Dr Mukshed Ahammed

Water-injection, oil production and water-supply pipelines are prone to pitting corrosion that may have a serious effect on their longer-term serviceability and sustainability. Typically, ob-served pit-depth data are handled for a reliability analysis using an extreme value distribution such as Gumbel. Available data do not always fit such monomodal probability distributions well, partic-ularly in the most extreme pit-depth region, irrespective of the type of pipeline. Examples of this are presented, the reasons for this phenomenon are discussed and a rationale is presented for the oth-erwise entirely empirical use of the ¿domain of attraction¿ in extreme value applications. This per-mits a more rational estimation of the probability of pipe-wall perforation, which is necessary for asset management and for system-sustainability decisions.

Capacity reduction factors (¿) for flexure, shear and axial loadings are derived for reinforced concrete (RC) structures based on a reliability-based calibration of the Australian Concrete Structures Standard AS3600. The structural reliability analysis considers the bias and variability of material properties, dimensions, loads and model error. The target reliabilities (ßT) are selected based on consideration of past practice and Australian and international standards. The capacity reduction factors (¿) for the new code AS3600-2009 are selected using the most recent statistical parameters for material strengths for 20¿100 MPa strength concrete using Class N (ductile) 500N reinforcement. The reliability-based calibration found that the ¿-factor can be increased from 0.80 to 0.85 for members in bending, and increased from 0.60 to 0.65 for axial loading of short (stocky) columns where the ratio of the live load to the dead load is at least 0.25. No changes are recommended for shear or torsion, at this time, or for slender columns; further research is needed to better refine the design models for these cases and reduce the variation in their model error. The proposed increases in capacity reduction factors will result in up to an 8.3% increase in design strength that, in turn, provides efficiency in the use of materials. The proposed changes provide for modest savings in greenhouse gas emissions.

Assessment of the reliability of design models developed for Australian Standards is of paramount importance for determination of public safety. Poorly calibrated models and safety factors can lead to overly safe and uneconomic construction or, worse, to an insufficient level of safety. This study investigates the reliability of the models used in the Australian Concrete Structures Standard AS3600¿2009 for the design of beams and slabs in bending and shear and columns under combined bending and axial loading. The study is in two parts; in Part I, strength and variability of over 20,000 concrete cylinders cured under standard conditions and tested at 28¿days are statistically analysed. The data were collected from all cities and regional areas in Australia and for all concrete strength grades; similarly, variability of reinforcement product is analysed. Next, reliable databases of laboratory tests for beams, slabs and columns were established for members failing in flexure, shear and compression and model errors, and their variability, determined for the code design models. It is concluded that improvements in the production of concrete and of bar products, over time, have led to reduced variability in their materials properties¿with potential for increasing code strength reduction factors and, thus, reducing excessive conservatism in design. This is assessed in Part II of this study.