The　modal　superposition　response　spectrum　method　(RSM)　has　been　widely　applied　in　seismic　analysis　and　the　design　of　aboveground　structures.　The　RSM　considering　SSI　has　evolved;　however,　most　methods　use　substructure　models　to　simulate　SSI　effects.　The　direct　method　can　simulate　the　SSI　effect　more　accurately　but　is　rarely　used　in　the　RSM　because　the　high　degree　of　freedom　required　for　soil　significantly　reduces　the　efficiency　of　modal　analysis.　To　address　these　challenges,　first,　an　efficient　analysis　technique　is　proposed　that　is　based　on　iterative　static　analysis.　This　method　can　efficiently　obtain　the　effective　frequencies　and　modes　of　aboveground　structures　from　the　SSI　system.　Subsequently,　the　effects　of　the　site　shear　wave　velocity　and　different　boundary　locations　on　the　structural　effective　frequencies　in　the　SSI　system　are　investigated　by　the　proposed　method.　Finally,　the　effective-modes　superposition　RSM　is　proposed　for　seismic　response　analysis　of　aboveground　structures　while　considering　the　SSI.　Examples　are　provided　to　demonstrate　the　effectiveness　of　the　effectives-mode　superposition　RSM　in　the　seismic　analysis　of　aboveground　structures.