Implicit in most analysis programs is the assumption that the structure is not subjected to any load until it is completely built. This is probably a reasonable assumption for live, wind and seismic loads and other superimposed loads. However, in reality, the dead load of the structure is continuously being applied as the structure is being built. In other words, the lower floors of a building are already stressed with the dead load of the lower floors before the upper floors are constructed. Engineers have long been aware of the inaccurate analytical results in the form of large unrealistic beam moments in the upper floors of buildings because of the assumption of the instantaneous appearance of the dead load after the structure is built. In many cases, especially for taller buildings because the effect is cumulative, the analytical results of the final structure can be significantly altered by the construction sequence of the building. Situations that are sensitive to the effects of the construction sequence include, among others, buildings with differential axial deformations, transfer girders involving temporary shoring, and trussed structures where segments of the truss are built and loaded while other segments are still being installed. ETABS has an option whereby the user can activate an automated sequential construction load case. This procedure allows the structure to be loaded as it is built, story by story.  Typically, you would do this for the dead load pattern and use the analytical results from the sequential construction load case in combination with the other load cases for the design phase.