Question
How do I model a steel plate in LimitState:GEO?
Solution
A steel plate can be modelled in LimitState:GEO in several different ways:
Modelling a plate using an 'Engineered element' material
This method is generally the easiest. You can model the plate by assigning an engineered element material to the boundary where the plate lies. Simply create a new material and give it an 'Engineered element (1D)' type.
If you want the plate to act as a rigid element, select 'Sheet pile wall (rigid)' as the application. If you want the plate to yield, choose 'Sheet pile wall (can yield at vertices)' as the application and assign an appropriate value of Mp.
Assign the new material to the boundary and also add a suitable derived plate / soil interface material to permit a realistic interaction.
Lastly, if you are modelling a yielding plate you will need to add vertices at regular intervals along the boundary to act as potential hinge positions.
More about Engineered Elements is given in Section 9.5 of the Theory and Modelling Guide. Note that this method does not account for the self-weight of the plate as part of the analysis.
If you wish to include the weight of the plate in the analysis, or if at any time you wish to model a structural element with depth (e.g. steel beam, reinforced concrete slab etc) you can also do this:
Modelling a yielding element using solids with hinging interfaces
You can also model structural elements as solids as having a finite depth. Draw a series of solid elements separated by interfaces at which plastic hinges can form. For the solids themselves, make sure they have the correct depth and assign them a rigid material with a self-weight. For the interfaces between the solids, assign a cutoff material with appropriate tensile and compressive limiting yield stresses.
You will need to also ensure that sufficient nodes are present along the interfaces between the solids. These provide potential positions for the neutral axis of the stress block over the depth of the beam (i.e. not just at the top and bottom faces - see Figure 1). Set the Nodal Density to your preferred value and then click Analysis > Preview Nodes. On the internal interfaces of the beam you should aim for 8 or more nodes to give a reasonable result. If you do not see this many nodes, adjust the Baseline Nodal Spacing on these interfaces.
Figure 1 - Provision of nodes along a boundary to act as potential hinge poitions.
Lastly, do not forget to assign plate / soil interface properties to the boundaries in contact with the surrounding soil.
Examine the two example files demonstrating these methods. The same problem is being modelled in both cases. The Mp value in the plate example is 48.125 kNm/m width, whilst the beam example uses a cutoff material with a limiting stress of 77000 kN/m2/m width (Mp = yield stress*(b*d2)/4, hence yield stress = 4*Mp/(b*d2) = (4*48.125)/(1*0.052) = 77000 kN/m2/m).
The soil / plate interfaces have been assigned multipliers of 0.5 on c and tan phi and the self weight of the solids in the 'Beam' example has been set to zero to allow comparison. You can see that the results are very similar to each other (within 0.5%). The discrepancy arises as the beam model is more sophisticated and can account for the longitudinal stresses induced in the beam due to friction along the base. If you remove friction from the interface material by setting the multipliers to zero then the calculated adequacy factors are almost identical.
If you want to see what would happen if using a rigid plate, set the Mp value of the Plate material to 1e+30.