Design criteria for the seismic rehabilitation of buildings

Abstract

SUMMARY A methodology was presented by the author in a previous paper for the development of design criteria for buildings. This methodology was based on the principles of structural reliability and structural dynamics. This paper illustrates how many elements of this methodology have been implemented and extended to develop design criteria for reinforced concrete and steel buildings by special blue ribbon committees selected by the City of Los Angeles Building Department. This paper presents the design criteria developed by these committees and discusses some of the more contentious issues. Reinforced concrete buildings built prior to 1976 have performed very poorly in recent earthquakes. They can certainly be identified as having a special life safety concern in the event of the occurrence of a major earthquake. This concern is not recent. Perhaps the first clear documentation of this concern was the proposed new design approach presented in the book entitled Reinforced Concrete Design Book by Blume, Newmark and Corning published in the early 1960s. Since one generation of structural engineers has been educated since the publication of this book and since numerous failures and near collapses of reinforced concrete frame buildings occurred during the Northridge and Kobe earthquakes, now is the time to take action. The building department of the City of Los Angeles has on numerous occasions been a leader in design criteria development. Consistent with this past tradition of excellence the city formed a special committee of structural engineers to document damage to concrete buildings during the Northridge earthquake and also to advise them on courses of action. Design criteria for the seismic rehabilitation of existing reinforced concrete buildings located in the City of Los Angeles were developed by the members of this committee. The acknowledgement section of this paper identifies the committee. This draft was based on and extended the methodology presented in a 1992 The Masonry Society Journal paper entitled ‘A philosophy of a reliability based limit state design approach’. The author as a member of this committee was asked to write the first draft for these new seismic rehabilitation criteria. The new design criteria were entitled ‘Evaluation and rehabilitation criteria for concrete structures’ and were approved by the committee in June 1994. The 1994 Northridge earthquake also presented structural engineers with an unexpected new class of hazardous existing buildings. These buildings are defined as ‘special moment resisting