Design of steel structures with semi-rigid connections

This dissertation examines the behaviour of semi-rigid joints and its purpose is twofold: it intends to provide a reason for their existence, and examine their effects on the structural response of steel structures of which these joints form part. The chosen approach is hinged on six chapters. The first chapter deals with the different definitions and concepts that are related not only to semi-rigid joints but also to all types of joints in general. Following this, after the joints are classified by various criteria such as their rotational stiffness and resistance, certain conclusions are presented as far as the large variety of choices that these joints afford in their application. The second chapter in this dissertation refers to the structural properties of joints. The structural properties of joints are used in the interpretation of their response. These properties may be expressed as a function of the bending moment M and the rotation φ to a greater or lesser extend depending on each specific property. The M-φ relationship is depicted by the curve M-φ that is the main topic of the third chapter. The ex-ante specification of the M-φ curve is of paramount importance for the reliable analysis of steel structures that involve semi-rigid joints. This type of evaluation is a basic prerequisite for the correct modelling of the joints. At the same time, the significance of joint modelling in combination with modern analytical methods, play a pivotal role in the development of a very extensive worldwide research activity whose ultimate goal is the improvement of steel structure design. The methods used in this ex-ante specification of the Μ - φ curve are based not only on models but also on laboratory experiments and are part of the third chapter of the present dissertation. The relationship between the semi-rigid joints and the steel frames of which these joints are part forms the fourth chapter in this dissertation. The bending deformation of the joints affects the structural response of the frames, as it “lowers” the bending moment diagram of the beams and increases the critical buckling length of the beams and columns. In addition, the bending deformation has an adverse effect on the lateral stability of the frames, making them more flexible. The contribution of the joints can be of greater or lesser importance and that is in direct relationship to the type of frame of which these joints are part. It is also related to the type of analysis that is being implemented. This chapter thus deals with the methodology that inter-relates the frame classification, the methods of their analysis and the types of joints. The fifth chapter of the dissertation presents numerical applications from the analysis and design of steel structures. Using an example from the field, a set of three frames whose only difference is the type of joint used in them is analyzed and designed. Three types of joints are used in this example: rigid, semi-rigid and hinged. After the analysis and design is complete, the double-sided semi-rigid joint in the first storey of the corresponding frame is isolated and the analytical procedure for determining the rotational stiffness, the design moment resistance and the classification according to the Annex J of Eurocode 3 is performed. Following this, a fine meshed three-dimensional finite element model is developed to describe this specific joint. Based on this model, a series of parametric analyses is conducted. Finally, a comparison between the Eurocode 3 - Annex J predictions and finite element model results is undertaken, before the presentation of general conclusions on the positive influence of the semi-rigid joints on reducing the cost of the structures. The fifth chapter can be treated as the core of this dissertation as its contribution is twofold: a) evaluation of Eurocode 3 predictions with respect to the calculation of the moment resistance and rotational stiffness of semi-rigid joints, and b) usefulness in implementing semi-rigid joints on steel frames. This chapter represents this dissertation’s main original contribution to scientific knowledge on the field of semi-rigid joints. In the sixth chapter, the dissertation closes with the presentation of conclusions drawn from research conducted, and the formulation of recommendations for future research and, in a more general sense, for activity in the area of steel design using semi-rigid joints