How do you estimate reinforced concrete beam flexural capacity?

Start with the tension force.

For a singly reinforced rectangular section, the simple screen assumes the tension reinforcement reaches its entered yield strength. The tensile force is therefore T = A_sf_y. Equilibrium with the equivalent concrete compression block gives its depth a = T / (α₁f_c'b).

The lever arm turns force into resistance.

When the block depth is less than the effective depth, the internal lever arm is d - a/2. The nominal resistance is T(d - a/2), and the tool multiplies it by the entered resistance factor φ before comparing it with the entered design moment.

Factors are deliberately visible.

Stress-block and resistance factors vary with the design code, material strength, strain state, and project rules. CivilKit keeps α₁, β₁, and φ as inputs instead of hiding a jurisdiction-specific choice inside the result.

Do not mistake capacity for a complete beam design.

• Minimum and maximum steel, ductility, compression reinforcement, and strain limits need a code-specific check.
• Shear, torsion, anchorage, development, laps, cover, spacing, crack control, deflection, fire, durability, and construction sequence are outside this screen.
• The effective depth and demand must come from the actual bar arrangement, load combinations, analysis model, and project information.