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Application of limit design to high-strength aluminum alloy beams Katramadakis, Tony

Abstract

The theory of limit design originally was developed for structural steel construction. Tests carried out on mild steel beams and frames are in agreement with the theory. Unfortunately a limited number of tests have been carried out on other ductile materials such as light alloys. Therefore more tests are required in order to investigate whether the theory of limit design is also applicable, with or without modification to aluminum alloys. The failure mechanism predicted in limit design materializes in steel frames not only because steel is very ductile but also because steel has strain hardening. Aluminum alloys exhibit very little strain hardening. In the research described here there were two objects. The first object was to investigate the applicability of limit design to aluminum alloys. The second object was to check experimentally the theory of inelastic bending. Three load tests were carried on continuous beams made of aluminum alloy to see if the mechanism condition was attained before failure of the beam. Moments and deflections predicted by the theory of inelastic bending were compared against measurement of beam moments and deflections. The theory of inelastic bending considers the effect of strain hardening. Tables of unit function derived from the stress-strain diagram of aluminum alloy (65S-T6) are presented so that they may he used when the theory of inelastic bending is applied. The first test failed prematurely due to crippling of the compression flanges. In the second and the third test the mechanism condition of limit design was reached shortly before failure of the tension side of the beam under the load point by fracture. Thus the type of failure indicates that not all structures will achieve the mechanism condition. The failure load and the ratio of moments at failure, as predicted by the theory of inelastic bending was equal to 15.53 Kips and 1.13 respectively. Test results indicated a failure load of 16 Kips and a ratio of moments at failure equal to 1.1. The load-deflection curves were the same as the curves from the theory. At failure the deflection under the load was 5.57 inches compared to computed theoretical deflection of 5.46 inches.

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