Steel Buildings in Europe

Part 2: Concept Design 2 - 20 3.1 Pitched roof portal frame A single-span symmetrical portal frame (as illustrated in Figure 3.2) is typically of the following proportions:  A span between 15 m and 50 m (25 m to 35 m is the most efficient)  An eaves height (base to rafter centreline) of between 5 and 10 m (7,5 m is commonly adopted). The eaves height is determined by the specified clear height between the top of the floor and the underside of the haunch.  A roof pitch between 5  and 10  (6° is commonly adopted)  A frame spacing between 5 m and 8 m (the greater frame spacings being used in longer span portal frames)  Members are I sections rather than H sections, because they must carry significant bending moments and provide in-plane stiffness.  Sections are generally S235 or S275. Because deflections may be critical, the use of higher strength steel is rarely justified.  Haunches are provided in the rafters at the eaves to enhance the bending resistance of the rafter and to facilitate a bolted connection to the column.  Small haunches are provided at the apex, to facilitate the bolted connection 1 3 4 5 6 7 2 1 Eaves 2 Roof pitch 3 Apex 4 Rafter 5 Eaves haunch 6 Apex haunch 7 Column Figure 3.2 Single-span symmetric portal frame The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the span. The length of the haunch means that the hogging bending moment at the “sharp” end of the haunch is approximately the same as the maximum sagging bending moment towards the apex, as shown in Figure 3.3.