BDK warning 198 - No convergence of cross-section classification for a welded Class 4 I-section

SOFiSTiK Teddy
1

Hello,

I am performing an EN 1993 stability verification of a welded thin-walled I-section using AQUA and BDK.

The cross-section dimensions are:

  • Total height: 3750 mm*

  • Web: 3665 x 28 mm*

  • Top flange: 1000 x 35 mm*

  • Bottom flange: 1200 x 50 mm*

  • Steel grade: S355*

The material and cross-section are defined as follows:

+PROG AQUA

NORM EN 199X-200X

STEE NO 1 TYPE S CLAS 355 TMAX 80

SECT 1 MNO 1 FSYM NONE BTYP BEAM TITL "PRS"

PLAT '1' YB -500 ZB -3732.5 YE 0 ZE -3732.5 T 35 MNO 1 FIXB FREE FIXE 14

PLAT '2' YB 500 ZB -3732.5 YE 0 ZE -3732.5 T 35 MNO 1 FIXB FREE FIXE 14

PLAT '3' YB 0 ZB -3732.5 YE 0 ZE -25 T 28 MNO 1 FIXB 17.5 FIXE 25

PLAT '4' YB -600 ZB -25 YE 0 ZE -25 T 50 MNO 1 FIXB FREE FIXE 14

PLAT '5' YB 600 ZB -25 YE 0 ZE -25 T 50 MNO 1 FIXB FREE FIXE 14

END

For the stability verification, I use BDK with automatic cross-section classification:

+PROG BDK

ECHO FULL FULL

ECHO CLAS EXTR

CTRL MTHD 1

CTRL TAB YES

CTRL LCST 951

LC NO 15000

CVA CB 0 CY FIX CZ FIX DX FIX DY 0 DZ 0

CVE CB 0 CY FIX CZ FIX DX FIX DY 0 DZ 0

CTRL CURV AUTO SCL AUTO

MEMB REF SLN 1 TYPE LTB

END

BDK always returns the following warning:

+++++ warning no. 198 in program BDKDES

No convergence of cross-section classification, please check the cross-section effective properties!

The cross-section is Class 4. If I force the cross-section classification using SCL 3, the BDK calculation runs. However, I am only using this as a diagnostic test because the original cross-section is Class 4.

  1. Is the thin-walled cross-section correctly defined, particularly the FIXB and FIXE values for the outstand flanges and the internal web?

  2. Should BDK normally be able to determine the effective properties of this Class 4 section automatically using SCL AUTO?

  3. If the automatic classification does not converge, is the recommended solution to define a separate effective cross-section in AQUA using TYPE NEFF and use that section for the BDK verification?

Thank you for your help.

Esteban

2

This warning means that the program was unable to find convergence internally, and you should therefore check the calculated effective cross-section properties again. These convergence problems can occur when the cross-section is classified as SCL 4.
Unfortunately, we cannot offer you a quick solution for these convergence problems.

You can use the following workarounds:

  1. Check the calculated cross-section properties again using a hand calculation. If the results are correct, you can simply ignore this warning.
  2. You can use a greater plate thickness to avoid cross-section class 4.
  3. Use SIG NEFF: With SIG NEFF, you can define the effective cross-sectional area yourself.
    You can find a tutorial on this here: Cross-section class 4 concept - SOFiSTiK Tutorials 2025
3

Thanks a lot.

Esteban