640 publications from this institution
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. Earlier studies have shown that corrosion and pitting are not linear functions of exposure period. This implies that for maximum pit depth the Frechet distribution rather then the usual Gumbel distribution is more appropriate for reliability analysis. The reasons for and the implications of this are discussed.
It is known that nutrients can increase corrosion loss and increase the severity of pitting corrosion, both for short and for longer term exposures. However, the quantitative relationships involved are not well-defined nor is the effect of high nutrient loadings well-understood. Nutrient loadings for natural coastal seawater can occur through run-off of excess fertilizers and waste products from adjacent agricultural lands, either as a result of fertilizers applied for crop improvement or as a result of animal husbandry. It is known also that relatively high amounts of oxidised nitrogen, such as in nitrites, can inhibit corrosion. Herein a report is given of observations for highly polished mild steel coupons 25 mm x 25 mm in size exposed to different concentration levels of fertilizer. The coupons were exposed continuously for up to 546 days to low velocity streams of natural seawater in 200 litre tanks. There was no intervention in the first tank but the other two were dosed periodically with agricultural fertilizer, with the third tank being dosed at twice the rate of the second tank. For the first three months corrosion losses were similar in all three tanks but then began to diverge. After about twelve months corrosion loss in the second and third tank was around 90% and 75% respectively of that observed in natural seawater and this continued for the rest of the experiment.
No abstract is provided for this article.
The response of an initially curved cantilever strut with a lumped mass at the tip subject to arbitrary axial impact loads is described, as a model for the dynamic response of piles being driven into the ground. The differential equation of motion defining the lateral movement of impact loaded cantilever struts is derived. This equation includes the effect of inertial resistance to motion provided by both the strut self mass and the mass of a pile hammer. The probable shape of force pulses produced during pile driving were obtained from observations by others and approximated by an equivalent trapezoidal force pulse. The differential equation of motion was solved for this force pulse using the Runge Kutta technique and for physical parameters corresponding to those of a slender steel pile which was observed whilst being driven. The resulting solution was found to agree well with observed pile behaviour during driving in that the magnitude of lateral displacements were very small. It is also shown that the magnitude of lateral displacements reduces as the mass of the pile hammer increases, due to inertial effects. In principle, the theory presented can be used to determine the maximum slenderness ratio for a cantilever pile so as to keep the lateral deflections within a prescribed value for a given pulse shape. (Author/TRRL)
Major water supply distribution networks in many cities consist of buried cast iron pipes. Often the pipes are internally cement-lined. For these the most severe external pitting corrosion is important because this may cause leakage and eventual structural failure. Herein representative data is reported for 10 pipes, ranging in age from 34 to 129 years and from 200 to 600 mm diameter that are part of a larger data set. The pipes examined were selected by the water utilities from several critical locations. Pit depths over 1–2 m lengths of pipes were measured and examined and soil properties measured. The morphologies of pitting around the pipes were found to be very different, with often much greater pit depths and larger pit areas at the base of the pipes. There were also considerable differences along the length of individual parts of pipes. When plotted on a Gumbel plot for the maximum pit depths, the deepest pits showed a different Gumbel trend compared with the majority of pit depth maxima. Theoretically, this indicates that different pitting mechanisms are involved. This possibility is explored by reference to earlier work on the long-term pitting corrosion of steels in sea water. Comments are made also about empirical fitting with other extreme value distributions, as sometimes suggested.
Initiation of pitting of mild steel in seawater usually is attributed to anode-cathode reactions set up by the small manganese sulphide (MnS) inclusions present in most steels. On the other hand there is now considerable evidence that microbiological factors are involved in the early corrosion of mild steel in natural seawater. Presented herein are the results of a two year investigation of whether microbiological factors are likely to have a significant role in the pitting of mild steel exposed to seawater. In the experiment, natural seawater was pumped from a Port Stephens tributary and split into two streams. One stream flowed slowly through a tank in which 25 mm x 25 mm polished steel coupons were suspended and was then released back to the estuary. The second stream was passed through a series of filters and sterilized using ultra-violet (UV) lamps prior to entering a similar but sealed tank, also fitted with UV sterilization, where identical coupons were suspended, before being returned to the estuary. Coupons were removed at various time intervals and examined both visually and using a scanning electron microscope (SEM). In all cases the metal surfaces show a considerable difference in topography between those recovered from natural and those from sterilised seawater. Since the only significant difference between the exposure conditions was the removal of living microbiological material it is concluded that microbiological factors are responsible, directly or indirectly, for the more severe pitting observed on the coupons exposed to natural seawater.
