Side-Face Blowout Strength of Two-Layer Headed Bars Embedded in Exterior Beam-Column Joints

The side-face blowout area of two-layer headed bars in an exterior beam-column joint could be overlapped by each headed bar and, therefore, the anchorage strength of the individual headed bar could be reduced. However; ACI 318-19 and previous studies did not consider the effects of layer-to-layer spacing. To evaluate the anchorage strength of two-layer headed bars in the exterior beam-column joint, a simulated exterior beam-column joint test was conducted with 43 mm (No. 14) headed bars of 550MPa (80 ksi) of design yield strength. The effects of layer-to-layer spacing, embedment length, and side cover on the anchorage strength of two-layer headed bars were experimentally estimated. Degradations of the anchorage strength due to overlapping failure area are very similar to the degradation of the side-face blowout strength of the group anchor subjected to tension, and the side-face blowout strength of two-layer headed bars can be predicted by using a spacing factor: A design equation for the development length of two-layer headed bars is proposed by applying the spacing factor developed in this study into a design equation for the single-headed bar developed from a previous study.