This　paper　proposes　closed-form　approximations　of　the　axial　force-bending　moment　interaction　diagrams　for　reinforced　concrete　columns　and　concrete-filled　steel　tubes　with　circular　cross-section.　For　reinforced　concrete　columns,　this　simplified　formulation　is　obtained　by　assuming　an　elastic-perfectly　plastic　behavior　of　the　reinforcing　steel　and　the　parabola-rectangular　strain-stress　relation　for　the　concrete　in　compression.　As　regards　concrete-filled　steel　tubes,　full　plasticity　conditions　are　considered.　Interestingly,　a　single　closed-form　expression　is　provided　for　both　cases,　wherein　only　one　parameter　(which　depends　on　the　mechanical　ratio　of　the　reinforcing　steel)　is　calibrated　by　means　of　a　numerical　optimization　procedure.　The　accuracy　of　the　formulation　is　demonstrated　through　several　numerical　applications.　Finally,　a　simple　design　procedure　based　on　the　approximated　interaction　diagrams　is　developed.　©　2016　Elsevier　Ltd