Extreme events (i.e. terrorist attacks, vehicle impacts, explosions, etc.) often cause local damage to building structures and pose a serious threat when one or more vertical load-bearing components fail, leading to the progressive collapse of the entire structure or a large part of it. Since the beginning of the 21st century there has been growing interest in the risks associated with extreme events, especially after the attacks on the Alfred P. Murrah Federal Building in Oklahoma in 1995 and on the World Trade Center in New York in 2001. The accent is now on achieving resilient buildings that can remain operational after such an event, especially when they form part of critical infrastructures, are occupied by a large number of people, or are open to the public. This paper presents an ambitious review that describes all the main advances that have taken place since the beginning of the 21st century in the field of progressive collapse and robustness of buildings.
Ambient vibration measurements and 3-D nonlinear time-history numerical modeling are used to assess the retrofitting measures conducted in a 6-story unreinforced masonry building (URM) built in the end of the 19th century in Switzerland. Retrofitting measures were taken in order to improve the soundproofing and possibly the seismic performance of the building. Reinforced concrete (RC) footings were added under the walls and horizontal steel beams were added to link the walls together with a RC slab at each floor, though the wooden beams were left in place. Several ambient vibration recordings were performed before, during and after the retrofitting work in order to monitor the evolution of the dynamic behavior of the structure. Moreover, numerical models representing the state of the building before and after the retrofit work have been developed to perform nonlinear dynamic analyses using various ground motion records.
The evaluation of the seismic vulnerability of monumental buildings is a difficult task and presents significantly higher level of complexity if compared to the case of new or current existing structures. This is due to the inherent uncertainty characterizing ancient buildings, regarding structural characteristics and constructive techniques, material properties, damages due to past actions, which should be properly handled in their seismic assessment.