Steel Joints

The steel frame structures with traditional joints with connecting parts as bolts and welds are designed based on forces resulting from plane frame analyses. The spatial behaviour of the structure due to the loading is most times neglected because the resistance of the traditional joints against these resulting forces is normally sufficient. The wind loading on a building can produce tensile forces in the joints, while eccentric loading on the floor can produce torsional moments on the joints. Traditional joints have by themselves sufficient resistance against these tensile forces and torsional moments. Because bolts in holes, designed primarily for shear and or bending moments, are almost always capable of carrying these “secondary” forces and moments. These forces are not “secondary” but primary forces and moments due to the spatial behaviour of the structures under the loading actions and most times neglected in design. In designing new concepts for plug and play joints the designer needs to be explicitly aware of these forces and moments, which can implicitly be neglected in designing traditional joints. Also, loading that can appear in the onsite construction phase produces most times tension and torsion in the beams and joints. The main task of the designer is to design the joints such that robustness to prevent premature and progressive collapse can be demonstrated explicitly. In traditional design, it is assumed that the joints are stiff and strong and that the forces and moments in the structure are determined using the linear-elastic theory. It is assumed that the joints were stiff, it needs to be checked whether the joints are stiff. In many cases in practice, this is neglected. The strength of the joints is adjusted as per the requirement. As a result, most joints have low deformation capacity. Last but not least, fabrication costs are very high. In the latest designs, the joints are considered as structural components such as columns and beams with properties as stiffness, strength and deformation capacity. These structural properties of the joints are incorporated into the design on the same level as those of columns and beams. The joint layout is influenced by fabrication considerations and considerations for easy and safe construction on-site. The structural safety verification of all components is dependent on the design method used to determine the distribution of forces and moment in the structure a. In case that the linear elastic theory is used, the beams need to be checked for strength and lateral-torsional buckling, the columns need to be checked for strength, beam-column stability and the connecting parts of the joints to have sufficient strength to transfer bending moments, shear forces and tensile forces resulting from the linear elastic theory adjusted for the second-order effects. b. In case that the elastic-plastic-non-linear theory is used, the beams and columns need to be checked for lateral-torsional buckling only and the joints need to be checked to have sufficient deformation (in fact rotation) capacity.