The main purpose of this research is to investigate the effectiveness and feasibility of semi-active control systems for structural protection during severe earthquake loading. However, the research reported herein also involves analytical studies on the effect of adding viscous damping to the second and fourth quadrants of the force-displacement curve, and laboratory and field testing of a fibre-optic gyroscope (FOG) for measuring rotations in civil engineering structures. The concept of the 2-4 viscous damping is introduced to reduce the response of single-degree-of-freedom (SDOF) systems subjected to harmonic and earthquake excitations. This concept involves the addition of structural viscous damping to the second and fourth quadrants of the force-displacement graph. Time-history analyses and response spectra for various SDOF systems are carried out to assess the effect of adding 2-4 viscous damping. The analytical results indicate that the addition of 2-4 viscous damping is beneficial for reducing the harmonic and seismic response of a wide range of SDOF systems. A newly developed semi-active resettable device is proposed to reduce the seismic response of a one-fifth scale structure. The device is investigated as part of a resettable tendon system installed in the structure. Nonlinear dynamic analyses are performed to determine the optimal configuration of the resettable tendon in the structure. Several shake table tests are performed on the structure equipped with two resettable devices. The dynamic characteristics of the structure and the devices are described. Various earthquake records at different levels of intensity are used during the seismic testing. Different control laws are employed to manipulate the hysteretic behaviour of the devices. The results of the shake table tests validate the effectiveness of the resettable devices to reduce the seismic response of structures. Analytical studies are performed to determine the optimal utilization of the resettable devices in a twelve-storey reinforced concrete building. The seismic performance of the structure is discussed in relation to the number and distribution of the devices. Inelastic time-history analyses are carried out to assess the effectiveness of the devices to reduce the seismic response of the building. The impact of various tendon arrangements and different control laws on the earthquake response is investigated. Relevant issues for the implementation of the resettable devices in actual building systems are identified. Finally, a new measurement concept based on the use of the fibre-optic gyroscope is proposed to measure rotation rates, rotations, displacements and inter-storey drifts of civil engineering structures. FOGs are compact, easy to install and, unlike conventional linear potentiometers, do not require a fixed reference frame to operate. Measurements recorded during the seismic testing of the one-fifth scale structure and displacement measurements at the Sky Tower in Auckland validate the suitability of the FOGs for applications in civil engineering.