Welded joints are often employed in high-rise buildings and grid-shell structural systems but on-site welding of cut steel tubes for spatially complex frame systems is problematic, time-consuming, and difficult to ensure quality. Innovative prefabrication combining topology optimization (TO) using the Solid Isotropic Material Penalization Method (SIMP) and additive manufacturing (AM) provides an innovative solution for materially efficient mass production of repetitive joints. Optimized AM joints can be bolted to other members on-site, which promotes construction efficiency, and overcomes the barriers of on-site welding. In this paper, tensile tests are conducted to assess the material properties of mild and stainless steel materials used in different manufacturing processes. Welded and 3D-printed joints using different materials are then investigated to maximize the mechanical properties by taking full advantage of AM and TO. A customized non-contact 3D-DIC (Digital Image Correlation) technique based on an open-source tool is applied to analyze and visualize the strain distribution in planar tubular and topologically optimized joints. The AM joint exhibits a uniform stress distribution which avoids stress concentrations and the innovative configuration of the optimized joint has good energy absorption resulting in the protection of the central core region of the joint.