Seismic analysis, behavior, and retrofit of nonductile reinforced concrete frame buildings with viscoelastic dampers

Many reinforced concrete (RC)) frame buildings constructed in the United States during the 1950s through 1970s were designed for gravity loads only using the non-seismic design provisions of the ACI-318 building code and ACI-315 detailing manual. The seismic performance of these RC frame buildings is unsatisfactory with brittle (non-ductile) failure modes that are controlled by poor reinforcing details including reinforcement with inadequate anchorage details, and joints and critical regions with light confinement and inadequate shear reinforcement. In particular, the columns of these non-ductile frame buildings have inadequate strength and ductility. Therefore, these buildings require retrofit to provide good seismic performance.; The use of viscoelastic (VE) dampers to improve the seismic performance of non-ductile RC frame buildings is investigated in this research. Three prototype non-ductile RC frame buildings are studied: a five-bay by five-bay three-story building, an eleven-bay by three-bay seven-story building, and a five-bay by five-bay twelve-story building. Properly designed VE dampers increase the damping in these buildings, resulting in less lateral drift, base shear, and overturning moment during earthquake loading. Two constitutive models for VE materials with frequency and temperature dependence are investigated: the generalized fractional derivative model and the generalized Maxwell model. A two-node VE damper finite element based on the generalized Maxwell model and implemented using the Newmark-β integration scheme is developed. The element was implemented into the PC-ANSR computer program for use in rigorous nonlinear time history analyses of structures with VE dampers. The advantages of the generalized Maxwell model over the generalized fractional derivative model are: an unconditionally stable numerical integration scheme, an accurate temperature rise model, less computational effort, and easy implementation. A simplified preliminary linear analysis approach is also developed for use in the design of structures with VE dampers. Both the rigorous and simplified analysis approaches are verified with comparisons to experimental test results.; Retrofit of non-ductile RC frame buildings using VE material with very high damping properties is hindered by the unfavorable sensitivity of the stiffness of these VE materials. An extensive parametric study was conducted using the simplified linear analysis approach to investigate the effect of VE material characteristics (damping and temperature sensitivity) on seismic behavior of non-ductile RC frame buildings retrofit with VE dampers. A series of nonlinear time history analyses of the prototype buildings at different temperatures were also performed using a total of eight earthquake ground motions. It was found that dampers that use VE materials with high damping and a stiffness that is highly sensitive to temperature change may not perform effectively over a range of temperatures that are anticipated in a building. Instead, VE materials with lower damping and less temperature sensitivity are shown to be more effective. A range of VE material properties desirable for the seismic retrofit of non-ductile RC frame buildings is suggested.