Reached total force Fy- Prescribed displacement over a footing on piles example.

Question

Dear all, I'm trying to find the pressure applied over a footing on piles that leads to a displacement of 0.025 m. The method of Prescribed displacement is used and the reached total force Fy is checked once the calculation is done as shown in the following figures: 
 
 In addition, a curve was drawn showing the vertical displacement at the center of the footing vs Fy: 
 
 However, when a cross-section is made just under the plate, the total normal stress is significantly different: 
 
 I hope that you can answer my inquiries: 
 1- Phases window shows Fy as a force in kN whereas the unit in the curve is kN/m, which one is true? 
 2- What forces should we add to the total normal stress in order to find Fy? is it the minimum Axial force in both piles (that are 181.6 & 179.9 kN)? 
 3- In which case can we take Fy as the bearing capacity of the whole foundation? 
 4- In the case of a rectangular footing of width B without piles, once Fy is extracted for a displacement of 0.025 cm, how can I convert it to a line-load that if I replace the prescribed displacement with it will result in the specified displacement? is this line load (kPa) the bearing capacity of a (B x 1 m) footing? 
 5- Finally, is the value of Fy extracted an ultimate load or an allowable load? (qu or qall in bearing capacity equations). 
 Thanks in advance!

Dennis Waterman · Accepted Answer

Dear Mohamad, 
 1) They're both true .... it's just that in the Phases window it has already been multiplied with the slice thickness of 1m (or 2*pi if it's an axisymmetric model). But considering it's plane strain using the unit kN/m is probably better. 
 2) Indeed you should add the pile forces. However, not the minimum axial force, but the axial force at the pile head as that is the node where the prescribed displacement is applied and thus where the reaction is found. 
 3) In case of failure 
 4) The easiest way is to simply divide Fy over the width of the footing. However, you will get entirely the same results unless the footing is extremely stiff. When using a prescribed displacement you force the footing to settle perfectly equal as if the footing is infinitely stiff. If in reality it is not infinitely stiff with a line load the footing will bend a bit and thus your settlements will vary a bit over the footing. 
 5) The value of Fy is the reaction force to the prescribed displacement. It depends on whether the value is taken at failure or not, what you consider failure and whether you used (partial) load and material factors if you want to consider it an ultimate load, design load, allowable load or something else. That is not up to the software to decide, but up to the user. 
 With kind regards, 
 Dennis Waterman

Reached total force Fy- Prescribed displacement over a footing on piles example.

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