ECONOMICS OF SEISMIC DESIGN FOR REGULAR RCC BUILDINGS

Abstract

The objectives of any analysis and design procedures are safety and economy. To withstand in this competitive world as a designer one should be able to design the structures more economically and in a safe manner. Of course we can achieve safety very easily as there are so many factors we are using for design loads and for material strengths. But if we consider the economics of structure there are so many codal provisions which are confusing the designers. If we follow the codal provisions blindly some times it may lead to uneconomic design of structures. Designer can not survive if he don't have idea about economics of the structural designs. There is big confusion in the designers in the use of ductile detailing of buildings. No doubt there will be increase in cost if we use ductile detailing of buildings as per IS: 13920-1993 code but if IS: 13920-1993 is used then IS: 1893-2002 (part-1) allows us to design for 40% less earthquake loads. So, whether or not it would more economical to design for ductility is not obvious. This study aims to answer this question for common buildings. For this purpose, five buildings designed for construction in 16 Crops Nagrota to house army personnel, were taken an designed as Ordinary Moment Resisting Frame (OMRF) buildings and also as Special (ductile) Moment Resisting Frame (SMRF) buildings. The amount of steel reinforcement and concrete in the two cases were then compared to find the relative economics of a ductile designs vis-à-vis ordinary designs. it was found that in all cases the amount of steel reinforcement and concrete was significantly more for the ductile design. Because this increase was predominantly in the form of increase in size and steel reinforcement in column members, we concluded that the SMRF buildings have greater lateral load carrying capacity than OMRF buildings; even though SMRF buildings were designed for 40% less lateral loads. To find the lateral load capacity of SMRF buildings a virtual work method was devised. It was found that the lateral load capacity of one storey SMRF building was four times that of a similar OMRF building. This building, if designed as an SMRF, would not be required to manifest the ductility it was provided with even if a DBE level earthquake was to occur. Clearly, designing a one storey iii building as an SMRF is unnecessary. It was then designed to study the variation of lateral load capacity with increase in number of stories. It was found that with increase in number of stories the over strength of SMRF design with respect to OMRF designs reduced. For 11-storied building the lateral load capacity of SMRF design was equal to that of OMRF design. From this study we concluded that it would be economical to design vertically regular and plan-symmetric low-rise buildings as OMRFs rather than as SMRFs. In the present work four regular RCC buildings have been analyzed and designed using both IS: 1893-2002 (part-1) and IS: 13920-1993 codal provisions. The variations in concrete and steel quantities are presented. Difference in costs is also tabulated. Variation of response reduction factor value based on the capacity method of analysis, with the increase in the number of stories of building was observed. The performance of one of the buildings (Ironing shed) is checked by using RAM PERFORM-3D software. iv