Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Civil Engineering

First Adviser

Ricles, James

Other advisers/committee members

Pakzad, Shamim; Wilson, John; Kurama, Yahya

Abstract

Numerous rate-dependent damping devices have been recently developed to enhance the seismic performance of structural systems and mitigate the effects of seismic hazards. Real-time hybrid simulation (RTHS) is a powerful and cost-effective method of testing that can be used to evaluate the performance of structures with rate-dependent damping devices. In a RTHS the damping devices can be modeled as an experimental substructure in the laboratory while the remaining part of the structural system is modeled analytically. This experimental technique enables different levels of seismic hazard to be considered by applying a large number of ground motions to the structure without the need to repair or replace the test specimens as long as the damage is confined to the analytical substructure.RTHS of reinforced concrete (RC) structures has been a challenge due to modeling complexities and a lack of appropriate integration algorithms and actuator delay compensation methods. To obtain accurate and reliable test results using RTHS, it is essential that the analytical substructure is accurately modeled and the restoring forces of this substructure are computed within the integration time step. This study demonstrates that a flexibility-based fiber element is capable of capturing nonlinearities and limit states that can occur within a reinforced concrete test specimen loaded under quasi-static cyclic loading. Therefore, this fiber element is a good candidate to be used for performing RTHS of reinforced concrete structures under strong ground motions, where the RC structure has nonlinear behavior with softening characteristics that include pinching as well as stiffness and strength deterioration. An issue arises when using flexibility-based fiber elements in a RTHS because there is no guarantee that the element state determination process can be completed within the integration time step since this process for the element requires iterations to converge. In order to ensure the completion of the state determination within the integration time step, a flexibility-based fiber element with a fixed number of iterations and carry over of the element residual forces to the next load step is proposed for the RTHS of RC structures. The accuracy of this fiber element is assessed by adopting the results of a quasi-static loaded RC test specimen and use the element for modeling the RC specimen. The effect of different parameters including number of iterations, carrying over of the element residual deformations to the next load step, and the size of displacement increment on the accuracy of the flexibility-based fiber element are investigated.Explicit integration schemes for integrating the equations of motion are generally more computationally efficient and require no iteration on the solution, making them suitable for performing RTHS. It is shown that it is essential to use controlled numerical damping possessed by an unconditionally stable explicit integration algorithm to achieve convergence within the flexibility-based fiber elements and successfully conduct the RTHS of RC structures. Accurate actuator control is another challenge to perform a successful RTHS, because it affects the stability and accuracy of the simulation. Actuator delay compensation can magnify the high frequency oscillations in the response of the flexibility-based fiber element that prevents the element from converging and can make the RTHS unstable. An adaptive compensation method (ATS) is developed and used to compensate for actuator delay and amplitude errors between the target and measured displacement of the experimental substructure in order to accurately impose the target displacement on the experimental substructure. To experimentally validate the performance of the flexibility-based fiber element with a fixed number of iteration, RTHS of a prototype 3-story RC building subjected to earthquake ground motions with different hazard levels is conducted. The building has perimeter moment resisting frames (RC-MRF) and steel braced frames equipped with nonlinear elastomeric dampers (DBF). The symmetry in the floor plan enables only one perimeter RC-MRF, one DBF, and the gravity load resisting system in one-quarter plan of the building to be considered in the RTHS. In the simulations the DBF with the elastomeric dampers is modeled as the experimental substructure while the remaining parts of the building (RC-MRF, seismic mass, gravity load system, inherent damping) are modeled as the analytical substructure. A ductile RC-MRF is designed in accordance with the International Building Code (IBC) (2009) and ACI-318 (2011). The analytical substructure of the RC-MRF is created using flexibility-based fiber elements with appropriate element integration method to capture the softening behavior of RC members under strong earthquake ground motions. The inherent damping of the complete hybrid system is modeled using a form of non-proportional damping based on the mass and initial stiffness of the system to appropriately model the damping forces. The combined use of flexibility-based fiber elements with a fixed number of iterations, actuator compensation and an explicit integration algorithm that possess controlled numerical damping is shown to enable exceptional RTHS results to be achieved.

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