FEM Modelling

Question

Good day members. I have a doubt and I would like to have this solved. 
 So, what is the minimum distance between the model and the boundaries when modeling with any FEM software, do you have any referencethat may have some basic theories about this topic? 
 A small model may have any influence on the output results? How? Any reference? 
 
 Thanks in advance.

Dennis Waterman · Accepted Answer

Dear Cristina, 
 It depends a bit on the type of analysis you're doing, particularly seismic analysis needs far larger models than static analyis. But as a rule of thumb if you look at the construction zone D x W (depth, width of an excavation, embankment, foundation etc) and you take the largest dimension of that area, then use a model with in all directions at least 3x times that largest dimension. So for an embankment that largest dimension is usually the width W, so use 3*W on either side of the embankment &aacute;nd also below the embankment. Under specific conditions a smaller model may be good enough, but there is not really such a thing as a too large model, but there can be a too small model. If the model is too small it may underestimate deformations and overestimate factor of safety and/or failure load. 
 The idea is that a boundary is so far away from the construction zone that stresses close to the boundary remain (almost) unchanged from the initial stresses during the construction. The lower boundary may experience a stress increase that is max 10% of the applied load. With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Hi Cristina, We have gathered a lot of this information as rules of thumb in a presentation. In this presentation we refer to the following literature, so maybe you want to use them too: 
 Potts D.M., Zdravkovic L. (2001). Finite element analysis in geotechnical engineering &ndash; Application . Thomas Telford, London. 
 Mei &beta; ner H. (2002). Baugruben &ndash; Empfehlungen des Arbeitskreises 1.6 &ldquo; Numerik in der Geotechnik&rdquo;, Abschnitt 3 , Geotechnik 25, 44-46. 
 Schweiger H.F. (2002). Musterl &ouml; sung und Parameterstudie f &uuml; r dreifach verankerte Baugrube , Geotechnik 25, 101-109. 
 Ruse N.M. (2003). R &auml;umliche Betrachtung der Standsicherheit der Ortsbrust beim Tunnelvortrieb . PhD thesis. Institut f&uuml;r Geotechnik. Universit&auml;t Stuttgart. 
 Vermeer P.A., Wehnert M. (2005). Beispiele von FE- Anwendungen &ndash; Man lernt nie aus. In: FEM in der Geotechnik (ed. Grabe et.al.). Technische Universit&auml;t Hamburg-Harburg. 
 Brinkgreve R.B.J, Bakker K.J., Bonnier P.G. (2006). The relevance of small-strain stiffness in numerical simulation of excavation and tunnelling projects . In: NUMGE 2006 (ed. Schweiger). Taylor & Francis, London. 133-139. 
 With kind regards, 
 Dennis Waterman

FEM Modelling

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