# D1.L.1.1.5 Combined Interaction Check

Members subjected to both axial compression and bending stresses are proportioned to satisfy

$\frac{{f}_{a}}{{F}_{a}}+\frac{{C}_{mz}{f}_{bz}}{(1-{f}_{a}/{F\prime}_{ex}){F}_{bx}}+\frac{{C}_{my}{f}_{by}}{(1-{f}_{a}/{F\prime}_{ey}){F}_{by}}\le 1.0$

and

$\frac{{f}_{a}}{0.60{F}_{y}}+\frac{{f}_{bz}}{{F}_{bz}}+\frac{{f}_{by}}{{F}_{by}}\le 1.0$

when
f_{a}/F_{a} >
0.15, otherwise

$\frac{{f}_{a}}{{F}_{a}}+\frac{{f}_{bz}}{{F}_{bz}}+\frac{{f}_{by}}{{F}_{by}}\le 1.0$

It should be noted that during code checking or member
selection, if f_{a}/F_{a}
exceeds unity, the program does not compute the second and third part of the
formula, because this would result in a misleadingly liberal ratio. The value
of the coefficient Cm is taken as 0.85 for
side-sway and 0.6 -
0.4·(M1/M2), but not less than 0.4 for no side-sway.

Members subjected to both axial tension and bending stress are proportioned to satisfy

$\frac{{f}_{a}}{0.60{F}_{y}}+\frac{{f}_{bz}}{{F}_{bz}}+\frac{{f}_{by}}{{F}_{by}}\le 1.0$