The　direct　displacement-based　design　(DDBD)　is　based　on　the　equivalence　between　the　nonlinear　hysteretic　response　of　a　structure　and　a　simple　linear　oscillator　with　equivalent　viscous　damping　(EVD).　Typically　the　EVD　is　calibrated　for　different　types　of　structures　as　a　function　of　the　ductility.　However,　those　simple　relationships　exhibit　a　huge　dispersion　and　uncertainty　since　the　specific　properties　of　the　structural　system　are　not　considered.　This　work　proposes　an　original　approach　for　estimating　the　EVD　of　infilled　reinforced　concrete　(RC)　frames.　We　focus　on　a　ductility　demand　corresponding　to　an　ultimate　limit　state　and　we　investigate　the　effect　of　the　properties　of　infilled　RC　frames　on　the　EVD.　A　data-driven　hysteresis　model　proposed　by　the　authors　in　a　previous　research　is　implemented　in　OpenSees　to　represent　the　nonlinear　response　of　infill　panels.　The　optimal　EVD　at　the　ultimate　limit　state　is　computed　using　an　extensive　series　of　time-history　analyses　(THAs)　on　a　dataset　of　14　infilled　RC　frames.　Afterwards,　an　empirical　correlation　law　for　the　estimate　of　the　optimal　EVD　is　calibrated　as　a　function　of　the　geometrical　and　mechanical　properties　of　infilled　RC　frames.　This　formula　is　meant　to　be　a　practical　tool　for　a　preliminary　design　of　infilled　RC　frames.　A　comparison　with　the　EVD　estimated　by　quasi-static　tests　and　the　EVD　of　bare　frames　is　made.　The　methods　and　results　presented　in　this　work　may　be　the　basis　for　more　robust　predictions　of　the　EVD　of　infilled　RC　structures.　©　2023　Elsevier　Ltd