Nonductile Concrete Frames

The Northridge earthquake struck the greater Los Angeles area during the early morning hours of January 17, 1994. The earthquake was responsible for approximately 60 deaths, more than 9,000 injuries, and an estimated $20 billion in damages. Significant ground shaking occurred over a wide area and exceeded design code values in many locations. Numerically, most of the damage was to wood-frame residences, but upwards of 200 concrete buildings were red-tagged. The Northridge earthquake was the first big test of pre-1980 concrete buildings and post-1980 buildings designed using updated code provisions following the 1971 San Fernando earthquake. The 1971 San Fernando earthquake exposed the deficiencies of the building codes in place at the time, particularly related to concrete. The collapses of the Olive View Medical Center and the Veterans Administration Hospital that occurred as a result of the San Fernando earthquake are famous examples of the hazards posed by “nonductile” concrete (NDC) buildings. Several of these NDC buildings collapsed or were severely damaged during the Northridge earthquake as well, including the Kaiser Permanente Office Building (Figure 1) and Saint John’s Hospital. This article discusses building code provisions for concrete structures, the performance of non-ductile concrete frame structures in the Northridge earthquake, associated changes made to the building code after, and retrofit ordinances being considered today for existing non-ductile concrete buildings.