2021 |
Melchers RE, Ahammed M, 'Estimating the long-term reliability of steel and cast iron pipelines subject to pitting corrosion', Sustainability (Switzerland), 13 (2021) [C1]
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. Ty... [more]
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.
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Nova |
2016 |
Stewart MG, Foster S, Ahammed M, Sirivivatnanon V, 'Calibration of Australian Standard AS3600 concrete structures part II: reliability indices and changes to capacity reduction factors', Australian Journal of Structural Engineering, 17 254-266 (2016) [C1]
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... [more]
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.
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Nova |
2016 |
Foster SJ, Stewart MG, Loo M, Ahammed M, Sirivivatnanon V, 'Calibration of Australian Standard AS3600 Concrete Structures: part I statistical analysis of material properties and model error', Australian Journal of Structural Engineering, 17 242-253 (2016) [C1]
Assessment of the reliability of design models developed for Australian Standards is of paramount importance for determination of public safety. Poorly calibrated models and safet... [more]
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.
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Nova |
2010 |
Melchers RE, Ahammed M, Jeffrey RJ, Simundic G, 'Statistical characterization of surfaces of corroded steel plates', Marine Structures, 23 274-287 (2010) [C1]
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Nova |
2009 |
Ahammed M, Melchers RE, 'A convenient approach for estimating time-dependent structural reliability in the load space', Probabilistic Engineering Mechanics, 24 467-472 (2009) [C1]
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Nova |
2006 |
Ahammed M, Melchers RE, 'Gradient and parameter sensitivity estimation for systems evaluated using Monte Carlo analysis', Reliability Engineering & System Safety, 91 594-601 (2006) [C1]
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2006 |
Melchers RE, Ahammed M, 'Statistical characterization of corroded steel plate surfaces', Advances in Structural Engineering, 9 83-90 (2006) [C1]
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2004 |
Melchers RE, Ahammed M, 'A Fast Approximate Method For Parameter Sensitivity Estimation In Monte Carlo Structural Reliability', Computers and Structures, Vol 82 55-61 (2004) [C1]
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2003 |
Melchers RE, Ahammed M, Middleton C, 'FORM For Discontinuous And Truncated Probability Density Functions', Structural Safety, Vol. 25 304-313 (2003) [C1]
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2002 |
Melchers RE, Ahammed M, 'Gradient Estimation For Applied Monte Carlo Analyses', Reliability Engineering And System Safety, Vol. 78 283-288 (2002) [C1]
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2001 |
Melchers RE, Ahammed M, 'Estimation of failure probabilities for intersections of non-linear limit states', Structural Safety, 23 123-135 (2001) [C1]
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1998 |
Ahammed M, 'Probabilistic estimation of remaining life of a pipeline in the presence of active corrosion defects', International Journal of Pressure Vessels and Piping, Vol. 75 321-329 (1998) [C1]
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1997 |
Ahammed M, Melchers RE, 'Probabilistic analysis of underground pipelines subject to combined stresses and corrosion', ENGINEERING STRUCTURES, 19 988-994 (1997)
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1997 |
Ahammed M, 'Prediction of remaining strength of corroded pressurised pipelines', INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING, 71 213-217 (1997)
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1996 |
Ahammed M, Melchers RE, 'Reliability estimation of pressurised pipelines subject to localised corrosion defects', INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING, 69 267-272 (1996)
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1996 |
Ahammed M, Melchers RE, 'Reliability of underground pipelines subject to corrosion - Closure', JOURNAL OF TRANSPORTATION ENGINEERING-ASCE, 122 408-408 (1996)
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1995 |
AHAMMED M, MELCHERS RE, 'PROBABILISTIC ANALYSIS OF PIPELINES SUBJECTED TO PITTING CORROSION LEAKS', ENGINEERING STRUCTURES, 17 74-80 (1995)
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1994 |
AHAMMED M, MELCHERS RE, 'RELIABILITY OF UNDERGROUND PIPELINES SUBJECT TO CORROSION', JOURNAL OF TRANSPORTATION ENGINEERING-ASCE, 120 989-1002 (1994)
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1992 |
Melchers RE, Ahammed M, 'Linearisation and offshore fatigue reliability', Probabilistic Mechanics and Structural and Geotechnical Reliability, Proceedings of the Specialty Conference, 5-8 (1992)
In determining the force effects due to wave action on offshore structures it is conventional to use an equivalent linearisation to convert wave velocities to forces via Morison&a... [more]
In determining the force effects due to wave action on offshore structures it is conventional to use an equivalent linearisation to convert wave velocities to forces via Morison's equation. Different linearisations have been suggested. However, it is not, a priori, clear which is appropriate when structural fatigue is to be estimated. A (theoretical) criterion is introduced herein and examined in relation to a simple offshore structure.
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1992 |
Melchers RE, Ahammed M, 'Equivalent linearisation and fatigue reliability estimation for offshore structures 531-536 (1992)
The force effects due to wave action on offshore structures are conventionally determined using Morison's equation. This is a nonlinear relation in velocity, conventionally r... [more]
The force effects due to wave action on offshore structures are conventionally determined using Morison's equation. This is a nonlinear relation in velocity, conventionally represented by an equivalent linearisation for converting wave velocities to forces, necessary to allow structural dynamic analysis by frequency domain methods (the most commonly applied approach). The suggested approaches for the linearisation can be shown to be of essentially two classes; those equivalent to Borgman's approach and those equivalent to Bolotin's. In general, it is not clear, a priori, which is more the appropriate. Nor is it clear whether these classical linearisations are relevant, particularly when structural fatigue is to be estimated (e.g., through the Palmgren-Miner rule). A criterion for derivation of a linearisation constant under fatigue considerations can be derived relatively simply. Unfortunately the criterion is not easily applied except by iteration. A sensitivity study has been performed for a simple but realistic offshore structure to estimate the relative importance of correctly estimating the linearisation constant.
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