A.M. Reinhorn

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...

Seismic Fragility of Suspended Ceiling Systems

Full-scale dynamic testing of suspended ceiling systems was performed to obtain fragility data suitable for performance-based assessment and design. On the basis of the fragility data derived from testing, (1) the use of retainer clips improves the performance of ceiling systems in terms of loss of tiles, (2) including recycled cross tees in the suspension grid increases the vulnerability of the ceiling systems, (3) undersized (poorly fitting) tiles are substantially more vulnerable than properly fitted tiles, and (4) the use of compression posts improves the seismic performance of ceiling systems for the limit states of minor and moderate damage. Fragility curves are provided for four damage states.

Gravity-Load-Designed Reinforced Concrete Buildings--Part I: Seismic Evaluation of Existing Construction

The seismic performance of nonductile reinforced concrete frame buildings in regions of low to moderate seismicity is evaluated. Several significant aspects of nonductile detailing are modeled using rotional simplifications of expected member behavior at critical sections to facilitate a complete inelastic time history analysis of the system. The detailing configurations included in the analysis are: discontinuous positive flexural reinforcement, lack of joint shear reinforcement, and inadequate transverse reinforcement for column core confinement. Seismic evaluations of three-, six-, and nine-story buildings are carried out under low- to moderate earthquake excitations. The essential parameters of the response are presented with a view to identifying vulnerability of such buildings to a potential seismic design event

Inelastic Response of R/C Structures with Viscoelastic Braces

The addition of viscoelastic braces in structures for vibration reduction has been proposed and implemented in the past decade in metal scaled models of full-scale structures. Viscoelastic braces can provide energy dissipation, while the structure remains elastic. In reinforced concrete structures, the seismic response is usually inelastic, which is often accompanied by permanent deformations and damage. The addition of viscoelastic dampers can dissipate energy at the early stages of cracking of the concrete elements and reduce the development of damage. With proper selection of dampers, this damage can be substantially reduced or even eliminated. However the addition of viscoelastic dampers may stiffen the structure unnecessarily producing increased inertial forces and base shears when subjected to seismic motion. The quantification of the influence of viscous damping and elastic stiffness properties of dampers during the inelastic response of reinforced concrete structures is the subject of this investigation. Models for analysis of inelastic response with damage indexing for reinforced concrete structures that include viscoelastic braces are developed and calibrated using experimental data produced by shaking table tests. These models are then used to determine the variation of expected damage in the presence of damping and quantify the hysteretic energy dissipation along with the damping energy.

Gravity Load-Designed Reinforced Concrete Buildings--Part II: Evaluation of Detailing Enhancements

The seismic performance of a reinforced concrete (RC) building designed primarily for gravity loads was presented in the first part of this investigative study. Since the source of most of the structural damage is attributed to nonseismic details in the beam-column joint area, it was decided to examine the effects of improving these details in a marginal way, so that the seismic performance could be enhanced without resorting to a full seismic design. An extensive parametric study of the original buildings with refined detailing characteristics is carried out to ascertain effectiveness of the techniques to improve seismic resistance of gravity load-designed buildings. The important feature of the study is that the buildings are not redesigned for lateral forces, but only that detailing in critical regions is altered to achieve improved performance. This study may also be viewed indirectly as an inquiry into the effectiveness of retrofit strategies, should they be considered in real practice. The investigation also includes a cost evaluation of the various detailing schemes.

A computational tool for evaluation of seismic performance of reinforced concrete buildings

Abstract A special-purpose computational tool is developed to evaluate the inelastic seismic response of reinforced concrete buildings. A macromodel approach is used to analyze a discretized building composed of parallel frame-wall systems interconnected by transverse elements. The macro-behavioral models include the essential hysteretic characteristics of reinforced concrete sections and also account for the effects of distributed plasticity. All developments are incorporated into a computer code, IDARC. The program performs a series of tasks to enable a complete evaluation of the structural system: (a) monotonic analysis to establish the collapse mechanism and base shear capacity of the structure; (b) quasi-static cyclic analysis under force or deformation control; (c) transient dynamic analysis under horizontal and vertical seismic excitations; (d) reduction of the response quantities to damage indices so that a physical interpretation of the response is possible. The program is built around two graphical interfaces: one for preprocessing of structural and loading data; and the other for visualization of structural damage following the seismic analysis. The program can serve as an invaluable tool in estimating the seismic performance of existing reinforced concrete buildings and for designing new structures within acceptable levels of damage.

Seismic Response of a 1:6 Reinforced Concrete Scale-Model Structure With Flexible Floor Diaphragms

A comprehensive investigation to study the effect of flexible floor diaphragms on the inelastic seismic response of reinforced concrete buildings is presented. The study used a combined experimental and analytical approach, the details of which are described. The paper presents extended descriptions and results of an experimental study of the shaking-table response of a scaled model structure, and its pertinent component tests. The correlation between the analytical predictions obtained using the developed model and the exoerimental responses is examined.

Adaptive negative stiffness: A new structural modification approach for seismic protection

Yielding 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 re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is “mimicked” without any, or with minimal permanent deformation or yielding in the main structure. As a result, the main structural system suffers less accelerations, less displacements and less base shear, while the ANSS “absorbs” them. This paper presents comprehensive 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. A companion paper presents the NSD and its mechanics in detail.

NONLINEAR DYNAMIC ANALYSIS OF MULTIPLE BUILDING BASE ISOLATED STRUCTURES

Abstract In long base isolated buildings the superstructure may consist of several parts separated by narrow thermal expansion joints. In such cases, neighboring bearings which support adjacent buildings are connected together at their top so that they form a common isolation and prevent impact at the isolation level. The analysis of such buildings constitutes the problem of a multiple base-isolated structure with a common basemat and isolation system. This situation can not be analyzed with the existing algorithms which are capable of analyzing only a single base-isolated structure. The torsional characteristics of the combined system are different than those of individual buildings on individual isolation systems. Hence, the combined system of several buildings on a common isolation system needs to be analyzed in its entirety rather than analyzing each building with its isolation system separately. In this paper an analytical model and an algorithm to analyze multiple buildings on a common isolation system are presented. Verification of the accuracy of the algorithm by comparison with results obtained using a general purpose finite element program are presented. A multiple building base-isolated structure is analyzed and the results are used to demonstrate the importance of analyzing the combined system as against analyzing individual buildings.

Active Stochastic Control of Seismic Structures

In a continuing effort to determine the feasibility of applying optimal control to building structures, a comprehensive experimental study was carried out using a standardized structural model under base excitation produced by the Earthquake Simulator at SUNY/Buffalo. Based upon computer simulations and experimental results, this paper presents a comparison of effectiveness of several optimal control algorithms using instantaneous optimal criteria, including time delay compensation. All investigations were done under similar conditions to permit a systematic evaluation of efficiency. Comparisons were also made between analytical and experimental results and between instantaneous control algorithms and classical closedloop linear feedback. Conclusions drawn regarding relative merits of the control algorithms considered under varying conditions are presented herein.