A design procedure for tied braced frames

Abstract

The paper deals with the analysis of the seismic behaviour and design of tied braced frames (TBFs). The behavioural properties of TBFs are described and a comparison drawn with standard eccentrically braced frames. A design procedure is then proposed that aims to achieve optimal collapse seismic behaviour, i.e. a global collapse mechanism characterized by uniform plastic rotations of links. The procedure is based on the displacement-based approach so as to achieve direct and efficient control of the peak ground acceleration of collapse. Applications are carried out on systems with different numbers of storeys and lengths of links to obtain confirmation of the accuracy of the design hypotheses and methodologies. Popov et al. [4], aware of the possibility of such deficiencies in the seismic behaviour of traditional eccentrically braced frames (EBFs), recommended the adoption of link strength values close to the internal forces obtained by modal analysis. However, despite the effectiveness of the proposal in a general sense, the seismic response of high-rise EBFs is often still far from being satisfactory [1,2].In order to produce a more intense and reliable participation of links in the global inelastic behaviour of eccentrically braced structures, other researchers have suggested the modification of the traditional scheme of EBFs by means of the addition of vertical ties that connect the corresponding ends of links between floors to each other, e.g. see [5]. However, at the moment no design procedure exists, which guarantees the objectives of the capacity design in tied braced structures.The purpose of this paper is to analyse the behavioural potentiality of tied braced structures and, in particular, to propose a design procedure that complies with capacity design principles. The design procedure aims at limiting the inelastic deformations to links and ensuring almost complete exploitation of the dissipative capacity of all links. OBSERVATIONS ON THE SEISMIC BEHAVIOR OF TIED BRACED FRAMESFrom the structural point of view tied braced frames (TBFs) may be conceived as constituted by braced sub-structures coupled by means of horizontal elements (links) generally undergoing shear and flexure [4]. As the flexural resistance at the base of the braced sub-structures is either virtually null or null, tied braced systems derive the capacity to resist overturning moments from the coupling action of links. Links are generally endowed with high plastic rotation capacity as well as stable and large hysteresis loops (see, e.g. [6-15]).If capacity design principles are fulfilled, the presence of ties certainly significantly improves the seismic behaviour of eccentrically braced structures. Indeed, yielding of one or more links does not cause large concentrations of plastic rotations upon successive increments in the global deformation. Unlike traditional EBFs, tied braced structures develop such behaviour because their storey stiffness is supported by the axial stiffness of braces, columns and ties rather than by the shear and f...

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