The Collapse of New RC Buildings During the March 4, 1977 Vrancea Earthquake

Abstract eng:
The seismic hazard of the Romanian territory is determined by the well known Vrancea seismic region, characterized by a high rate of large earthquake occurrence in a narrow focal volume confined to an area of about 30 x 70 sq kilometers. During the last century, four major Vrancea earthquakes happened (1940, 1977, 1986, 1990), the second one leading to a disastrous impact on the Romanian territory. 36 buildings collapsed, 4 among them being new buildings at that time, designed according to the seismic legislation in force. This paper summarizes the behavior of these four RC buildings during this earthquake, showing their main structural deficiencies that contributed to their collapse. All these RC buildings were designed and constructed prior to having a recording of a strong earthquake in Romania and prior to the introduction of provisions for ductile response in seismic design codes (nonductile concrete buildings). After this seismic event, the need for improvement in collapse assessment technology for existing nonductile concrete buildings has been recognized as a high-priority. Once more, it was demonstrated that the failure of such buildings involves partial or complete collapse, together with substantial life and property losses. The most instructive building collapse was that of a new computing center, built in 1967. It was a 3 storey building, comprising a central structure with service towers at both ends, structurally separated from the main building that collapsed, while the service towers did not. The second of the four buildings was a residential block of flats whose structural system was RC cast-in-place structural walls. It was designed in 1972 and completed in 1974. The block consisted of six sections, developed over a length of 225 m, separated by small joints. The height regime of the building consisted of a basement, a ground floor and ten stories. During the 1977 earthquake the last section of the block completely collapsed by cross overturning, rotating relative to the ground structural floor. The third building that makes subject of the paper is also a block of flats that was completed in the period 1961-1962. The height regime of the building consisted of a basement, a ground floor, eight complete stories and a partially retracted top floor. Along its length, the building was divided in three sections. Its structural system consisted in a flexible first storey with RC columns and a more rigid system with cast-in-place RC structural walls for the rest of the building. During the March 4, 1977 earthquake the west part of the building, on a length of about 10 meters, was dislocated, its subsequent demolition being required. The last case refers to a building with mixed structural system, consisting in moment resisting frames and masonry walls. During the earthquake the ground floor – a weak and soft storey – “disappeared”. All of these case studies will be presented in such a manner, so that everyone can understand what happened.

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