Probabilistic capacity and seismic demand models and fragility estimates for reinforced concrete buildings based on three-dimensional analyses

• Build seismic demand and capacity models for RC buildings based on 3D responses.
• Find the traditional 2D fragility analysis significantly underestimates the fragility of RC buildings.
• Analyze the factors affecting the seismic fragility of RC buildings.

This paper presents bivariate fragility estimates for reinforced concrete (RC) buildings accounting for their three-dimensional (3D) response to earthquake ground motions conditioning on spectral accelerations in the two horizontal directions. The fragility estimates are conducted using the demand and capacity models typically for the 3D responses. The demand models expressed in terms of drift are developed as functions of the spectral accelerations in the two horizontal directions. The demand prediction is compared in a probabilistic framework with the capacity estimates. The proposed capacity models for five performance levels consider the strength and stiffness degradation under the bi-axial loading. The proposed approach for the fragility estimates considers the uncertainties involved in the spectral acceleration components and capacity variation. The proposed approach is illustrated considering a typical 3-story RC building and results are compared with those from a traditional two-dimensional approach. The results indicate that the two-dimensional approach tends to significantly underestimate the fragility.