As shown in the figure, the hollow cylinder, with a wall thickness of 0.4 m, is placed in the soil, and the prescribed displacement in the z direction is applied to its bottom to be +0.001 m. The resultant force Fz should be positive, but the result is negative. Why is this ?Is it because the bottom node of the cylinder is shared with the soil? Can I solve this problem by adding an interface at the bottom?
Dear Xin Fang,
I would expect so, but with the details given, I cannot explain why.
I did run a simple model with a prescribed displacement, and the sign was correct, as you described.
You mention that the sign was wrong, but is the value incorrect, too?
Note that an interface should be added if you want to include the soil-structure interaction in your simulation. Typically concrete vs soil/rock does require interfaces, but on the other hand I do not really know all the details of what you are trying to achieve.
If you want us to assist you with this question, it is good to check the project directly. Then, one of our support engineers can help you in detail: https://bentleysystems.service-now.com/csp
Dear Stefanos,
Yes, the value of the result is also wrong. My purpose is to find a total Fz that can make the soil at the bottom of the cylinder not compressed ( so I applied a smaller upward surface displacement + 0.001m to the bottom of the cylinder ). In a word, the cylinder is only subjected to total Fz and side friction resistance without end resistance. I tested three cases : a ) The bottom of the cylinder is in direct contact with the soil below and no interface element is added, and the result is negative. b ) An interface element with Rinter = 0.01 is added, and the result is positive and close to the ideal value. c ) An interface element with Rinter = 1 is added, and the result is still negative. I want to know if I apply an upward surface load or surface displacement to the bottom of the cylinder and do not add an interface element, does it mean that the cylinder and the soil at the bottom will move up at the same time, but in fact, I only want to move the cylinder up a small distance. Can you give me some advice?
Please take a look at my question at your leisure, thank you!
I really want to know how to apply a prescribed surface displacement to the bottom of the cylinder, so that the part of the soil in contact with the bottom of the cylinder does not move up at the same time.
The Rinter is a factor that reduces shear stiffness, cohesion, friction angle and dilatacy (if applicable). It was never meant to be used with values lower than 0.5 as it was mostly implemented to work on loading conditions where there is resistance between the soil and the structure.
If you want to apply a reduced behaviour, I would go for a Custom material used for the Interface option, in which you can configure the properties directly. In this case the Rinter = 1.00 as you will manually adjust the properties you want.
Since this is project related questions, please consider opening a new Case for support if you need further assistance.
Then, one of our support engineers can help you in detail: https://bentleysystems.service-now.com/csp
Answer Verified By: xin Fang