Previous　studies　on　reinforced　concrete　(RC)　beam-column　subas-semblies　under　a　column　removal　scenario　are　helpful　to　under　-stand　the　load-resisting　mechanisms　of　RC　structures　against　progressive　collapse,　but　most　of　these　studies　failed　to　simulate　actual　boundary　conditions,　which　were　simplified　as　fixed　bound-aries　to　allow　sufficient　development　of　the　load-resisting　mecha-nisms.　These　studies　were　unable　to　reflect　the　response　of　joints　and　side　columns　under　progressive　collapse.　To　fill　this　gap,　an　experimental　program　on　six　half-scale　beam-column　subassem-blies　with　joints　and　side　columns　was　designed　and　tested　to　fully　understand　the　effects　of　boundary　conditions　on　the　structural　behavior　of　RC　planar　frames　against　progressive　collapse.　Three　subassemblies　were　specially　designed,　while　the　other　three　were　ordinarily　designed　to　quantify　the　benefits　of　special　detailing.　The　test　results　show　that　the　effects　of　boundary　conditions　on　the　development　of　load-resisting　mechanisms　are　marginal,　whereas　the　effects　of　special　detailing　are　significant.　Specifically,　speci-mens　under　a　middle-column　removal　scenario　and　a　penultimate-column　removal　scenario　develop　similar　compressive　arch　action　(CAA)　capacities　and　catenary　action　(CA)　capacities.　The　CAA　capacity　dominates　the　load　resistance　of　specimens　with　ordinary　detailing.　In　contrast,　the　CA　capacity　governs　the　load　resistance　of　specimens　with　special　detailing　mainly　due　to　the　larger　areas　of　longitudinal　reinforcing　bars　and　the　greater　rotation　capaci-ties　of　beam　ends.　However,　boundary　conditions　can　greatly　affect　the　failure　mode　of　specimens　with　ordinary　detailing.　Finally,　an　analytical　study　was　performed　to　demonstrate　the　contributions　of　axial　force　and　shear　force　to　load　resistance.　According　to　test　results　and　analytical　analyses,　RC　frames　with　special　detailing　have　sufficient　rotational　capacity　to　develop　adequate　tie　forces　to　resist　progressive　collapse.