A buckling-restrained brace (BRB) is a structural brace in a building, designed to allow the building to withstand cyclical lateral loadings, typically earthquake-induced loading. It consists of a slender steel core, a concrete casing designed to continuously support the core and prevent buckling under axial compression, and an interface region that prevents undesired interactions between the two. Braced frames that use BRBs – known as buckling-restrained braced frames, or BRBFs – have significant advantages over typical braced frames. [1]
The concept of BRBs was developed in Japan by Nippon Steel at the end of the 1980s [2] and was known by its trademark name of Unbonded Brace. It was first installed in the United States in 1999, in the Plant & Environmental Sciences Building in U.C. Davis. [3] In 2002, both CoreBrace LLC and Star Seismic LLC were incorporated, [4] [5] and began competition with Nippon in the BRB design market. BRB usage is currently accepted, with its design regulated in current standards, throughout the world.
Three major components of a BRB that can be distinguished are its steel core, its bond-preventing layer, and its casing.
The steel core is designed 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. The core consists of a middle length that is designed to yield inelastically in the event of a design-level earthquake and rigid, non-yielding lengths on both ends. The increased cross-sectional area of the non-yielding section ensures that it remains elastic, and thus plasticity is concentrated in the middle part of the steel core. Such configuration provides high confidence in the prediction of the element behavior and failure.
The bond-preventing layer decouples the casing from the core. This allows the steel core to resist the full axial force developed in the bracing, as designed.
The casing – through its flexural rigidity – provides lateral support against the flexural buckling of the core. It 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.
Because BRBs achieve a high level of ductility and stable, repeatable hysteresis loops, BRBs can absorb significant amount of energy during cyclic loadings, such as an earthquake event.
Preventing buckling leads to similar strength and ductile behavior in compression and tension, illustrating the envelope of the hysteresis curves, also referred as a backbone curve. This curve is considered as an important basis of practical design. The beneficial cyclic behavior of the steel material can therefore be extrapolated to an 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 behavior of structures built with BRBs. [6] [7] [8] Depending on the configuration of braces, the building codes in the United States [9] allow the use of a response modification factor up to 8, that is comparable to special moment resisting frames (SMRFs); a higher response modification is associated with greater ductility, and thus enhanced post-yielding performance. 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.
The purpose of buckling-restrained braces is to dissipate lateral forces from columns and beams. Therefore, the connection of the braces to beams and columns can greatly affect the performance of the brace in the case of a seismic event. Typically, the brace is attached to a gusset plate, which in turn is welded to the beam and/or column that the brace will be attached to. Usually three types of connections are used for BRBs:
In addition to the connection type, the details of the connection can also affect the transfer of forces into the brace, and thus its ultimate performance. Typically, the brace design firm will specify the proper connection details along with the brace dimensions.
Comparative studies, as well as completed construction projects, confirm the advantages of buckling-restrained braced frame (BRBF) systems. [10] BRBF systems can be superior to other common dissipative structures with global respect to cost efficiency for the following reasons:
Buckling-restrained braces have energy dissipative behavior that is much improved from that of Special Concentrically Braced Frames (SCBFs). Also, because their behavior factor is higher than that of most other seismic systems (R=8), and the buildings are typically designed with an increased fundamental period, the seismic loads are typically lower. This in turn can lead to a reduction in member (column and beam) sizes, smaller and simpler connections, and smaller foundation demands. Also, BRBs are usually faster to erect than SCBFs, resulting in cost savings to the contractor. Additionally, BRBs can be used in seismic retrofitting. Finally, in the event of an earthquake, since the damage is concentrated over a relatively small area (the brace's yielding core), post-earthquake investigation and replacement is relatively easy. [11]
An independent study concluded that the use of BRBF systems, in lieu of other earthquake systems, produced a savings of up to $5 per square foot. [12]
Buckling restrained braces rely on the ductility of the steel core to dissipate seismic energy. As the steel core yields, the material work-hardens and becomes stiffer. This work hardening can represent increases in the expected force of up to 2x the initial yield force. This increased stiffness decreases the building's period (negating some of the initial increases) and increases the expected spectral acceleration response requiring stronger foundations and connection strengths.
Buckling restrained braces rely on ductility and generally must be replaced after usage during a major earthquake.
Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made structures. Structural engineers also must understand and calculate the stability, strength, rigidity and earthquake-susceptibility of built structures for buildings and nonbuilding structures. The structural designs are integrated with those of other designers such as architects and building services engineer and often supervise the construction of projects by contractors on site. They can also be involved in the design of machinery, medical equipment, and vehicles where structural integrity affects functioning and safety. See glossary of structural engineering.
Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.
In structural engineering, a shear wall is a two-dimensional vertical element of a system that is designed to resist in-plane lateral forces, typically wind and seismic loads.
A tie, strap, tie rod, eyebar, guy-wire, suspension cables, or wire ropes, are examples of linear structural components designed to resist tension. It is the opposite of a strut or column, which is designed to resist compression. Ties may be made of any tension resisting material.
