640 publications from this institution
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 multi-phase mean-value model previously proposed for modelling the marine immersion corrosion of low carbon and low alloy structural steels is examined for application to fresh and brackish waters. Use is made of field data for brackish and fresh waters available in the literature, supplemented with data or estimates for water temperature, pH, hardness and nutrient levels. It is shown that the data exhibits consistency with the model and that it is a function of average water temperature. Corrosion in brackish and fresh waters corrosion depends on water hardness, pH and nutrient levels, with higher pH levels and lower water hardness associated with higher aerobic levels of corrosion but these are not significant for anaerobic corrosion. In the anaerobic phases 3 and 4 of the model, the available data and associated trends are interpreted as showing that elevated levels of nutrients produce higher rates of corrosion. Conversely, these phases showed very low rates of corrosion for fresh waters with very low nutrient levels. Consistent with basic corrosion theory and with laboratory observations, salinity by itself is not a clear distinguishing characteristic. The model provides a new approach to interpreting the available data for corrosion in fresh and brackish waters. It permits plausible explanations for previous apparently inconsistent observations for corrosion in brackish waters. Finally, it reinforces the need for full and detailed reporting of corrosion testing programs, including details of precise timing, location, orientation and environmental conditions including means and variations in water temperatures, DO, salinity, pH, water hardness, carbonates and various nutrients.
No abstract is provided for this article.
Lower bound methods have had little application in shell design. The paper presents optimum lower bound solutions for a circular cylindrical tank unrestrained at the top, and either fixed or hinged around the base. Both the optimization method of Rozvany and of Marcal, Prager, and Shield give the same results. A numerical comparison of optimized lower bound design, elastic design, and uniform isotropic yield-line analysis is made. Savings in reinforcement volume of 20% to 25% can be obtained in certain cases. The effects of prescribed minimum amounts of reinforcing and of cracking below working load are analyzed.
No abstract is provided for this article.
It is proposed that for immersion corrosion conditions extreme pit depths are those resulting from immediate pit initiation and pit growth without significant metastable activity. This is ...
The plates that will be analyzed herein have their fibers stressed either in tension on compression in each face of the plate, and it is assumed that the matrix material of the plate has no inherent strength except in shear. The problem analyzed in this paper concerns the optimal fiber layout in a corner of a fiber reinforced plate supported by two simply supporting edges meeting at an angle 20 > 90°. It would appear that there are two distinct possibilities for the optimal fiber layout in the corner, depending on the assumptions made, and the interpretation of the boundary conditions. An attempt is made in this note to study the implications of these alternatives.
This paper provides an overview of recent research into the modelling of the loss of material for structural steels continuously immersed in seawater. Until recently the available engineering literature in this area has been able to provide only very general information, perhaps sufficient for conservative approximations suitable for the design of new structures. However, it is insufficient to predict the likely loss of steel with time. It is even less suitable to predict the additional losses that may occur for already partly corroded structures. Field trials to assess the likely 'engineering' corrosion rates are expensive, take a long time and may be misleading. An alternative approach is to use the basics of corrosion mechanics to developed engineering models for prediction of corrosion. A recently developed multi-phase model for corrosion loss with time of exposure is presented. This model, already accepted in the corrosion literature, indicates that metal oxidation as conventionally associated with corrosion is only one part of the overall process. In the longer term corrosion induced by anaerobic bacteria is the governing mechanism. This has major practical implications for the assessment of infrastructure. Various research findings for the effect of water temperature, dissolved oxygen levels, nutrient pollution, depth, water velocity and water salinity are reviewed.
The process of structural design has received very little attention from the research community despite its centralness in day to day structural engineering. Much of the research which has been done has been carried out in Australia over a number of years and with the cooperation of many structural engineers. The present paper reviews the reasons for being engaged in this relatively unusual research activity and provides an overview of the work to date. A review is given of the incidence and type of errors made in basic design tasks such as calculations, table look up and table interpolation, and how these errors relate to results obtained for more comprehensive tasks such as loading determination and member design. One of the outcomes of the work has been to suggest areas in certain current design codes which are prone to misinterpretation by practitioners. Preliminary results related to design computation checking and inspection are also reviewed. This work potentially has major implications for quality assurance programmes (A).
