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

Abstract

The performance based seismic design (PBSD) aims at satisfying the specified performance objectives corresponding to the different levels of seismic hazard, while moving away from the concept of a single design (or, operating) basis earthquake motion The most fundamental problem of PBSD is to grasp the nature of seismic loading and performance of building structures. In this context, energy concept has been considered as the principal loading performance evaluation. Identification and quantification of demand (seismic input) and capacity (output) under the earthquake loading in terms of energy components have undergone critical reappraisal in the recent past due to robustness of the approach and evolution of computational facilities. The study presented herein, aims at investigation of performance based seismic design of steel building frameworks using energy based evaluation. 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. Quantification of inelastic energy during severe earthquake ground motions has been subject of the recent know how techniques, 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 in dynamic loading. Input seismic energy has been investigated using the present state of art. An 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. Sequence of hinge formations are directly recorded through the variation of energy slope of energy capacity curve. 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. 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 energy 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 uponthe 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. Further, damage indices available in the literature are reassessed and extended using the energy response parameters as obtained during this study. Interpretation of the normalized cumulative energy and the cumulative ductility are useful tools for present performance based seismic design evaluation. Number of yield excursion cycles during severe seismic demands is investigated for the critical members using the time history IV analysis results. Such a number of yield excursion cycles under reversal of stresses are employed for performance evaluation for the successive ground motions Usage ratio is the basic attribute used for correlation of the component level performance at the global performance through extension of performance limit states. Steel building fames have been taken from the existing literature in order to investigate the objectives of the present study. For the performance evaluation of these building frameworks, a series of Nonlinear static pushover and Nonlinear time history analyses have been carried out in RAM Perform 3D. Detailed procedures of modeling for nonlinear analysis as desirable for performance evaluations are followed, using guidelines of FEMA-273, FEMA-356 and FEMA-350 and the background ofRAM Perform 3D. A strategy in advance can be made for developing path for the consumption of input seismic energy under earthquake ground motions. With the understanding, how much energy is consumed and what amount of input seismic energy is left for dissipation is possible through counter balancing the input and output total seismic energy. Using active and passive damping devices along with the base isolation is attractive for controlling the severe earthquake ground motions. Strain energy, whether it is elastic strain energy or inelastic strain energy is internal energy. Any change to these two energy is responsible for damage to the structure.A relation in between strain energy and cumulative inelastic energy developed in this research program has been investigated and found they have close relations. Number for loop and the number of yield excursion cycles are accessible using hysteretic and elastic strain energy relation. Severity of damage may be identified and quantified through the number of hysteretic loops, number of yield excursion cycles and the normalized hysteretic energy. Intended life of a building structure under earthquakes may be predicted using the residual energy capacity after successive earthquakes; however, such issues require further investigations.