Steel Buildings in Europe

Part 11: Moment Connections 11 - 7 The resistance of the bolt row nearest the centre of compression is reduced as a first step, and then the next row, until the total design resistance in the tension zone is no more than the compression resistance, or the web panel shear resistance. Reducing the bolt row resistance in this way is satisfactory, as the design approach presumes a plastic distribution of bolt forces. As an alternative to reducing the resistance in the tension zone, stiffeners can be provided to increase the design resistance of the web panel in shear, and the web in compression. Once the effective design tension resistances have been calculated, by reducing the potential resistances if necessary, the design moment resistance of the connection can be calculated, as the summation of each bolt row tension resistance multiplied by its lever arm from the centre of compression, i.e.:   r r r h F M t ,Rd j,Rd (as given in § 6.2.7.2 of EN 1993-1-8) The centre of compression is assumed to be in line with the centre of the compression flange. 1.7 Weld design EN 1993-1-8 § 6.2.3(4) requires that the design moment resistance of the joint is always limited by the design resistance of its other basic components, and not by the design resistance of the welds. A convenient conservative solution is therefore to provide full-strength welds to components in tension. When components are in compression, such as the bottom flange of a haunch, it is normally assumed that the components are in direct bearing, and therefore only a nominal weld is required. If the joint experiences a reversed bending moment, the weld will be required to carry some tension force, and this should be considered. 1.7.1 Tension flange welds The welds between the tension flange and the end plate may be full strength. Alternatively, common practice is to design the welds to the tension flange for a force which is the lesser of: (a) The tension resistance of the flange, which is equal to b f t f f y (b) The total tension force in the top three bolt rows for an extended end plate or the total tension force in the top two bolt rows for a flush end plate. The approach given above may appear conservative, but at the ultimate limit state, there can be a tendency for the end plate to span vertically between the beam flanges. As a consequence, more load is attracted to the tension flange than from the adjacent bolts alone. A full strength weld to the tension flange can be achieved by:  a pair of symmetrically disposed fillet welds, with the sum of the throat thickness equal to the flange thickness, or  a pair of symmetrically disposed partial penetration butt welds with superimposed fillet welds, or