Building Structures with Damping Systems: From Research to Design Practice

A. S. Whittaker1 and M. C. Constantinou2

1Professor, Department of Civil, Structural and Environmental Engineering, State University of New York, Buffalo, New York, 14260; PH (716) 645-2114; FAX (716) 645-3733; email: awhittak@acsu.buffalo.edu. 2Professor and Chairman, Department of Civil, Structural and Environmental Engineering, State University of New York, Buffalo, New York, 14260; PH (716) 645-2114; FAX (716) 645-3733; email: constan1@eng.buffalo.edu

Abstract

Research and studies on seismic damping devices and systems started in earnest in the United States in the 1980s. In less than 15 years, damping devices moved from the university laboratory into mainstream design practice. Damping devices, damping systems, analytical tools, design procedures and codes and guidelines have been developed and deployed in building structures. The paper summarizes briefly the research work and describes in some detail the guidelines and codes of practice now available for implementing damping devices in buildings.

Introduction

Conventional or non-protected earthquake-resistant buildings rely on significant inelastic action (energy dissipation) in selected components of the framing system for survival in design and maximum earthquake shaking. For the commonly used special moment-resisting frame, inelastic action should occur in the beams near the columns and in the beam-column panel joint: both zones form part of the gravity-load-resisting system. Inelastic action results in damage, which is often substantial in scope and difficult and costly to repair. Damage to the gravity-load-resisting system can result in significant direct and indirect losses.

The desire to avoid damage to components of gravity-load-resisting frames in buildings following the 1989 Loma Prieta and 1994 Northridge earthquakes spurred the development of passive energy dissipation systems. Passive metallic yielding, fluid viscous and viscoelastic damping devices are now available in the marketplace, both in the United States and abroad. Soong and Dargush (1997), Constantinou et al. (1998) and Hanson and Soong (2001) describe these and other types of passive dampers. The primary objective of adding damping devices to building frames has been to focus the energy dissipation during an earthquake into disposable elements specifically designed for the purpose of dissipating energy, and to substantially reduce or eliminate energy dissipation in the gravity-load-resisting frame. Since energy dissipation or damping devices do not form part of the gravity-load-resisting system they can be replaced after an earthquake without compromising the structural integrity of the building frame. Recent installations of fluid viscous dampers and unbonded (buckling restrained) braces in high-performance structures are shown in Figure 1.

a. unbonded brace b. unbonded brace application

c. fluid viscous damper d. fluid damper application

Figure 1. Supplemental damping devices and installations

The following two sections of this paper summarize a) research work on damping devices, and b) guidelines and codes of practice now available for implementing damping devices in buildings. Emphasis is placed on design practice and code and guideline development because there is insufficient space to describe the research work in any detail.

Research

Studies on the use of damping devices for building structures began in New Zealand in the early 1970s with a focus on metallic yielding damping (energy dissipation) devices. Information on some of these studies can be found in (ATC, 1986). In the United States, work began in earnest on development and deployment of dampers in the mid-1980s at the University of California at Berkeley, the University of Michigan and the University at Buffalo. Aiken et al. (1993) and ATC (1993) summarize the studies in the United States in the period from the mid-1980s through the early 1990s. Most of these studies addressed fluid viscous dampers, metallic yielding dampers and viscoelastic dampers. Since the mid-1990s, research work in the United States on dampers has focused on the development of a) fluid viscous dampers (e.g., Constantinou and Sigaher, 2000; Sigaher and Constantinou, 2003) and

unbonded (buckling restrained) braces, and b) analytical tools, design procedures, guidelines and codes for deployment of damping devices in buildings (see the following section).

Numerous journal papers, books and primers have been published on the subject of supplemental damping devices. Some of these papers, books and primers are listed in the References. The interested reader should consult these references for detailed information on the research work and studies that underpin the codes of practice and design guidelines described in the following section.

Design Practice and Codification

Guidelines for the implementation of energy dissipation or damping devices in new buildings were first proposed by the Structural Engineers Association of Northern California (SEAONC) to provide guidance to structural engineers, building officials, and regulators who were tasked with implementing such devices in building frames (Whittaker et al., 1993). These guidelines were prepared in response to the increased interest shown in damping devices following widespread damage to building frames in the 1989 Loma Prieta earthquake in Northern California and the emergence of vendors of damping hardware. The intent of the authors of that document was to direct the dissipation of earthquake-induced energy into the (disposable) damping devices and away from components of the gravity-load-resisting system, thereby reducing repair costs and business interruption following severe earthquake shaking.

The SEAONC guidelines were developed on the basis that the primary lateral-force-resisting system (that is, the lateral system exclusive of the damping devices) met the strength and drift requirements of the 1991 Uniform Building Code. Three methods of analysis were prescribed for the implementation of damping devices: response-spectrum analysis, linear response-history analysis, and nonlinear response-history analysis. The linear procedures could only be used with velocity-dependent (e.g., viscous or viscoelastic) dampers in building frames that would remain elastic for design-basis earthquake shaking. Nonlinear response-history analysis was mandated for the implementation of displacement-dependent dampers and for yielding frames incorporating velocity-dependant dampers. Detailed procedures for the prototype testing of damping devices were also presented in the SEAONC guidelines.

