Seismic Braced Frames Design Concepts and Connections

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.

The Structural Design of Tall and Special Buildings, 2013

The collapse probability of ductile and non-ductile concentrically braced frames was investigated using nonlinear dynamic response analysis. Two buildings with three and nine stories located in Boston and Los Angeles, respectively, were designed and subjected to ground motions from the areas. In Boston area, three-story and nine-story buildings were designed as ordinary concentrically braced frame with response modification reduction factor R equal to 3 1/4 to be considered as non-ductile structural systems; comparatively, in Los Angeles area, three-story and nine-story buildings were designed as special concentrically braced frame with response modification reduction factor R equal to 6 to be considered as ductile structural systems. In order to evaluate the performance of ductile and non-ductile concentrically braced frames in moderate and severe seismic regions, ATC-63 would be used as reference to assess the seismic behaviors. Evaluation approach recommended by ATC-63 was adopted, and hundreds of nonlinear dynamic analyses were performed. Through alternating the scale factors of designated ground motions, median of structural collapse intensity was presented for each structure. By observing the results of statistical performance assessment, the seismic performance of the systems was evaluated, and some observations are made based on the study.

Buckling restrained bracing systems are effectively being used to reduce seismic response during recent years. But often configurations of these systems are ignored while evaluating their behavior. Aim of this study is to consider the effect of different bracing patterns on the behavior of Buckling Restrained Braced Frames (BRBFs). Multi-storey braced frames with different patterns are modelled. The main variables of this study are the bracing pattern. The effects of bracing patterns on seismic behavior of buckling restrained braces are also considered. The best bracing pattern for a specific bay width and storey height in terms of seismic response are suggested finally.

2011

Concentrically braced frames (CBFs) are stiff, strong structures that are suitable for resisting large lateral loads. Special CBFs (SCBF) are used for seismic design and are designed and detailed to sustain relatively large inelastic deformations without significant deterioration in resistance. Current AISC Seismic Design Provisions aim to ensure the brace sustains the required inelastic action, but recent research showed that current SCBF design requirements lead to variable seismic performance, unintended failure modes, and limited deformation capacity. To improve the seismic response of SCBFs, a balanced design procedure was proposed. The premise of the design methodology is to balance the primary yield mechanism, brace buckling and yielding, with other, complementary ductile yielding mechanisms, such as gusset plate yielding. This balance process maximizes ductile yielding in the frame thereby maximizing the drift capacity of the frame. Further, the undesirable failure modes are balanced with the yield mechanisms and the preferred failure mode, brace fracture, to ensure that the frame fails in the desired manner. To achieve the objectives of the design methodology namely maximum drift capacity, and adherence to a desired yield and failure hierarchy, rational resistance checks and appropriate balance factors (β factors) are used to balance each yield mechanism and failure mode. These factors were developed, validated, and refined using the measured results from an extensive test program. An SCBF connection design example to illustrate the application of the balanced design method and to demonstrate differences from the current AISC design method is presented in an appendix.

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This paper highlights research being conducted to identify ground motion and structural characteristics that control the response of concentrically braced frames, and to identify improved design procedures and code provisions. The focus of this paper is on the seismic response of three and six story concentrically braced frames utilizing buckling-restrained braces. A brief discussion is provided regarding the mechanical properties of such braces and the benefit of their use. Results of detailed nonlinear dynamic analyses are then examined for specific cases as well as statistically for several suites of ground motions to characterize the effect on key response parameters of various structural configurations and proportions. _________ 1.

