Improving the seismic performance of structures with Direction and Displacement Dependent Viscous dampers.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Modern structures demand greater protection from natural hazards, such as strong winds and severe earthquakes. Structures have traditionally been designed to sustain significant sacrificial damage to absorb and dissipate the input energy, while preserving life safety. However, this approach causes significant direct and indirect economic cost, which leads to long term societal costs.
Instead of damaging the main structural elements to absorb energy, supplemental energy absorbing dissipation devices can be incorporated to protect structures, creating low damage structures. In particular, devices capable of respectably dissipating energy without requiring inspection, repair or replacement are highly desirable in this role. Fluid viscous dampers are one well-known, highly repeatable damping device with numerous experimental and analytical investigations. However, while viscous dampers can reduce displacement demand, they can increase the overall base shear demand for nonlinear structures or with high levels of added damping, as they provide resistive forces in all four quadrants of the force-displacement hysteresis loop.
This thesis presents analytical and experimental studies on improving seismic structural performance using novel Displacement and Direction Dependent (D3) viscous devices. These proposed devices offer the adaptability of semi-active devices in an entirely passive device design, and thus include the high reliability and low complexity of passive devices. A number of structural applications such as linear, nonlinear, and rocking systems, utilising D3 devices are described and analysed.
A distinguishing feature of this research is the novel design of a large-scale D3 device developed and experimentally validated. This design dramatically extends the capabilities of viscous devices by readily manipulating the device response to structural demands. In particular, the unique ability to use these devices to reshape or sculpt structural hysteretic behaviour in a fully passive device offers significant new opportunities in low damage structures with dissipation devices, which were previously only possible using much more costly, complex, and less robust, active or semi-active devices.
Time history analysis of linear structures and rocking systems with bi-linear elastic hysteresis, shows response reductions in both displacement and base-shear demand are only available with the 2-4 control method and devices, which dissipate energy only for motions towards equilibrium. These results indicate the robustness of simple 2-4 viscous dampers could be used to better mitigate structural response damage and potential foundation damage. These results extend prior results with semi-active, stiffness based devices to passive, velocity based dissipation devices.
To enable guidelines for adding a 2-4 device into the design procedure, damping reduction factors (RFξ) are developed, as they play an important role in design and thus provide a means of linking these novel devices to standard design procedures. Three methods are presented to obtain damping reduction factors and equivalent viscous damping of a structure with a 2-4 semi-active viscous damper. In the first method, the relationship between RFξ and the damping of a structure with the 2-4 viscous devices can be obtained by calculating the area enclosed by the forcedeformation diagram. The second and third methods are a modified version of the Eurocode8 (EC8) formula for damping reduction factors and smoothed results from time-history analysis, respectively. Finally, a simple method is proposed to incorporate the design or retrofit of structures using 2-4 viscous D3 dampers and standard design approaches.
Given the potential and link to standard design procedures, the D3 device design concept is presented and experimental tests undertaken on a prototype device. Sinusoidal displacement inputs provide a range of velocity inputs and device forces used to characterize the damping behaviour of the prototype and illustrate the ability to provide controllable viscous damping in any single or multiple quadrant(s) of the force-displacement response. Performance is characterized in term of device design dimensions and parameters. The overall results provide a proof-of-concept for a new class of relatively low cost passive device that enable customized hysteretic behaviour for any given structural application.
The overall outcomes of the thesis are experimentally validated in combination via the seismic performance of a 1/2 scale, two storey steel frame building with passive 2-4 D3 dampers subjected to uni-directional shake table testing.
Performance in mitigating structural response and foundation demand are assessed by evaluating base shear, maximum drift and acceleration. The test results show very good agreement with the nonlinear time history analysis and a numerical structural model.
Overall, this research presents a methodology for designing, testing and applying this new generation of viscous damping devices in enhancing seismic structural performance. The results show the ability to obtain simultaneous reductions in displacement, base-shear and acceleration demand for nonlinear and linear structures using passive 2–4 D3 viscous fluid dampers. This device is entirely passive and provides a unique retrofit option that would not require strengthening of columns and foundations.