M.Sc Student | Netzer Itamar |
---|---|

Subject | Optimal Design of Passively Damped Outriggers Considering Perimeter Column Flexibility |

Department | Department of Civil and Environmental Engineering |

Supervisor | Professor Oren Lavan |

Full Thesis text |

The interest in earthquake design for
tall buildings has grown tremendously in recent years due to seismic events
such as the Tohoku-Oki earthquake in 2011 in Japan. In the last decade, a new
structural model known as the *damped outrigger* has been gaining ground
as an efficient structural model to withstand the seismic excitations. By utilizing
damping technologies in strategic locations, it is possible to mitigate
responses of interest efficiently. Therefore, the goal of this work is to find
the optimal design for the seismic design of the damped outrigger.

This thesis presents a formal optimization methodology for the damped outrigger problem under seismic excitations. The work is divided into two main stages, analysis and optimization. An efficient analytical model is developed for the coupled connection between the perimeter columns, with their different stiffness measures, and the vertically placed dampers. A Maxwell model is utilized to describe the series connection in each of the structures' floors. Employing the developed model achieves a reduction in the computational effort for the both the analysis and the optimization. Once the model is properly defined, the governing equations of motion are formulated in the state space notation where the state variables are the horizontal displacements, velocities and damping forces. The seismic excitation is represented by a stochastic approach, utilizing an adjusted Kanai-Tajimi filtered white noise input. Finally, the root mean square responses are derived by solving the governing equations using Lyapunov's equation.

Formulation of the gradient based optimization problem has the damper size, outrigger stiffness and damped outrigger locations as the design variables. A novel damping distribution function is introduced to represent the number of damped outriggers. The objective is to minimize a cost-related function while limiting the structural behaviour to an acceptable degree. Taking into account several different responses as design criteria is done by formulating a single aggregated constraint in an efficient manner for the optimization. The adjoint analytic method is adopted to carry out the gradient based optimization.

Finally, demonstrating usage of the optimization methodology is shown in two design examples for a 50 story, 200 m high, building. The first case seeks a reduction of 25% of the maximal interstory drift of the bare (undamped) structure. It is seen that as the perimeter columns become more flexible, the need is to overcome this lack of stiffness. In turn, this leads to higher values of the outrigger stiffness and to lowering the location of the damped outriggers. The second case accounts for both the interstory drifts and the absolute acceleration, where both are restricted to 75% of the maximal respective responses of the bare structure. Limiting the acceleration in the top floors is an important criterion for tall buildings in its own right, especially considering performance based design. Looking at the results of the two cases leads to the understanding that the perimeter column flexibility has a great influence on the behaviour of the structure in general serves as a reminder of the importance that optimization methodologies, such as this one.