The Polish NA recommends the equations in Annex B of PN-EN 1993-1-1 to calculate these interaction factors. The current implementation of EC3 BS in STAAD.pro uses the method in Annex B by default. The proposed implementation of the Polish NA will also use Annex B for Cl.6.3.3 checks.

The Polish NA also gives two additional simplified checks. This implementation will provide for these additional checks as well. However as they are intended as optional checks, by default, the program will not perform these checks. However, the user can invoke these checks by using the PLG parameter. Refer to D5.C.6 Design Parameters

If the value of the PLG parameter is set to 1, the following two checks will be performed as per Cl. NA.20.(2) and NA.20(3) respectively:

• Cl. NA.20.(2): The following condition will be checked

  n/ χ and + C_my m_y/ χ_LT + C mz m with ≤ 1- Δ₀ (I = y or z)

  where

  n	=	NEd/NRd
  my	=	max My,Ed (+ Δ My, Ed)/My, Rd; mz = max M,Z Ed (+ Δ M , Ed)/MZ Rd,
  χ and	=	buckling factor
  χLT	=	LTB factor
  Cm	=	moment factor from table B 3 of PN EN 1993-1-1
  Δ0	=	correction factor (estimation of maximum reduction) and will be worked out as: Δ0 = 0,1 + 0,2 (wi – 1), przy czym wi = Wpl,i/Wel,i , or Δ0 = 0,1 – in case of class 3 and 4 sections.

• Cl. NA.20.(3): This condition will only be checked for circular hollow sections.

  n/χ_i + [(k_ii m_i)² + (C_mj m_j)²] 1/2 ≤ 1 (i,j =y,z)

  where

  k	=	the interaction factor from table B.1 of PN-EN 1993-1-1
  n, m, and Cmj	=	as previously described

If the PLG parameter has been set to 1, the maximum among the following ratios will be taken as being critical for Cl 6.3.3:

• 6.3.3: Eqn6.61
• 6.3.3: Eqn6.62
• NA.20(2) and
• NA.20(3)

If however PLG has been set to 0 (or not specified at all), the program will ignore the last two checks in the list above.