Seismic performance of reinforced concrete frames with nominal ductility

Abstract

<p>An analytical and experimental study is undertaken in this thesis to evaluate the seismic performance of Nominally Ductile Moment Resisting Frames (NDMRFs) designed to conform to the current Canadian provisions. Buildings of different heights and typical configurations are considered. The frames of such buildings are designed as nominally ductile frames for the combined gravity and seismic loading in accordance with the National Building Code of Canada, NBCC 1990. The designed frames are then analyzed statically under monotonically increasing static lateral loading to study their force-deformation characteristics and order of hinge formation among members. Following the static analysis, the frames are analyzed dynamically under earthquake excitation. Several deficiencies are observed in the seismic performance of NDMRFs designed strictly following the minimum Code requirements. First, the interstorey drifts are substantial which implies excessive non-structural damage. Second, the beam ductility demands are much larger than what is implied in the Code. Third, the beam shear demands can exceed the capacity of the provided stirrups. An experimental investigation is carried out to evaluate the reliability of the concrete shear resistance when the beams are subjected to ductility demands similar to those obtained in the dynamic analysis. To ensure no beam shear failure in this type of frame, a procedure is proposed to modify the factored design shears in the beams of NDMRFs to be more reflective of the demand shears. The modified design shears calculated based on the proposed procedure are shown to be in good agreement with the response values. Therefore, shear failure in the beams of NDMRFs can be avoided if the capacity of the provided stirrups is based on the modified design shears with the concrete contribution to the beam shear resistance neglected. Finally, an alternative design procedure, based partially on the capacity design philosophy, is proposed for the design of gravity-dominated frames in seismic regions. Reinforced concrete frame design using the proposed procedure can avoid many of the deficiencies observed in the seismic performance of NDMRFs designed strictly following the minimum Code requirements, and is therefore a preferred design procedure to enhance seismic performance. (Abstract shortened by UMI.)</p>