The Earthquake Engineering Online Archive

Rehabilitation of nonductile RC frame building using encasement plates and energy-dissipating devices

Sasani, Mehrdad; Bertero, Vitelmo V.; Anderson, James C.

PEER-1999/12, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 1999-12, 296 pages (400/P33/1999-12)

The main objective of the study has been to develop a general seismic rehabilitation and upgrade approach for existing nonductile reinforced concrete (RC) moment frame buildings. A review of the design and as-built drawings of such existing buildings reveals that one of the main weaknesses is the lack of transverse (shear) reinforcement in the beam-column joints. Thus, it was decided to attempt an upgrade approach centered on the use of a steel jacket placed around each of these joints. Furthermore, it was considered desirable that the study be conducted on an existing instrumented building that had already been subjected to at least moderate earthquake ground motions (EQGMs) so it would be possible to calibrate the analytical models and computations involved in the analyses of its response and degree of damage. The building selected is a 15-story (13 stories above ground and a 2-story basement) RC building that was designed in 1964 and constructed in 1965. While the 13 stories above grade consist of a RC moment frame structural system, the two levels below ground are enclosed by structural walls. After the 1971 San Fernando earthquake, the building was instrumented with 15 accelerometers. The building experienced damage during both the San Fernando earthquake and the 1994 Northridge earthquake. From the study, it was concluded that the conventional strategy of increasing the stiffness, strength, and ductility (toughness) of the as-built structure would not lead to an efficient economical solution because it leads to a significant increase in the stiffness and strength demands. Because of the observed weaknesses of the existing structure, it was clear that, no matter what upgrading strategy was used, it was necessary first to use the traditional strategy of increasing the toughness of the critical regions of the structure using what appeared to be one of the most promising techniques: the use of steel jacketing and/or encasement of the members (at least in their critical regions) by using the minimum practical steel plate thickness. Once this increase in toughness was achieved, it was necessary to control the demands by using an efficient innovative approach that, for this particular structure, appeared to be the use of energy-dissipating devices that increased its effective damping up to about 30% in order to control the response to the probable severe near-field EQGMs that could occur at the site of the building.

Available online: http://peer.berkeley.edu/publications/peer_reports/reports_1999/9912.pdf