fix: area computation on Generic Section

[talledodiego]

This fix an issue mentioned in PR #113: the area should be the sum of surface + reinforcement area.

This code now returns the expected value:

# imports
from shapely import Polygon
from structuralcodes.materials.concrete import ConcreteEC2_2004
from structuralcodes.materials.reinforcement import ReinforcementEC2_2004
from structuralcodes.geometry import SurfaceGeometry
from structuralcodes.sections._reinforcement import add_reinforcement_line
from structuralcodes.sections import GenericSection

# Materials
concrete = ConcreteEC2_2004(25)
reinforcemnet = ReinforcementEC2_2004(fyk=500, Es=200000, density=7850, ftk=550, epsuk=0.07) 

# Create section
poly = Polygon(((0, 0), (350, 0), (350, 500), (0, 500)))
geo = SurfaceGeometry(poly, concrete)
geo = add_reinforcement_line(geo, (50, 450), (300, 450), 12, reinforcemnet, n=3)
geo = add_reinforcement_line(geo, (50, 50), (300, 50), 20, reinforcemnet, n=6)
sec = GenericSection(geo, integrator='marin')
# Print gross properties
print(f"area = {round(sec.gross_properties.area*1e-2)} cm2")
print(f"area_reinforcement = {round(sec.gross_properties.area_reinforcement*1e-2)} cm2")
print(f"area_surface = {round(sec.gross_properties.area_surface*1e-2)} cm2")

and the output is:

area = 1772 cm2
area_reinforcement = 22 cm2
area_surface = 1750 cm2

[mortenengen]

I think I get it, but I am not sure if I find it intuitive.

For regular reinforcement embedded inside the concrete section, I would expect the area to be computed as the gross area of the concrete section. However, increased accuracy could be obtained by considering the concrete area as the gross area minus the reinforcement area, but this difference is usually neglected. We also rarely consider subtracting the reinforcement when calculating the area moments of the concrete. For external reinforcement, on the other hand, the total area should be the sum of the two.

Could you please elaborate a bit?

[talledodiego]

You are right!
With the last commit c06b47c the API has been improved removing area_surface result from the gross properties. Now area contains the area of all surfaces while area_reinforcement contain area of all point geometries.

[mortenengen]

Good clarification.