<abstract> <p>An important structural component for cavity brick and masonry-veneer construction are wall ties. Typically, they are galvanized steel, sufficiently strong to provide continuity for transmission of direct and shear forces. However, field observations show they are prone to long-term corrosion and this can have serious structural implications under extreme events such as earthquakes. Opportunistic observations show corrosion occurs largely to the internal masonry interface zone even though conventional Code requirements specify corrosion testing for the whole tie. To throw light on the issue electrochemical test for 2 grades of galvanized ties and 316 stainless steels combined with three different mortar compositions are reported. Most severe corrosion occurred at the masonry interface and sometimes within the masonry itself. Structural capacity tests showed galvanized ties performed better than stainless steel ties in lieu of stainless steel R4 class ties presenting significantly greater relative losses of yield strength, ultimate tensile strength and elongation structural capacity compared to R2 low galvanized and R3 heavy galvanized tie classes.</p> </abstract>
Time-variant structural reliability problems arise when the structural resistance may change with time (such as in structural deterioration) or when the system is acted upon by two or more stochastic load processes. This paper reviews current trends in using sampling based techniques to solve such problems.
The corrosion of reinforcement bars and prestressing strands in concrete structures such as bridges and coastal and harbour facilities is a significant maintenance and renewal issue. Mostly such structures are considered to have useful lives of 2-3 decades, but, surprisingly perhaps, there are a considerable number of structures still in existence with much longer useful lives, some showing little evidence of reinforcement corrosion despite being either exposed directly to hostile marine environments or very likely to have been made with seawater as mixing water. A small number of the latter, and a recent review of the performance of many (>300) of such structures is given. That review did not support the conventional thinking that modern cements were responsible for the poorer longterm durability of modern structures. Instead it was proposed, on the basis of electrochemical arguments and observed experience, that the properties of the aggregates is a key factor. To test this hypothesis a series of exposure tests were commenced several years ago in which seawater was used as mixing water and with different aggregates, including those rich in calcium-carbonate, which the conventional wisdom suggests lowers the concrete pore solution sufficiently to initiate reinforcement corrosion. Preliminary results from these tests are described and it is shown that calcium carbonate concretes, even with seawater, do not lead to early corrosion initiation. Results for the effect of higher cement content, water cement ratio and the presence or absence of salt water in the concrete mix also are presented.
Improvements on the approximate solutions for the standard multinormal or multivariate normal distribution function based on the first order structural reliability concepts are proposed. The numerical accuracy and efficiency of three methods of obtaining the solutions are compared. For parallel systems problems, it is shown that the suggested first order multinormal (FOMN) approach produces results significantly closer to the exact answer than either those of the crude or the improved FOMN solutions. The performance of the improved and general FOMN approaches when applied to high reliability series systems appears to be as good as or even better than those obtained by the well-known second order bounds.
No abstract is provided for this article.
Microbial induced corrosion (MIC) of reinforced concrete sewer piping and manholes is a significant issue in Australia and overseas costing water authorities hundreds of millions of dollars annually. It is anticipated that as the country's sewer infrastructure ages the problem will become more severe. Over the last 4 years an ARC and industry funded research project has been undertaken with the aim of building a mathematical model to predict the corrosion of concrete as a function of exposure time and environmental and operational conditions. After almost 4 years of field trials in Sydney, Melbourne and Perth sewers a detailed understanding of the evolution of the corrosion process has emerged and a phenomenological model has been developed. The present paper describes the study findings and their implication for pipe service life prediction.
Seabed 'parking' of steel pipelines is a common practice in the oil and gas industry. However, despite the use of corrosion inhibitors and biocides internal corrosion remains an issue. So-called 'wet parking' may show severe pitting corrosion, often at the 6 o’clock position and usually attributed to microbiologically influenced corrosion, since high concentrations of various bacteria often are detected. The usual response, dosing with biocides, may not be not effective. It also can have environmental consequences if biocide is eventually or accidentally released. Current practices and options for parking of offshore pipelines are reviewed briefly. Recent findings for internal corrosion inside water injection pipelines are then considered as these have implications for 'wet' parked systems. After initial oxygen controlled corrosion longer term corrosion can occur under anaerobic conditions and this has implications for corrosion during periods of stagnant conditions and for corrosion under deposits of rusts and other debris. These corrosion mechanisms can be enhanced by microbiologically influenced corrosion but only if necessary nutrients are available. Acceptably low levels of corrosion can be achieved inside 'wet' parked pipelines with the use of seawater with low concentrations of particulate matter and low concentrations of microbiologically-critical nutrients.
Pitting can be a critical form of corrosion for mild and high strength low alloy structural steels subject to saline environments. For pipelines, tanks and nuclear and other containers perforation is the main design criterion. The present paper outlines a recently developed model for the progression of ‘maximum’ observed pit depths, based on new field trials using conventional coupon testing. The variability of the maximum pit depth and its changing behaviour with time of exposure is an important aspect for structural reliability studies. The most common approach uses the Gumbel distribution. However, recent data for mild steel reveals inconsistencies. These are described and have been noted also for pitting in some other metals. It is proposed that proper account has not been taken of sample inhomogeneity and, more importantly, that extreme pits are unlikely to be independent events, even asymptotically, as required by extreme value theory. A much simpler approach based on structural systems reliability theory is proposed. This has important practical implications for extrapolation to larger areas of steel plate or for long-term exposures, such as for nuclear containers.
