Considerable progress has been made in the theory of structural reliability in the last 15-20 years, particularly in relation to the analytical treatment of information. Much of this has gone relatively unnoticed by the engineering profession, even though the potential for structural reliability assessments in certain practical problems is now significant. Structural reliability theory has been applied, of course, to structural design code calibration, but has a much wider potential. Particularly in the offshore industry and in large projects it is being used where risk must be specifically considered and where to do so may yield considerable design and construction cost savings. In this paper the basic concepts of structural reliability theory will be reviewed and related to the standard and type of information usually available in engineering studies. An overview will also be given of the various calculation techniques now available (A).
For modern mild and low-alloy structural steels under marine immersion conditions, pitting corrosion typically is not considered an important issue. However, the literature shows that in t...
‘Directional simulation in the load space (DS-LS)’ is a simulation-based technique used to perform reliability analysis of structures subjected to time-invariant or time-variant random loads. To perform DS-LS a location must first be chosen for an ‘origin of simulation’. The origin may be positioned in either the safe or failure region of the load space, and its precise location (with respect to these regions) influences the DS-LS formulation needed to evaluate reliability correctly. The current formulation requires the origin to be positioned in the safe region. However, even for simple structures, the ‘exact’ location of the safe and failure region is not always known explicitly ‘a priori’. Modifications to allow for the possibility of positioning the origin not only in the ‘safe’ region but in the ‘failure’ region are proposed in this paper. Some numerical examples involving one or more stationary continuous Gaussian loads and the simulation of directions by ‘Monte Carlo’ and ‘the hyperspace division method’ are presented to demonstrate the validity of the proposed formulations. Some comments on convergence are made.
No abstract is provided for this article.
The 45 year old Sorell Causeway Bridge was demolished in 2002 due to concerns regarding the corrosion of prestressed tendons. Three beams of varying condition were salvaged from the demolition process and were relocated to the Structural Laboratory at the University of Newcastle. The beams were examined for load capacity and load deflection behaviour, as well as being subject to very detailed investigations. This paper provides the first set of findings from initial investigations carried out in November-December 2005, in particular providing comment on the load characteristics of the beams and instances of corrosion observed. (a) For the covering entry of this conference, please see ITRD abstract no. E214936.
No abstract is provided for this article.
Given that human error, particularly in design, causes a significant proportion of structural failures, it is of interest to develop a design task model that simulates the effect of human error in design. The development and analysis of a model for a typical design task (loading calculations for sizing a steel portal frame building) are described in the present paper. It is shown that human error in design loading calculations leads to a loss of structural safety. A comparison of the results of the design task model with available survey data for the same task suggests that such models can realistically represent the effect of human error on the modelled process.
The higher early corrosion rate of mild steel coupons under elevated nutrient levels (in particular total nitrogen) in seawater is examined using recently reported and new one-month empirical observations of corrosion losses. These are compared with field observations of the variation with exposure time of the bacterial and the iron sulfide contents in rust layers. Together these observations are interpreted using previously reported laboratory observations, including the marine bacterial survival strategy of colonizing (metal) surfaces in nutrient-poor ocean waters and the known heterogeneity of surface biofilms that permits the development of niches of anaerobic sulfate-reducing bacteria (SRB) activity that eventually become incorporated in the rust layers. It is proposed that this mechanism provides an explanation for the observed increased levels of short-term corrosion under elevated levels of nutrient exposure. A more detailed first estimate of the effect of nutrients on the calibration par...
No abstract is provided for this article.
