1 INTRODUCTION Damage from blast loads can occur in two sepa- rate stages. Firstly, the initial blast impacting on the structure may cause the structure to fail due to large shear stresses generated by higher order modal re- sponses. This failure occurs when structures are in the direct vicinity of the blast, or are the subject of the blast. As this situation directly depends on the size and proximity of the blast load, the structures usually cannot be saved by use of various devices. The second method by which structures can fail due to blast loads occurs after the initial impact, in the structures free vibration response to the blast. Struc- tures can literally shake themselves to pieces. This second failure mode usually occurs in structures that are near by blast loads, but not directly affected by the full force of the initial blast. Structures can sur- vive the initial blast, but the free vibration response induced by the blast can cause the structural failure due to the displacement of the structure (Dhakal, R.P. et al 2003). If this displacement can be reduced, the likelihood of failure is also reduced. An ideal method of reducing a structures dis- placement during its free vibration period is to use semi-active devices. Resetable devices non-linearly alter the stiffness, rather than the damping, of the structure with the stored energy being released as the compressed fluid reverts to its initial pressure. These emerging devices have been studied extensively for seismic structural control (Barroso, LR et al 2003) and have the ability to re-shape hysteretic behaviour in some implementations (Mulligan et al 2005). The amount of displacement in the free vibration phase of the structures response depends heavily on the size of first displacement peak resulting from the blast load. Hence a tendon is used to reduce the magnitude of the initial displacement, that is, the first displacement peak. A resetable device is used to reduce the subsequent free vibration displacement, reducing the size of the free vibration phase. The tendon and the resetable device acting in tandem will therefore reduce the likelihood of failure due to dis- placement. Semi-active management of structures subjected to high frequency ground excitation C.M. Ewing, R.P. Dhakal, J.G. Chase & J.B. Mander Departments of Mechanical and Civil Engineering, University of Canterbury, Christchurch ABSTRACT: 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 vibra- tion phase involves localised high frequency modes with small displacement and large accelerations, thus in- ducing a large shear force. Therefore, even if a structure is strong enough to survive the initial ground excita- tion, 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 combina- tion 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 cre- ates a solution suitable for both new and retrofit applications.