Semi raft design etabs

[Mohammddcharara]

Hello engineers , I have a question that never had an answer to it . While designing semi raft which is haveing a small thickness raft foundation with isolated footings in each columns that needs a more thickness and moment. This will make the footings work as continuous strips in Etabs and safe which will make the moment to be doubled if the load is huge (400 tons for example) is this actually true ? Or we should consider that the isolated part in the raft with more thickness in still working as isolated? If yes is there a way to release it and make it happen in etabs and safe? Thank you !

 

[HTURKAK]

You may model the raft with FEM and the soil with WINKLER springs and compare the results for hard soil and soft soil. . If you harden the springs ( for stiff soil ) , the thickened portion under the columns , will approach spread ftg. If you soften the springs , the raft will behave like a plate , shell element .

 

[Mohammddcharara]

You may model the raft with FEM and the soil with WINKLER springs and compare the results for hard soil and soft soil. . If you harden the springs ( for stiff soil ) , the thickened portion under the columns , will approach spread ftg. If you soften the springs , the raft will behave like a plate , shell element .

How can i do that exactly in etabs or safe ? And is there a reference so I can read more since I never heard about that . As in etabs usually my foundation calculation is based on adding subgrade modulus is it the same approach you are talking about?

 

[Ammaniya]

Honestly, it's a bit of a gray area. A thicker section might act more like a continuous strip, but it really depends on how the loads distribute. I'm not sure if SAFE or ETABS can accurately model that; might need to experiment with different setups.
Good luck.