Minimum tensile reinforcement according to NEN EN 1992-1-1

When calculating the reinforcement in a beam according to NEN EN 1992-1-1 (National Annex of the Netherlands), it seems as if Diamonds doesn’t show the minimum reinforcement (= 93mm² in the beam on the left).

Beam calculated according to EN 1992-1-1 [- -]

Same beam calculated according to EN 1992-1-1 [NL]

However, Diamonds is correct! NEN 1992-1-1:2005 §9.2.1.1 states that the minimum tensile reinforcement should be calculated as:

is function of a minimum reinforcement ratio (see referencces below). This is the minimum reinforcement ratio that can be consulted in the material library of Diamonds. Nothing special until here.

In the case of concrete with lower strength classes, concrete structures are sometimes made heavier for practical reasons than necessary for the strength. To prevent oversized structures from being unnecessarily heavily reinforced, has been introduced in the Netherlands. equals 1.25 times the ULS-reinforcement.

The combination of both and causes the minimum reinforcement to equal zero:

• In the middel of beam above, you would need 236mm² reinforcement in ULS FC.
  = 93mm², derived from a calculation according to Eurocode without national annex.
  = 1.25 x 236mm² = 295mm²
  
  is decisive. But because < 236mm², no minimum reinforcement is shown.
  
• At the end of beam above, you would need 0 mm² reinforcement in ULS FC, because the bending moment is zero.
  = 93mm², derived from a calculation according to Eurocode without national annex.
  = 1.25 x 0mm² = 0mm²
  
  is decisive. But because =0 mm², no minimum reinforcement is shown.

Only when an element is lightly loaded, will be added. The beam below requires 44mm² in ULS F

Only in case of lightly loaded beams, will be added. For example: the beam below requires 44mm² reinforcement in ULS FC combination. Multiply that by 1,25 and you get 55mm². The minimum reinforcement is not a constant line, it follows the shape of the bending moment.

Note: literature (see references below) warn of the consequences of the rule. If overloaded, brittle failure will occur.

References:

• Braam, C. R., Lagendijk, P., Dees, W., & Centrum, C. E. B. (2010). Constructieleer gewapend beton 2: studieboek voor het HBO, de opleidingen Bouwkunde en Civiele Techniek.
• Braam, C. R., Lagendijk, P., Dees, (2025). Constructieleer gewapend beton 2: studieboek voor het HBO, de opleidingen Bouwkunde en Civiele Techniek.