Joints in steel structures

The steel frame structures with connecting parts as bolts and welds are designed on the basis of 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. In fact, 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, in order to prevent premature and progressive collapse, can be demonstrated explicitly. The distribution of forces and moments in the structure due to the loading is due to the strength and stiffness distribution in the structure. Thus it can be said that the stiffness, strength and rotation capacity, together with those of the structural components like beams and columns, produce these forces in the joints. The designing of the joints include the connecting parts are of direct influence on the level of forces and moments in these joints. Joining components such as columns and beams together is construction, while designing is making choices for components taking the structural properties such as strength and stiffness into account. In traditional design the joints are stiff and strong and that the forces and moments in the structure are determined using the linear-elastic theory. In many cases in practice, this is neglected. The joints strength is adjusted to the level needed. As a result, most joints have low deformation capacity. Last but not least, fabrication costs are very high. In modern design, the joints are 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 the fabrication considerations and considerations for easy and safe construction on-site. The structural safety of all components depends on the design method used to determine the distribution of forces and moment in the structure.