Interaction curve

The interaction curve is determined numerically in order to determine the capacity of the composite cross-section with regards to an interaction of internal forces (N_Ed + V_Ed + M_Ed). Dependent on the load, the position of the neutral axis is changed and this leads to obtaining different values of compressive and tensile areas in composite members. Therefore this concludes to a different capacity calculated from the strain distribution.

In reality this curve is a 3D-diagram, but SCIA Engineer displays only two sections (N_Ed+M_y,Ed & N_Ed+M_z,Ed) of that 3D-interaction diagram in accordance with EN 1994-1-1.

Simplified interaction curve of EN 1994-1-1

EN 1994-1-1 defines the interaction curve by means of calculating 4 points (A-B-C-D) and linearly interpolating the points between them.
This leads to less capacity between those 4 points and thus to a lower resistance of the composite cross-section.

SCIA Engineer calculates the whole interaction curve (700 points) in order to obtain the composite cross-section resistance to a combination of internal force more accurately. However it still informatively displays the resistance values at those 4 points. Below an example of the interaction curve:

When comparing SCIA Engineer's result with a benchmark it can happen that both interaction curves do not match. In such cases it is often caused due to the fact that the benchmark doesn't take into account a strain limit while SCIA Engineer does (see material properties). When modifying the strain limit in the material properties you should get matching results for comparison.

Influence of high shear force

In case high shear force is present (V_Ed >0.5*V_pl,Rd) the yield strength of the structural steel section is reduced by means of the ρ-factor (eq. 6.5 of EN 1994-1-1).
This in turn influences the interaction curve as the structural steel (full section) uses a reduced yield strength.

In case the high shear force surpasses the shear resistance leading to a failing transverse shear check (Unity Check > 1.00) it also impacts the interaction curve. 
The consequence is that no interaction curve can be determined and no combined check can be executed as the reduction factor ρ would be 0 leading to a yield strength of 0 MPa for the structural steel section.

Internal forces that fall outside the interaction curve

In case the internal forces set (N_Ed+M_y,Ed or N_Ed+M_z,Ed) fall outside the interaction curve no combined check can be executed because these internal forces surpass the cross-section's capacity.

Concrete filled tubes of circular cross-section

Account is taken for the increase of concrete strength of concrete filled tubes of circular cross-section in case they fulfil both criteria mentioned under article 6.7.3.2(6) of EN 1994-1-1.
The interaction curve is in such cases determined by an increase of f_cd due to multiplication by a factor: