Assessment of Potential Seismic Damage to Residential Unreinforced Masonry Buildings in Northern Illinois

Ground acceleration estimates for northern Illinois are used to assess any damage that can occur to unreinforced masonry (URM) residential structures as a result of potential future earthquakes. These estimates are based on probable future activities in the New Madrid Seismic Zone and are published by the U.S. Geological Survey for exceedance probabilities equal to 0.10, 0.05, and 0.02 per 50 years. Accelerations corresponding to higher exceedance probabilities (1.0 and 0.25 probability in 50 years) are obtained from simplified seismic hazard analyses. Damage is estimated based on fragility curves available for URM buildings. The probability of damage is then used along with the inventory of buildings to arrive at estimates of the percentage and the number of buildings that may suffer damage of various levels (ranging from nonstructural damage to total collapse). The study shows that considering p = 1 exceedance probability in 50 years (i.e., a 50 year return period), the maximum acceleration for the area will be about 0.017g. This acceleration causes slight damage to about 1% of buildings and nonstructural damage to about 4% of them. With an acceleration of 0.057g, about 1% of URM buildings will experience severe damage, 5% moderate damage, 17% slight damage, and 53% nonstructural damage. This acceleration is based on p=0.1 exceedance probability in 50 years. A very rare event at p=0.05 exceedance probability in 50 years corresponds to 0.101g acceleration and may cause collapse in about 1.5% of buildings, severe damage to 4%, moderate damage to 16%, slight damage to 42%, and nonstructural damage to 84% of URM buildings.