Equivalent length for FE anchor and N2N anchor

Question

Hey! 
 I've always been a bit confused about how to set the equivalent length of FE and N2N anchors. 
 What I understand by far is that a FE anchor usually used when it's symmetrical excavation and the equivalent length is half of the excavation length. While a N2N anchor can be used to as an alternative when the structure is supported in multiple directions and the equivalent length of the N2N anchor needs to be adjusted accordingly. Is this correct? 
 Does it mean, if I have a non-symmetrical excavation, for example in the figure below, I should use N2N anchor instead of FE anchor (or is it possible to use FE anchor as well)? And for this N2N anchor, I don't need to consider anything regarding equivalent length? 
 Thanks a lot for help!

Kalyani Singh · Answer

Hi Xuyan, 
 The equivalent length of a fixed end anchor is indeed considered as half of the excavation length. However in case of node to node anchor PLAXIS considers the out-of-plane spacing between the anchors and accordingly the axial stiffness is considered. Now coming to the second part of the question you can practically use both the methods which should give the same result. However there are few points which should be considered as a good modelling approach. A fixed end anchor is a point element which is attached to one side and fixed at the other side so it cannot move. But node to node anchor is a two noded elastic spring element which has some fixity in the middle. So if for any reason the excavation moves then it will also move accordingly in the model which is not the case for fixed end anchor. So it depends on you what kind of behaviour you expect in this case of excavation in the model. 
 For more detailed explanation you can refer the reference manual - Chapter 5- Modelling loads and structures and some links are attached here which will be useful. 
 . https://communities.bentley.com/products/geotech-analysis/w/wiki/46137/manuals---plaxis 
 (+) PLAXIS 2D Tutorial 06: Dry excavation using a tie back wall - GeoStudio | PLAXIS Wiki - GeoStudio | PLAXIS - Bentley Communities 
 
 Regards 
 Kalyani Singh

Dennis Waterman · Answer

Dear Xuyan Liu, 
 I'm a bit puzzled by your question as you speak over the equivalent length of a Node-to-Node anchor .... but Node-to-Node anchors don't have an equivalent length, they have a real length (which is the length you draw). The reason that we have to specify the equivalent length for a Fixed-end anchor is because we do no draw the real anchor length, we only specify a spring and thus the user must specify what real length that spring represents. Since for a Fixed-end anchor the end point that is not connected to our model is fully fixed (hence the name Fixed-end anchor) we have to specify the real length from our structure to a point where we can consider our real anchor to behave like fully fixed - and that point is the axis of symmetry, not the other side of the excavation. Therefore the equivalent length should be half the excavation width. This also explains why we call it "equivalent length" and not "real length", if we would call it "real length" you may think you have to specify the full excavation width, which would not be correct. 
 With kind regards, 
 Dennis Waterman