640 publications from this institution
This technical note concerns the estimation of the outcrossing rate of nonstationary Gaussian vector processes from a convex polyhedral limit-state surface enclosing the origin. The theoretically most attractive approach for this type of problem in time-dependent structural reliability analysis is to employ the concept of first-passage probability in stochastic-process theory. Because the use of the generalized Rice formula poses some difficulty, the outcrossing problem is formulated in another way; i.e., using the original Rice formula directly. This situation allows the multidimensional integral to be reduced to a one-dimensional integral. This then allows the formulation (which applies also to smooth nonstationary processes) to be extended to time-dependent domain boundaries. An example is given to illustrate the method.
No abstract is provided for this article.
Developments in numerical methods and higher-speed computing are providing rapid advances in the analysis and design of complex structures—so rapid that it can be difficult for engineers to keep abreast of this expanding knowledge. Modeling Complex Engineering Structures provides an overview of cutting-edge developments in computational theory and techniques as currently applied in various fields of structural analysis, in the United States and around the world. Individual contributors discuss research and practice in diverse but related applications such as high-rise buildings, lightweight long-span roofs, bridges, geotechnical structures, aging nuclear structures, offshore structures, automobiles, aircraft, and ships.
Aspects of formulating safety formats for the design and assessment of structures subject to deterioration processes are discussed. With basis in the load and resistance factor design (LRFD) safety format focus is given to highlight the interrelation between the design values for resistance variables and the statistical characteristics of the predicted degradation. The effect of inspections and maintenance on the safety of deteriorating structures is illustrated and it is shown how this effect may be taken into account when determining the design values for resistance variables. (A) For the covering abstract see ITRD E121600.
No abstract is provided for this article.
Microbiologically influenced corrosion (MIC) of reinforced concrete sewer systems is a major issue for waste water authorities world-wide. Global repair costs are estimated at billions of dollars per year. Advanced stages of corrosion may trigger structural failure with potentially serious operational, economic and societal consequences. The fundamental issues involved in concrete sewer deterioration are reasonably well-understood but much less developed is the quantitative prediction of the rate at which deterioration is likely to occur under particular operating environments. An outline is given of a current industry-sponsored research project that aims to develop a phenomenological model of the sewer corrosion process for use in estimating the reliability and the expected remaining service life of concrete sewers. The model will be based on corrosion science but to ensure relevance to use in industry it will be calibrated to field observations and specific laboratory testing. This will also involve historical records and the experience of the industry participants. Parameters of particular interest include H2S levels, temperature and humidity and the effect of odour control measures.
This paper describes the condition of a reinforced concrete balustrade consisting of some 1000 individual beam elements all exposed similarly to the hostile marine environment of the North Sea at Arbroath, Scotland since 1943. A comparison is made of the condition of the original construction with the condition of repairs carried out in 1968 and in 1993. It is shown that the 1943 construction shows very little corrosion-induced cracking and little rust staining even though it does not appear to be of high construction quality. Only a very low percentage of the balustrade beams have been replaced. In contrast the beam installed in 1968 and later in 1993 show very considerable and large concrete cracks directly attributable to the corrosion of the longitudinal reinforcement, even though the concrete is of a higher quality and density. A detailed condition survey and statistics of crack sizes are presented in the paper. It is found that the higher corrosion resistance of the 1943 concrete is generally consistent with the concrete electrical resistivity measurements but the degree of corrosion of the reinforcing bars is inconsistent with chloride penetration measurements. The results are compared with the very few observations available in the literature for ageing concrete structures in marine environments. The results cast doubt on the conventional wisdom that chloride content at the reinforcement correlates well with reinforcement corrosion.
The cause of the high corrosion losses below the low tide level on older steel piling (usually called Accelerated Low Water Corrosion – ALWC) is incompletely understood but usually linked to microbiological influences. Vertical 3–6m long steel strips were exposed through the tidal zone at 13 different marine sites. After only 1–3years good correlation was found between ALWC and local seawater average concentration of dissolved inorganic nitrogen, a necessary nutrient for microbiological activity. These results allow prediction of the likelihood of long-term ALWC through measurement of either short-term corrosion loss profiles or average nutrient levels.
This paper presents an overview of a research program aimed at obtaining basic data about errors committed in some of the tasks that typically occur in structural engineering design. Such information is of interest in attempting to better understand the design process itself and thus in attempts to mathematically model it and to improve it.
A phenomenological model for general corrosion of mild, low-alloy steel under near-surface, at-sea immersion conditions was proposed earlier. The model was set in a probabilistic framework. The present paper deals with estimation of the various uncertainties associated with corrosion assessment from field data and the estimation of model bias. Uncertainties arise from inherent variability in corrosion processes, from variability in material properties, from corrosion response to environmental conditions, and in the environmental conditions themselves. Model uncertainty is also involved, as is model bias. In general, there is an acute shortage of appropriate data in the literature since individual coupon results have only seldom been reported. Results from recent field tests carried out on the Eastern Australian seaboard provide possibly the most complete variability information to date and are applied herein. An example is given to illustrate the application of the derived uncertainty information to a corrosion-time relationship and the associated uncertainty probability bounds.
