Stiffness characterisation of lattice joints with CHS chords and longitudinal branch plates

In braced frames and lattice structures, the connection between longitudinal branch plates and circular hollow section (CHS) members is a key detailing issue. While these joints often exhibit low stiffness leading to significant deformations of the tube wall, current design provisions, such as Eurocode 3 Part 1–8 and CIDECT guidelines, address only the plastic resistance providing no specific rules for estimating their initial stiffness. In order to fill this gap, this paper proposes analytical formulations to predict the initial stiffness of CHS-to-longitudinal plate T-joints. Experimental results available in the literature are first used to validate a finite element (FE) model for both T- and X-joint configurations with longitudinal branch plates. The validated model is then employed in a parametric study on 65 different configurations subjected to monotonic tension and compression. On this basis, two semi-empirical design equations are derived from a closed-form solution obtained using Clapeyron’s theorem and are calibrated and verified against the numerical database. Both formulations provide accurate stiffness predictions, with mean predicted-to-numerical ratios close to 1.0 and coefficients of variation of about 7–8%, while one equation offers a particularly simple form suitable for design practice.