Retrofit of Reinforced Concrete Moment Frames in NZ Using Dual Supplemental Damping

The repair and seismic strengthening of a mid-1990s building structure in Christchurch, New Zealand, using fluid viscous dampers and buckling-restrained braces provides a case-study for mixed-system retrofit to improve the seismic performance of ductile reinforced concrete moment frames. The building was designed and detailed to essentially current standards but was very flexible and utilised a precast hollow-core floor system with thin topping slab which is vulnerable to deformations associated with moment frames, as demonstrated in recent New Zealand earthquakes. This paper highlights a simple displacement-based approach to setting retrofit design targets and performance verification. The retrofit using viscous dampers provided an efficient means to reduce ground motion-induced deformations and accelerations. The structural benefits to the rest of the project included no need for foundation strengthening and floor diaphragm strengthening being limited to only the perimeter hollow-core units. Development of the final design and viscous damper specification involved close coordination with the manufacturer to optimise the damper design, fabrication tolerances and site installation methods.