2008 Higher Degree Theses

The following postgraduate students were awarded higher degrees for their theses in 2008.

Doctor of Philosophy

Helen Chow, Analysis of Piled-Raft Foundations with Piles of Different Lengths and Diameters

Jurgen Becque, The Interaction of Local and Overall Buckling of Cold-Formed Stainless Steel Columns

MPhil / Master of Engineering Research (MER)

Damien Koen, Structural Capacity of Light Gauge Steel Storage Rack Uprights

Helen Chow
Analysis of Piled-Raft Foundations with Piles of Different Lengths and Diameters
Supervisor: John Small


In recent years, there have been an increasing number of structures using piled rafts as the foundation to reduce the overall and differential settlements. For cases where a piled raft is subjected to a non-uniform loading, the use of piles with different sizes can improve the performance of the foundation. Extensive research work has been performed in the past to examine the behaviour of piled rafts. However, most of the research was focused on piled rafts supported by identical piles, and the use of non-identical piles has not received much attention.

In this thesis, the behaviour of piled rafts supported by non-identical piles is examined by the use of a computer program APRILS based on the finite layer and finite element methods. The finite layer method is used for the analysis of the layered soil system. The application of this method to different shapes of loadings is presented and has been shown to be in good agreement with the theoretical solutions. The finite element method is used for the analysis of the raft and piles. Full interaction between raft, piles and soil which is of major importance in the behaviour of piled rafts is considered in the analysis. Among the four different types of interaction present in the piled raft foundation, the interaction between piles plays an important role.

Interaction between non-identical piles subjected to both horizontal and vertical loadings is examined. It is found that for a pair of piles under consideration, if the diameter and length of one of the piles is a multiple of the diameter or length of the other pile, the reciprocal theorem holds.

Program APRILS can be used for the analysis of piled rafts subjected to horizontal and vertical loadings in which the base of the raft is treated as being rough and slip along the pile-soil interface is allowed in the analysis. By having a rough raft base, there is a significant effect on the vertical movement of the raft due to the lateral resistance of the soil and the piles. For piled rafts embedded in a non-homogeneous layered soil, the modulus of each layer of soil is used in the computation and accurate solutions are obtained without the use of an averaging technique.

The performance of piled rafts is affected by several factors such as the soil conditions, pile dimensions and arrangements. Detailed examinations of these factors have shown that the displacement of the raft and the proportion of load carried by piles are significantly affected by them. For piled rafts subjected to non-uniform vertical or horizontal loads, the use of non-identical piles can improve the performance of the piled rafts. For vertically loaded piled rafts, longer piles are preferred to be used underneath the heavily loaded region while for horizontally loaded piled rafts, larger diameter piles are more preferable.

Several field cases are studied in this thesis, and they show that the solutions obtained from program APRILS are in good agreement with field measurements. The use of the insitu test results and back-analysis is used to obtain the correlation between the modulus of the soil and field test results for laterally loaded piles.


Download Helen Chow's thesis (3.0 MB PDF)

Juergen Becque
The Interaction of Local and Overall Buckling of Cold-Formed Stainless Steel Columns
Supervisor: Kim Rasmussen


The objective of this research is to investigate the interaction of local and overall flexural buckling in cold-formed stainless steel columns.

Literature study exposes a lack of understanding of this subject and a need for experimental data, particularly on the local-overall interaction buckling of stainless steel open sections. Two separate experimental programs were therefore carried out. The first program included 36 tests on pin-ended lipped channel columns. Three alloys were considered: AISI 304, AISI 430 and 3Cr12. The specimens were designed to fail by local-overall interaction buckling in the inelastic stress range, thus highlighting the non-linear behaviour of stainless steel. Half of the specimens were tested under a concentric load. The other half had the load applied with a nominal eccentricity of Le/1500. The test results demonstrate the imperfection sensitivity of local-overall interaction buckling and illustrate the shift in effective centroid in pin-ended columns with singly symmetric cross-section.

The second experimental program studied local-overall interaction buckling in 24 pin-ended stainless steel I-section columns. The specimens consisted of plain channels connected back-to-back using sheet metal screws. Two alloys were considered: AISI 304 and AISI 404. Local and overall imperfections were carefully measured in both experimental programs.

Extensive material testing was carried out on the alloys employed in the experimental program, in order to determine tensile and compressive material properties, anisotropic parameters and enhanced corner properties.

A detailed finite element model is presented, which includes non-linear material behaviour, anisotropy, increased material properties of the corner areas and local and overall imperfections. The model was verified against the two aforementioned experimental programs and against additional data available in literature on stainless steel SHS columns. The model yielded excellent predictions of the specimen failure mode, ultimate strength and load-deformation behaviour.

The finite element model was used to generate additional data for stainless steel columns with lipped channel, plain channel, SHS and I-shaped cross-section, failing by local-overall interaction buckling. The parametric studies covered the practical ranges of overall and cross-sectional slenderness values.

The Australian/New Zealand, European and North American standards for stainless steel were evaluated using the available data. The comparison reveals an inability of the design codes to properly account for the interaction effect as the cross-sectional slenderness increases. Predictions are unsafe for I-section columns with intermediate or high cross-sectional slenderness.

A direct strength method is proposed for stainless steel columns, accounting for the local-overall interaction effect. The method offers a simple design solution which fits within the framework of the current Australian and North-American standards.

Download Jurgen Becque's thesis - Volume 1 (5.5 MB PDF)
Download Jurgen Becque's thesis - Volume 2 - Appendices (15.4 MB PDF)

Damien Koen
Structural Capacity of Light Gauge Steel Storage Rack Uprights
Supervisor: Kim Rasmussen

This report investigates the down-aisle buckling load capacity of steel storage rack uprights. The effects of discrete torsional restraints provided by the frame bracing in the cross-aisle direction is considered in this report. Since current theoretical methods used to predict the buckling capacity of rack uprights appear to be over-conservative and complex, this research may provide engineers an alternative method of design using detailed finite element analysis.

In this study, the results from experimental testing of upright frames with K-bracing are compared to finite element predictions of displacements and maximum axial loads. The finite element analysis is then used to determine the buckling loads on braced and un-braced uprights of various lengths. The upright capacities can then be compared with standard design methods which generally do not accurately take into account the torsional resistance that the cross-aisle frame bracing provides to the upright.

The information contained in this report would be beneficial to engineers or manufacturers who are involved in the design of rack uprights or other discretely braced complex light gauge steel members subject to axial loads.


Keywords: Steel storage racks, cold-formed steel structures, perforations, experimental testing, finite element analysis, design, buckling capacity.

Download Damien Koen thesis - (4.5 MB PDF)