How Do Buckling-Restrained Braced Frames Survive Big Quakes?

Buckling-restrained braced frame design uses braces with a yielding steel core restrained against buckling by an outer casing, giving stable energy dissipation in both tension and compression. Design covers brace sizing and testing, gusset and connection design for the overstrength forces, and capacity design of beams and columns per AISC 341. The system's genius is its honesty: instead of pretending braces don't buckle, it builds a brace that physically can't. That single change converts the braced frame from a stiff-but-brittle system into one of the most ductile systems in the code.

The governing philosophy is capacity design — the braces are the designated fuses, and everything else must be stronger than the braces at their maximum possible force. The engineer sizes the braces for the design seismic forces, then designs the gussets, beams, and columns for the braces' adjusted strengths, which account for strain hardening and the higher compressive capacity of the restrained core. These overstrength forces are substantially larger than the design forces, and they flow through every connection in the frame. Beam and column design in a BRBF has its own subtleties. The beams in a chevron configuration see large unbalanced vertical forces when one brace yields in tension and the other in compression — the beam must carry that imbalance. Columns accumulate the brace overstrength forces from every floor above, so lower-story columns in a tall BRBF can be surprisingly large. None of this is difficult, but it all has to be done; a BRBF designed with ordinary braced-frame habits will have under-designed connections and columns.

BRBFs are unforgiving of shortcuts — the system's performance depends on the hierarchy of yielding being exactly right. My verification checklist. The non-negotiables in buckling-restrained braced frame design.

• Brace qualification: confirm the selected BRBs are tested and qualified per AISC 341 for the application
• Connection overstrength: design every gusset for the adjusted brace strengths, not the design forces
• Chevron beam unbalanced loads: check beams for the tension-compression imbalance in every configuration
• Column accumulation: carry brace overstrength forces down through every column to the foundation
• Deformation compatibility: verify the frame's expected drifts against cladding, stairs, and nonstructural systems