Modal analysis of a riveted lattice beam for improved retrofitting operations

Wrought‐iron and mild‐steel riveted lattice beams are well‐represented in the 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. During retrofitting operations, those beams are often reinforced to prevent them from buckling. However, the assessment of the buckling risk depends strongly on the modelling assumptions regarding the out‐of‐plane rotational stiffness of the riveted joints. This paper therefore presents an experimental and numerical investigation aiming at quantifying this joint stiffness. A 3m‐long riveted lattice beam, dating from 1930, was collected from a demolition site. An experimental modal analysis was carried out and the measured natural frequencies and mode shapes were used to calibrate a finite‐element model of the beam. A parametric study suggests that the model with clamped connections, i.e. with high out‐of‐plane rotational stiffness, is in better agreement with the experimental results. This result could lead to a less conservative buckling risk assessment and less extensive strengthening measures.