Keywords MINIMUM, REINFORCEMENT, SOLUTIONS, FLAT, SLABS, METHODS, BENDING, COLUMNS, IDEALISED, IDEALISATION, POINTS, SUPPORTS, FINITE, STRIP... Show All
This chapter provides an overview of the approaches that can be used to determine structural reliability based analytic safety assessment for steel structures corroding in marine environments. Corrosion and pitting of structural steels used in marine pipelines and for off-shore mooring chain are important factors in continued integrity and service performance and are best considered in a reliability framework. For steel structures fatigue, corrosion and various combinations are likely to be very aggressive. The actual corrosion losses and maximum pit depths develop in a strongly non-linear fashion with increasing exposure period. In corrosion engineering it is usual to think in terms of a 'corrosion rate'. This assumes that corrosion continues at a constant rate with time. Steel composition has been found to have little effect on the development of pitting corrosion in marine immersion conditions. Importantly, marine corrosion can be influenced by the involvement of microorganisms particularly in nutrient polluted waters.
The short-term immersion corrosion of mild and low alloy steels in seawaters is known to be proportional to the concentration of dissolved oxygen (DO) in the bulk water. Longer-term corrosion is a function of the activity of sulphate-reducing bacteria and is influenced by the concentration of nutrients in the bulk water. These influences are examined in more detail for the corrosion of steels in the brackish waters of the River Thames and for several immersion corrosion sites on the Eastern Australian seaboard and in the North Sea. The published data sources were supplemented with plausible assumptions about environmental conditions. New interpretations of the data are provided based on the previously published model for immersion corrosion. For waters with negligible salinity and sulphate levels early corrosion loss was shown to depend on the dissolved oxygen content of the waters, and later corrosion loss was a direct function of nitrogenous nutrient (pollution) levels. This also applies to longer-term corrosion.
An electrode array probe has been specially designed and demonstrated for in situ monitoring and visualising multiple localised corrosion processes and mechanisms, including crevice, weldment and pitting corrosion, occurring simultaneously on marine steel structures. It enabled the probing of time-dependent development of multiple localised corrosion processes, mechanisms and kinetics, thereby facilitating more in-depth understanding of the initiation, propagation and kinetics of localised corrosion of steel marine structures.
For infrastructure applications in marine environments, the eventual initiation of corrosion (and pitting) of steels (and other metals and alloys) often is assumed an inescapable fact, and practical interest then centres on the rate at which corrosion damage is likely to occur in the future. This demands models with a reasonable degree of accuracy, preferably anchored in corrosion theory and calibrated to actual observations under realistic exposure conditions. Recent developments in the understanding of the development of corrosion loss and of maximum pit depth in particular are reviewed in light of modern techniques that permit much closer examination of pitted and corroded surfaces. From these observations, and from sometimes forgotten or ignored observations in the literature, it is proposed that pitting (and crevice corrosion) plays an important role in the overall corrosion process, but that longer term pitting behaviour is considerably more complex than usually considered. In turn, this explains much of the, often high, variability in maximum depths of pits observed at any point in time. The practical implications are outlined.
Aluminium alloys have excellent corrosion resistance to a wide variety of exposure conditions. Usually they corrode by pitting rather than by uniform corrosion. For infrastructure applications long-term corrosion behaviour is of interest. The relatively limited long-term pitting data that is available shows that maximum and average pit depths do not follow the power law function as conventionally assumed but tend to follow a bimodal trend with exposure time. This is consistent with the bimodal trends observed previously for corrosion mass loss of aluminium alloys. Most likely it is the result of the accumulation of corrosion products over the pit mouths, leading to the gradual development of localised anoxic conditions within pits. In turn this permits the development within the pits of anoxic autocatalytic conditions, consistent with established theory for pitting corrosion of aluminium. It also is consistent with observations of hydrogen evolution from pits. The implications of this for practical applications are discussed.
Available evidence suggests that design checking can significantly reduce the incidence of structural failures resulting from human errors in the design process. Since “real-world” experimentation is not possible, it is of interest to develop a model to simulate the effects of human error on a typical design task and also to model typical checking procedures. The development and analysis of these models is described in the present paper. The design task considered is that of member design for a rafter of a steel portal frame building. It was found that checking efficiencies between 0.6 and 0.9 are most effective in increasing structural reliability and that, often, only two separate design checks are necessary to virtually eliminate the incidence of structural failure due to human error.
