Direct Answer
Eccentricity in a structural connection is the distance between the line of applied force and the centroid of the bolt group or weld group resisting that force. Eccentric connections create bending (moment) in addition to the primary shear or tension, increasing the demand on the extreme fasteners. Connection eccentricity must be accounted for in connection design and sometimes in detailing geometry.
Full Explanation
Common eccentric conditions: (1) A beam reaction applied at a distance from the supporting column face — the eccentricity creates additional moment on the column web. (2) A bracket welded to a column face with load applied at the bracket's workpoint — the horizontal distance creates a moment on the weld group. AISC Manual Table 8-4 through 8-11 provide elastic method and instantaneous center of rotation method solutions for eccentric weld groups. Eccentric bolt groups are solved with Table 7-6 through 7-13. The detailer must verify that connection geometry (specifically, the distance from the bolt group centroid to the load application point) matches the engineer's design assumptions.
What This Means for Your Shop
If the detailer modifies a connection geometry to avoid a field clearance issue — say, shifting a bolt group 1" from the theoretical position — the eccentricity changes, which may affect the connection's capacity. Any geometric modification to a connection must be reviewed by the engineer.
Common Mistakes
Shifting a shear tab or bolt group without engineering review because 'it's just an inch.' An inch of additional eccentricity on a heavily loaded shear tab can reduce bolt capacity by 5–15%.
Recommended Process
When detailing requires a departure from the connection schedule geometry (to avoid a clearance issue, accommodate a fabrication constraint, etc.), submit an RFI to the engineer identifying the revised geometry and requesting confirmation that it is acceptable.