Study Of Staircase Effect On Seismic Performance Of Multistoried Frame Structure

Abstract

In RC frame buildings, there are mainly two structural systems, Primary structural system and secondary structural system. The primary structural system to resist lateral load are beams and columns. Besides, primary structural system, some elements also contribute to lateral load resistance. These elements fall in the category of secondary systems. Secondary system can be structural secondary like staircase, structural partition etc and non-structural secondary like storage tanks, machinery etc A special case of structural secondary members which are normally designed for non seismic force are concrete staircase. There exist a large number of reinforced concrete (RC) buildings that are gravity load designed and constructed in actual seismic areas. Many of these structures were constructed in areas that are not considered seismic at the construction time or although they were located in seismic areas at that time, the earlier codes did not include seismic provisions or may have specified lower levels of seismic loads. Due to the high cost of replacement, many old structures are still in service far beyond their design life. Besides, gravity load designed structures may perform in a no-ductile manner with dangerous modes of failure. Before the 1980’s the design of the structure, both in seismic and in non seismic area, did not consider the presence of the stair, although the stair offers a higher strength and stiffness influencing considerably the distribution of seismic forces. It is well known that the stair could be a vulnerable part of the structure attracting the seismic action; in the meanwhile its stiffness could preserve the structure from collapse if it was adequately designed and built. If the stair is not well designed it can lead the structure to collapse, in particular if only gravity loads are considered into the design or the reinforcement detailing is not adequate.