Direct Answer
An RBS connection, also called a 'dogbone,' is a prequalified seismic moment connection where a portion of the beam flange is intentionally cut away (reduced) near the column face. This weakens the beam at a controlled location, ensuring that the plastic hinge (ductile yielding) forms in the beam away from the column during a seismic event — protecting the more critical column and weld region from inelastic demand.
Full Explanation
AISC 358 Chapter 5 defines RBS geometry: a circular radius cut is made in both flanges at a specific distance from the column face. The reduction is typically 40–50% of the flange width. After the RBS cut, the beam is shop-welded to the column flange with CJP welds at the full flange width (not the reduced section). The RBS cut must be smooth — rough edges create stress concentrations that can initiate cracks. CNC plasma or router cutting produces the required smooth profile.
What This Means for Your Shop
RBS connections require: CNC-quality cuts (rough torch-cut RBS profiles fail inspection), CJP flange welds with D1.8 compliance, and UT inspection of the CJP welds. The RBS geometry must be precisely located — too close to the column or too far changes the plastic hinge location.
Common Mistakes
Cutting the RBS with a manual torch rather than a CNC plasma. AWS D1.1 and D1.8 require that the finished surface meet roughness requirements that manual cutting cannot consistently achieve.
Recommended Process
Specify CNC plasma or router cutting for all RBS profiles. After cutting, check surface roughness. Grind any rough areas to meet the AWS surface roughness requirements. Inspect and document before proceeding with CJP welding.