The assessment of an existing RC framed structure: a case study on a collapsed building

The total collapse of a building in L’Aquila, Via D’Annunzio (D’Annunzio Street), located at about 6 km from the epicenter of the earthquake of April 6 2009, is here analyzed. The reinforced concrete moment-resisting frames were designed in the 1961 according to the Italian Seismic Code of 1937, and failed with a “pancake-type collapse”, with a very high death toll of 13 casualties. At the beginning of 2013, the first Author was appointed by the legal authority to investigate the reasons for the collapse. Studies were partly based on the on-site investigations performed during the summer 2009, including tests on the concrete properties, analysis of the ground properties and exam of parts of the collapsed structure recovered from the debris. Due to the lack of a complete set of design blueprints, the dimensions and positions of the columns, as well as the geometry of the reinforcement, were obtained by an on-site series of measurements during 2013. The range of variation of natural periods and modal shapes depending on the modeling assumptions have been determined through numerical analyses. The seismic excitation at the site, determined from the earthquake records and the ground properties coming from down-hole tests, has been provided in terms of time history and response spectrum. When all the factors affecting the seismic behavior are taken into account, the collapse of the building can be explained; the collapse mechanism resembles the modal shape of the first mode. The flaws of the original design, brought to light with this study, can be assumed as typical for the design time and provide clear indications on the critical points to be checked when assessing an RC frame of the sixties.