The existing safety culture within structural engineering has been highly successful and is likely to remain so. However, it is argued herein that it is no longer completely sufficient. It lacks the ability to interface adequately with risk assessment procedures used for other facilities in society. Such procedures require complete analysis of all sources of risk and of the various potential consequences. These are consequences to society rather than simply ‘structural failure’. Moreover, the probabilities involved are estimates for expectations rather than the nominal probabilities used to calibrate limit state design codes. The theme is illustrated with two examples: one dealing with the safety assessment of an existing road bridge and the second with potential structural fatigue failure of a large pressure vessel for liquefied petroleum gas storage. These examples show that structural safety cannot, for major structures, be divorced from more general safety analyses. Some comments are made also about the World Trade Center and related risk rationality.
Despite the importance of proper organizational structure and procedures in the successful implementation of civil engineering projects, little attention appears to have been given to this by the profession. This apparent lack of interest is most notable when examining cases of project failure. Only the few inquiries into tragic and very expensive failures have been concerned with organizational matters as distinct from purely technical problems. Four cases in which organizational matters were of importance are briefly described. A review is made of efforts (principally in sociology) to determine desirable properties of organizations and the related but limited work in the building industry. This is compared to the requirements of civil engineering projects and a suggested approach for research is outlined. It is also suggested that complementary studies might follow a pathological approach to organizational study, although this would require a greater openness in accident investigation.
Four aspects of structural systems reliability are considered. These are: (a) bounds on structural systems reliability for systems loaded with a sequence of independent loadings. It is found the the Ditlevsen bounds for a structure under one set of loads can be extended to a sequence of loadings; (b) the use of the static theorem of plasticity to evaluate system reliability using first order, second moment theory. It is shown that unless all equilibrium equations are considered, the correct solution for system reliability will not be obtained; (c) the use of the static theorem of plasticity to evaluate system reliability using a simplification obtained by assuming that plastic moment capacities are perfectly correlated. It is demonstrated that this is not a gross assumption for a simple structure; (d) an approximate systems reliability calculation procedure based on integrating in the load space using the observation that the hyper volume over which integration is to be performed is usually fairly regular in shape. This then leads to two suggested avenues of approach for simplified analysis. (Author/TRRL)
This paper considers the role of structural reliability theory in the wider context of (i) achieving structural safety in practice and (ii) as part of safety assessment for engineering and other projects more generally, including its relationship to achievement of adequate safety in the community generally. It notes that the rules for the design of structures have advanced from 'Factor of Safety' to more rational procedures and these now include the widespread use of structural reliability theory for calibrating design codes. While this is an admirable achievement from a structural engineering viewpoint, and can be considered as a 'stand-alone' matter, increasingly there is an expectation that safety matters in structural engineering must be seen also in a wider perspective, that is in the context of societal expectations for structural engineering. As a result 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 have recently been outlined and are reviewed herein. It is argued also that structural engineering safety and reliability measures must be compatible with procedures adopted by other potentially hazardous industries.
Elementary monte-carlo sampling as a technique to artificially replicate an experimental program has been used successfully for relatively simple problems, provided the failure probability to be calculated is relatively large. The present paper reviews the elementary monte-carlo method and describes a little known, but improved approach known as importance sampling (a).
