Gross section:The cross sectional area without deducting the holes and without considering the weakening of the cross section by the holes.Stabilisation problems are holistic in nature and can be calculated using the gross section area.Net section: the cross-section area of the holes is deducted, and the weakening of the cross-section by the holes is considered.Tensile strength problems are localised problems and are generally calculated using the net section area.Effective cross-section: The effective area of the cross-section considering the strength after buckling but without deducting the holes.Generally, for cold-formed steel with large width and thickness, the effective area is used to consider the local strength after buckling.Effective net section: consider the strength after bending and deduct the effective area of the cross-section of the hole, compressive strength problems should be considered both the weakening of the cross-section of the strength of the hole, but also to consider the local strength after bending, – – Generally used in the calculation of the effective net cross-section.

1, net section, used in the strength test.Remember that the strength test refers to the strength of a cross-section of the test, so to use the actual cross-section of the cross-section, that is, the net cross-section.It is also equal to the overall cross-section (gross cross-section) of the cross-section minus the cross-section of the holes in the cross-section.
Gross cross-section, used in the overall stability test.The overall stability check is relative to the whole member, and the member of the cross-section, boundary conditions, etc. are related.The weakening of a local section has little effect on the overall stability.Therefore, the gross cross-section is used here, i.e., the weakening of some holes in the cross-section is ignored.
3, the effective cross-section, is relative to the thin-walled members (width-thickness ratio or height-thickness ratio of the plate).Plate is too thin, the pressure will occur when the local buckling, so that the whole section can not be used to carry.Therefore, the specification of such thin-walled members, made the corresponding simplification, that part of the cross-section (effective cross-section) can be like ordinary plate to force, while the other parts of his role is not considered.
4, effective net cross-section, the effective cross-section minus the effective cross-section range of holes in the cross-section, is used in the strength of thin-walled compression in the calculation, tensile without local buckling, so still use the net cross-section.
In summary.

Ordinary steel members.
Strength test —- net section
Stability test —- gross section
Thin-walled steel members.
Tensile strength calculations – net sections
Compressive strength calculations —- Effective net section
Stability calculations ——— effective cross section