will, within the space of one or two decades, start to show modest or even serious deterioration due to reinforcement corrosion unless special care is taken to prevent or reduce the rate of entry of aggressive chlorides. Usually the life prediction is based on the rate of ingress of chlorides ions. However, careful review of many older reinforced concrete structures shows that some have survived decades despite very high chloride levels and little or no protective measures or special additives. Conversely, there are cases were reinforcement corrosion is evident despite considerable concrete cover and high concrete quality. A review is given of more than 300 cases for which corrosion initiation and corrosion progression took many years to occur. Ccnsiderable differences were found in the time to corrosion initiation and in time to active corrosion. Figure 1 gives an example. A small number of brief case studies are given, together with an extended discussion of the possible reasons why some reinforced concrete structures show much better long-term durability than others. It is argued that long-term durability depends on pH reserves (i.e. alkalinity) and this can be provided by aggregates such as limestone and non-reactive dolomites. This observation is consistent with corrosion science principles. It is concluded also that calcium carbonate by itself (such as caused by 'carbonation') does not lower the concrete pH immediately adjacent to the reinforcement to permit corrosion initiation. The additional leaching of alkalies must occur before corrosion can initiate. These findings potentially have important practical implications for the prediction of the life of reinforced concrete structures. This matter is currently under investigation.
Conference paper written by Robert E. Melchers and Mark G. Stewart presented at IABSE Symposium: Towards a Better Built Environment - Innovation, Sustainability, Information Technology, Melbourne, Australia, 11-13 September 2002.
There is remarkably little quantitative information in the literature regarding the marine corrosion expected for steel exposed at different levels in the tidal region. There are no data that describe the effects of orientation, shielding, and water wave action. Coupons exposed at two locations on the eastern Australian seaboard produced corrosion profiles with elevations that are generally consistent with data reported in the literature. Estimates made of the statistical variability in corrosion losses showed a typical coefficient of variation around 0.02 to 0.05. Coupons boldly exposed between the mean high tide and the average mean tide levels were found to have a higher degree of corrosion for orientations associated with more aggressive exposure conditions. This is attributed mainly to differences in solar radiation and in weather conditions. There is little effect of orientation at higher and lower elevations. Where the waters and the coupons are subject to wave action such as from shipping, the mean tide water level for effective corrosion tends to be raised, in the case described, by about 0.25 m.
Thin-walled structures are relatively complicated structural systems governed by many parameters. These include the dimensions, the material properties and imperfection measures. In principle all of these are not known with certainty at the design stage, and even after construction some degree of uncertainty remains. The present paper reviews briefly how such uncertainties are considered in modern structural reliability theory to estimate nominal probabilities of failure. An example application will be given to illustrate the process of estimating the probability density function for the failure stress of stiffened plates subject to axial forces.
The influence of atmospheric conditions on the corrosion of steel and its associated microbial community was studied. Surface analysis revealed greater localized corrosion in steel exposed to near-ocean atmospheres with high chloride deposition compared to inland and subalpine sites. High-throughput sequencing analysis of corrosion products showed that dissimilar microbial communities and dominant species were deposited on steel in the different atmospheres. Close to the ocean, Brevundimonas diminuta were predominant whereas Clostridium and Pseudomonas species dominated for inland sites with agricultural or forestry activities. Bacillus and Enterococcus were dominant for sites close to a fertilizer plant and a sewage treatment plant, respectively. Actinobacteria species dominated at sub-alpine conditions. Results from this study indicate that microbial communities on corroding steel exposed to atmospheric conditions are the result of deposition of locally-generated aerosols. Acid-producing activity and exopolysaccharide (EPS) production was widespread and rapidly detected in microbial cultures from all the exposure sites. Sulphate-reducing bacteria were not detected in this study. These results suggest that acid production and EPS synthesis can be important mechanisms for microbial corrosion of steel under atmospheric conditions.
