This　study　employs　a　multiscale　numerical　approach　to　establish　models　of　reinforced　concrete　(RC)　beams　under　different　structural　spatial　constraints　to　investigate　the　influence　of　structural　spatial　constraints　(beam　end　constraints　and　adding　cast-in-place　floor　slabs)　on　the　yield　mechanism　of　frame　structures.　It　quantitatively　analyzed　the　influence　of　end　restraints　and　cast-in-place　floor　slabs　on　the　shear　capacity　and　stiffness　of　RC　beams.　It　revealed　the　mechanisms　by　which　end　restraints　and　cast-in-place　floor　slabs　affect　the　shear　behavior　of　RC　beams　and　compared　the　simulated　results　with　the　load　capacity　calculated　results　according　to　current　codes　of　various　countries.　The　study　found　the　following:　1)　Beam　end　restraint　conditions　significantly　affect　the　beams＇　failure　mode,　whereas　cast-in-place　floor　slabs　have　little　effect.　2)　Changing　the　beam　end　restraint　from　simple　to　fixed　supported　dramatically　increases　the　stiffness　and　shear　capacity　of　the　beams.　Fixed-end　beams　can　have　stiffness　and　shear　capacity　up　to　3.49　times　and　2.66　times　higher,　respectively,　compared　to　simply　supported　beams.　3)　Adding　cast-in-place　floor　slabs　significantly　increases　the　shear　capacity　and　stiffness　of　the　beams.　Adding　cast-in-place　floor　slabs　to　simply　and　fixed　supported　beams　can　increase　the　shear　bearing　capacity　of　the　beams　by　1.50　times　and　1.39　times,　respectively,　and　increase　the　stiffness　by　2.68　times　and　1.63　times,　respectively;　4)　The　prediction　of　the　shear　bearing　capacity　of　fixed　supported　beams　and　beams　with　cast-in-place　floor　slabs　in　various　national　codes　is　exceptionally　conservative.　The　simulated　values　of　fixed-supported　plus　cast-in-place　slab　beams　with　a　shear-span　ratio　of　1.0　are　4.00　times,　4.47　times,　6.16　times,　and　5.18　times　higher　than　the　calculated　values　in　Chinese,　American,　Canadian,　and　European　codes.