In the mid 1990s, the Federal Emergency Management Agency (FEMA) funded the development of guidelines for the seismic rehabilitation of buildings. New methods of seismic analysis and evaluation were presented in FEMA 273 and 274 (ATC, 1997). The four new analysis methods presented in FEMA 273 and 274 were the Linear Static Procedure, the Linear Dynamic Procedure, the Nonlinear Static Procedure, and the Nonlinear Dynamic Procedure. All four methods were displacement based and all directly or indirectly made use of displacement-related information for component checking. (As such, the FEMA 273 and 274 procedures represented a paradigm shift in the practice of seismic design because the focus of

analysis, design, and evaluation shifted from forces to deformations.) Actions in components of a building frame were characterized as either deformation-controlled (for ductile actions such as bending moments in beams) or force-controlled (for brittle actions such as shear forces in columns). Rotation limits for deformation-controlled actions were presented in the materials chapters of FEMA 273 for comparison with rotation demands estimated using the displacement-based methods of analysis. Strength limits were established for force-controlled actions using procedures similar to those in codes and manuals of practice.

Two important components of FEMA 273 and 274 (ATC, 1997) were the guidelines and commentary for implementing damping devices in retrofit construction. In these documents, damping devices were grouped into two categories, namely, displacement-dependent devices and velocity-dependent devices, with the assumed hysteretic shapes of Figure 2: parts a. and b. for displacement-dependent devices and parts c. and d. for velocity-dependent devices. These categories are similar to those adopted in the 1993 SEAONC guidelines and in the Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA, 2000).

Force

Displacement

Force

Displacement

a. metallic yielding damper b. friction damper

Force

Displacement

Force

Displacement

c. viscoelastic damper d. fluid viscous damper

Figure 2. Hysteresis loop shapes for damping devices

Linear and nonlinear procedures for analyzing buildings with damping devices were prepared for FEMA 273 and 274 in a format consistent with that presented for conventional construction. Use of the linear procedures was limited to building frames responding essentially in the elastic range in the scenario earthquake. For frames expected to undergo substantial inelastic action in that earthquake, FEMA 273 required the user to adopt the nonlinear static or nonlinear dynamic procedures, which represent the technically superior methods for evaluating the inelastic behavior of

building frames. Two methods of nonlinear static analysis of buildings with damping devices were presented in FEMA 273 and FEMA 274. Both methods are most suitable for evaluating the response of existing buildings equipped with damping devices but neither method is particularly suitable for sizing either conventional components (e.g., beams and columns) or damping devices for new construction. Also, because these two methods made use of nonlinear analysis, neither was compatible with the elastic-analysis-based procedures of then modern seismic codes for new buildings such as the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (BSSC, 1997). Further, the restrictions on the use of the linear procedures of FEMA 273 and FEMA 274 to essentially linear framing systems meant that these procedures could not be readily directly adopted for use in the 2000 NEHRP Recommended Provisions (BSSC, 2001) for which substantial inelastic response was anticipated in design-basis and maximum-capable earthquake shaking.

In 1997, Technical Subcommittee 12 of the Building Seismic Safety Council was tasked with developing analysis, design, and testing procedures for damping systems and devices for inclusion in 2000 NEHRP Recommended Provisions. The resultant provisions were required to be 100-percent consistent with those presented in 2000 NEHRP Recommended Provisions for conventional construction. The equivalent lateral force and modal analysis procedures for damped buildings that were developed are based in large part on the procedures of FEMA 273 and 274 but assumed that a) the collapse mechanism for the building is a single-degree-of-freedom mechanism so that the drift distribution over the height of the building can be reasonably estimated using either the first mode shape or another profile such as an inverted triangle, b) the building is analyzed in each principal direction with one degree-of-freedom per floor level, c) the nonlinear response of the building can be represented by an elastoplastic relationship, and d) the yield strength of the building can be estimated by either simple plastic analysis or using the specified minimum seismic base shear and values of the response modification (R), the reserve strength of the framing system ( 0Ω ), and the deflection amplification ( dC ) factors presented in the 2000 NEHRP Recommended Provisions.

Ramirez et al. (2000, 2002a, 2002b and 2003) and Whittaker et al. (2003) provide the technical basis for the analysis and design procedures for damping systems presented in Appendix to Chapter 13 in the 2000 NEHRP Recommended Provisions and Chapter 15 of the recently published 2003 NEHRP Recommended Provisions (BSSC, 2004). These studies were based on nonlinear-response history analysis of buildings equipped with displacement- and velocity-dependent damping devices subjected to far-field earthquake shaking. Procedures for calculating the effective damping and effective period, and higher mode damping ratios for buildings equipped with yielding damping devices and nonlinear viscous damping devices were developed. Based on the data presented in Ramirez et al. (2000), the minimum design base shear force for the primary lateral-force-resisting system of a building incorporating a damping system could be reduced by up to 25% for a level of

performance identical to that of a conventional framing system with no damping system1.

Conclusions

Damping devices and systems have moved from the university laboratory into mainstream design practice in less than 15 years. Damping devices, damping systems, analytical tools, design procedures and codes and guidelines have been developed and deployed in building structures in this relatively short period of time.

Acknowledgements

Many academicians and expert design professionals have contributed to the development of damping devices and damping systems, the development of analysis tools and procedures for structures incorporating damping devices and systems, and the preparation of guidelines and codes of practice for implementing damping devices and systems in buildings, bridges and infrastructure. Key contributors include Dr. Ian Aiken, Mr. Robert Bachman, Professor Gary Dargush, Professor Robert Hanson, Mr. Martin Johnson, Professor Kazuhiko Kasai, Professor James Kelly, Dr. Charles Kircher, Professor Nikos Makris, Professor Douglas Nims, Professor Oscar Ramirez, Professor Andrei Reinhorn, Dr. Roger Scholl, Professor Larry Soong, Mr. Douglas Taylor, Professor Panos Tsopelas, Professor K.-C. Tsai and Professor Akira Wada.

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