A.A. Sarlis

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 ...

Simulated Bilinear-Elastic Behavior in a SDOF Elastic Structure Using Negative Stiffness Device: Experimental and Analytical Study

AbstractThe acceleration and base shear of structures during strong ground motion can be attenuated by achieving bilinear-elastic behavior without any permanent displacement—also referred to as “apparent weakening.” The negative stiffness device (NSD), used in this study, exhibits nonlinear-elastic negative stiffness behavior; by adding NSD to the elastic structure, the resulting structure-device assembly behaves like a bilinear-elastic structure. Peak acceleration and base shear experienced by the structures can be reduced by adding the negative stiffness device, and the additional deformations caused by the reduced stiffness can be contained by adding a viscous damper. This paper presents the experimental study on a three-story fixed-base structure (3SFS), acting as a single-degree-of-freedom (SDOF) system (because of bracing in the top two stories), that demonstrates the concept of apparent weakening in elastic structural systems. Two NSDs and a viscous damper are installed in the first story of 3SFS. ...

Apparent Weakening in SDOF Yielding Structures Using a Negative Stiffness Device: Experimental and Analytical Study

AbstractThe peak deformation, acceleration, and the base shear experienced by the structures can be reduced by simulating yielding in an elastic system—also referred as apparent weakening. The negative stiffness device (NSD), used in this study, exhibits nonlinear-elastic negative stiffness behavior; by adding NSD to the elastic structure (primary structure), the resulting structure-device assembly behaves like a bilinear-elastic structure. In an elastic structure, the acceleration and base shear experienced by the structure can be reduced by adding the negative stiffness device, and the additional deformations caused from the reduced stiffness can be contained by adding viscous dampers. Previously, the authors have carried out experimental studies to demonstrate the effectiveness of apparent weakening in elastic structures, but little is known about the behavior of these systems when the primary structure itself yields. This paper focuses on the issues that may emanate with the addition of NSD to the sys...

Performance Evaluation of Negative Stiffness Devices for Seismic Response Control of Bridge Structures via Experimental Shake Table Tests

A newly developed passive device that provides negative stiffness has been implemented within a quarter-scale highway bridge model and subjected to seismic loading via shake table testing. Details of the experimental results and their comparison with numerical simulations under a wide range of ground motions are presented. In addition, performance indices were developed to systematically evaluate the relative performance of different isolation system configurations that employ combinations of positive and negative stiffness as well as various levels of damping. Further, the influence of boundary conditions (rigid versus flexible bridge piers) on the effectiveness of employing negative stiffness devices has been evaluated.

Experimental Shake Table Testing of an Adaptive Passive Negative Stiffness Device within a Highway Bridge Model

The implementation of a mechanical negative stiffness device (NSD) within a reduced-scale highway bridge model and its performance under seismic loading conditions is evaluated via shaking table tests. Four different isolation system configurations are considered: isolated bridge (IB), IB with viscous dampers, IB with NSDs, and IB with viscous dampers and NSDs. In addition, two bridge pier configurations were considered: one with flexible piers (mimicking a middle span of a multi-span bridge) and one with braced piers (mimicking a single span bridge supported on abutments). The main feature of the NSD is a large pre-compressed spring, which can push the structure away from its initial undeformed position and thus induce negative stiffness behavior. The experimental results clearly demonstrate the effectiveness of the NSDs in limiting the seismic response of the bridge and provide validation of numerical simulation results wherein numerical models of the bridge model components were calibrated via system identification testing.

Negative stiffness device for seismic protection of structures: Shake table testing of a seismically isolated structure

AbstractThe concept of apparent weakening by adding true negative stiffness to a structure has been previously introduced by the authors in order to reduce simultaneous drifts, accelerations, and displacements in a structure without yielding or permanent deformation in the main system. A novel negative stiffness device (NSD) that generates true negative stiffness has been developed, built, and tested and has been previously described by the authors in terms of operation and analytical and numerical modeling. This paper presents results that represent proof-of-concept for weakening with the use of the NSD based on the shake table testing of a 3-story seismically isolated structure, equipped with these devices complemented by viscous dampers. The NSD is shown to have a significant effect on the superstructure response by reducing floor accelerations, story drift, and the base shear and upon the addition of dampers, also results in a reduction in isolator displacements. Moreover, this paper provides validati...

Performance Assessment of a Highway Bridge Structure employing Adaptive Negative Stiffness for Seismic Protection

A negative stiffness device has been tested within a quarter-scale highway bridge model on the seismic shaking table at the University at Buffalo Network for Earthquake Engineering Simulation (NEES) site. Based on the experiments, numerical models have been developed, calibrated, and used to simulate the response of the bridge under a wide range of ground motions. In addition, performance indices have been developed to systematically and quantitatively evaluate the relative performance of different isolation system configurations that employ combinations of positive and negative stiffness as well as various levels of positive damping. Further, the influence of boundary conditions (rigid versus flexible bridge piers) on the effectiveness of employing negative stiffness devices has been evaluated. Finally, concepts for graphical interpretation of the performance indices are presented and used to demonstrate the degree to which employing negative stiffness may be beneficial in improving the seismic response of bridge structures.

