Compliant base for dynamic analysis

Question

Good morning, 
 
 I am wondering if the compliant base at the horizontal base of the model is capable or not to dissipate also downward waves with both horizontal and vertical direction (model with horizontal line displacement for the horizontal component of the ground motion applied at the base and fixed vertical displacement at the base). 
 
 Has anyone tried to run a dynamic analysis applying, instead of the fixed vertical displacement for the base, a vertical accelerogram with zero amplitudes? 
 
 Thank you in advance, 
 
 Andrea Balia

Dennis Waterman · Accepted Answer

Dear Andrea, 
 As I already explained above Model A doesn't dissipate vertical waves and model B does. 
 The reason Model A doesn't dissipate is because you've set the Displacement_y to Fixed. If you would set it to Free it would dissipate vertical waves. 
 With kind regards,

Dennis Waterman · Answer

Dear Andrea, 
 The compliant base will indeed dissipate downward waves. In fact, that is the main reason why the compliant base boundary condition exists since a "regular" boundary conditions with a prescribed displacement cannot disspitate downward waves due to the prescribed displacements. 
 With respect to part 2 of your question....I have never tried, but I wouldn't immediately see the reason for it. If this is related to your question whether a compliant base will dissipate downward waves: yes it will, always. Independent of whether a vertical acceleration is assigned or not and whether that vertical acceleration is zero or has values. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Jamal, 
 The waves measured contains both upwards and downward motion waves, but in the project only the upward motion should be applied. We assume that the upward motion is half the total measured signal, hence u = 0.5. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Andrea, 
 No, the results will not be identical, because it's not the same boundary condition. 
 Applying a zero displacement or applying a zero acceleration is not the same.... Be aware that applying a zero acceleration does not mean that the acceleration will necessarily be zero during the entire calculation as the compliand base transforms the acceleration into a load (stress) and that load is applied, not the acceleration itself (which is the whole idea behind a compliant base). So in case of applying a zero acceleration the boundary may move due to internal waves reaching the boundary which will then be dissipated. But if you apply a zero displacement the boundary will act as a hard reflective boundary. 
 This is exactly why a compliant base was "invented"....to avoid this hard reflection. So it would be strange to then overrule that again by setting the boundary to fixed. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Jamal, 
 If you have data that is indeed only upward motion data you can use u=1.0. I'm not so sure though that PEER data is upward motion data, so you would have to check that. 
 With kind regards, 
 Dennis Waterman

Compliant base for dynamic analysis

Top Replies