Static Axial–Flexural Behavior of RC Beam-End Plastic Hinges Under Spatial Frame Effects

Abstract

Reinforced concrete (RC) beams undergo axial elongation during inelastic flexural deformation. In frame structures, this elongation is restrained by adjacent columns and slabs, inducing axial compression and beam overstrength, thereby amplifying force demands on columns and joints. Although previous studies have examined beam elongation restraint in planar frames and slab participation in isolated beam–column subassemblies, the combined restraint provided by columns and continuous floor slabs in spatial frame systems remains insufficient. This study conducted static vertical loading tests on twelve restrained RC beams within a three-dimensional frame system. The beams were detailed to reduce the mid-span flexural resistance, allowing the beam-end plastic hinges to contribute predominantly to the response. Compared with the unrestrained beams, the restrained frame beams developed more pronounced flexural–shear crack patterns in the beam-end plastic hinge regions and were more prone to premature concrete crushing before tensile rebar yielding under stronger spatial restraint. Higher spatial restraint also limited the tensile strain development of slab rebars, reducing the effective overhanging flange width. The restraint-induced axial compression ratios ranged from 0.13 to 0.46, and the combined effect of axial compression and slab contribution increased the beam-end flexural strength by 57% to 225%. Compared with the tensile contribution of slab rebars, restraint-induced axial compression was the dominant source of strength enhancement, accounting for 64% to 86% of the total enhancement. These findings highlight the need to consider spatial restraint in RC beam design.