How does Diamonds calculate the shear reinforcement

Diamonds uses the Variable Strut Inclination method as described in EN 1992 §6.2.

The inclination of the concrete strut is expressed with the angle θ and can vary between 1 ≤ cot(θ) ≤ 2,5 (45° ≤ θ ≤ 21,8°). Diamonds assumes θ is at least 31° (reason: increasing anchoring length). The inclination is increased when the concrete compression stresses in the compression strut are too high.

The inclination of the stirrups is expressed with the angle α. α can be 90° (vertical stirrups) or 45° (stirrups under 45°). If the compression strut is still insufficient with stirrups under 90°, Diamonds will switch to stirrups under 45°. Stirrups under 45° are represended in Diamonds as vertical stirrups + longitudonal reinforcement left and right (NL: lijfwapening). the vectorial sum of both is equivalent to stirrups under 45°.

Example:

Beam 4m simply supported on both sides, 200x400mm, concrete cover 40mm, C25/30, 25kN/m dead load, 10kN/m life load (Diamonds-file in the attachement).

From equation 6.13 in EN 1992-1-1