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CivilKits/Calculators/Structural/Design/Hong Kong/RC beam flexure

structural / Hong Kong concrete

Hong Kong RC beam flexural resistance.

A bounded resistance screen for a rectangular singly reinforced beam using the Buildings Department concrete-code stress block.

implemented code subset

One transparent internal force couple.

This screen follows the Code of Practice for Structural Use of Concrete 2013 (2020 Edition), with the Buildings Department amendments published in February 2022, June 2023, and April 2024. The edition and amendment state was checked on 12 August 2026.

Equations used

T = 0.87fyAsTension force in the reinforcementx = T / (0.405fcub)Neutral-axis depth from the equivalent 0.45fcu blockz = min(d − 0.45x, 0.95d)Lever arm, with the clause 6.1.2.4(a)(v) capMRd = 0.87fyAszFlexural resistance reported by the screen

These are the direct resistance form of the assumptions in clause 6.1.2.4(a) and the design formulae in clause 6.1.2.4(c). The numbered demand form is K = M/(bd2fcu) (Equation 6.7), K′ = 0.156 for this no-more-than-10%-redistribution subset (Equation 6.8), z = [0.5 + √(0.25 − K/0.9)]d (Equation 6.10), and As = M/(0.87fyz) (Equation 6.12).

Variables: b is beam width, h is overall depth, d is effective depth, As is tension-steel area, fcu is characteristic concrete cube strength, fy is characteristic reinforcement yield strength, x is neutral-axis depth, z is the internal lever arm, and MRd is in kN·m after unit conversion.

Declared limits

  • 20 ≤ fcu ≤ 45 MPa and fy = 500 MPa in this release.
  • Rectangular, normal-proportion, singly reinforced beam; no axial force, compression steel, flange, prestress, or deep-beam behaviour.
  • The neutral axis must satisfy x ≤ 0.5d for the selected fcu and redistribution basis. K must not exceed K′ = 0.156.
  • Minimum tension steel is screened using Table 9.1; the 4% gross-area maximum is screened using clause 9.2.1.3.
  • Shear, torsion, anchorage, development, laps, cover, spacing, crack width, deflection, fire, durability, construction sequence, and lateral stability remain separate checks.

April 2024 adds a strut-and-tie route for non-flexural components and a deep-beam reference; that route is intentionally outside this page. The February 2022 and June 2023 amendment items do not alter the flexural equations implemented here.