Negative Stiffness Device for Seismic Response Control of Multi- story Buildings

Weakening and damping of structures has proven to be an effective method for mitigating the structures response. This approach has drawn further attention after the invention of negative stiffness device (NSD), developed by the authors. Preliminary analytical and experimental studies reported on the NSD have revealed that by adding the NSD to a single story structure the base shear demands and peak acceleration of the main structure are reduced significantly and the inter-story deformations are contained by adding a passive damper. In this paper an analytical study is carried on an inelastic multistoried shear building to demonstrate the effectiveness of placing NSDs and dampers at multiple locations along the height of the building. It has been shown that by placing a NSD in a particular story the superstructure above that story can be isolated. It has also been shown through simulation studies that the NSD will limit the amount of energy transmitted to the superstructure from the ground excitation. Essentially, NSD acts as a vibration isolator. Large base deformations is one major limitation in base-isolating the structural systems but using NSDs this can be overcome as the isolation is achieved over the height of the building and not confined to the base. It has been shown through the simulation studies that by placing NSDs in all the lower storys the acceleration of the superstructure and base shear can be reduced significantly without affecting the drifts. Simulation results of a nine-storied 1:3 scale inelastic shear building subjected to periodic ground motion and Kobe fault normal ground motion

A New Structural Modification Approach for Seismic Protection Based on Adaptive Negative Stiffness Device: Conceptual Analysis

Column forces, displacements and accelerations experienced by the structure during strong ground motions can be reduced by weakening and (or) softening the structure and adding a supplemental damper. Although this approach proved to be promising analytically, the concept of “structural strength reduction” leads to inelastic behavior and large permanent deformations in the main structural system. In this paper a new concept is developed to emulate weakening 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” with reduced inelastic excursions in the main structural system. This is achieved through an adaptive negative stiffness system (ANSS), which is a combination of NSD and a damper. Engaging the NSD at an appropriate displacement (simulated yield displacement), that is well below the actual yield displacement of the structural system, will result in a composite structure-device assembly that behaves like a yielding structure. The NSD has a re-centering mechanism thereby avoiding permanent deformation in the composite structuredevice assembly unless, the main structure itself yields. Essentially, a yieldingstructure is “mimicked” without any, or with minimum yielding and permanent deformation in the main structure. In summary, the main structural system undergoes less acceleration, less displacements and less base shear, while the ANSS “absorbs” them. This paper presents the working principle and details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers is presented.

Control of inelastic structures by weakening and damping

Control of structures can be achieved by adding suitable control devices such as actively controlled actuators, strengthening and stiffening elements, and/or adding passive damping devices. However, the control demands often require reducing the induced forces in structures, and eliminating essential structural elements and masses that contribute to generating inertial forces during earthquakes. Recently, Reinhorn and his colleagues developed design and implementation concepts that weaken the structural system and reduce the induced forces at the expense of increased deformations, while correcting and controlling such increases with supplemental damping. This results in an improved behavior, in particular when applied to existing structures, as well as when introduced into new construction. The concept and implementation was studied by the author’s team theoretically using control methods and experimentally using structural models with weakening, or softening, devices and simulated earthquakes. The implementation of such a concept requires particular attention to and balance of safety and stability. The presentation will introduce the concept, the development of weakening components (such as rocking columns), the innovation of true negative stiffness devices, the theoretical and experimental verification of the concept using simulated earthquakes in the laboratory, and the development of design procedures using active control theories. Andrei M. Reinhorn, Ph.D., PE Professor Emeritus, University at Buffalo, State University of New York Andrei M. Reinhorn is a professor emeritus at the University at Buffalo (UB) who was involved in education, research and consulting in structural dynamics with applications to earthquake engineering, wind effects and extreme loads engineering. He is a graduate of the Technion – Israel Institute of Technology (BS 1968, PhD 1978) followed by an academic career at UB spanning over thirty years. Professor Reinhorn conducted research in evaluation and design of building structures experiencing inelastic deformations near collapse. He also developed modeling and solution techniques for structural control and base-isolated structures. Computational platforms 3D-BASIS and IDARC developed by him and his coworkers are widely used by academics and design professionals around the world. He pioneered experimental structural control that brought the experimentation from small scale laboratory implementations to full scale real-life realization of controlled structures using active tendon systems in Japan. He was one of the pioneers in defining the disaster resilience of communities and its quantification, using basic principles of process control. Most recently, he developed new approaches to analysis of structures using the State Space Approach (SSA) and Mixed Lagrangian Formulation (MLF). He developed integrated computing and experimentation methods, which are in the forefront of hybrid simulation techniques. As one of its designers and founders, he directed one of the largest laboratories in the US, The Structural Engineering and Earthquake Simulation Laboratory (SEESL) at UB. Professor Reinhorn was awarded the 2011 ASCE Nathan M. Newmark Medal. He has received numerous other awards, including the 2007 SUNY Chancellor’s Award for Excellence in Scholarship and Creative Activity, 2006 UB “Exceptional Scholar” Sustained Achievement Award, 2005 ASCE/CERF Charles Pankow Award for Innovation, and 1998 AGC-Build San Diego Award, for work related to applications of structural control. More information can be found at http://civil.eng.buffalo.edu/~reinhorn/ Date: Friday, April 5, 2013 Time: 12:00 PM – 2:00 PM Location: 140 Ketter Hall, North Campus, University at Buffalo Webcast: http://civil.eng.buffalo.edu/webcast Technical Questions: seeslwebcast@gmail.com Refreshments will be served!