Buckling restrained brace

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Buckling restrained braces (BRBs) are composed of a slender steel core continuously supported by a concrete casing in order to prevent buckling under axial compression. The core and the casing are decoupled to prevent interaction between them.

The concept of BRBs was developed in Japan at the end of the 1980s. It appeared in the United States after the Northridge earthquake in 1994 and it is now accepted with its design regulated in current standards as a displacement dependent lateral load resisting solution. As earthquake awareness among engineers is enhanced by evolving European standards, the need for economical solutions providing adequate resistance for new structures is now increasing in Europe and precipitating the use of BRBs there.

Components[edit]

Three major components can be distinguished in the cross-section:

  • steel core
  • bond-preventing layer
  • casing

The bond-preventing layer decouples the casing from the core. Accordingly, the axial load of the brace is transmitted by the steel core only, while the casing - through its flexural rigidity – provides the proper lateral support against flexural buckling of the core.

The steel core is to resist the full axial force developed in the bracing. Its cross-sectional area can be significantly lower than that of regular braces since its performance is not limited by buckling. Along its length the core can be divided into three parts: the middle, so-called yielding length and the rigid, non-yielding parts on both ends. Increased – and typically stiffened – cross-section of the non-yielding part ensures that it remains elastic, and thus plasticity is concentrated in the middle part of the steel core. Such configuration provides high confidence in prediction of the element behaviour and failure.

The casing is typically made of concrete filled steel tubes. The design criterion for the casing is to provide adequate lateral restraint (i.e. rigidity) against the steel core buckling.

Characteristics of buckling restrained braces[edit]

As indicated by the achieved ductility and stable, repeatable hysteresis loops, BRB can absorb significant amount of energy during cyclic loadings, such as an earthquake event.

It can be observed that preventing buckling leads to similar strength and ductile behaviour in compression and tension, illustrating the envelope of the hysteresis curves, also referred as backbone curve. This curve is considered as an important basis of practical design.

In accordance with the above observations, the beneficial cyclic behaviour of the steel material is extrapolated to element level and thus to the overall structural level: an extremely dissipative structure can be designed using BRBs.

Experimental results prove the ductile, stable and repeatable hysteretic behaviour of structures built with BRBs. Depending on the configuration of braces, the building codes (e.g. AISC) in the United States allow the use of a behaviour factor (response modification factor) up to 8, that is comparable to special moment resisting frames. Thus, the seismic load applied to the structure is efficiently reduced, which results in smaller cross sections for the beams and columns of the braced frames, smaller demands on the connections and, most importantly, the loads on the foundation are drastically decreased.

Connections[edit]

Usually three kinds of connections are used for BRBs:

  • welded connection
  • pinned connection
  • bolted connection

Advantages[edit]

Comparative studies (e.g. Dasse report and Star Seismic Europe report) as well as completed construction projects confirm the advantages of Buckling Restrained Braced Frame (BRBF) system. BRBF can be superior to other common dissipative structures with global respect to cost efficiency due to the following reasons:

  • superior ductile and energy dissipative behaviour,
  • low seismic loads (due to high behaviour factor and usually increased fundamental period),
  • easy-to-control structural behaviour,
  • smaller member sizes (columns, beams),
  • smaller and simpler connections,
  • larger efficient plan area of the building, which also increases the real estate value,
  • lower demand on foundations – specially, the arising tension loads are drastically decreased,
  • easy and fast erection providing significant time savings,
  • easy to adopt in seismic retrofitting,
  • easy post-earthquake investigation and replacement if needed.

Reference structures[edit]

References[edit]

  • S. Hussain, P. V. Benschoten, M. A. Satari, S. Lin: Buckling Restrained Braced Frame Structures: Analysis, Design and Approvals Issues
  • Merritt, S., Uang, Ch.M., Benzoni, G., Subassemblage testing of Star Seismic buckling-restrained braces, Test report, University of California, San Diego, 2003.
  • AISC 341-05: Seismic Provisions for Structural Steel Buildings
  • L. Calado, J. M. Proenca, A. Panao, E. Nsieri, A. Rutenberg, R. Levy: Prohitech WP5, Innovative materials and techniques, Buckling Restrained Braces
  • Dasse Design Inc.: Cost Advantages of Buckling Restrained Braced Frame Buildings. San Francisco, 2009.
  • EN1998-1:2005, Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings, CEN.
  • European standard EN 15129:2009: Anti-seismic devices
  • L. Dunai: Type testing of Buckling Restrained Braces according to EN 15129 – EWC800 – Final report, 2011.
  • Bonessio, N., Lomiento, G., Benzoni, G., (2011). "An experimental model of buckling restrained braces for multi-performance optimum design". Seismic Isolation and Protection Systems, Vol. 2, No. 1, pp. 75–90. doi:10.2140/siaps.2011.2.75

External links[edit]