**Abbreviations**

Area of concrete core cross-section (*Ac*), area of steel tube cross-section (*As)*, coefficient of variation (COV), depth of rectangular steel tube (*D*), depth of steel stiffener (*dst*), modulus of elasticity for concrete (*Ec*), modulus of elasticity for steel (*Es*), compressive strength of concrete cube at 28 days (*fcu*), characteristic concrete strength of 0.67*fcu* (*fck*), ultimate strength of steel (*fu*), yield strength of steel (*fy*), steel tube buckling stress (*Fcr*), moment of inertia for concrete tube cross-section (*Ic*), moment of inertia for steel tube cross-section (*Is*), initial flexural stiffness of composite section (*K<sup>i</sup>* ), serviceability level of the flexural stiffness of the composite section (*Ks*), effective length of the specimen (*M*), ultimate bending moment capacity (flexural strength capacity; *Mu*), nominal bending moment (*Mn*), plastic limit bending moment (*M<sup>p</sup> = Mu*), yield limit bending moment (*My*), mean value (*MV*), steel tube/C-section thickness (*t*), steel stiffener thickness (*tst*), width of rectangular steel tube (*W*), effective width of rectangular steel tube (*Weff*), effective width/distance between two internal stiffeners and/or between the edge of the tube and the first internal stiffener (*weff*), distance of N.A. from the top flange of the tube for the Compact section (*yc*), distance of N.A. from the top flange of the tube for the Noncompact section (*yn*), distance of N.A. from the top flange of the tube for the Slender section (*ys*), slenderness ratio of steel tube cross-section (*λ*), slenderness ratio at the compactness limit (*λp*), slenderness ratio at the noncompactness limit (*λr*), maximum limit of the slenderness ratio (*λlimit*), stiffened slenderness ratio (*λst = weff/t*).

### **References**

