The　crack　development　of　two　cast-in-situ　RC　frame　structures　subjected　to　lateral　loading　was　investigated.　Then　the　develop　reasons　were　analyzed.　The　investigation　indicates　that　the　existence　of　monolithic　slab　in　reinforced　concrete　frame　structure　overstrengthened　the　actual　flexural　capability　of　longitudinal　beams.　Therefore,　the　participation　of　monolithic　slabs　should　be　made　in　consideration　in　structural　design　to　achieve　the　strong　column　weak　beam　philosophy.　©　(2011)　Trans　Tech　Publications.