AbstractDue to a lack of test data on large-diameter headed bars, design provisions limit the bar diameter of headed bars to 36 mm or less. In exterior beam–column joints with two-layer headed bars as beam bars, if the layer-to-layer spacing of the headed bars is narrow, the side-face blowout area of an individual headed bar is overlapped. Existing design provisions do not consider the effects of layer-to-layer spacing. In this study, 34 simulated exterior beam–column joints were tested to examine the effects of bar diameters over the limitation of existing design provisions using large-diameter (41- and 51-mm) headed bars. The side-face blowout strengths of the two-layer headed bars were experimentally investigated through simulated exterior beam–column joints with layer-to-layer spacing of 2db (two times the bar diameter). The test results showed that the side-face blowout strengths had the same characteristics for bar diameters from 22 to 51 mm. For the two-layer headed bars, crack propagation and failure mode were the same as those for single-layer headed bars, but due to the overlap of the blowout area, the strength of the individual headed bar was reduced. A new model was proposed to predict the side-face blowout strengths of large-diameter high-strength headed bars in both a single layer and in two layers by incorporating a spacing factor for an individual headed bar into a model from the literature. A total of 173 test results from this study and previous studies were compared with predictions from the proposed model, and the average and coefficient of variation (COV) of test-to-prediction ratios were 1.08 and 17.4 %, respectively.