STORY BASED DUCTILITY MODELS FOR DISPLACEMENT BASED. DESIGN OF STEEL FRAMES

Mohammad Safi

Department of Civil Engineering, Power & Water University of Technology, Tehran, P. O. Box 16765-1719, Iran,

E-mail: msafi@pwit. ac. ir

Abstract

Estimation of ductility demand distribution through the height of the structure is a very hard task for seismic design engineers working on performance based design of buildings. In this paper a modified direct displacement based design procedure has been proposed. In this method the design force distribution among the height of the structure is obtained based on various ductility demand distributions derived from modal characteristics of the structure and mathematical formulations. The method has been applied to the moment steel frames in low, medium and high rise buildings and the results of various ductility distributions have been compared. The plastic mechanism has also been modeled and the efficiencies and deficiencies of each have been discussed through various numerical examples. The effect of yield mechanisms and ductility demand patterns for various building types on the equivalent SDOF parameters have been investigated compared to the time history analysis results to find the sensitive parameters.

Introduction

The purpose of Performance Based Design is to design the structure with sufficient and proportioned stiffness and strength in the structural members so as to develop inelastic action in the ductile designed members and to have appropriate over strength in the brittle members. Then the structure must be checked so that the demands do not exceed the existing capacities. This is best performed using a set of nonlinear dynamic analyses under earthquake with appropriate characters. Different design methods have been proposed based on performance criteria such as, Capacity Spectrum Method [1,2,3], N2 Method [4,5], Energy Based Methods and Displacement Based Design (DBD) Methods. In the last four decades the idea of DBD has been introduced and developed by different researchers started by introducing the concept of substitute structure [6]. This idea has been adopted for a direct displacement design of SDOF and MDOF reinforced concrete bridges [7,8,9]. Capacity Spectrum Method and the N2 Method have also been used to create other direct DBD procedures [1,2,3,4,9,10]. In all of these researches, seismic demand is specified as either a displacement spectrum or an acceleration-displacement response spectrum. Generally nonlinear inelastic behavior of a structural system can be accounted for either by an equivalent elastic response spectrum or an inelastic response spectrum. The former is associated with effective viscous damping and the latter is directly constructed based on relations between reduction factors and ductility.

In this paper the direct DBD method is briefly reviewed for multi story steel buildings. In this method the design force distribution among the height of the structure is obtained based on various ductility demand distributions derived from modal characteristics of the structure and mathematical formulations. The method has been applied to the steel braced frames with concentric and eccentric bracing systems in low, medium and high rise buildings. The plastic mechanism for each system has also been modeled and the efficiencies and deficiencies of each have been discussed through various numerical examples. The effect of yield mechanisms and ductility demand patterns for various

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M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 355-365.

© 2006 Springer. Printed in the Netherlands.

building types on the equivalent SDOF parameters have been investigated compared to the time history analysis results to find the sensitive parameters. A design displacement spectrum has also been created for the parametric study based on the Iran earthquakes. Various factors affecting the dynamic response have also been provided in the procedure. It has been shown that this method is capable of predicting the response of braced frames especially high rise buildings in an efficient and robust manner.