The modelling of both short- and long-term corrosion loss and maximum pit depth is increasingly of interest to engineers and others interested in predicting the remaining life of coastal and ocean infrastructure. Traditional models are demonstrably of poor quality. A more refined modelling approach applicable both to corrosion loss and to pitting of mild steel in marine environments is described. A crucial aspect is that explicit consideration is given to the influence of microbial activity. The model is unique in that it considers the governing corrosion process to change with increased exposure time. The rationale for this approach and some of the key findings are described. It is supported by observations of the corrosion response to changes in metal alloying and to water pollution effects. The model also permits more rational explanations to be offered for the effects of water velocity, depth of immersion and seawater salinity. Recently the effect of anaerobic bacteria on very early corrosion has been re-examined and an explanation offered in terms of the influence of water pollution on the initial corrosion rate. Applications to pitting corrosion of mild steel has led to new interpretations of pit data and new interpretations for the probability distributions associated with maximum probable pit depth. The model has been calibrated using an extensive re-examination of literature data and extensive use of specially commissioned field tests.
"The History of the Theory of Structures: From Arch Analysis to Computational Mechanics." Australian Journal of Structural Engineering, 8(3), p. 259
Channelling corrosion of steel pipelines used for offshore water injection pipelines can be severe. Both microbiologically influenced corrosion and under-deposit corrosion are suspected. Half-pipe steel specimens with mixed deposits of magnetite, calcium carbonate and sea sand exposed to near-deoxygenated seawater for 180 days showed more severe pitting corrosion under the deposits than elsewhere. Less severe under-deposit pitting occurred for seawater treated with filtration and ultraviolet (UV) irradiation. Nitrate addition, used in practice to control bacterial H2S production in oil reservoirs, resulted in the most aggressive localized corrosion under the deposits. This has implications for water injection pipeline management.
Phenomenological modeling of the long term general corrosion of mild and low alloy steel specimens under marine conditions is considered, using weight loss as a function of time. A conceptual model for immersion corrosion, tidal corrosion and atmospheric corrosion under marine conditions is proposed. The model uses accepted theories for short term surface corrosion and employs modern understanding of the action of bacterial colonisation of the surfaces of specimens, including the development of anaerobic conditions. Kinetic, diffusion, nutrient and anaerobic components of the model are identified and mathematical descriptions given. The model is compared to some data available in the literature. Some observations are made about data requirements for further development of models of the type proposed.
The importance of being able to estimate frame deflections accurately is described and the effect of connection flexibility on deflection estimates shown with the aid of a simple example. A number of tests conducted to obtain moment- rotation curves for typical bolted extended end plane connections are described and discussed. It is evident that for the connections considered, present design recommendations are conservative for strength. However, the degree of importance of flexibility as a matter for connection design requires further investigation. (Author/TRRL)
A transcript of a discussion held regarding the assessment of buildings and structures following the Newcastle earthquake of 1989. Topics include building regulation, stereoplotters, the Newcastle Workers Club, and building materials.
In the first-order second moment (FOSM) reliability analysis, the gradients of the limit state function with respect to the basic random variables need to be calculated. For complex structures, these calculations can often be performed with the use of the probabilistic finite element (PFE) routines. However, for practical problems with a large number of basic random variables, the computation of the gradients is usually very expensive. This is particularly true in the FOSM analysis since the computation of the gradient vector is required to be repeated at each iteration step in the optimization algorithm. A new formulation, which reduces CPU time in computing the gradient vector without affecting the accuracy, is developed in this paper. For a simple example, direct comparison of the new technique with other methods that it is able to significantly reduce the CPU time and storage space requirements.
Some steel water injection pipelines used to increase the yield from oil production reservoirs in the offshore oil industry have experienced severe channelling corrosion at the 6 o′clock position. The examination of field observations suggests both under-deposit corrosion (UDC) and microbiologically influenced corrosion (MIC) are likely to be associated with the phenomenon. Long-term laboratory experiments are described to distinguish the relative contributions of the presence of deposits, MIC and nitrate addition to the formation of channelling corrosion. Half-pipe steel specimens were exposed to different simulated test environments up to 365 days. The analysis of pitting morphology and pitting depth shows the synergistic effect of MIC and under-deposit corrosion led to severe localized corrosion. Nitrate addition caused most severe localized corrosion. Extreme value distribution examination shows Gumbel function is not appropriate to describe all the pit depth data for 1 year exposure. The results have implications for the corrosion management in industrial practice.
