Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 55 11 CONNECTIONS The major connections in a portal frame are the eaves and apex connections, which are both moment-resisting. The eaves connection in particular must generally carry a very large bending moment. Both the eaves and apex connections are likely to experience reversal in certain combinations of actions and this can be an important design case. For economy, connections should be arranged to minimise any requirement for additional reinforcement (commonly called stiffeners). This is generally achieved by:  Making the haunch deeper (increasing the lever arms)  Extending the connection above the top flange of the rafter (an additional bolt row)  Adding bolt rows  Selecting a stronger column section. The design of moment resisting connections is covered in detail in Single-storey Buildings. Part 11: Moment connections [5] . 11.1 Eaves connections A typical eaves connection is shown in Figure 11.1. In addition to increasing the moment resistance of the rafter, the presence of the haunch increases the lever arms of the bolts in the tension zone, which is important if the connection carries a large bending moment. Generally the bolts in the tension zone (the upper bolts under conventional gravity loading) are nominally allocated to carry tension from the applied moment, whilst the lower bolts (adjacent to the compression stiffener) are nominally allocated to carry the vertical shear, which is generally modest. Because the portal frame members are chosen for bending resistance, deep members with relatively thin webs are common in portal frames. A compression stiffener in the column is usually required. The web panel of the column may also need reinforcing, either with a diagonal stiffener, or an additions web plate (referred to as a supplementary web plate) The end plate and column may be extended above the top of the rafter, with an additional pair of bolts. The end plate on the rafter is unlikely to require stiffening as it can simply be made thicker, but it is common to find that the column flange requires strengthening locally to the tension bolts. Stiffeners are expensive, so good connection design would minimise the need for stiffeners by judicious choice of connection geometry. Under a reversed bending moment, it may be necessary to provide a stiffener to the column web at the top of the column, aligned with the top flange of the rafter.