NON-LINEAR STATIC PUSHOVER ANALYSIS OF A G+2 STOREY REGULAR RCC BUILDING

Abstract

When the structures are regular and of small height, simple linear static analysis or dynamic analysis (response spectrum analysis) methods are better to get accurate solutions. But as one aims for high-rise structures and irregularity is introduced within the structure, linear static analysis does not yield optimum results. Moreover linear static analysis method assumes that the material property lies within linearity zone i.e. elastic zone. It does not consider the redistribution of moments and concept of plastic zone. Hence the design is conservative. For small regular structures, it is safer to be conservative in analysing and design. But as we go for high-rises, if we are conservative, this will affect the cost of the project and may work out to be significant. Also the fact remains that the linear static analysis will not be able to predict the actual behaviour of the structure. Hence, to overcome this difficulty of analysing the complex behaviour of such reinforced concrete structures, enhanced analysis methods known as non-linear static analysis and non-linear dynamic analysis methods have been developed. By performing analysis using these methods, nowadays engineers can predict the actual behaviour of the structure and make optimum designs. Apart from the above, the devastation caused due to collapse of structures during earthquakes was primarily due to constraints in the linear static and dynamic analysis methods. To overcome this limitation, either a non-linear static or dynamic analysis is desirable. The best and most accurate method for this purpose is the non-linear dynamic analysis as it incorporates the non-linearity and the dynamic effects. But the non-linear dynamic analysis which is better known as the Time History Analysis requires the selection and employment of an appropriate set of ground motions followed by effectively analysing the data to produce ready-to-use results. To perform these activities, the time required for even simple structures will be very high. Hence, due to its simplicity and less time requirement, the structural engineering profession has been utilising the non-linear static analysis procedure which is also known as the pushover analysis. Modelling for such analysis requires the determination of the non-linear properties of each component in the structure, quantified by strength and deformation capacities which depend upon modelling assumptions. Pushover Analysis is carried out for either user-defined nonlinear hinge properties or default hinge properties, available in some programs based on FEMA-356 and ATC-40 guidelines. Many papers/journals provide the hinge properties for several ranges of detailing while the programs may implement averaged values. The user needs to be careful; the misuse of default-hinge properties may lead to unreasonable displacement capacities for existing structures. Plastic hinge length and transverse reinforcement spacing are assumed to be effective parameters in the user-defined hinge properties. These parameters have considerable effects on the displacement capacity of the frames. An increase in the amount of transverse reinforcement improves the displacement capacity. This dissertation aims to evaluate the performance of an existing four storey RCC frame hospital building located in zone–V as per parameters given in ATC-40. Since the Hospital building was constructed more than 50 years ago, the construction was guided by the parameters of older prevalent version of the respective codes i.e. IS1893:1984, IS456:1978, etc. which have now been revised and updated. It was required to check the actual behaviour of the structure and point out its performance level. iv Accordingly, hinge properties were generated for each member. Also as the structure was a low rise structure and the first mode was the dominating mode, hence the lateral load was applied on the structure in pattern similar to the first mode shape. Finally, the results of pushover analysis viz. pushover curves and capacity spectrum were conducted in both orthogonal directions to obtain the respective performance points.