68 2.5.2 Modelling the Construction Process (Modelling the Construction Process) For simple bridges which are constructed from Class 1 or Class 2 sections, it is not necessary to consider the detailed build-up of stresses in the bridge as it is constructed. Such structures have sufficient ductility to redistribute stresses within the cross-section. It is therefore only necessary to check the adequacy of the structure at each stage in construction and to ensure that the completed structure can carry both fixed and variable actions, i.e.dead plus live loads. Larger structures are usually constructed of Class 3 or 4 sections and it is not safe to assume that redistribution of construction stresses can occur. It is therefore necessary to model in some detail the build up of construction stresses throughout the construction process. Figure 21 shows the sequence for a typical three-span composite highway bridge. Each case is analyzed elastically and the stresses are summed at critical cross-sections.