Improving performance of reinforced concrete connections using Sigma 8 couplers and normalized threads

This study introduces a novel mechanical splicing system for reinforced concrete (RC) structures, combining a Sigma 8 coupler with a post-threading normalization process to enhance seismic performance and reliability. While mechanical couplers increasingly replace lap splices, residual stresses from cold-forged threading are often overlooked and can reduce ductility and energy dissipation. The proposed system addresses these issues through two innovations: (1) a Sigma 8 (olive-shaped) coupler designed to reduce stress concentrations and enable smoother load transfer, and (2) a normalization process applied after threading to relieve internal stresses and restore mechanical consistency. Experimental testing on spliced bars compared normalized and non-normalized specimens, focusing on tensile strength, ductility, slippage, and failure modes. Non-normalized bars exhibited an average decrease of 12%–15% in yielding-point strength, whereas normalized specimens showed a marked reduction in residual slip. Finite element modeling of RC column-to-foundation joints assessed the structural impact of normalized threads using Concrete Damage Plasticity (CDP) and a tie constraint for the coupler–bar interface. The model captured how thread treatment influences global behavior, including cracking, stiffness, and load transfer. Numerical simulations matched experimental results for normalized specimens, and highlighted a reduction in capacity and yielding strength of 15%–30% in non-treated bars. Results confirm that normalization significantly improves the performance of threaded bars and that the coupler preserves structural integrity under demanding conditions. This system provides a reliable, constructible, and high-performance solution for seismic and high-load applications, offering quantifiable gains in slip control and resilience while addressing limitations in current practice. • Couplers. • Mechanical response to seismic loading. • Reinforced concrete structures. • Columns. • Ductility. • Damping ratio.