Soil-Structure Interaction with PLAXIS (Tech Preview)

RAM Elements can export a structural information file to perform the Soil-Structure Interaction analysis in PLAXIS and then import structural solution result files to get the analysis results for the load conditions analyzed in PLAXIS or create bi-linear springs in case other load conditions are required to be analyzed considering the soil-structure interaction at RAM Elements side. All files in this wiki can be found here.

The workflow is described as follows:

1. Export structural information to PLAXIS

Let’s consider the concrete building Concrete.etz which is required to be analyzed using Soil-Structure interaction as there is some deep foundations planned due to the poor soil conditions.

To export a structural information file, press the PLAXIS Soil-Structure interaction button in the Export tab.


This option opens the Create a structural information file dialog where it is required to select the load conditions to be included in the files and the option to create the information file in json format if it is required to check the information sent to PLAXIS.

 Once all load conditions required to be sent to PLAXIS are selected, the OK button will create a ZIP file with the *.ssi file extension. 

2. Analyze soil-structure interaction in PLAXIS

To use the structural information files in PLAXIS, the *.ssi file needs to be unzipped in a folder. It is important to notice that for each load condition selected a different file with the name of the model and the load condition ID has been created. Each of these files contain the complete geometric and load information from the load conditions selected.

Let’s consider the PLAXIS FoundationModel.p3d that has been created modeling soil and the deep foundation conditions below. Notice that PLAXIS automatically rotates the RAM Elements structural model to have Z axis up; therefore, there is no need to rotate the model in RAM Elements side.



In order to import the structural model information, it is required to use the Import super element option.

This will open the Import super element dialog where the structural information files need to be selected.

As there are three structural information files, then three super elements were created.


From each super element a structural model needs to be generated. Select each super element and create the structural model using the option below.

This will create three structural models, one for each super elements imported.

Before analyzing the model, it is required to set the shared domain between the structural model and the soil model. The shared domain is defined as the contact points or planes between the structural model and the soil model. In the soil model, the shared domain is defined as the polygon that represents the pile raft. Therefore, we need to define this polygon as the shared domain. In the command line we need to enter the following commands.

add StructuralModel_1.SharedDomain Foundation
add StructuralModel_2.SharedDomain Foundation
add StructuralModel_3.SharedDomain Foundation

These will define Foundation as the shared domain.


Then, create three different phases in the Staged construction tab and enable the Structural models, one in each construction stage. Finally, Calculate the stages (Run the analysis).

Once the Soil-Structure Interaction analysis is done in PLAXIS, it is required to get the structural results from the PLAXIS model folder. These are easy to locate as they have the name of the structural information file plus the analysis step with the result extension as below.

As these files can be numerous, RAM Elements requires to group all of them in a zip file with the ssi extension. Therefore, using any available zip tool, save all the result files in a zip file with the ssi extension. In this case the results zip file is saved as FoundationModel.p3dat.ssi

3. Import structural solution from PLAXIS

Once that the analysis results are available, the next step is to import these to RAM Elements. There are two options to import the analysis results which are shown in the Export tab below.

Having the model Concrete.retx opened, the first option to Import PLAXIS Soil-Structure Interaction results is the “Direct approach” that directly reads the result files from PLAXIS and post-process them to get the internal forces and stresses.

This option opens the Import results from PLAXIS dialog where it is required to browse the *.ssi (zip) file with the structural results. Once this is selected all results will be available to match them with the corresponding load condition in the model. Notice that there might be several load step results for each load condition being the last step the one that contains the results for the full load step.

Once each load condition has been matched with a result file press OK. This will automatically enable all analysis results options for the selected load conditions.


Now, it is possible to check analysis result for the load conditions analyzed in PLAXIS.

The second option is to Import PLAXIS Soil-Structure Interaction Springs also called the “Substructure approach” that creates bi-linear springs in all nodes in contact with the soil. This option opens the Import supports from PLAXIS results dialog where it is required to browse the *.ssi (zip) file with the structural results. All Displacement vs Force data is shown for all nodes in contact with the soil for all DOFs. The dialog has the option to considered Compression only springs and to include rotational springs. Once all support nodes and DOFs have been checked, the button OK assign these to the model support nodes.

Finally, an analysis can be performed in RAM Elements to get the Soil Structure Interaction for all load conditions in the structural model using the springs defined from the analysis results from PLAXIS.