Dimitrios G. Lignos, Ph.D. Assistant Professor McGill University Montreal, Quebec, Canada

Daniel Mauricio Moreno-Luna Ph.D. Student Stanford University Stanford, California, United States

Sarah L. Billington, Ph.D. Associate Professor Stanford University Stanford, California, United States

Experimental and Analytical Validation of an Innovative Seismic Retrofit System for Existing Steel Moment-Resisting Frames

Recent breakthroughs in fiber reinforced concrete technology have made it possible to utilize High Performance Fiber Reinforced Cementitious (HPFRC) materials for structural applications including seismic retrofit. 

For this purpose, an innovative retrofit system that consists of HPFRC infill panels that are acting as energy dissipation fuses has been developed. The seismic performance evaluation of the infill panel retrofit system was validated through large scale hybrid simulation testing series of a 2-story steel moment-resisting frame, designed based on older
seismic provisions in California. These tests were conducted at the Network for Earthquake Engineering Simulation (NEES) facility at the University of California at Berkeley. The proposed
retrofit system was proven to be able to reduce the absolute maximum story drift ratios as well as the residual deformations of the retrofitted steel moment resisting frame relative to bare frame performance.

This paper summarizes the experimental validation of the proposed HPFRC retrofit system. The available experimental data is also utilized for the development of phenomenological analytical models that are implemented in the OpenSees simulation platform and permit the seismic assessment of retrofitted steel moment resisting frames in a Performance Based Earthquake Engineering context.

1. INTRODUCTION

Recent earthquakes around the world have demonstrated that steel moment frame structures designed based on older seismic provisions (UBC 1994) might be seismically deficient due to premature fracture of beam-to-column connections at fused zone or column flange often noted as “divot” zone (FEMA 351). Further important facilities such as hospitals that have been designed in the 1980s and it is likely that their seismic vulnerability is high, should remain operational to minimize human casualties and economic losses after a major earthquake.

Seismic retrofit is an economic solution to upgrade the existing infrastructure. In 1990s the Federal Emergency Management Agency (FEMA) and the State of California developed a program for seismic rehabilitation strategies summarized in FEMA 356 (2000). The main strategies of these rehabilitation techniques in the United States typically involve (1) global modification of the structural system and (2) local modifications of isolated components of the structural and nonstructural system (e.g. steel, fiber reinforced composite jackets, beam-to-column connection modification improvements).

The emphasis was on global modification techniques that involve: (a) the use of reduced steel thickness shear walls (Bruneau 2005) that allow shear buckling and act as an energy dissipation mechanism; (b) ductile fuse elements that move plastic deformation ways from beam-to-column connections (Leelataviwat et al. 1998); (c) self-centering systems (Ricles et al. 2001, Christopoulos et al. 2002, 2008, Tremblay et al. 2008); (d) buckling restrained braces that primarily enhance the strength of  the structural system (Clark et al. 1999, Wada et al. 1998, Lopez et al. 2002, Uang and Kiggins 2003); (e) passive energy dissipation devices including various types of dampers (Constantinou and Symans 1992, Soong and Spencer 2002, Kasai et al. 2010). 

Recently, infill panels have become popular for structural applications including seismic retrofitting. Jung and Aref (2005) using polymer matrix composite (PMC) infill panels showed that damping as well as lateral resistance of existing steel frames can be increased. Using recent breakthroughs in fiber reinforced concrete technology, Kesner and Billington (2005) utilized High Performance Fiber-Reinforced cementitious (HPFRC) composites to seismically retrofit vulnerable steel moment frame structures. 

This system was redesigned and evaluated extensively through an experimental program that included a large number of infill panel component tests (see Olsen and Billington 2009, Hanson and Billington 2009) and a series of large-scale hybrid simulation tests of a 2-story steel moment frame designed in 1980s in California. These tests were conducted at the Network for Earthquake Engineering Simulation (NEES) facility at University of California at Berkeley. This paper discusses the experimental and analytical validation of the proposed retrofit system for existing steel moment frame structures subjected to earthquakes.

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