Keywords: aluminium alloys, rotational capacity, cross-sectional classification, local slenderness, strain hardening, local buckling.

The rotational capacity is one of the most important parameters characterising the inelastic behaviour of metallic members. In particular, it is required when inelastic calculation methods are employed, allowing for the development of complete plastic mechanisms of the whole structure. Besides, high rotational capacity is required when plastic strain-concentration occurs as well as when a high-energy dissipation capability of the structure is necessary. The evaluation of rotational capacity has been extensively studied, especially in the field of steel structures. On the basis of such studies, modern design codes provide simplified rules for assessing the capability of cross sections to behave inelastically. Classification systems for cross sections are therefore proposed, based upon fixing suitable slenderness limits for plate elements composing the cross-section. In particular, the width-to-thickness ratio and the conventional elastic stress limit are recognised as the only governing parameters. This approach is excessively simplified because the collapsing mode of a bending member could be strongly affected by other factors such as member lateral torsional buckling, moment gradient in the beam, sectional area ratio, interaction between the slenderness parameters of individual plates constituting the section and, in case of aluminium, material strain hardening.

Aiming at improved knowledge of inelastic behaviour of aluminium beams, a general research project has been recently undertaken. In particular, an experimental investigation dealing with several extruded aluminium profiles subjected to moment gradient loading has been carried out at the Norwegian University of Science and Technology [1,2]. Subsequently, a numerical investigation concerning the assessment of rotational capacity of aluminium rectangular hollow cross-sections has been worked out at the University of Naples Federico II in cooperation with the University of Chieti [3]. It has been concluded that the influence of above factors, which are disregarded in the codified approach, is not always negligible. In particular, material strain hardening provides a remarkable effect producing variations of rotational capacity in a similar range of local slenderness. The current paper focuses on the determination of the influence of each one of the abovementioned parameters (local slenderness , strain hardening , sectional area ratio , moment gradient , flange-to-web slenderness ratio and web restraint ) on the non-linear response of extruded I shaped aluminium beams. A wide parametric analysis is therefore carried out by using a sophisticated numerical model implemented in the implicit FEM code ABAQUS/Standard. This model, which has been used also in previous studies [2], has been largely calibrated against experimental tests [2]. The obtained results allow several important remarks to be drawn: (1) According to the current codification, the most important parameter affecting the rotational capacity of aluminium beam is the slenderness of elements constituting the cross-section; (2) The simplified classification system proposed by EC9, since no allowance is made for all governing parameters, is remarkably conservative; (3) Scatter with codified approach is higher in case of non-heat-treated alloys due to the effect of the material hardening, which is not suitably accounted for. In fact, AA 6082 Temper T4, which actually is a not-heat-treated alloy, in plastic range, behaves significantly better than AA 6082 Temper T6, which is fully heat-treated, and this contrasts what is presently stated by the European provisions.

Anyway, it seems that a simplified classification system for cross-sections based on the relevant slenderness of compressed elements and the hardening properties of the material could be of concern for codification purpose, especially when account is made for the sole stable part of the rotational capacity provided that the hardening effect is accounted for correctly. Therefore, a simplified method for the evaluation of the rotational capacity of aluminium members, accounting for material hardening and local slenderness variation, is proposed. The comparison with numerical results obtained by means of the FEM model shows that, if the secondary parameters ( , , , ) are kept constant, the proposed procedure is conservative, providing values of rotational capacity in good agreement with those obtained by means of numerical simulations. On the other hand, it has been pointed out that in same cases the influence due to the secondary parameters could be rather important (i.e. higher than 20-30%). In these cases, which really correspond to unusual member geometrical configurations, the correct evaluation of the rotational capacity should account for the effects of such secondary parameters as well, with the consequence to lose the simplicity of the approach.

1

L. Moen, O.S. Hopperstad, M. Langseth, "Rotational capacity of aluminium beams subjected to non-uniform bending � Part I: Experiments", Journal of Structural Engineering, 125(8), 910-920, 1999. doi:10.1061/(ASCE)0733-9445(1999)125:8(910)

2

L. Moen, G. De Matteis, O.S. Hopperstad, M. Langseth, R. Landolfo, F.M. Mazzolani, "Rotational capacity of aluminium beams subjected to non-uniform bending � Part II: Numerical Model", Journal of Structural Engineering, 125(8), 921-929, 1999. doi:10.1061/(ASCE)0733-9445(1999)125:8(921)

3

G. De Matteis, L. Moen, M. Langseth, R. Landolfo, O.S. Hopperstad, F.M. Mazzolani, "Cross-sectional classification for aluminium beams � A parametric study", Journal of Structural Engineering, 127(3), 271-279, 1999. doi:10.1061/(ASCE)0733-9445(2001)127:3(271)

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £125 +P&P)