PERFORMANCE BASED SEISMIC DESIGN OF STEEL FRAME BUILDING USING ENERGY BALANCE CRITERION

Abstract

The most fundamental problem in carrying out Performance Based Seismic Design (PBSD) is to grasp the nature of seismic loading on building structures. In this context, energy concept has been considered as the principal loading for seismic input and evaluation of capacity. For the present study, energy balance criterion for performance based seismic design is presented. Such criterion is promising for development of cumulative damage indices under seismic loading. The study presented herein, aims at investigation of performance based seismic design of steel building frameworks using energy based evaluation. Energy based seismic design is based on the principle of balancing the input seismic energy through the energy absorbing/dissipating capacity of the structure. Using the energy balance equation, the amount of various energy capacities is quantified. Initial input seismic energy is consumed by the structure as elastic strain energy; a part of input seismic energy is dissipated as viscous damping energy, while the structure is elastic. Kinetic energy of the mass, along with elastic strain energy constitutes vibration energy. At the end of earthquake ground motion, this energy gets dissipated as viscous energy. Identification and quantification of inelastic energy in severe earthquake ground motions has been subject of the recent state of art, since earthquake resistant design allows damages for larger ground motions, because meeting corresponding demand through strength is not economically viable as well as higher strength does not warranty the better performance at the same time larger earthquake is rare to occur during the design life of a structure. In the study, the issue of performance based seismic design is reassessed for multi-performance objectives. The existing performance objective formats are discretized events on the possible damage spectrum under varying earthquake ground motions in space and time. Recent trend of research in this regard accepts the potentiality of energy based design as a better approach for further formulation and development of design aids, which are close to damages even better than displacement approach. Thus, energy based seismic design is viewed as an effective design approach for assessment of performance evaluation and further formulation of PBSD for new design decisions. Nonlinear static pushover analyses are popular for performance evaluation using force/displacement controlled/modal procedures. The aim of nonlinear static pushover analysis is to estimate the displacement in spite of the earthquake load imposes reversal of stresses due to its simplicity in comparison to the nonlinear dynamic loading. Input seismic energy has been investigated using the present state of art. An 111 algorithm of energy based capacity curve has been developed using the conventional pushover analysis procedures through some example building frames evaluated under varying earthquake ground motions. Energy based capacity curve is the plot of energy along ordinate and displacement along abscissa. Sequence of hinge formations are directly recorded through the variation of energy slope of energy capacity curve. Simplification of energy based capacity curve results into better damage assessment. Plotting conventional base shear and the corresponding energy capacity curve on the same ordinate and displacement on abscissa shows a clear relationship corresponding to various performance levels. Such a capacity curve for a particular structure is an index for its use under varying earthquake ground motions for evaluating capacity in terms of energy. Knowing the actual displacement, energy capacity may be evaluated using the developed capacity curve. In developing energy based design approach and assessing the damage potential of structures, distribution of input seismic energy among its components: strain energy, kinetic energy, damping energy and hysteretic energy, are required. This study is focused on the accuracy for input seismic energy evaluation and further distribution of the energy among its components in order to formulate some pattern of damage pattern as required for the performance assessment, during performance based seismic design. The present performance levels are disctretized and are used in such way that one performance level has no relation with the other, however all performance levels are inherently associated with the each other. During this study it has been demonstrated through energy based relations under varying demands are related with each other in a definite fashion, which provide the support that the damage spectrum are continuous, and not discretized. In this regard a relation between elastic strain energy and inelastic energy has been established using suitable assumptions. Elastic strain energy and the inelastic strain energy (hysteretic energy) have close relationship, since they represent the internal configuration of structure. A simple algorithm in between these two energy parameters has been developed as the content of this research program. Hysteretic energy, which is the outcome of the energy dissipated through yielding, depends upon the size and number of loops. Cumulative hysteretic energy under reversal of stresses due to varying earthquake ground motions, which is the major task of performance based design evaluation that can be viewed through the elastic strain energy since hysteretic energy is closely related. Further, damage indices available in the literature are reassessed and extended using the energy response parameters as obtained during this research program. Interpretation of