A　complete　shear　strength　model　is　proposed　to　define　the　relationship　between　seismic　shear　strength　and　lateral　drift　ratio　of　circular　concrete　columns.　The　proposed　bi-linear　model　comprises　the　initial　shear　strength　branch　and　the　shear　strength　degradation　branch,　and　it　can　trace　the　degradation　of　shear　strength　along　with　lateral　deformation.　The　formula　to　assess　the　initial　shear　strength　is　derived　from　the　equilibrium　of　the　forces　acting　on　the　primary　shear　failure　plane　and　Mohr-Coulomb　failure　criteria　for　concrete,　while　the　degradation　slope　of　shear　strength　is　associated　with　a　factor　indirectly　representing　the　contribution　by　the　so-called　dowel　action　of　longitudinal　reinforcements.　Another　feature　of　the　proposed　model　is　that　it　can　be　used　to　directly　calculate　the　seismic　shear　strength　of　circular　columns　without　transforming　circular　section　into　an　equivalent　rectangular　or　square　section.　In　order　to　calibrate　the　proposed　model　and　verify　its　reliability　and　accuracy,　eighty-eight　relatively　large　scale　circular　concrete　columns　that　many　researchers　have　reported　failing　in　shear　are　collected.　These　previous　tests　cover　a　wide　range　of　structural　factors　such　as　concrete　strength,　axial　load　ratio,　yield　strength　and　amount　of　transverse　as　well　as　longitudinal　reinforcements,　and　shear　span　ratio.　Comparisons　between　the　experimental　results　and　the　calculated　ones　indicate　that　the　proposed　model　can　predict　the　seismic　shear　strength　and　trace　the　shear　strength　degradation　till　large　deformation　more　accurately　than　previous　models.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.