Importance sampling as a special technique in Monte Carlo probability integration has been shown to be a highly efficient and rather unrestricted method. Non-Gaussian and dependent random variables and nonlinear limit functions can be treated relatively easily and with reasonable rates of convergence. A major draw-back, however, is the need to identify so-called “interesting” or “important” regions for integration. Reference to first-order second-moment (FOSM) methods may help, as well as numerical maximization routines applied. Each involves certain difficulties. An alternative procedure, based on directing and correcting the importance sampling function as sampling is carried out, is presented herein. In particular it is possible to have a multi-modal sampling function.
There have been few reports of structures that could be examined in detail for long-term behaviour and performance. One opportunity arose in 2002 when the Sorell Causeway bridge, a 45-year-old, 457 metres long, 34 span, pre-stressed concrete bridge in Tasmania, Australia, was demolished. This followed concerns about its safety in the light of the increasing size and frequency of occurrence of longitudinal cracks in the webs of the beams, the cracks appearing to follow the trajectory of the post-tensioning tendons. Corrosion was suspected. Pre-demolition testing had not provided sufficient information for estimates to be made of the remaining capacity of the bridge. After demolition, three bridge beams were selected for very detailed evaluation. The load capacity of the most severely damaged beam was only half that of the apparently sound beam. It failed catastrophically. The more seriously damaged beams showed severe loss of ductility. Progressive failure of the strands in the tendons was recorded in each case. The corrosion of the conventional reinforcement was consistent with expectations. However, some of the pre-stressing strands showed severe localised corrosion with cross-section losses between 75% and 100% but with little evidence of conventional oxidation rust products. Green rusts and ferrous chlorides were observed. The apparently sound beam also showed severe localised corrosion of strands.
The application of reliability theory for slip circle analysis of earth and rockfill dams is described using the First-Order Second-Moment method and stresses calculated from non-linear finite element analyses. The limit state function was formulated from the definition of a finite element safety factor based on shear stress. Representative statistical data for the fill materials were obtained from analyses of experimental and field data of the material properties in three existing dams. A reliability analysis was carried out for the upstream slope for the "end-of-construction" stage of an earth and rockfill dam. Six slip circles were used in the analysis, and comparison of their probability of failure and safety factor showed that the order of criticalness of the slip circles predicted by the latter can be misleading. In addition, the overall system probability of failure is close to that of the most critical circle. Sensitivity studies were carried out on the six slip circles to investigate the effects of progressive introduction of correlation between the properties.
A major problem for the management of oil and gas pipelines is corrosion influenced by microorganisms particularly, bacteria. The present study focuses on the influence particularly of the sulfate reducing bacteria on the internal corrosion of water injection pipelines. The research is based on reports from long-term observations of several water injections pipelines from the North Sea continental shelf. Observation data taken into consideration are pigging operation information, composition of corrosion products and their amount, corrosion rates and pipe geometry, identified biological consortia, water chemistry and process parameters. Observations regarding water quality and mitigation methods are made also. The distribution of corrosion including pitting and ‘features' along the pipeline as well as the localization of these in relation to the orientation of the surfaces, is considered. This includes different corrosion patterns along the pipeline and the relative severity of six o'clock corrosion. These observations are used to make correlation estimates between severity and location of corrosion and service history and the local environmental conditions, where this information is available. The correlations are used to develop a clearer view of the proportion of biocorrosion contributing to the total corrosion in water injection pipelines. Additionally, an assessment is made of the efficiency of mitigation procedures such as biocide treatments and pigging operations. The paper provides possible explanations for different rates and spatial patterns of corrosion for water injection pipelines.
The so-called rigid connections in real frames are seldom rigid, and instead contribute to the deflection behaviour of the frame. In conventional practice this is usually ignored. The present paper is a first step in the analysis of the significance of connection flexibility, allowing for both elastic and plastic behaviour of frame members. It is shown that existing formulae for prediction of connection behaviour of bolted end plated connections are inadequate and that connection flexibility can make a significant contribution to frame deflection. However, this contribution varies in magnitude with load level and frame flexibility. A wide spectrum of realistic frames needs to be examined to assess the importance of connection flexibility for designers. (Author/TRRL)
For estimating the likely service life of brittle material pipes with exterior corrosion pits and subject to hydrogen embrittlement crack propagation from the pits is of interest. Similarity of pattern of fatigue cracking from these pits under pressure fluctuations to that under monotonically increasing pressure is referred to. This similarity permits the effect of hydrogen on the cracking rate to be formulated. The depth of cracks initiated from surface pits is estimated as a function of pipe service time, based on the proposed formulation for Hydrogen-Assisted Fatigue Cracking.
Structures are usually composed of many members and connections, each with individual properties. When the member properties and the loadings applied to the structure are specified in statistical terms, the determination of structural system reliability becomes a problem of multi- fold integration. Since direct integration is feasible only for extremely simple structures, several alternative approaches to the problem have been suggested, either through simplifying the integration process itself or through simplified reliability theory. These approaches are discussed in the context of use of computer programs and computer usage. (Author/TRRL)