The reliability analysis of multi-membered structural systems such as steel tubular offshore platforms (jacket-type) has proved to be a challenging task. One matter of particular importance is the modelling of applied wave loading. For the structural analyses used in reliability estimation, one single wave loading system only is most commonly considered. Real wave trains, however, reach the various structural components at different times. This may have an effect on the sequence of member failure and hence on structural failure. This can, in turn, affect the calculated probability of failure for the structure as a whole. The importance of this possibility is investigated in the paper for a reasonably realistic but idealized offshore structure.
A procedure, formulated in the space of the load processes, is described for estimating the reliability of structures subject to multi-parameter time-varying loading. For most realistic reliability problems the load process space is of low order. As a result, the required multidimensional integration is significantly simplified. The proposed approach also has well defined steps. As a result, there is increased transparency and reduced problems of integration instability and non-convergence. Both loads and resistances are described in terms of random variable parameters and time dependent structural resistances can be considered. Numerical examples are given to illustrate the proposed method. Example applications are given for a fixed base rigid-plastic portal frame subjected to time dependent loads and resistances. Linear and non-linear limit state equations and Normal and non-Normal distribution of the random variables are considered and compared, in some cases, to the results evaluated using Monte Carlo simulation.
Improvements can be made to importance sampling by recognising some of the essential features of the time-independent structural reliability problem. Thus non-interesting regions in the integration domain can be deleted from consideration using a priori information and updating techniques. These can also be used to obtain the point(s) of maximum likelihood, a central parameter for importance sampling in cartesian co-ordinates.
Corrosion loss data from the 1957 NBS study for steels buried in a large variety of soils and exposed over 12–17 years are re-interpreted, supported with estimates for scatter in the data. This shows that losses predominantly are consistent with the bi-modal trending pattern also previously observed for steel exposed in a variety of other environments. For short-term and low time of wetness exposures the trends are consistent with mode 1 of the bi-modal trend. These reinterpretations should permit development of better understanding of the factors important for short- and-long term corrosion trends and for improved modelling and prediction.
The reliability analysis of off-shore structures under dynamic non-Gaussian wave, wind and water current loading is considered using an outcrossing approach and directional simulation in the space of the load processes. To circumvent the non-Gaussianity of the load processes for which results for mean outcrossing rate are not generally available, the “sample-specific linearization method” is proposed. This method is based on well-known results for Gaussian processes and uses the functional relationships between the non-Gaussian wave, wind and water current loads and the corresponding velocities. The proposed method is applied to a simple model off-shore structure. The structure is analyzed also to obtain probability of failure by means of Monte Carlo simulation and using time domain simulation of the load processes. This was found to be extremely expensive of computer time. The results were found to compare very favourably with those obtained by the proposed method, both in terms of accuracy and computation required.
The cost of injuries and “accidents” to an organisation is very important in establishing how much it should spend on safety control. Despite the usefulness of information about the cost of a company's accidents, it is not customary accounting practice to make these data available. Of the two kinds of costs incurred by a company through occupational injuries and accidents, direct costs and indirect costs; the direct costs are much easier to estimate. However, the uninsured costs are usually more critical and should be estimated by each company. The authors investigate a general model to estimate the above costs and hence to establish efficient safety control. One construction company has been a pilot for this study. By analysing actual company data for three years, it is found that the efficient safety control cost should be 1.2–1.3% of total contract costs.
Directional importance sampling is considered in the original space of loads when the loads are highly correlated and/or highly out of proportion. Under these conditions, the joint probability density function contours are elongated and the application of unbiased directional sampling may result in poor and unstable results. It is shown that a major improvement can be achieved by applying importance sampling to the directional sampling technique. In this way better and quite stable results can be obtained. Also, it is shown that uncertainties in structural strength may affect directional sampling in load space. Some illustrative examples are given.
