Innovative strategies for seismic retrofitting of steel and composite structures L Di Sarno 1 and A S Elnashai 2 1 University of Sannio, Benevento, Italy 2 University of Illinois, Urbana USA Summary This paper reviews briefly traditional rehabilitation methods and provides a detailed discussion of design issues along with the advantages and the disadvantages of innovative strategies for seismic retrofitting of steel and composite structures. Special emphasis is placed on base isolation and supplemental damping devices. The viability and cost-effectiveness of these strategies are assessed on the basis of multiple limit states within the framework of performance-based design. A number of parameters which govern the selection of the intervention devices are investigated. Key words: retrofitting; steel structures; composite structures; base isolation; dampers Prog. Struct. Engng Mater. (in press) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pse.195 Introduction Recent earthquakes, such as the Northridge (January 17, 1994, California, USA), Hyogoken-Nanbu (January 17, 1995, Japan) and Chi-Chi (September 21, 1999, Taiwan) caused unexpectedly high levels of damage of steel and composite structures. Damage was varied and widespread, ranging from residential buildings to highway bridges and lifelines[1]. In California the most severe effects were imposed on low-rise steel buildings, typically three-storey apartments with a ground level garage, second-storey living quarters and third-storey bedrooms[2]. In Japan, columns, braces, column bases and beam-to-column connections of old and new steel buildings sustained damage[3], and 1067 old buildings}constructed with bundled light-gauged sections for columns and trusses for beams}collapsed or were damaged beyond repair[4]. A further 100 new structures, consisting of two to five storeys, collapsed, and 332 were severely damaged. In Taichung, (Taiwan) some high-rise dual systems and welded MRFs suffered localized yielding and damage to architectural features[5]. Observation of these earthquake effects prompted a number of research projects in seismic retrofitting (see, for example Di Sarno & Elnashai[6]). The work carried out has shown that structural deficiencies in existing steel and composite buildings may be remedied by the application of several retrofitting strategies, either traditional or innovative. While local and global interventions are effective for enhancing Review Article Terminology A b = bonded area B D = damping-dependent coefficient at design displacement B D = damping-dependent coefficient at maximum displacement C = damper coefficient D = displacement E loop = energy dissipated for cycle of loading E D = energy dissipated for cycle at de- sign displacement E M = energy dissipated for cycle at maximum displacement F = force F y = damper yield force F E = elastic force in the structure g = acceleration due to gravity G 0 = shear storage modulus G 00 = shear loss modulus k eff = effective stiffness k 0 eff = effective stiffness at peak displace- ment N = brace force N b = brace buckling force N g = brace tensile force at slip N l = damper force at slip S D = design 5% damped spectral accel- eration for 475-year return period earthquake S M = design 5% damped spectral accel- eration for 2475-year return per- iod earthquake R = force reduction factor T D = effective period of a seismically isolated structure at design displacement T M = effective period of a seismically isolated structure at maximum displacement V = base shear V b = design base shear W = seismic weight of the structure b = equivalent viscous damping b eff = effective equivalent viscous damp- ing b D = effective equivalent viscous damping at design displacement b M = effective equivalent viscous damp- ing at maximum displacement D = displacement; D p = peak displacement D ’ = velocity o = natural circular frequency. Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. (in press)