Buckling Resistant Braced Frames

Technology Triumphs (Modern Steel Construction, September 2005) describes the use of "buckling restrained braces" in steel frame construction. These composite steel members are used as diagonal, lateral force resisting members in a steel braced frame. In comparison to conventional steel members, BRBs have significantly greater energy absorbing capacity and result in a structure better able to resist extreme seismic events.

BRBs are comprised of three main parts:
1) An inner steel member acts as the primary loadbearing component. This element is intended to resist the axial forces--both tension and compression--that are generated in the brace when the structure is subjected to lateral forces.

2) An outer steel tube surrounds the inner member.

3) Mortar fills the space between the outer tube and inner member. A coating on the inner member prevents bonding between the inner member and the mortar so that the inner member remains free to slide within the mortar.

Under the influence of lateral forces, a building frame naturally distorts, with columns tending toward out of plumb, and normally rectangular column/beam bays becoming parallelogram shapes. In the case of a braced frame structure, this distortion is resisted by  diagonal braces within the frame which experience axial tension or compression under these conditions. Under the extreme loading  that may accompany a major seismic event, such braces may be subjected to repeated cycles of stress reversals, well beyond the elastic limits of the material.

In the case of conventional steel bracing, the ability of the bracing members to to resist seismic forces under such conditions is limited by the tendency of the member to fail in buckling. Once buckling occurs, the member's load resisting capacity becomes compromised, and it may no longer be able to effectively resist additional stress cycles.

In contrast, with buckling restrained braces--as the name suggestions--the combined effect of the outer steel tube and the mortar infill is to restrain the brace against buckling failure. Under high compressive forces, the BRB will yield plastically, but not undergo gross geometric instability. In this way the brace remains intact and capable of absorbing additional cycles of stresses.

First developed in Japan, BRBs are gaining acceptance in North America as well. In comparison to conventional structural steel bracing members, the major benefit of using BRBs in a braced frame are its greater strength and energy absorbing capacity with less weight. Additional benefits may include simplification of bracing member connections, and reduced foundation loads.

More Info
Seismic framing technology and smart siting aid a California community college (Tech Brief, Architecture Record, 08.05) describes the use of buckling restrained braces in the San Bernadino Vallue College of California.