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2004 Higher Degree Theses

The following postgraduate students were awarded higher degrees for their theses in 2004

Doctor of Philosophy

Demao Yang
Compression Stability Of High Strength Steel Sections With Low Strain-Hardening
Supervisor: Prof Greg Hancock

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis.

This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled "Compression Stability of High Strength Steel Sections with Low Strain-Hardening".

For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes.

A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement with the experimental results.

Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models.

As expected from the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities.

The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian/New Zealand Standard for Cold-Formed Steel Structures (AS/NZS 4600) and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel.

Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

Download Demao Yang's Thesis (10 MB PDF)

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Viet Nguyen
Ground Water Pumping Analysis and its Influence on Pile Foundations
Supervisor: Prof John Small

This thesis presents a study of the behaviour of soil due to excessive ground water pumping or excavation with pumping of ground water to dry excavation sites and the influence of these factors on piled foundations. Numerical analysis has been used to determine soil movements in both the vertical and horizontal directions due to pumping of ground water. The area that is most influenced by pumping of ground water is also investigated. Since pumping of ground water or excavation causes an influence on structures and utilities in the vicinity of the pumping area, a study of the behaviour of piled foundations that are subjected to both vertical and horizontal soil movements has been carried out. A combined technique that uses both finite element and boundary element methods has been used for these analyses. The main objectives of this study can be summarised as follows:

(a) To investigate lowering of the water table due to excessive pumping of ground water for living and industrial purposes. Two case studies of ground water pumping in Hanoi, Vietnam are examined and a comparison between the measured vertical soil movements and the numerical analysis is carried out. The horizontal soil movement due to ground water pumping is also obtained from the numerical analysis.

(b) Excavation with lowering of the ground water table is investigated. Undrained excavation and partially drained excavation (excavation with pumping of ground water) analyses have been carried out to investigate the influences of both excavation and lowering of the ground water table on soil movements.

(c) Combined technique using both the finite element and the boundary element methods is used to study piled foundation behaviour where the piles are close to the area of ground water pumping or excavation with ground water pumping.

Numerical analysis has been applied to the analysis of a series of reported case studies. The good agreement between the measured data and the analysis results show that the finite element methods used can predict field behaviour.

Two case studies are analysed where soil movements due to ground water pumping occurred. In this analysis, lowering of the ground water table and vertical soil movements over several years have been analysed and compared to the measured data. Three analyses of excavations, which were reported and published, have been conducted. Among them, one excavation with lowering of the ground water table has been carefully investigated. As may be expected, lowering of the ground water table significantly increases the settlements of the ground surface behind the retaining wall. Numerical analysis was shown to be an effective means of predicting soil movements due to excavation with lowering of the ground water table behind retaining walls. Two case studies of the behaviour of a pile next to an excavation have been considered. One involves results from a centrifuge tests and the other is a full-scale case study.

Numerical methods have also been used to conduct parametric studies of the soil behaviour due to lowering of the ground water table or due to excavation with lowering of the ground water table behind a retaining wall. Behaviour of piled foundations in the area of influence has also been explored. The results can be used to predict the behaviour of piled structures that may be affected by lowering of the ground water table so that an effective management scheme can be implemented to avoid potential damage.

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