Analysis stages
In GSA, analysis stages allow you to work on a subset of a model and assign different properties and constraints to this subset.
Example: In a tall building it may be useful to consider stages that correspond to the structure at quarter, half and three-quarter heights. These can be defined in different stages within a single whole model.
Defining a stage
There are three options in the Data explorer used to define stages. These can be accessed by going to the Explorer pane > Data > Analysis stages:
Stage definition
This defines a list of elements that are included in this analysis stage. At its most basic this is all that is needed to define an analysis stage. This can also define a creep coefficient that applies to all the concrete elements in the model, and a option to lock releases on elements. When defining a stage for footfall analysis it can be good modelling practice to lock releases.
Stage properties
It can be useful to assign different properties to a section for different analysis stages. For example in the final model a concrete beam may be considered as a tee section of beam and slab acting compositely, but at an earlier stage is is modelled simply as a rectangular beam.
Stage materials
Stage materials allow different material properties to be associated with an element in the analysis stage. This is typically to allow creep to be specified for some of the concrete materials, or to vary the creep for different parts of the model.
Heirarchy
A parameter specified on the material will override a change specified in the properties.
A parameter in the properties will override a change specified in the stage definition.
Constraints
All constraints apply to a list of stages and by default, apply to all stages. However, it is possible to change constraints and restraints for a particular stage, for example to apply temporary props at a stage during construction.
Analysis
Stage is specified on the first page of the Analysis wizard. Each stage is independent of the others so no locked in stresses are carried over from one stage to the next. However, when running a dynamic response analysis - for example, response spectrum analysis - the selected stage must correspond with the stage used in the modal analysis from which it derives.
Viewing stages
In the graphic window, go to the Display drop down and select Stage. Choose your stage, to view elements in the graphic display. Elements in excluded stages are shown as ghost elements.
Select the Labels and display methods tool: 
In the Display methods tab, tick 'By initial stage' to specify how your elements are coloured.