It’s been dubbed the Dunning Thornton Damper, designed in response to the specific requirements for the seismic strengthening of the Harbour City Centre.
Modern earthquake code prohibits the use of concentric steel bracing in any situation where post-elastic performance is required. Instead, Eccentric K-Frames or Buckling Restrained Braces are now preferred.
But in the case of the Harbour City Centre, which had existing steel-frame structure and concentric bracing, the Dunning Thornton Damper provided a bespoke conversion.
In mechanics, “hysteretic” means that in the damping of a vibrating system, the retarding force is proportional to the velocity and inversely proportional to the frequency of the vibration.
“In short,” Mr Thornton says, “it describes the ability of the material — in this case, steel — to continue to absorb the energy from repeated cycles of seismic displacement and, thus, provide the structure with the required resilience.”
When the UCs experience axial loads, the gap between them would open and close according to the tension and compression (see Figure 1).
However, the pins have to flex (bend) as this occurs, and it is these flexural actions that absorb the energy. The bearings on which they are mounted allow them to rotate up to 10mm on either side of centre (see Figure 2).
Inside the splice plates, the round heads of the pins are in contact with brass bushes which extrude under pressure and pop the thin keeper plates. The seismic energy is thus dissipated with only axial deformation of the structural steel braces.