Structural Response and Design of Aluminium Alloy Members

Georgantzia, Evangelia (2022) Structural Response and Design of Aluminium Alloy Members. Liverpool John Moores University, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

6xxx series aluminium alloys, widely known as structural alloys, are characterised by a wide variety of advantages, such as high strength-to-weight ratio, ease of fabrication, high degree of workability, great durability, excellent electrical and thermal conductivity, high corrosion resistance and recyclability and attractive appearance at their natural finish. The aforementioned advantageous features have contributed to increased usage of aluminium alloys in structural applications, where their application can allow for a reduction of the total structural weight.
Despite the benefits of structural aluminium alloys, a comprehensive literature review conducted herein, revealed that there are still limitations in their design which forces the designers to favour more conventional materials. This is related to the fact that the current design specifications are based on limited amount of experimental and numerical results, whilst sometimes adopt similar principles to their steel structure counterparts, without sufficient consideration of the differences between the two materials. This practice leads to inaccurate strength predictions, which are opposed to an economical and efficient design philosophy. However, additional research work can lead to modifications of the existing design codes and potentially increase structural engineers’ confidence towards a more frequent employment of aluminium alloys as primary structural material.
The literature review has identified a gap in knowledge on the structural performance of bare tubular, concrete-filled tubular and channel sections. To this end, a series of experimental tests combined with finite element (FE) modelling studies is conducted to investigate the compressive and flexural performance of bare tubular, concrete-filled tubular and channel sections. Material testing including tensile tests on coupons and compressive tests on concrete cubes is carried out to determine the mechanical properties of the examined aluminium alloy and concrete, respectively. Upon material testing, 22 fix-ended stub column tests are executed to study the cross-sectional response of bare tubular, concrete-filled tubular and channel cross-sections. The same types of cross-sections are also employed to perform 24 pin-ended column tests to investigate their minor-axis buckling behaviour. Moreover, 9 bare tubular and 4 concrete-filled tubular cross-sections are tested under three-point bending, whilst 5 bare tubular and 14 channel cross-sections are tested under four-point bending to quantify their moment resistance and rotational capacity. Finally, 5 two-span continuous beam tests employing bare tubular cross-sections are also executed to estimate the rotational capacity and the potential for moment redistribution of aluminium alloy indeterminate beams.
Subsequent parametric studies are carried out to supplement the experimentally obtained data sets providing a deeper understanding about the structural response of the considered cross-sections. Particularly, an extensive numerical modelling study consisting of 47 FE models is performed to investigate further the cross-sectional response of channel cross-sections. 133 in total parametric studies are also undertaken to generate additional structural performance data for the buckling behaviour of bare tubular, concrete-filled tubular and channel cross-sections. Moreover, the flexural behaviour of channel cross-sections under four-point bending configuration is better clarified through 140 additional numerical analyses. Finally, the experimental results for the bare tubular cross-sections obtained from the three- and four-point bending tests as well as the two-span continuous beam tests are utilised to generate 108 results aimed to extend the pool of performance data for aluminium alloy indeterminate structures.
Following, the results obtained from the testing programme in conjunction with those generated from the parametric studies are used to examine the influence of various parameters on the behaviour of aluminium alloy structural elements. Moreover, the experimental and numerical ultimate strengths are utilised to assess the applicability and accuracy of the existing design specifications with particular emphasis on current European Standards, i.e., Eurocode 9 (EC9) and Eurocode 4 (EC4). The applicability of the Continuous Strength Method (CSM) and Direct Strength Method (DSM) to aluminium alloy structural elements are also evaluated herein. Particularly, revised buckling curves are proposed for Class A aluminium alloy bare tubular and channel pin-ended columns improving the strength predictions by 12% and 5%, respectively. A strength increase in the range of 23% to 93.1% of the concrete-filled tubular members is captured compared to their bare counterparts. Moreover, in absence of codified criteria for composite aluminium-concrete cross-sections and members, the present study proposes adopting the European design formulae for composite steel-concrete cross-sections and members, i.e., EC4, replacing the material properties of steel by those of aluminium alloy. In addition, the DSM is suggested for the design of aluminium alloy channel sections and members subjected to concentric compression providing improved strength predictions by 13% and 7%, respectively. This study also suggests revised EC9 Class 2 and Class 3 slenderness limits for outstand elements under stress gradient. A modified plastic effective width method is also recommended for the design of slender aluminium alloy channel sections subjected to minor axis bending offering 52% more accurate strength predictions than those of EC9. Finally, this study concludes that employing the plastic design concept and particularly the plastic hinge method and the CSM in case of 6082-T6, 6063-T5 and 6061-T6 aluminium alloy indeterminate structures, 20% more accurate strength predictions could be achieved than those resulting from global elastic analysis.
The design recommendations suggested in the present study are in line with the observed structural response and thus providing quite accurate and consistent strength predictions towards a more safe and economically efficient design process.

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