Abstract: Experimental results of eight full-scale beam-to-column connections are presented. Two dimensional finite element fracture models are used to simulate the fracture behaviors of the connections which are commonly applied in steel moment resisting frames. Mode I elastic stress intensity factor (K_I) and inelastic J-integral (J_I) at the crack tip are calculated to provide quantitative estimates of fracture toughness demands of the connections with different design details, such as weld access hole types, initial flaw size & locations, weld backing bars, fillet reinforcement and other configurations. The elastic analyses indicate that the measure of removing the backing bar and reinforcing a fillet weld, is effective to reduce the toughness demand. The inelastic analyses show that the initial crack at weld heat affected zone (HAZ) causes larger toughness demands than the crack at column flange and weld interface when the weld is over matched. The analyses have confirmed the observation in former connection tests that the fractures are more likely to occur at the beam bottom flange weld HAZ area where extensive plastic strains accumulate resulting in low material toughness. The evaluated moments by fracture model analyses agree well with the ultimate moments obtained by the connection tests. This study provides valuable references for the fracture resistant design of beam-to-column connections in steel moment resisting frames.