Dear,
It seems that double angle back to back as per EC3 works only for S235? For same sections and S355 it reports slenderness class 4.
Right?
The design of slender double angle sections (class 4) is not implemented in STAAD.Pro as per EC3.
It is not a slender section. Please try with S235.
STAAD SPACE INPUT WIDTH 79 UNIT METER KN JOINT COORDINATES 1 0 0 0; 2 8 0 0; MEMBER INCIDENCES 1 1 2; SUPPORTS 1 PINNED 2 FIXED BUT FX MY MZ DEFINE MATERIAL START ISOTROPIC STEEL E 2.05e+08 POISSON 0.3 DENSITY 76.8195 ALPHA 1.2e-05 DAMP 0.03 TYPE STEEL STRENGTH RY 1.5 RT 1.2 END DEFINE MATERIAL MEMBER PROPERTY EUROPEAN 1 TABLE LD L150X150X15 SP 0.015 CONSTANTS MATERIAL STEEL ALL LOAD 1 LOADTYPE None TITLE LOAD CASE 1 JOINT LOAD 2 FX -10 PERFORM ANALYSIS PRINT STATICS CHECK PARAMETER 1 CODE EN 1993-1-1:2005 SGR 2 ALL CHECK CODE ALL FINISH
I think it's slender. If you would use the 150X150X18 instead of 150X150X15 then that double angle would be designed even with steel S355