2012

Special concentrically braced frames (SCBF) are stiff, strong and economical lateral-load resisting systems, which can sustain large inelastic deformation if properly detailed. Historically, experimental research on the seismic response of braced frame research has focused on the cyclic and monotonic responses of isolated components, such as braces or gusset plate connections. However, these components do not work in isolation, and recent research shows that accurate evaluation of their seismic performance requires consideration of the complete system. A collaborative research program with investigators from National Center for Research on Earthquake Engineering (NCREE) in Taiwan, and the Universities of Washington (UW), California, and Minnesota was undertaken to investigate the full system response of SCBFs. The research results presented herein focus on two three-story SCBFs that were tested at the NCREE laboratory. The specimens evaluated a new design approach for midspan gusset plate connections. The two specimens had HSS or wide-flange braces in combination with framing members and connections typical of those used in a three-story building in regions of high seismicity. Composite, concrete slabs were placed on each story. The tests were designed using a recently proposed design method to balance the desired yield mechanisms and form yield hierarchy. The results demonstrate that multi-story SCBFs exhibit good inelastic seismic performance with proper design detailing. Together with prior test results, the test specimens advanced design recommendations for SCBFs, which result in thinner, more compact corner gusset plate connections, a rational method of dimensioning mid-span gusset plates, and a balanced-design procedure for enhanced ductility.

This Steel Technical Information and Product Services (Steel TIPS) report provides information and technologies on seismic behavior and design of base plates in concentrically braced frames. These base plates are primarily subjected to axial load of downward compression and uplift in combination with shear force. Normally, compared to axial load and shear force, bending moments in these base plates are small and ignored in design. Chapter 1 is an introduction to base plates in ordinary and special concentrically braced frames and outlines the seismic design and detailing issues. Chapter 2 provides information on the performance of base plates in concentrically braced frames during past earthquakes as well as the tests conducted in laboratories. Chapter 3 discusses provisions of the AISC Seismic Provisions (2005) that are applicable to base plates in ordinary (OCBF) and special concentrically (SCBF) braced frames. It also discusses seismic and gravity design issues in terms of strength, stiffness, and ductility under downward compression and uplift. Chapter 4 provides economical and efficient detailing suggestions for base plates in concentrically braced frames.

Earthquake Engineering & Structural Dynamics, 2020

Buckling-restrained braces (BRBs) have proven to be very effective in improving the seismic performance of existing and new structures. They provide strength, stiffness and add energy dissipation to the structure. However, being BRBs characterized by a low post-elastic stiffness, their use may lead to residual deformations hindering the building's reparability and to excessive cumulative ductility demand possibly compromising the residual capacity of BRBs. To overcome these drawbacks, BRB frames (BRBFs) can be coupled with moment-resisting frames (MRFs) to form dual systems. If properly designed, MRFs acting as backup frames allow the control of the residual drifts and the optimization of the performance of the BRBs. The contribution of this study is to provide insights into the performance and residual capacity of BRBFs-MRFs dual systems and to shed light on the influence of the main BRB's design parameters. To this end, a nondimensional formulation of the equation of motion is introduced for a single degree of freedom system, and an extensive parametric study is performed for a set of natural ground motion records with different characteristics and scaled to various intensity levels. This allows the investigation of a wide range of configurations, considering different levels of the relative strength and ductility demand of BRBFs and MRFs, and obtains useful information for their design. Finally, two case study frames, modeled as two-dimensional nonlinear multi-degree of freedom systems, are analyzed, and the results were compared to those obtained from the nondimensional formulation to show the capabilities and the limitations of the adopted methodology and of the SDOF approximation.

Earthquake Engineering & Structural Dynamics, 2000

This paper is concerned with the design of steel frames using friction damped slotted bolted connections (SBCs) in the diagonal braces. A dynamic model is developed to describe the behaviour of a single-degree-offreedom (SDOF) steel frame that uses bilinear hysteretic behaviour for the damper. This model is generalized to MDOF systems. A novel algorithm for displacement reversal in the transition from slip to stick is presented. It uses numerical noise for its success. A design procedure that attains the sti!ness of the individual braces and their elongation at the threshold of activation is then applied to a 10-storey steel frame. This design process is a two-phase iterative procedure that converges quite fast.

Engineering Structures, 2003

Some results are highlighted in this paper from a research effort being undertaken to identify ground motion and structural characteristics that control the earthquake response of concentrically braced steel frames and to identify improved design procedures and code provisions. The focus of this paper is on the seismic response of three and six story concentrically braced frames utilizing buckling-restrained braces. A brief discussion is provided regarding the mechanical properties of such braces and the benefits of their use. Results of detailed nonlinear dynamic analyses are then examined for specific cases as well as statistically for several suites of ground motions in order to characterize the effect on key response parameters of various structural configurations and proportions. 