How to model a tieback wall in a FEA program?
How to model a tieback wall in a FEA program?
(OP)
I am a new engineer at a structural engineering firm. My first 3 projects have all been walls. First a cantilever, Second a Tieback, Third a soil nail. The 60% design I submitted for the tieback needs to be updated for our next submittal. I did some rudimentary calcs for the tieback wall by hand but am wanting to run a more in depth analysis. The problem is that I'm not quite sure how to model it. At first, it seemed simple. Create a mesh of kirchoff plate elements, apply the loads on the backside (Earth, Water, LLS, Seismic, etc.) and then pin the structure where the anchor heads are located. But as I got to thinking about this, this is not correct. The anchors are post-tensioned to 60kips. I would somehow need to apply a 60 kip force at each of the anchor head locations. The question then is how do I support the structure? Where do I apply the restraints and how? Just to be clear, I am only interested in modeling the wall itself and have no interest in the soil interaction with the tiebacks. That configuration has already been chosen by the geotechnical engineer. I need to get my internal bending moments so that I can design the reinforcing in the wall. Any advice would be greatly appreciated.






RE: How to model a tieback wall in a FEA program?
I suppose you could model your tiebacks as pins and make sure the reactions at these pins don't exceed 60k. The drawback to this is your model won't correctly indicate the stresses in the front face when tensioning is applied.
RE: How to model a tieback wall in a FEA program?
The anchors should be examined for punching shear, as for columns, and this may determine the wall thickness. Again, uniform thickness will usually keep costs lowest. Formwork and labor is most of the expense of concrete constructon, not the amount of concrete or reinforcement.
The weight of the wall should be placed on a strip footing (or other foundation) to avoid laterally loading the tiebacks. There will be a friction component between wall and soil, but this could vary significantly with changes in soil moisture and with creep of the anchors.
RE: How to model a tieback wall in a FEA program?
Thank you for the detailed information. I did already model it as a two-way slab with column-strips and everything. I was wanting to compare these results with a FEA model to see how accurate or overly-conservative my calculations were. In my opinion, modeling it as a flat plate with the anchor heads treated as columns is not correct since the tiebacks are post-tensioned. I feel as if the stress concentrations around the lock-off zone will be much greater. The geotech gave me loading configurations for the soil, water, seismic, etc. but these were decided independently of the stress in the tie back bar. You are probably correct, I am complicating this problem but I want to feel confident that my design is accurate.
Splitrings,
Yes, post tensioned tieback bar tensioned against some initially poured stressing blocks with steel anchor plates. No, geotech has not given me wall displacements, just loads. I would like to model it with springs but I'm not sure about the stiffness to apply to the springs. I definitely agree with your last comment.
RE: How to model a tieback wall in a FEA program?
RE: How to model a tieback wall in a FEA program?
But if you want to know the load distribution on the wall you need to have an interest in the soil interaction with the wall, even if you just model the tiebacks as point loads. You could adequately model this with some "soil springs" if you knew how stiff to make them, but since you don't (and published typical figures can be highly misleading) why not model the soil with plate elements in a 2D plane strain FE analysis?
Or get the geotech engineer to do it and provide you with a load distribution on the wall.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/