Force and Deformation Demands of Bolts in Steel Bolted Bracket Moment Connections

The behavior of pretentioned bolts at bracket-to-column in steel bolted bracket moment connections was investigated through detailed 3D non-linear finite element analysis. Analysis results indicated the significant increase of axial force in the bolt was caused by prying action due to column flange local bending. Tension force in the bolt was noted up to 166% of the initial bolt pretension, which corresponded to 125% of the nominal strength of the bolt. In addition, local deformation at bolt threads resulted in high strain demand at nut-to-column flange, which is the potential hazard of bolt fracture. A subsequent parametric study demonstrated that the strain concentration in the bolt could be shifted away from the critical location to the unthreaded shank by simply adding a 25 mm thick gang washer on the back side of column flange. But the tensile force demand in the bolts could not be reduced with additional gang washer plates. Reducing initial bolt pretension (i.e., sung-tight condition) does not contribute to the reduction in bolt tension force demand as well.