Michael C. Constantinou

Semi-active control systems for seismic protection of structures : a state-of-the-art review

As passive structural control systems begin to see an increased acceptance within the earthquake engineering community, strong research efforts have been shifted towards the development of semi-active structural control systems. To place semi-active control systems within a proper frame of reference, this paper begins with a qualitative description and comparison of passive, active, and semi-active control systems for protecting structures subjected to earthquake induced ground motion. A detailed literature review of semi-active control systems is then provided which provides references to both theoretical and experimental research but concentrates on describing the results of experimental work. Specifically, the review focuses on descriptions of the dynamic behavior and distinguishing features of various systems which have been experimentally tested both at the component level and within small-scale structural models. The semi-active systems which are reviewed include stiffness control devices, electrorheological dampers, magnetorheological dampers, friction control devices, fluid viscous dampers, tuned mass dampers and tuned liquid dampers. The review clearly demonstrates that semi-active control devices have the potential for improving the seismic behavior of full-scale civil structures.

SEISMIC TESTING OF A BUILDING STRUCTURE WITH A SEMI‐ACTIVE FLUID DAMPER CONTROL SYSTEM

This paper describes shaking table tests of a multi-storey scale-model building structure subjected to seismic excitation and controlled by a semi-active fluid damper control system. The semi-active dampers were installed in the lateral bracing of the structure and the mechanical properties of the dampers were modified according to control algorithms which utilized the measured response of the structure. A simplified time-delay compensation method was developed to account for delays within the control system. The results of the shaking table tests are presented and interpreted and analytical predictions are shown to compare reasonably well with the experimental results.

Elastic and Inelastic Seismic Response of Buildings with Damping Systems

The effect of damping on the response of elastic and inelastic single-degree-of-freedom systems was studied by nonlinear response-history analysis using earthquake histories that matched on average a 2000 NEHRP spectrum on a stiff soil site for a region of high seismic risk. New displacement reduction factors for levels of damping greater than 5% of critical are presented. New equations to relate inelastic and elastic displacements in the short-period range, for levels of damping greater than 5% of critical, are presented. The technical basis for reducing the minimum design base shear in damped buildings by a maximum of 25%, from that required for the corresponding undamped building, is derived based on comparable levels of damage in both the damped and undamped buildings.

EXPERIMENTAL STUDY OF FPS SYSTEM IN BRIDGE SEISMIC ISOLATION

An experimental study of a seismically isolated and a comparable non-isolated bridge is presented. The bridge model featured flexible piers, weighed 158 kN and was tested on a shake table with an array of real and simulated seismic motions with peak acceleration in the range 0.1-1.1 g. When isolated, the bridge deck was supported by four spherically shaped sliding bearings (known as Friction Pendulum System or FPS bearings) with friction coefficient under dynamic conditions in the range 0.07-0.12. The experimental results demonstrated a substantial improvement in the ability of the isolated bridge to sustain all levels of seismic excitation under elastic conditions.

Modeling triple friction pendulum bearings for response-history analysis

There are currently no applicable hysteresis rules or nonlinear elements available in structural analysis software that can be used to exactly model triple Friction Pendulum bearings for response-history analysis. Series models composed of existing nonlinear elements are proposed since they can be immediately implemented in currently available analysis software. However, the behavior of the triple Friction Pendulum bearing is not exactly that of a series arrangement of single concave Friction Pendulum bearings—though it is similar. This paper describes how to modify the input parameters of the series model in order to precisely retrace the true force-displacement behavior exhibited by this device. Recommendations are made for modeling in SAP2000 and are illustrated through analysis of a simple seismically isolated structure. The results are confirmed by (a) verifying the force-displacement behavior through comparison with experimental data and (b) verifying the analysis through comparison to the results obtained by direct numerical integration of the equations of motion.

Negative Stiffness Device for Seismic Protection of Structures

AbstractStructural weakening and addition of damping is an approach previously proposed for the reduction of seismic forces and drifts in the retrofit of structures. It is also used in the design of new buildings with damping systems. While this approach is efficient, it does not significantly reduce and may even amplify inelastic excursions and permanent deformations of the structural system during a seismic event. This paper describes a negative stiffness device (NSD) that can emulate weakening of the structural system without inelastic excursions and permanent deformations. The NSD simulates yielding by engaging at a prescribed displacement and by applying a force at its installation level that opposes the structural restoring force. The NSD consists of (a) a self-contained highly compressed spring in a double negative stiffness magnification mechanism; and (b) a gap spring assembly (GSA) mechanism which delays the engagement of negative stiffness until the structural system undergoes a prescribed disp...

Scissor-Jack-Damper Energy Dissipation System

Installation of damping devices has been limited to diagonal or chevron brace configurations until the recent development of the toggle-brace configurations. These configurations magnify the effect of damping devices, thus facilitating their use in stiff framing systems. This paper introduces the scissor-jack-damper system that was developed as a variant of the toggle-brace-damper systems, with the added advantage of compactness. The effectiveness of the scissor-jack configuration is demonstrated through testing of a large-scale steel-framed model structure on an earthquake simulator. Experiments showed that despite the small size of the damping device considered, the scissor-jack system provided a significant amount of damping and substantially reduced the seismic response of the tested structure. Response history and simplified analyses produce results that are consistent with the experimental results.

Adaptive Negative Stiffness: New Structural Modification Approach for Seismic Protection

AbstractYielding can be emulated in a structural system by adding an adaptive negative stiffness device (NSD) and shifting the yielding away from the main structural system, leading to the new idea of apparent weakening that occurs, ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a recentering mechanism that avoids permanent deformation in the composite structure-device assembly unless the main structure itself yields. Essentially, a yielding-structure is mimicked with no, or with minimal, permanent deformation or yielding in the main structure. As a result, the main structural system suffers ...

Evaluation of Simplified Methods of Analysis of Yielding Structures with Damping Systems

The 2000 NEHRP Provisions include simplified methods of analysis of inelastic buildings equipped with linear viscous, nonlinear viscous, and hysteretic damping devices. These methods are based in part on the data presented in the paper. Nonlinear response-history analysis was used to validate the methods. The derivation of the 2000 NEHRP simplified methods for calculating the maximum acceleration and maximum velocity in damped framing systems are presented. These simplified methods produce exact or conservative estimates of peak displacement and peak acceleration, and reasonable estimates of peak velocities. The estimated velocities are within 25% of the average values obtained from response-history analysis for a broad range of periods and damping ratios. Velocity correction factors are provided to calculate peak velocity given the pseudo-velocity.

Experimental study of bridge seismic sliding isolation systems

An experimental study of a seismically isolated and a comparable nonisolated bridge is presented. The bridge model featured flexible and stiff piers and, in certain cases, different isolation system properties at the two piers. Designs for application in areas of low to moderate seismic activity and designs for application in areas of strong seismic activity and all types of soil conditions were tested. All systems consisted of sliding bearings and rubber restoring force devices. Moreover, fluid viscous dampers were utilized in a system specifically designed to withstand the strong and long period level 2 Japanese design motions while transferring a force to the substructure not more than one-third of the deck weight and allowing bearing displacements less than 200 mm in prototype scale. The analytical modelling of the systems and a design example are presented.