Assessment of the rotational stiffness of single-riveted joints in a steel lattice girder by modal analysis

Wrought-iron and mild-steel riveted lattice girders are common structural elements in the French metallic construction heritage of 1850–1930. Many rafters and purlins of roof structures of train sheds or market halls are made of flat and angle bars assembled with single-riveted joints. The assessment of the mechanical properties of those joints is key to sensitive and targeted retrofitting operations. Despite its influence on the buckling risk of lattice girders, the out-of-plane rotational stiffness of the joints has not been investigated yet. To fill this gap, experimental and numerical investigations based on modal analysis techniques were carried out on a riveted steel lattice girder dating from 1930. The measured natural frequencies and mode shapes were used to update a finite-element model of the girder. The results suggest that the single-riveted joints exhibit a rigid behaviour in the out-of-plane direction. When numerically varying the rotational joint stiffness, two indicators seem to be relevant, one based on the study of mode shapes, and the other one linked to the presence of additional numerical modes compared to the experimental measurements. Furthermore, a numerical study was conducted to assess the potential use of the findings to develop a modal-based damage detection method for riveted joints. With only one measurement point on each lattice web member, an indicator named ΔECOMAC, derived from the enhanced coordinate modal assurance criterion (ECOMAC) proposed in the literature, appears capable of detecting and localising one or several joints presenting a loss of stiffness. Overall, this paper shows that experimental modal analysis can be an efficient tool for assessing single-riveted joints in old steel lattice girders. •Single-riveted joints are typical of riveted lattice girders.•The out-of-plane rotational stiffness of these joints needs to be assessed.•Two modal indicators were proposed to assess the average joint stiffness.•Healthy riveted joints exhibit a rigid behaviour for out-of-plane rotation.•A modal indicator was developed to detect and localise damaged riveted joints.