when we are going to model any problem in fea we have to choose which element type we have to use, like 1D , 2D or 3D element, we can solve all type of problems using 3D elements but for sake of simplicity and ease to work we can simplify the problems using 1D or 2D.
1. Complex parts has to be meshed as 3d elements
2. Its surface has uniform thickness hence can be meshed as 2d elements
3. Frame composed of uniform section bars hence can be meshed as 1D
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