What Is an Eccentrically Braced Frame (EBF)?
An eccentrically braced frame is a seismic lateral system where the braces connect off-center, creating a short beam segment — the 'link' — between the brace and the column (or between two braces). In an earthquake, the link yields in shear, absorbing energy like a structural fuse while the braces and columns stay elastic. EBFs combine the stiffness of braced frames with ductility approaching moment frames — the sophisticated choice for high-seismic steel buildings.
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How the link works
The eccentric connection forces earthquake energy into the short link segment, which is detailed to yield in shear (short links) or flexure (long links). Shear-yielding links are the preferred design: stable, predictable hysteresis with enormous energy dissipation. After the earthquake, the damaged link can be replaced — the rest of the frame is designed to stay elastic. This replaceable-fuse concept is the EBF's signature advantage.
Advantages and trade-offs
High ductility (R=8, matching special moment frames and BRBFs) with the architectural openness of braced frames — the bracing stays out of the way of most openings because the eccentricity creates natural door and window zones. Stiffer than moment frames (less drift), more ductile than concentric braced frames. Trade-offs: the link detailing is exacting (stiffeners, no penetrations or attachments in the link, lateral bracing of the link), and the frame analysis must capture the link's behavior accurately.
Design requirements
AISC 341 governs: link length limits by section (shear links vs. flexural links), stiffener spacing and detailing, bracing of the link against lateral-torsional buckling, and capacity design of everything outside the link (braces, columns, and connections designed for the link's overstrength — the fuse must be the weakest element). Special inspection covers the link detailing strictly — a penetration or attachment in the link voids its qualification.
The bottom line
EBFs use a yielding link beam as a replaceable seismic fuse — R=8 ductility with braced-frame stiffness and openness. Detail links per AISC 341 with no penetrations; capacity-design everything else.
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