The bi-modal characteristic of general or 'uniform' corrosion, or corrosion as measured by mass loss, as a function of exposure time now has been demonstrated extensively for various steels including low alloy, chromium and weathering steels, cast irons and, more recently also for aluminium alloys and for copper alloys. Also, it usually is independent of the environment. Further, it also occurs for the trend in maximum pitting as a function of time. This wide applicability demands explanation. Previously it was argued that the bi-modal character is the result of a change, at the metal-environment interface, from predominantly oxygen-rich corrosion conditions to predominantly locally anoxic corrosion conditions. It was argued also that pitting plays an important role in this process. Herein these concepts are extended from accepted theory for pitting under nominally oxic conditions that, within pits, become anoxic and causes the pitting process to become autocatalytic but eventually to lead to cessation of pit depth growth. There are two key outcomes. The first is the observation that pits grow episodically, that is, they grow in depth to a given limit, grow sideways and possibly amalgamate or coalesce, leaving plateaus of corroded surface in which new pitting can occur. The cycle time for this is faster under already anoxic conditions than earlier when it takes time for anoxic conditions to develop. The second outcome is that mass loss as a function of time is a function both of the pit depth and thus the pitting process and the increase in the number of pits with time. A considerable body of observational evidence is available to support these mechanisms. There also are well-developed theories that already explain the component mechanisms.
No abstract is provided for this article.
No abstract is provided for this article.
For containment structures, such as pipelines, risers and tanks, the most aggressive type of corrosion attack often is considered to be pitting corrosion since metal perforation can cause leakage of the contents and thus potentially have serious environmental and economic consequences. For welded pipelines there is the complicating factor that the heat-affected zone is known to be more prone to corrosion than either the weld zone or the parent metal. However, quantitative data for this effect are scarce. The present paper reports on new field studies performed at Jervis Bay over a total of 3.5 years of exposure for the welded zone of API X56 Spec 5L grade pipeline steel. The corrosion patterns are reported as well as the increase in corrosion mass loss and maximum pit depth with time. It is shown that the development of pit depth and mass loss with time is consistent with earlier observations for mild steel under immersion conditions. As expected, the data show a considerable difference between the losses and pit depths for the weld metal, the heat affected zone and for the parent metal. The potential reasons for this are discussed.
Elementary monte-carlo sampling as a technique to artificially replicate an experimental program has been used successfully for relatively simple problems, provided the failure probability to be calculated is relatively large. The present paper reviews the elementary monte-carlo method and describes a little known, but improved approach known as importance sampling (a).
Chlorides have long been held responsible for the initiation and progression of the corrosion of reinforcing steels in concrete structures, with higher concentrations assumed to cause earlier and more severe subsequent reinforcement corrosion. However, extensive field observations and detailed experimental results show that, in well-compacted, low-permeability concretes, reinforcement corrosion often does not occur even in the presence of high concentrations of chlorides. If corrosion does occur, it has been observed as pitting (and crevice) corrosion primarily at air voids in the concrete at the steel–concrete interface. Herein, it is shown that this is consistent with thermodynamic principles (Pourbaix) for the pitting of steel in practical concretes with high pH and air voids, irrespective of chloride concentration. Any subsequent corrosion becomes inhibited, in part through the formation of corrosion products. The experimental observations also show that there is a separate, concurrent process of the dissolution of calcium hydroxide and its leaching from the concrete. The rate of dissolution is accelerated proportionally to the concentration of chlorides. This is the primary mechanism for longer-term reinforcement corrosion, eventually producing circum-neutral pH at the steel and thereby setting up the thermodynamics permitting general corrosion. The findings question the relevance of a critical chloride concentration as an indicator of the commencement of reinforcement corrosion. Concrete permeability, remaining alkali reserves (or pH), and physical observation of evidence of rust damage are better indicators.
This article addresses the problem of reliability assessment of reinforced concrete (RC) bridges during their service life. First, a probabilistic model for assessment of time‐dependent reliability of RC bridges is presented, with particular emphasis placed on deterioration of bridges due to corrosion of reinforcing steel. The model takes into account uncertainties associated with materials properties, bridge dimensions, loads, and corrosion initiation and propagation. Time‐dependent reliabilities are considered for ultimate and serviceability limit states. Examples illustrate the application of the model. Second, updating of predictive probabilistic models using site‐specific data is considered. Bayesian statistical theory that provides a mathematical basis for such updating is outlined briefly, and its implementation for the updating of information about bridge properties using inspection data is described in more detail. An example illustrates the effect of this updating on bridge reliability.
No abstract is provided for this article.
