Compression

In general, there are two ways to calculate compression resistance of composite column member.

In case of Linear analysis and pure compression and composite setup setting "Simplified method using European buckling curves" is activated, the program will calculate the compression resistance using Simplified method using European buckling curves based on EN 1994-1-1, 6.7.3.5(2).

You can see the design flow of the check on the picture below:

In all other cases the interaction diagram is used.

 

Pure compression check according to European buckling curves

The check is executed according to EN 1994-1-1, 6.7.3.5(2), while the resistance of the structural steel is considered. A contribution from the reinforced concrete part of the beam is not assumed to be established.

with:

N_Ed - design normal compression force

N_pl,Rd - plastic resistance of the composite section according to EN 1994-1-1, 6.7.3.2(1), with f_yd determined using the γ_M1 given by EN 1993-1-1, 6.1(1)

χ - is the reduction factor for the relevant buckling axis y-y or z-z, given in EN 1993-1-1, 6.3.1.2 in terms of the relevant relative slenderness λ_rel.

α - imperfection factor depending on the buckling curve of a cross-section given by EN 1993-1-1, 6.3.1.2(2) Table 6.1 for the considered bending plane

K_e - correction factor taken as 0,6

N_pl,Rk - characteristic value of N_pl,Rd of the composite section given by EN 1994-1-1, 6.7.3.2(1), using characteristic strengths of materials

 

Compression resistances N_pl,Rd and N_pl,Rk for supported cross-section type

The plastic resistance of the composite section according to EN 1994-1-1, 6.7.3.2(1) for supported cross-sections:

Concrete encased (type a/aw) and partially concrete encased (type b/bw & c& d/dw):

 

Concrete filled tube (type e & f):

According to EN 1994-1-1, 6.7.3.2 (6), for concrete filled tubes of circular cross-section, account may be taken of increase in strength of concrete caused by confinement. In order to do that, two conditions have to be fulfilled: 

• 
• with

In case both conditions are fulfilled a note will be displayed and initial equation 6.30 is therefore replaced by equation 6.33 for plastic resistance to compression

with:

f_cc - increased concrete strength due to confinement

η_a - e/d ratio coefficient for structural steel

η_c - e/d ratio coefficient for concrete

 

if e=0:

η_a=η_ao

η_c=η_co

 

if 0< e/d ≤0.1:

Final compression resistance N_pl,Rd for this case is then taken as .

 

Pure compression is considered when only compression force is present possibly accompanied with shear force, no design bending moments from FEM analysis.