Ductility is an important limit state for the design of reinforced concrete beams. Its implementation varies considerably between design codes. This is investigated using reliability-based assessment with ductility defined by strain ratio. The modelling uncertainty for the ductility limit state typically is much greater than that for structural strength limit state. This is reflected in the corresponding reliability indices of limit state defined for ductility. Some of these could be considered unacceptably low.
This paper presents observations of carbonation and alkali loss for well-compacted reinforced concrete columns from the exterior and interior of a 60-y-old in-land building. Calcium carbonate formation was detected only in the outermost 10 mm to 15 mm. However, alkali loss and concrete pH reduction extended much further inward but reinforcement corrosion was not observed. Theoretical thermodynamic conditions dictate that corrosion initiation of reinforcement can result only from the long-term dissolution and loss by leaching of calcium hydroxide from the concrete matrix. These appear rate-limited by the barrier effect of carbonated concrete. These interpretations provide a new model for “carbonation” initiation of reinforcement corrosion. The results also show the potential for concretes to absorb atmospheric carbon dioxide (greenhouse gases) for an extended time without significant risk of reinforcement corrosion.
Structural risk is not the same as corrosion risk. Therefore once assessment of infrastructure confirms corrosion is active and the mechanism(s) of corrosion are understood this does not necessarily indicate an unacceptable structural risk. Asset managers, infrastructure owners and stakeholders normally indicate a required service life requirement. An implicit requirement is meeting structural and other safety requirements as defined by Australian or other Standards. Maintenance and/or remedial work activities are only supported for the asset to achieve desired functionality, serviceability, safety, operational and commercial outcomes for its required service life. The paper explores the differences between structural and corrosion risk and how each of these risk types should be considered in the assessment, management and maintenance of infrastructure to meet required service life, structural safety and risk requirements. Some examples are used to illustrate this theme.
Within the confines of absolute minimum weight design of plastic structures, it has generally been assumed that the relationship between material (cost) and the vector of internal actions is linear (or piecewise linear) that is, sandwich approximations, or essentially constant lever-arm assumptions for reinforced materials, have been employed. Where both axial load and moment act on the one cross-section, or moments are relatively large, such idealization of cost function is no longer consistent with actual behaviour. A nonlinear cost function model must be substituted. The cost-function for an idealized reinforced section under bending moment and axial load (tensile or compressive) is derived herein. Two examples of encastre reinforced beams with axial forces are considered. When the applied load consists of a transverse central point load the optimum design is found to accord with earlier results employing a simplified cost function. When the applied lateral load is uniformly distributed earlier results are only approached in the limit as the lateral load approaches infinity.
Samples of new and 70year old pre-corroded OPC concrete were exposed for up to 48months in 6 sewers throughout Australia. Corrosion losses at each site followed the bi-linear trend originally proposed by Wells and Melchers [1]. During an initial phase (lasting <2years) negligible loss of material occurs however once the surface pH=6 losses commence and accumulate linearly at a rate that is likely to remain constant over time. Corrosion rates were found to be sensitive to humidity but insensitive to concrete alkalinity. A first pass model which predicts the rate of concrete sewer pipe corrosion from a knowledge of local average sewer gas temperature, humidity and H2S concentrations was also developed. The equation predictions were in good agreement with rates determined from field observation and historical data.
To derive the extreme value statistics for maximum depth of pits in pitting corrosion, it is conventional to use the Gumbel distribution. However, this has been shown recently to be not entirely appropriate. Modern pitting theory considers that the pit population consists of two major sub-populations, ‘stable pitting’ that can produce extreme depth pits in longer-term exposures, and ‘meta-stable’ pitting. It is not appropriate to mix these two different populations. Also, because of lack of sufficient data at any one exposure time, it is conventional to combine data from different exposure times. Unfortunately the pit depth growth relationship commonly used for this purpose does not accord with actual observations. The reason is that for longer-term exposures sulphate-reducing bacterial activity is the main corrosive agent. Under these conditions some plausible assumptions show that the Frechet extreme value distribution is more appropriate than Gumbel to represent the maximum pit depth. A simple example demonstrates that the difference in maximum pit depth predicted by these two distributions is considerable. This could have serious practical implications for predicting long-term pitting corrosion failure. Also, the question of coupon size to optimize the amount of stable pitting is discussed.
No abstract is provided for this article.
Steel pipelines externally exposed to seawater pit more severely in heat-affected weld zones but longer-term quantitative data are scarce. Maximum pit depths and variability are reported for the longitudinal weld in API X56 Spec 5 L grade steel pipeline samples exposed continuously to natural Pacific Ocean seawater for 3.5years. For the first year maximum pit depths and pit depth variability for the different weld zones were very similar. Both became much greater subsequently, with the greatest increases for the heat affected zone. This is compared with steel composition and grain size and potential reasons for the observations discussed.
