IMPACT OF INCREASED CODE DESIGN BASE SHEAR ON SEISMIC BEHAVIOR OF REINFORCED CONCRETE MOMENT FRAME BUILDINGS WITH LONG PERIODS

摘要

Building Codes generally increase design seismic loads in an attempt to address a number of issues on the seismic design of buildings. For buildings having long vibration periods, the increase of design base shear force could have significant impacts on the design of lateral bracing system of buildings. Given the constraints lacing design engineers regarding member sizes relative to practical user limits, these increased forces may result in poorer, not better, seismic performance. This paper describes the impact of higher design lateral loads on the design of reinforced concrete moment-resisting frame buildings, with an emphasis on buildings with long periods. A building located in Southern California is analyzed using UBC 94 and 97 seismic base shear forces as an example to demonstrate the impacts. The design of the frame beam, the beam-column joint, and the frame column based on UBC 94 and 97 are compared and the impacts on the design are highlighted. The significance of axial load demand on the behavior and design of frame column is particularly emphasized. Several methods of estimating the axial load demand of frame columns are described and the results are compared. The results emphasize that the goal of improving seismic performance in buildings by increasing the design base shear may not be achieved in reinforced concrete buildings with long periods if the engineers simply react to the higher lateral load demand by increasing the reinforcing contents of the members.