RE: Plaxis 3D - How to model monopile for obtaining load displacement curves under lateral loads

Dennis Waterman — Mon, 28 Feb 2022 10:23:49 GMT

Dear Vidushi,

Whether something is solid or hollow you determine yourself based on the same model. If you model a cylinder and you would make the outside wall of the cylinder a plate element, inside you keep the soil and you don't do anything with the top and bottom surfaces it's a hollow pipe. If on the other hand you don't do anything with the surfaces forming the cylinder and inside you apply some kind of material representing concrete it will be solid. Same cylinder, different application.

Q1. You don't have to, but it depends on what you exactly want to model. A hollow cylinder open on both sides is just an extruded polycurve. A solid circular cylinder is an extruded polygon.

Q2. There are several ways. You could apply the load on the wall - it will be excentric, but for a lateral load that is OK, for am axial load you probably don't want that. So you could indeed create a surface and apply the load in the middle of that surface - but by creating the surface you define that end of the cylinder as closed, not sure if that is what you want. Alternatively, you could apply the load on the rim of the cylinder as circular distributed load.

Q3. To get bending moments and shear force diagrams you would indeed have to create a beam on the wall surface. Be aware that depending on where on the wall you would place the beam would give you difference bending moment and shear force distribution. Technically you should place the beam in the center line of the hollow pile....but that is of course a bit difficult because it's hollow. If you're only interested in structural forces of the pile as a beam, you may want to consider modelling it as a beam and not as a hollow cylinder.

Q4. As mentioned under Q3, that depends on what results you're interested in. If you're only interested in the bending moment and shear forces of the monopile as a beam, you could model it as a beam. But if you're interested in skin friction, bearing capacity etc. you may need to model it as a cylinder. Probably a solid cylinder with a beam in the center line (to get the structural forces) would be better than a hollow cylinder.

With kind regards,

Dennis Waterman