13.1 Design and construction 13.1.1 Portal type and structural action The singlestorey clearspan building is in constant demand for warehouses, factories and many other purposes. The clear internal appearance makes it much more appealing than a trussed roof building and it also requires less maintenance and heating. The portal may be of threepinned, pinnedbase or fixedbase construction as shown in Figure 13.1(a). The pinnedbase portal is the most common type adopted because of the greater economy in foundation design over the fixedbase type. Only pinnedbase portals are discussed here. Inplane the portal resists the following loads by rigid frame action (see Figure 13.1(b)): Dead and imposed loads acting vertically Wind causing horizontal loads on the walls and generally uplift loads on the roof slopes In the longitudinal direction the building is of simple design and diagonal bracing is provided in the end bays to provide stability and resist wind load on the gable ends and wind friction on sides and roof (see Figure 13.1(d)). 13.1.2 Construction The main features in modern portal construction shown in Figure 13.1 (c) are: Columns — uniform universal beam section; Rafters — universal beams with haunched ends usually of sections 30 40% lighter than the columns; Eaves and ridge joints — sitebolted joints using grade 8.8 bolts where the haunched ends of the rafters provide the necessary lever arm for design. Local joint stiffening is required;
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