I want to change the sectional stiffness of an element without changing the element.
You can use 'Stiffness Scale Factor' feature in 'Section Manager' to change the sectional stiffness.
In Properties tab > Section group > Section Manager, we can define the 'Stiffenss Scale Factor' to be applied to the sectional properties of line elements (Truss, Tension-only, Compression-only, Cable, Gap, Hook, Beam Element).
This feature can be used for reducing the flexural stiffness of each type of member. You can also assign Boundary group in the 'Stiffness Scale Factor' to define the sectional stiffness that changes with each construction stage during construction stage analysis. The feature can also be applied to Composite Section for Construction Stage. It might be duplicated when you apply the section stiffness variation factor using both in 'Section Manager' and 'Composite Section of Construction Stage'.
'Section Stiffness Scale Factor' is used only for displacement and member force calculations, and the original sectional properties are used for stress calculations. However, you can define an option in 'Main Control Data' to consider the 'Stiffness Scale Factor' to be used in stress calculations.
The 'Effective Width Scale Factor' feature is similar to the 'Stiffness Scale Factor' feature, and it is used for analysis considering the effective width of the section. The scaling factor applied in the 'Effective Width Scale Factor' is always used in stress calculations and is treated as a boundary condition, so you can use the stiffness considering the effective width from a specific construction stage during construction stage analysis.