Semi-active management of structures subjected to high frequency ground excitation
The structural response to high frequency ground excitations resulting from blast loading differs significantly from that of low frequency excitations such as earthquakes. Due to local mode vibrations in the structure the response is more complex with the maximum response generally occurring after the excitation has ceased. As a result, most of the motion takes place in the free vibration phase, which is dominated by low frequency modes with large displacements and small accelerations. However, the response in the forced vibration phase involves localised high frequency modes with small displacement and large accelerations, thus inducing a large shear force. Therefore, even if a structure is strong enough to survive the initial ground excitation, it may still be damaged in the significantly long free vibration phase. Non-linear structural analyses were carried out on a two storey reinforced concrete frame subjected to high frequency ground excitations. Several semi-active devices and device architectures were modelled, including passive tendon based solutions. The use of semi-active devices significantly reduces the free vibration response of the structure, thus reducing the extent of the structural damage generated and the likelihood of structural failure. In particular, the combination of these semi-active devices in a specialised 2-4 implementation with a yielding tendon fuse-bar provides protection against the initial pulse and the resulting free vibration without increasing shear forces. Thus it creates a solution suitable for both new and retrofit applications.