Stiffness reduction of cantilever I-beams through lateral torsional buckling for compliant beams

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Lateral torsional buckling (LTB) of I-beams is an established buckling phenomenon in civil and structural engineering known for causing structural failures. However when a buckling instability is present the structure contains a form of negative stiffness. Due to the observed tendency of I-beams subject to LTB to twist about their longitudinal axis it is suspected that such I-beams contain a form of negative rotational stiffness about their longitudinal axis. Rather than considering LTB as a failure mode for structures, this work aims to investigate the potential of cantilever I-beams subject to LTB having a reduced rotational stiffness about their longitudinal axis with minimal lateral deflection on demand by subjecting the beam to LTB. By minimising the lateral deflection the longitudinal twisting behaviour approaches the behaviour of a compliant rotational joint. The occurrence of negative rotational stiffness in the buckled state is verified and a parameter study is executed to evaluate the stiffness reduction and co-occurring lateral deflection by means of 1-dimensional finite element modelling. Finally an optimisation is performed to obtain a beam with minimal lateral deflection and a significant reduction in longitudinal rotational stiffness. The simulated behaviour is experimentally validated. The simulation and experiment showed that rotational stiffness can be significantly reduced through LTB. Additionally the experiment verified the significant reduction of lateral deflection for the optimised beam. Overall this work forms an initial step towards the use of reduced rotational stiffness of beams by exploiting lateral torsional buckling.