Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 64 13 DESIGN OF MULTI-BAY PORTAL FRAMES 13.1 General Most aspects of the behaviour and design of multi-bay portal frames are similar to single bay structures. This Section describes common types of multi-bay frames and highlights key points of difference. 13.2 Types of multi-bay portals 13.2.1 Valley beams and ‘hit’ and ‘miss’ frames In multi-span portal framed building, it is common practice to use valley beams to eliminate some internal columns. Most commonly, alternate columns are omitted and the valley of the frame is supported on a so-called valley beam spanning between the columns of adjacent frames, as shown in Figure 13.1. This arrangement is often referred to as ‘hit’ and ‘miss’ frames, the frames with columns being the ‘hit’ frames. Sometimes more than one column is omitted, though such schemes require very large valley beams and reduce the stiffness and the stability requirements of the structure, even where the remaining complete frames are used to stabilise the frames without columns. 1 1 3 2 1 Valley beams 2 Rafter 3 Valley beam and fabricated connection Figure 13.1 Valley beams Valley beams may be simply supported or continuous through the supporting columns. The choice will normally depend on the relative cost of a heavier beam for simply supported construction and the more expensive connection for continuous construction. Valley beams often form one or more rigid frames with the internal columns along the valley to provide overall structural stability at right angles to the frames. This avoids the use of cross bracing on the internal column lines, which is often unacceptable for the intended use of the building. Alternatively, a deep truss may be provided in the plane of the rafters, which spans between the external elevations. For long trusses on multi-span structures, it would be