Introduction to Buckling Restrained Brace Frames

In nearly all structures made with metals such as steel, the capacity of the columns and braces is governed by the buckling capacity of these elements. This results in an underutilization and inefficient use of the materials. There is a great economical interest in developing compression members (e.g. columns and braces) that would fail by yielding of the materials rather than buckling at a fraction of the yielding capacity!

Braces are frequently used in construction of steel frames. These are frequently made of steel profiles in various shapes such as angles, tubes, wide flanges, etc. As the building is subjected to lateral loads, these braces resist compression or tension loads depending on the direction of the lateral load. It is well known that the capacity of a steel brace is much higher in tension than in compression; in the latter case, buckling of the brace limits the capacity of the brace much sooner than the steel reaches its yield strength.

Braced frames are also an effective solution for limiting lateral displacement of building stories. Regardless of the arrangement of braces in braced frames (diagonal, chevron, etc.), the overall strength and stability of the lateral-force resisting system depends mainly on the performance of the structural braces. The buckling restrained brace frame (BRBF) is a highly ductile seismic-force resisting system intended primarily for special seismic applications. The principal advantage of the buckling restrained brace is that the brace does not buckle, so the brace strength is similar under compression and tension loading, which leads to significantly lighter framing members especially when compared to special concentric braced frames (SCBF). Another advantage of the buckling restrained brace frame is that the brace connections are relatively small and compact in comparison to the connections or special concentric braced frames.

Flat steel plates and/or bar materials are used to create a unique configuration that is made up of a yielding steel core made from steel plate or bar as the load resisting element. The yielding steel core is confined against buckling between steel web plates welded to two steel flange plates in an "I" shape configuration. To limit the deformation of the steel core the web plates are placed in close proximity to the steel core, with only a very nominal gap provided by natural unevenness of the steel material. Additional friction reducing material, a liner or a thin coating may be applied to the steel core contact surfaces and to the surrounding web members to reduce friction and facilitate movement of the steel core Specialized manufacturing equipment is utilized including automatic computerized plate cutting technology and automatic submerged arc welding equipment to effectively fabricate the brace. With the exception of a small weld or bolt located at mid-length to secure the core to the webs, the yielding steel core is not connected directly to the restraining elements in order to allow for independent movement of the load resisting core relative to the restraining brace elements.

The state of the art buckling restrained braces (BRB) currently available are designed primarily for high rise buildings and other structures where large lateral loads are involved, most commonly to resist lateral earthquake loads. Current state of the art buckling-restrained braces utilize conventional hot roll shapes, usually HSS tubes or pipe filled with mortar, concrete, or other non-compressible filler material to restrain the load resisting steel core against buckling. When conventional structural braces are subject to high axial forces the braces may reach various forms of local and global buckling that can lead to reduced strength and stiffness, and degraded performance, even collapse, especially under cyclic loading resulting from an earthquake. In contrast to conventional braces, the buckling-restrained brace exhibits stable and predictable behavior under cyclic loading. With these braces the impact of an earthquake can be absorbed or reduced, and the frame lateral displacement reduced to an acceptable level. The principle difference is in the unique arrangement of elements of the buckling-restrained brace assembly that will allow plastic deformation of its inner core while at the same time prevent buckling within the member or its end connections. Consequently, the continuously braced inner core element will elongate or compress during loading cycles and the brace will achieve nearly equal strength and stiffness under axial compression and tension loading.

To assure the above described behavior, the brace assembly must allow for free movement of the inner core with respect to the restraining apparatus along the brace length. This relative movement can be facilitated with a variety of friction reducing materials or coatings, or an air-gap

The concept of Buckling Restrained Brace (BRB) was introduced in the late 1990s (Lopes and Sabelli 2004; Uang and Nakashima 2003). Buckling Restrained Braced Frames (BRBF) offer an alternative to conventional braced frames which surpasses their energy dissipation capacity and thus is cost-effective. Conventional steel bracing element shows unsymmetrical behavior under cyclic loading as it is shown below: on one hand, it is characterized by high ductility in tension due to the ductile yielding material characteristics; on the other hand, its performance is governed by buckling under compression. The stability problem influences the overall cyclic response of the element, reflected by the cyclic degradation. Excluding the buckling phenomenon - that is the basic idea of BRB - leads to a balanced, extremely ductile and dissipative cyclic behavior as illustrated below.

There are three major components that can be distinguished in the cross-section: 1. steel core 2. bond-preventing layer and 3. casing As known to those skilled in the art, when a BRB is loaded in compression, as soon as the steel core buckles and moves laterally, it encounters the steel casing which stops this lateral movement. This forces the steel core to take more loads and buckle in the next higher mode of buckling. This process continues until finally the cross section reaches its compressive yield strength.