A　comprehensive　study　combining　experimental　tests　and　numerical　analysis　was　conducted　to　investigate　the　damage　mechanisms　of　reinforced　concrete　(RC)　structures　under　seismic　loads　primarily　acting　in　the　non-principal　axis　direction.　The　results　showed　that　joints　subjected　to　non-principal　axis　loads　experienced　significant　damage　at　the　column　ends,　undermining　the　＇strong　column-weak　beam＇　design　principle.　Using　the　validated　finite-element　model,　a　RC　frame　model　was　developed　and　dynamic　time-history　analysis　was　conducted,　providing　insights　into　the　damage　characteristics　of　the　structure,　focusing　on　plastic　strains,　inter-storey　drift　and　base　shear　forces.　The　findings　showed　that,　when　seismic　loads　act　along　the　non-principal　axis,　the　force　mechanism　of　the　structure　changes　substantially.　In　this　direction,　the　structure　exhibits　higher　load-bearing　capacity　and　increased　force　demands,　leading　to　significant　plastic　strains　at　the　column　ends　and　compromising　the　column　hinge　mechanism.　These　results　highlight　the　need　to　consider　the　influence　of　orthogonal-plane　beams　on　column-end　restraints,　as　horizontal　seismic　forces　in　the　non-principal　axis　direction　may　exceed　those　in　the　principal　axis　direction.　Therefore,　structural　damage　in　the　non-principal　axis　direction　should　be　considered　when　evaluating　and　designing　RC　structures　for　seismic　performance.