Concrete Shape Terminology

Fy	The yield stress of the reinforcing bars.
f'c	The 28-day concrete compressive strength.
f"c	The maximum concrete compressive stress.
eu	The ultimate concrete strain.
eo	The concrete strain at the maximum compressive stress.
e	Extreme compression fiber strain under the loading specified.
Maximum Stress Factor, k	Multiplier for the compressive strength, f'c, to obtain the maximum compressive stress, f"c.
Confinement Factor, g	The confinement factor used to calculate the compressive stress at the ultimate concrete strain.

Cracked Section Properties

Neutral Axis Offset (NA D)	Perpendicular distance from the neutral axis to the centroid of the cross-section, determined by the specified loading.
Neutral Axis Angle (NA a)	Inclination angle of the neutral axis, measured counterclockwise from the X-axis, based on the specified loading.
Tangent Stiffness	The stiffness of the section at the current state of stress and strain, i.e. the derivative of the moment with respect to the curvature.
Curvature	The curvature of the section under the applied loads.

Interaction Surface Points

Po, To	The axial only capacity of the section. Calculated based on a load that causes crushing of the concrete (compression) and/or yielding of the reinforcement (tension, compression).
Mox, Moy	The uniaxial moment capacity of the section for bending about the x and y-axes.
Pbx, Pby, Mbx, Mby	The balance points (i.e. transition points) on the Interaction Diagram for Axial-Moment interaction caused by bending about the x and y-axes.
Note: These extreme points on the Interaction Diagrams are modified by the Strength Reduction Factors (f) when specified

Initial Stiffness

E Initial	Secant stiffness of the concrete at low strains. Used as the base material for moment of intertia calculations.
Ix (Top. Comp.), Ix (Bottom Comp.)	Moment of inertia of the section when the neutral axis is parallel to the global X axis and the top or bottom of the section is in compression. Calculated at low load levels, before steel has yielded and the concrete is behaving nearly linearly.
Iy (Right. Comp.), Iy (Left Comp.)	Moment of inertia of the section when the neutral axis is parallel to the global Y axis and the left or right of the section is in compression. Calculated at low load levels, before steel has yielded and the concrete is behaving nearly linearly.
Note: Properties are transformed to concrete.

Uncracked Transformed Properties

Modular Ratio, n	The ratio of the modulus of elasticity of steel (Es) to the modulus of elasticity of concrete (Ec).
Area	Area of the cross section.
Xc, Yc	X and Y coordinates of the cross-section's centroid relative to the global origin.
Ix, Iy	Moment of inertia about the centroidal x and y-axes
Ixy	Product of inertia about the centroidal x and y-axes
Sx+, Sx-, Sy+, Sy-	Elastic section modulus about the centroidal axis (x or y) with respect to the extreme fiber in the positive or negative direction.
fcr	The Modulus of Rupture
Mcr Top, Mcr Bottom, Mcr Right, Mcr Left	The cracking moment for bending about the x and y axes. The Mcr Right corresponds to the right side of the section being in tension, etc.
Note: Applicable properties are transformed to concrete.