Use of Permanent Magnets to Improve the Seismic Behavior of Light-Framed Structures


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Virginia Tech


Light-framed wood structures generally have satisfied the life safety objective of the current seismic design approach. The main source of energy dissipation in such structures is the inelastic behavior of the connectors connecting framing and sheathing elements. Wood framed structures when subjected to strong ground excitations experience structural and non-structural damage which may incur large repair/replacement costs or may even render the structure out of service. Thus, it is very important to apply techniques to mitigate the seismic response of the light-framed structures and avoid large monetary losses.

It is proposed to use commercially available permanent magnets, incorporated in the form of passive friction dampers, to dissipate a part of input energy induced due to strong ground motions, thereby reducing the inelastic energy dissipation demand of the lateral load resisting system. The force of attraction between the permanent magnet and ferromagnetic material like steel was utilized to produce the required friction resistance. A sliding wall configuration consisting of flexible permanent magnets and steel plates sandwiched between the plywood sheets was analyzed for its effectiveness in mitigating the response of a two story wood shear wall structure. The structural analysis program SAP2000 was used to perform nonlinear dynamic analysis of the finite element models generated using the meshing algorithms incorporated into 'WoodFrameMesh'. Nonlinear link elements available in SAP2000 were used to model the friction between the flexible magnet sheet and the steel plate. The effects of various modeling parameters on the solution of the nonlinear analysis were studied so as to arrive at appropriate values to represent the friction problem. Also the friction damped structure was analyzed to study its forced and free vibration characteristics. Further, the responses of the friction damped structure and the undamped structure were compared when subjected to different ground accelerations. The response of the friction damped structure was also compared to that of the structure in which the proposed friction dampers were replaced by normal shear walls. A huge reduction in the response of the friction damped structure was observed when compared to the response of the undamped structure. The friction damped structure was also analyzed for different values of modal damping ratios. Over all about 60-80% of the input energy was dissipated by friction damping in all the cases. The slip resistance of a flexible permanent magnet sheet was also verified in the laboratory. Above all the magnetic properties of commercially available permanent magnets and the effects of strong permanent magnets on human health were also studied.



Friction dampers, Permanent magnets, Passive energy dissipation devices, Light