Boundary Conditions

Question 1

Affecting forces

Answer 1

-point loads: columns
-boundary stress: udl that represents raft foundation, usually translated to nodal F
-body force: gravitational loading
-construction: reduction of DoFs due to deactivation of elements is followed by an increase as the layers are built and the loads applied to previous layers
-excavation: removes nodal F that the body of soil applies, applying an equal an opposite one

Question 2

Affecting displacement

Answer 2

Prescribed displacements:
-required to stop rigid body movements
-strong material @ bottom of model has u=v=0
-line of symmetry at centre and side boundaries have u=0, Fy = 0 so no shear stress
-ground surface has Fy=0
-rigid prop has u = 0

Question 3

Affecting all

Answer 3

Local axis: allows BC to be applied @ an angle to the global axis

Springs: Replace elements that only hold axial F such as beams, columns and end bearing piles
* Assumes linear with K = EA

Tied DoFs: gives an equal but unknown displacement
* for rigid footings Fx = 0 for smooth or u = 0 for roof