ETABS is purely building software , you can design any type of building in etabs but you cannot design Retaining walls , basement walls , Underground water tank and Over heard water tank walls in it.
Where as for SAP , it can also be used to design buildings , but modeling buildings in SAP is tiresome work to do because you have to assign Columns in 3d view wherease in etabs you just have to assign columns in Plans i.,e 2d plan.
But in SAP you can design complex structures such as steel structures , Overhead water towers , Any type of Reinforced wall such as basement walls , retaining walls , UGWT and OHWT walls , you can also use SAP for bridge designing and offshore structures , You can design Pilecap and piles in SAP and Also raft can be designed in SAP
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