Structures may be subjected to both mechanical loads and imposed deformation. On one hand, the mechanical loads include, for example, the self-weight of the construction work, the action from normal use by person, furniture and movable objects, vehicles, snow, wind, execution, etc. On the other hand, when the deformation of a structure is restrained, imposed deformation occurs. The sources of deformation can be various, not only environmental conditions, such as temperature and humidity changes, but also chemical or physical actions, such as sulphate ingress or creep. If the shortening of a structure is restrained, the structure will be subjected to imposed deformation which results in tensile stress. Since the tensile strength of concrete is relatively low, concrete structures, such as tunnels, bridges and pavement roads, always suffer a high risk of cracking. Even if the concrete structure is prestressed, the tensile stress resulting from imposed deformation would consume the compressive stress in concrete and raise the risk of cracking. If the crack width exceeds the limit, leakage, corrosion and even structural failure may happen. According to the schematized 푁 - 휀 diagram of reinforced concrete, the development of cracking caused by imposed deformation and mechanical load are different. Suppose the cracking is caused by imposed deformation, there is a developing stage for cracking. It means the cracks caused by imposed deformation could be either fully or not fully developed. However, suppose the cracking is caused by mechanical loads, the cracks could only be fully developed. The stiffness of fully or not fully cracked members are different. The stress resulting from imposed deformation in a structure is related to the stiffness of the structure. Therefore, cracking has significant impact on the magnitude of stress resulting from imposed deformation. Therefore, when a structure is subjected to imposed deformation and mechanical load together, it is necessary to take the impact of cracking into account during structure design. The combination of imposed deformation and mechanic loads is referred to as combined actions. It is common to use FEM software to analyze the stress resulting from the combined actions during structure design when cracking has to be taken into account. However, FEM analysis only is not enough. It is also necessary to check the results calculated by FEM software to avoid mistakes, for example a wrong input.
There is a project called 'Approach Ring South, Groningen'. In the project, widened deck KW03.01 is subjected to a combination of imposed deformation and prestressing force. FEM software called SCIA is used to calculate the prestress consumption in the widened deck KW03.01. According to the data file of the project, 41% in maximum of the compressive stress resulting from prestressing is consumed when the structure is subjected to combined actions, which is much more than the engineering experience. As a result, a simple approach is required to check whether the prestress consumption in widened deck KW03.01 suits the expectation or not, where the prestress consumption is calculated by FEM software.