Keywords: bolted joint, metallic structure, bending stiffness, dynamic parameters, finite element model, experimental test.

Bolted joints are usually used to form connections between metallic components in buildings and machines. They can be pre-manufactured in the welding workshop without defects in the welding and then the beam components can be assembled with bolts in situ. Traditionally bolted joints are considered to be rigid elements in structural calculations; however for some of them it would be closer to reality to consider them as semi-rigid.

In the present paper, the bending stiffness of three joints was measured by means of flexural tests, and the results were compared to those calculated from a three-dimensional modelling of the joints. The types of modelled joints are: two kinds of beam-column joint and a column-column joint. The results of bending stiffness from the first and second test have similar values. The experimental values corresponding to the third test are higher than the theoretical ones. This is likely to be due to the existence of tolerances in the beams, columns and plates arising from the hot rolling manufacturing process.

In second step, the behaviour of a metallic structure has been modelled both by means of a two-dimensional rigid frame model (with rigid joints) and with a semi-rigid one. In the former case, the specific bending stiffness parameters estimated from the finite element model (semi-rigid model) of the joints were used. As it is well known, the natural frequencies of vibration of a structure depend strongly on the stiffness of the joints. Therefore in dynamic analyses of machines and buildings, a semi-rigid model appears to give more reliable results than a classical rigid one, where bolted joints are considered as rigid elements. In the structure analyzed using SAP 2000, the natural vibration frequencies calculated with the semi-rigid model are consistently lower than those obtained with the rigid one. The relative differences are of about 20%, 15% and 5% for the first, second and third natural frequencies, respectively. These values indicate that the influence of semi-rigid joints on the vibrational behaviour is more important for the lower natural modes.

The results reported in this paper show that the stiffness behaviour of a bolted joint can be accurately estimated by means of a three-dimensional finite element model, and that it is of paramount importance to take into account the stiffness behaviour of the joints in the dynamical analysis of structures.

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