Earthquake engineering is an interdisciplinary branch of engineering that designs and analyzes structures, such as buildings and bridges, with earthquakes in mind. Its overall goal is to make such structures more resistant to earthquakes. An earthquake engineer aims to construct structures that will not be damaged in minor shaking and will avoid serious damage or collapse in a major earthquake. A properly engineered structure does not necessarily have to be extremely strong or expensive. It has to be properly designed to withstand the seismic effects while sustaining an acceptable level of damage.
Steel frame is a building technique with a "skeleton frame" of vertical steel columns and horizontal I-beams, constructed in a rectangular grid to support the floors, roof and walls of a building which are all attached to the frame. The development of this technique made the construction of the skyscraper possible.
In structural engineering, a rigid frame is the load-resisting skeleton constructed with straight or curved members interconnected by mostly rigid connections, which resist movements induced at the joints of members. Its members can take bending moment, shear, and axial loads.
This is an alphabetical list of articles pertaining specifically to structural engineering. For a broad overview of engineering, please see List of engineering topics. For biographies please see List of engineers.
Steel Design, or more specifically, Structural Steel Design, is an area of structural engineering used to design steel structures. These structures include schools, houses, bridges, commercial centers, tall buildings, warehouses, aircraft, ships and stadiums. The design and use of steel frames are commonly employed in the design of steel structures. More advanced structures include steel plates and shells.
In structural engineering, the tube is a system where, to resist lateral loads, a building is designed to act like a hollow cylinder, cantilevered perpendicular to the ground. This system was introduced by Fazlur Rahman Khan while at the architectural firm Skidmore, Owings & Merrill (SOM), in their Chicago office. The first example of the tube's use is the 43-story Khan-designed DeWitt-Chestnut Apartment Building, since renamed Plaza on DeWitt, in Chicago, Illinois, finished in 1966.
A steel plate shear wall (SPSW) consists of steel infill plates bounded by boundary elements.
Buckling-restrained braced frame (BRBF) is a structural steel frame that provides lateral resistance to buckling, particularly during seismic activity.
In construction, cross bracing is a system utilized to reinforce building structures in which diagonal supports intersect. Cross bracing is usually seen with two diagonal supports placed in an X-shaped manner. Under lateral force one brace will be under tension while the other is being compressed. In steel construction, steel cables may be used due to their great resistance to tension. The common uses for cross bracing include bridge (side) supports, along with structural foundations. This method of construction maximizes the weight of the load a structure is able to support. It is a usual application when constructing earthquake-safe buildings.
Cold-formed steel (CFS) is the common term for steel products shaped by cold-working processes carried out near room temperature, such as rolling, pressing, stamping, bending, etc. Stock bars and sheets of cold-rolled steel (CRS) are commonly used in all areas of manufacturing. The terms are opposed to hot-formed steel and hot-rolled steel.
In structural engineering, a braced frame is a structural system designed to resist wind and earthquake forces. Members in a braced frame are not allowed to sway laterally.
The weld access hole or rat hole is a structural engineering technique in which a part of the web of an I-beam or T-beam is cut out at the end or ends of the beam. The hole in the web allows a welder to weld the flange to another part of the structure with a continuous weld the full width on both top and bottom sides of the flange. Without the weld access hole, the middle of the flange would be blocked by the web and inaccessible for welding.
Hybrid masonry is a new type of building system that uses engineered, reinforced masonry to brace frame structures. Typically, hybrid masonry is implemented with concrete masonry panels used to brace steel frame structures. The basic concept is to attach a reinforced concrete masonry panel to a structural steel frame such that some combination of gravity forces, story shears and overturning moments can be transferred to the masonry. The structural engineer can choose from three different types of hybrid masonry and two different reinforcement anchorage types. In conventional steel frame building systems, the vertical force resisting steel frame system is supported in the lateral direction by steel bracing or an equivalent system. When the architectural plans call for concrete masonry walls to be placed within the frame, extra labor is required to ensure the masonry fits around the steel frame. Usually, this placement does not take advantage of the structural properties of the masonry panels. In hybrid masonry, the masonry panels take the place of conventional steel bracing, utilizing the structural properties of reinforced concrete masonry walls.
Moment-resisting frame is a rectilinear assemblage of beams and columns, with the beams rigidly connected to the columns.
The endurance time (ET) method is a dynamic structural analysis procedure for seismic assessment of structures. In this procedure, an intensifying dynamic excitation is used as the loading function. Endurance time method is a time-history based dynamic analysis procedure. An estimate of the structural response at different equivalent seismic intensity levels is obtained in a single response history analysis. This method has applications in seismic assessment of various structural types and in different areas of earthquake engineering.
Pres-Lam is a method of mass engineered timber construction that uses high strength unbonded steel cables or bars to create connections between timber beams and columns or columns and walls and their foundations. As a prestressed structure the steel cables clamp members together creating connections which are stronger and more compact than traditional timber fastening systems. In earthquake zones, the steel cables can be coupled with internal or external steel reinforcing which provide additional strength and energy dissipation creating a damage avoiding structural system.