Effects of Vertical Irregularities on Seismic Behavior of Building Structures

Irregular buildings make up a large portion of the urban infrastructure. The presence of irregularities can be due to architectural, functional, and economical constraints. The main objective of this research is to improve the understanding of the seismic behavior of building structures with vertical irregularities. This is done by quantifying the effects of vertical irregularities in mass, stiffness, or strength on seismic demands. The seismic demands investigated are elastic story shears, overturning moments, and drifts as well as inelastic drift and energy dissipation demands. Furthermore, a procedure is developed for estimating the seismic drift demands for structures with such irregularities.

The effects of vertical irregularities in the distributions of mass, stiffness, and strength are considered separately and in combinations. The seismic response of irregular structures is assessed by means of elastic and inelastic dynamic analysis. A base case structure is developed, and cases that represent irregular structures are defined by modifying the vertical distribution of mass, stiffness or strength of the base case. Structures are represented by simple single-bay frame models in order to facilitate interpretation of the response and provide insight into the dynamic behavior. Seismic demands are evaluated in a statistical manner, based on the response of the structural models to a set of 15 ground motions. Ground motions recorded on rock or firm soils are used.

The results of the research reveal that the seismic response of building structures is more sensitive to stiffness irregularities than to mass irregularities of similar magnitudes. Strength irregularities have much larger effects on the seismic response than mass or stiffness irregularities of similar magnitudes. Even small changes in story shear strengths may lead to a significant change in seismic demands. The findings of this research emphasize the importance of implementing a design methodology in which inelastic deformation demands and capacities are explicitly incorporated in the design process.