EUROSTEEL 2008, 3-5 September 2008, Graz, Austria INITIAL STIFFNESS OF FLANGE BOLTED JOINTS AND THEIR INFLUENCE ON THE BEHAVIOUR OF STEEL CHIMNEYS Aleksander Kozlowski a , Andrzej Wojnar a a Rzeszów University of Technology, Civil Engineering Department, Rzeszów, Poland INTRODUCTION Flange bolted joints are the most popular type of connections of steel elements with large diameter circular hollow cross section. Essential feature of such connections is axisymmetric arrangement of the bolts in flange plate. Flange joints are used very often in structures such as chimneys, supports of elevated tanks, wind power plants, etc. The design codes and the analytical methods applied nowadays to calculate bearing capacity of these joints do not include the methods of calculation of their stiffness. Stiffness of flange bolted joints can influence on: - the horizontal displacements of the structures, - the natural period of the structures, - the parameters of vibration damping. Investigation of the influence of the stiffness of flange bolted joints on the natural period, critical wind velocity, number of cycles and the horizontal displacements of the steel chimneys has been presented in the paper. 1 MECHANICAL MODEL OF FLANGE BOLTED JOINT The general view of typical flange bolted joint with external flanges and its mechanical model are presented in Fig. 1. To create this model general rules of component method were used. Behaviour of the joint was modelled by set of individual springs, fixed in the perimeter of circle, which diameter is equal to diameter of structural shell of chimney (Fig. 1). The model consists of two types of components: - components in the tension zone, characterized by their stiffness k t and resistance R t , - components in the compression zone, characterized by their stiffness k c and resistance R c . Characteristics of the components were identified using experimental tests [1] and numerical FEM calculations. Influence of bolts, flange plate, type of welds were taken into account. The initial stiffness of flange bolted joints was calculated using Eq. (1) and the resistance of the joint was calculated according to Eq. (2). Based on the mechanical model of flange bolted joint, the computer software permitting to calculate the initial stiffness and resistance of flange bolted joint was created. More detailed description of the model was presented in [2, 3]. ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ] cos [ ] cos [ , , , , , ∑ ∑ - ⋅ + - ⋅ + + ⋅ + + ⋅ = i 1 2 m s 2 m s c n 1 2 m s 2 m s t ini j a i r 2 a r k a n r 2 a r k S δ δ (1)       ⋅ + + ⋅ + + = a N M M a N M M M Ed el R t c Ed t el R c el R j . . . . . . min (2) where: k t , k c – the initial stiffness of tension and compression component; M t,R,el , M c,R,el – the resultant bendig moment due to forces on tension, compression side of connection, calculated taking into consideration the resistance of tension, compression component; M t , M c – the bending moment on tension, compression side of connection, corresponding to bending moment M c,R,el , M t,R,el .