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Corrosion of mild steel in contact with both seawater and particulate media has important practical implications. It is considered herein for coupon exposures up to 2 y in 0.05 mm and 0.425 mm (nom.) granular media in natural Pacific Ocean seawater and in seawater dosed with calcium nitrate ostensibly to stimulate microbiological corrosion. The observed pit depths were consistent with an empirical bi-modal corrosion prediction model and overall lowest for smaller particle sizes and greatest without particles. Contrary to expectations, nutrient dosing tended to reduce pit depths. The observations are discussed in light of earlier observations and for relevance to practical implications.
In the past decade, there have been numerous advances in probability-based structural codes and codified
The serviceability design requirements of various structural elements and the provisions of building regulations, codes and standards are discussed in the paper. Results of the questionnaire survey of; a) engineers in Australia and England on the serviceability design requirements of structures, b) owners of buildings on their reaction to existing serviceability design provisions, as reflected in the building, and the cost implication of such serviceability requirements, and c) occupants of residential and commercial buildings on their reaction to any noticeable deflection and/or vibration of buildings, are summarised. The paper concludes by identifying the shortcomings in the current method of design for serviceability, by proposing a method for formulating realistic limits and an improved method of design for serviceability by computing the actual deflection and vibration of structural components and by recommending the preferred direction for future research.
No abstract is provided for this article.
No abstract is provided for this article.
This paper deals with the analysis of uncertain creep and shrinkage effects, and in particular the computation of the probability of serviceability failure for a reinforced concrete structure subjected to stochastic loadings. A sustained load of uncertain magnitude, a stationary Gaussian loading process, and a Poisson loading process are considered. To model the creep and shrinkage phenomena. Bažant‐Panula model is employed. The resulting time‐variant reliability problem is an upcrossing problem in stochastic process theory that is, in general, difficult or computationally demanding to solve. The problem can be simplified to allow prediction at end of a given time period, using the so‐called time‐independent reliability theory (e.g., the first‐order second‐moment method). However, this can be achieved only once the statistics of the load effects have been obtained by Monte Carlo simulation, but is considerably less demanding than a complex Monte Carlo solution. A numerical example is given to illustrate the method, and the results are compared with simulation results.
Although it is well known that moisture ingress in glass fiber reinforced polymers (GFRP) enhances the phenomenon of stress corrosion cracking in the fibers, and that this reaction is likely to proceed more rapidly at the weakest sites in the glass fiber surface, a fundamental law that would permit the valid extrapolation of stress rupture curves to long service lives is yet to be developed. As a result, design guidelines for glass fiber reinforced polymers components have been developed mainly on a prescriptive rather than of a performance basis. Based on the well established knowledge on the chemical behavior of glass and, in particular, that of glass flaws, a model that combines fracture mechanics, shear lag theory, and a probability model for flaw size is developed to describe the behavior of GFRP composites. The predicted results, although limited to rather idealized situations, are very encouraging. They suggest that, with only modest assumptions about material properties, it is possible to obtain mechanisms of GFRP breakdown, which correspond with observed experimental behavior.
This paper reports on some early observations in a more extensive field programme aimed at elucidating basic in-situ immersion corrosion behaviour characteristics for mild and low alloy steel exposed in coastal seawater regions. Five sites are described - two in Newcastle Harbour and three in Lake Macquarie (two in the entrance and one in Fennel Bay). Results are described for coupons recovered after one, three and six months exposure. Overall corrosion rates for the six months were found to be in the range 150 - 300 micrometres/year. Seasonal factors such as rainfall appeared to have a significant influence on the corrosion behaviour, particularly for the specimens at Fennel Bay. For the present observations, corrosion rates were found to increase with salinity, dissolved oxygen and water velocity but reduced with increasing biofouling. These results are not inconsistent with those previously reported - the present investigation is an attempt to obtain results systematically. (a) For the covering entry of this conference, please see IRRD abstract no. E200447.
