Modelling the collapse of a masonry tower during the 2015 Nepal earthquake with support from a video-camera recording

Abstract

During the April 25, 2015 Nepal earthquake, the Dharahara Tower, a 60 m high historical brick masonry structure, located in Kathmandu collapsed completely. The tower was initially built in 1832, having suffered a previous failure at mid-height in 1934. The 2015 collapse event was previously analyzed by various authors using numerical models, focusing on the likelihood of a bending failure, as it had happened during the 1934 earthquake. However, a video camera recording recently found is not consistent with this mode of failure, as it shows the top of the tower descending vertically more than 10 m, before the initiation of overturning. In the present paper, a discrete element model is used to analyze the potential collapse modes. The model, based on the mechanical properties obtained in experiments on similar local building materials, was subject to the seismic records measured in the accelerometric station closest to the site. The new numerical model shows that a different type of mechanism is possible, triggered by compressive failure at the base and progressing by shearing along an inclined plane through the lower part of the structure. The results are consistent with the video images and also with the geometry of the tower segment remaining at the site. Parametric analyses were performed to investigate the role of a long period pulse present in the seismic records on the observed damage and collapse.