Lessons from Building Damage Patterns During April 25, 2015 Gorkha Earthquake in Nepal

Abstract

The April 25, 2015 Gorkha Earthquake of Magnitude 7.8 in Nepal damaged about seven hundred thousand buildings. The main typologies of buildings in the affected area are stone masonry with mud mortar, some buildings with stone and brick masonry with cement/sand mortar and few reinforced concrete buildings with masonry infill. Among the damaged buildings, about 96% of the buildings were masonry and about 4% reinforced concrete buildings with masonry infill. This study conducted detailed damage assessment of over 150,000 buildings of different type of masonry and reinforced concrete (rc) buildings in Nepal. First, the buildings were classified according to different structural types like adobe, stone in mud, brick in mud, stone in cement, brick in cement, wood, bamboo, rc and others. Other important parameters like type of floors and roofs and occupancy of the buildings were noted before starting the detailed damage assessment of structural elements. Damage to overall building as well as to different structural/non-structural elements was categorized into four different categories mainly overall hazard, structural hazard, non-structural hazard and geotechnical hazard. The damage level to different structural/non-structural elements was assigned from insignificant damage to extreme damage in three categories considering the severity of damage like crack widths, delamination, tilting etc. In addition to the severity of damage, extent of damage to that particular element of different severity was also noted. Each type of damage with different severity was estimated in terms of extent like less than 1/3rd of the total area, 1/3rd–2/3rd and more than 2/3rd. Considering the damage severity and extent, overall damage grade to the building was assigned. Finally, based on the damage grade and extent of damage, recommendation for the building, either to demolish, repair and retrofit or just repair was recommended. This study further analyzes the main type of damage to different categories of the buildings and finds out critical factors to be considered for making them earthquake resistant. Existing traditional earthquake resistant elements like wooden bands and their effectiveness on earthquake safety of masonry buildings are further studied. It is found that, corner separation, diagonal cracking, out of plane failure, in-plane flexural failure and delamination are the main type of damage to masonry buildings while soft-story damage, joint failure, lap splice, columns shear failure, beam failure and infill walls failure are the main types of damages to non-engineered rc-buildings.