trong>This article describes analytical studies performed to explain the collapse of the Cypress Street Viaduct. The modes of failure of four types of bents are described. A strength analysis is presented, which gives estimates of the forces that were necessary to produce the failure of these bents. These estimates are computed based on the observed failure mechanism of each type of bent, on measured mechanical characteristics of the materials, and on simplified structural models. The overstrength of these bents, above the level of forces for which they were designed, is evaluated. Levels of acceleration required to produce the failure are given. From comparison of these levels of accelerations with those resulting from the elastic as well as the inelastic response spectra corresponding to ground motions recorded near the Viaduct, it is concluded that the demanded shear strength was considerably higher than the supplied strength and that bent type B8 was the weakest one and where the failure was probably initiated.trong>
