Seismic Braced Frames Design Concepts and Connections

Seismic design of steel braced frames in the modern building codes follow the capacity design approach where some of the members are obliged to dissipate energy whereas others are taken care to be protected. In this paper the seismic design methodologies used by European and American approaches for Concentric Cross Braced Frames (CCBF) and Eccentric Braced Frames (EBF) are highlighted. Synoptic tables for the design of such frames of the most advance seismic codes i.e., Eurocode 8 and the seismic provisions of American Institute of Steel Construction (AISC) are provided. Emphasizes are made on the provisions of Eurocode 8 both for CCBF and EBF. Finally, a general conclusion is drawn when dealing with CCBF and EBF.

Journal of Constructional Steel Research, 2008

Special concentrically braced frames (SCBFs) are commonly used lateral-load resisting systems in seismic design. In SCBFs, the braces are connected to the beams and columns by gusset plate connections, and inelastic deformation is developed through tensile yielding and inelastic post-buckling deformation of the brace. Recent experimental research has indicated that the seismic performance of SCBFs can be improved by designing the SCBF gusset plate connections with direct consideration of the seismic deformation demands and by permitting yielding in the gusset plate at select performance levels.

WIT Transactions on the Built Environment, 2005

Steel concentrically braced frames (CBFs) are stiff and strong and are an economical method to control damage during small, frequent seismic events. During severe, infrequent earthquakes, CBFs develop inelastic deformation through axial tensile yielding and post-buckling deformation of the brace. The inelastic deformation demands of the brace places severe force and deformation demands on the gusset plate connections. As such, the frame response depends on the connection response as well. To better understand the frame response of CBF systems, an analytical and experimental research study into the seismic performance of CBFs with an emphasis on better understanding the response and design of gusset plate connections is underway. The research examines the yield mechanisms and failure modes encountered in the design of gusset plate connections. To achieve a more balanced design which better distributes yielding throughout the system, a design procedure that provides economical design and good seismic performance by balancing the yield mechanisms and failure modes is proposed. Experimental research to evaluate the proposed procedures is underway. This experimental program is described, and initial test results are summarised. Complementary theoretical studies are used to establish mathematical models to estimate the connection properties and to perform parametric studies to better understand the influence of the connection on the frame response.

Buckling-Restrained Braced Frames (BRBFs) are a new steel seismic-load-resisting system that has found use in the western United States because of its efficiency and its promise of seismic performance far superior to that of conventional braced frames. The system is not yet addressed in the 2005 edition of the AISC Seismic Provisions for Structural Steel Buildings, but nevertheless a set of design provisions has been developed by AISC in conjunction with the Structural Engineer's Association of California. This report illustrates the seismic design of buckling-restrained braced frames; they are defined, and the provisions governing their design and required testing are explained. A summary of selected Buckling-Restrained Brace (BRB) testing performed to date is provided. Compliance with design requirements is explained through detailed component design of two typical BRBF configurations and development of testing protocols. A discussion of gusset-plate design and its influence on acceptable frame behavior is provided.

2004

Seismic response of braced frames largely depends on the response of braces. In high seismic zones in the United States, two systems are currently being used; special concentrically braced frames (SCBF) and concentrically braced frames with buckling restrained braces (BRCBF). Braces in SCBF systems develop their inelastic action through compressive buckling and tension yielding. Buckling restrained braces (BRB) are encased in an unbonded, stiff material that restricts brace buckling and they develop their ductility through compression and tension yielding. In both types of systems gusset plate connections are used to connect the brace to the framing elements. These different systems place different demands on the connection by the brace depends on the brace type. Braces in SCBF systems place large axial and out-ofplane rotational demands on the brace as a result of brace buckling. The stiffness of BRB can result in large in-plane moment and axial force demands. The connection design must account for these different demands.

Earthquake Engineering & Structural Dynamics, 2007

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.

Journal of Constructional Steel Research, 1990

This paper describes the mode of application of the two usual design philosophies--elastic and plastic--to braced frames with semi-rigid connections.

2013

He has performed extensive research on the seismic performance of steel and composite structures, and his research has focused on the seismic performance and rehabilitation of braced frames for the past several years. He is a member of AISC, Structural Engineering Institute (SEI), and Earthquake Engineering Research Institute (EERI), and serves on the ASCE 41 and AISC 358 standards committees. About the Review Panel The contributions of the three review panelists for this publication are gratefully acknowledged.

2013

A growing number of buildings in the U.S. are using buckling-restrained braces as the primary seismic lateral force-resisting system. At the University of California, Berkeley, buckling-restrained Unbonded Braces™ have already been used for both retrofit and new construction. Recently, as part of the validation of the design of buckling-restrained braced frames for a major new laboratory building at the campus, tests of large-scale braced frame subassemblies were performed. Three frame subassemblies, with Unbonded Braces™ in one chevron and two single-diagonal configurations, were subjected to design-level and beyond design-level cyclic lateral loading. The tests showed good behavior of the braces, and the results indicated a number of important considerations for the design of buckling-restrained braced frames and also of braced frames in general. The paper describes the laboratory building project, the design of the test subassemblies, and presents an overview of the findings from the test program and subsequent analyses.