Experimental and numerical investigation of large-scale truss complex chord joint under multi-axis loading

Truss structural systems have become ubiquitous in long-span roofs and bridge superstructures because they offer an exceptional strength-to-weight ratio. This study addresses the limited understanding of joints in large-scale trusses where the chord depth changes along the span and different web-member shapes converge at a single joint, referred to herein as a large-scale truss complex-chord (LSTCC) joint. Three full-scale LSTCC specimens were tested under the most adverse in-service loading condition, namely synchronous axial compression in all connected members, using a multi-axis loading frame capable of applying load vectors in three dimensions. The influences of chord-transition details and web-member cross-sections (H-section versus box-section) were investigated. The experiments revealed that (i) when an I-section web delivers load through its flanges, shear-dominated failure initiates in the joint at around 1.10 times full-section yield strength; (ii) replacing the web by a box section postpones joint failure to 1.30 times full-section yield strength; and (iii) adopting a smoother chord-transition detail further raises the capacity to 1.33 times full-section yield strength and markedly delays plasticity in the joint plates. A refined finite element model reproduced the observed failure modes and stress histories with high fidelity and was subsequently used for a parametric study. Numerical results demonstrate that gentler transition angles reduce stress concentrations, but eliminating the transition segment shifts plasticity to the chord with no net benefit. Besides, a slight thickness of joint area plates over chord members is sufficient to guarantee joint area integrity. The combined experimental and numerical evidence provides actionable guidelines for the safe and economical design of complex-chord joints in long-span steel trusses.