Influence of joint load-bearing behavior on the system-level response of reinforced-concrete truss-like beams: topology optimization basis

Abstract

Structural optimization can reduce material use and embodied carbon in reinforced concrete (RC) beams, but the removal of concrete creates truss-like load paths in which local joint behavior may strongly influence stiffness, force redistribution, and failure development. This study investigates the mechanical behavior of joints in reinforced concrete truss-like beams and evaluates how different joint geometries influence the transfer of moment, axial force, and shear between connected members. The research combines experimental testing, calibrated nonlinear finite element analysis (FEA), and isolated joint simulations. Two beam configurations, namely, Warren truss with 45° diagonals (W45) and Warren truss with verticals and 45° diagonals (W45-V), were tested under three-point bending and used to calibrate the numerical models. The calibrated FEA approach was then extended to four truss-like configurations: W45, Warren truss with 60° diagonals (W60), W45-V, and Pratt truss with 45° diagonals (P45). In addition, 66 isolated joint models were analyzed using unit rotation, axial displacement, and shear displacement to quantify moment, axial-force, and shear transfer between connected members. The results show that the investigated joints behave nonlinearly and asymmetrically, with clear stiffness changes after cracking. None of the joints behaves as an ideal hinge; instead, all configurations transfer moments, axial forces, and shear through semi-rigid joint action. W60 shows a stiffer and more direct force-transfer mechanism, W45-V provides improved redistribution through vertical members, and P45 exhibits the strongest directional dependence. Overall, member-only verification is insufficient, and simplified design should include semi-rigid joint behavior and local joint verification.