How Do Buckling-Restrained Braced Frames BRBF Yield in Tension and Compression Without Buckling

Structural steel design engineers and earthquake mitigation specialists design high-ductility, resilient seismic force-resisting systems for mid-rise and high-rise commercial buildings by installing Buckling-Restrained Braces (BRB) within structural steel framing bays, conforming to AISC 341 seismic provisions approved by the Standards Organisation of Nigeria (SON).

In conventional concentric braced frames (CBF), diagonal structural steel tube or wide-flange braces provide high lateral stiffness; however, under severe cyclic seismic loading, conventional braces buckle elastically or inelastically in compression long before their full yield strength is mobilized. After compressive buckling, subsequent tension cycles experience pinched hysteresis loops, rapid strength degradation, and catastrophic fracture at mid-span gusset connections.

Buckling-Restrained Braced Frames (BRBF) eliminate compressive buckling completely by decoupling the axial load-resisting function from the buckling-restraining mechanism. A BRB consists of a ductile, low-yield structural steel core plate (such as ASTM A36 or LYP225 steel) enclosed inside a continuous outer steel casing tube filled with high-strength cementitious mortar or concrete grout.

Crucially, an unbonding material or microscopic slip-layer gap (silicone or polymer tape) separates the inner steel core from the surrounding mortar. Under intense earthquake shaking, the inner steel core plate is completely free to expand and contract axially. When subjected to severe compressive forces, the heavy outer mortared casing provides lateral confinement that physically prevents the core plate from buckling, forcing the steel core to yield plastically in compression exactly as it does in tension, delivering full, symmetric, non-degrading hysteretic energy dissipation loops.

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