Experimental and analytical assessment of the influence of masonry fa?ade infills on seismic behavior of RC frame buildings

On May 11, 2011 at 6:47p.m., an M5.1 earthquake took place in Lorca at a depth of 4 km, in the Region of Murcia, south of Spain, which resulted in nine fatalities and 324 injuries and major built environment damage. Approximately 80% of the residential buildings were affected with various degrees of damage, 5% with severe structural damage and 13% with moderate structural damage, in addition to the damage of infrastructure and public buildings. Reconnaissance and expert analyses revealed that one of the main causes of damage during the Lorca earthquake was the interaction between the non-structural elements and the seismic-resisting framing of the buildings. Studies on this interaction, that started in the 1950 s, led to the development of a wide range of macro-models that simulate the stiffness of the masonry infills using one or more equivalent diagonal strut(s). Following an extensive literature review, 12 mathematical expressions have been compiled to estimate the horizontal stiffness of the equivalent strut that simulates the behavior of masonry infills that make up a typical fa?ade of many modern buildings. Two full-scale tests under horizontal cyclic displacement reversals have been carried out on two 5x3m concrete frames: one frame with masonry infill simulating a regular fa?ade; and the other is a bare control frame. The comparison of both tests enabled quantifying the stiffness contribution of the fa?ade infill and identifying the simplified expression that best fits the horizontal stiffness. Using this expression, a realistic seismic design of a prototype building in Lorca, Spain, is conducted under the typical assumption of most modern building codes to consider masonry infills as non-structural elements, and consequently the designed building was seismically assessed considering the influence of the infills. Results of the assessment are in accordance with the real damage patterns observed during the Lorca earthquake reconnaissance.