Cross Project - Soil Structure Interaction [QG-16]

To perform a soil-structure interaction analysis using an isolated substructure model, follow these steps.

Throughout this article, your existing model (superstructure + substructure) is referred to as the Parent Project, and the isolated substructure model is referred to as the Child Project.

1. Transfer Model

1	In the Parent Project, navigate to Organization → Cross-Project Data → Shared Substructure. Select the support line above the substructure you want to model.
2	Open a new All-in-One Bridge Workflow project. This will be the Child Project, which will include the substructure, soil layers, soil set, etc.
3	Navigate to the External References Workflow and open Project Objects. Click the three-dot menu and select “Add Projects…”. A selection box will appear, displaying all your projects. Select the Parent Project. Click the three-dot menu again and select “Select Shared Objects…”. A selection box will appear, listing the shared objects defined in the selected project. Select the Shared Substructure object created in the first step.
4	The Child Model is now ready to be created. Click the three-dot menu again and select “Sync Project…”. Click the Zoom Extent button. The entire substructure beneath the selected support line should now be visible.
5	This method not only copies the project but also establishes a reference to the Parent Project. As a result, any changes affecting the substructure in the Parent Project can be updated in the Child Project by simply repeating Step 4.

2. Transfer Results

Follow the steps below to transfer results from the Parent Project to the Child Project.

1	In the Parent Project, navigate to Organization → Cross-Project Data → Analysis Results → Bottom of Column Forces. For more details please refer to Bottom of Column Forces page. Select the column(s) that belong to the substructure. Click the three-dot menu and choose “Extract Results” to extract the forces.
2	In the Parent Project, navigate to Organization → Cross-Project Data → Shared Substructure and ensure that “Transfer Result Extractions” is set to YES. The results in the Parent Project are now ready to be synced in the Child Project.
3	In the Child Project, navigate to the External References Workflow and open Project Objects. Click the three-dot menu again and select “Sync Project…”. The extracted results can now be viewed under Project Analysis Results.
4	Similar to the model, the analysis results are also referenced. Any changes in the analysis results can be transferred by repeating Step 1 and Step 3. This way, manually copying the results is avoided.

The following sections describe the pre/post processes for Soil-Structure Interaction in OpenBrIM. Changes will be made in the Child Project.

3. Introduce Soil Structure Interaction Definitions

Follow the steps below to define Soil-Structure Interaction (SSI) in the Child Project.

1	In the Child Project, navigate to the workflow you are using
2	Navigate to Substructure → Soil-Structure Interaction in the Child Project. Introduce the Soil Layer and Soil Set. For more details please refer to Soil Structure Interaction page. The soil definitions are now ready to be assigned to the Pile/Pile Group.
3	Navigate to Substructure → Pile → Single Pile/Pile Group. In the FEA tab, select “Nonlinear Spring from the Soil Set” and choose the created Soil Set. Also, ensure the real/exact length of the piles is assigned to the “Pile Length/Section” input. For more details please refer to Single Pile and Pile Layout pages.
4	Ensure that Pile Objects' Sync is disabled. Since changes are necessary in the Pile/Pile Group object to assign soil definitions, disable the sync for these objects by clicking “Disable Sync” in the three-dot menu. This way, when the Child Project is synced to the Parent Project, changes made in the pile objects will not be overridden by the Parent Project’s pile objects. Note that when sync is disabled for objects, a lock icon will appear at the bottom right of the object's name. Additionally, in the workflow tree, these objects will be displayed with a purple badge.
5	Create the Construction Stages and ensure the Nonlinear option is set to YES. In this example, since the Bottom of Column force is extracted, constructing only the Piles and Footing is sufficient.
6	Navigate to Loading → Loads → Water Load → Buoyancy. In OpenBrIM, the buoyancy force is applied using this library component. Ensure that you have created this component and assigned it to a separate construction stage.
7	Assign the Bottom of Column force. Using OpenBrIM’s parametric features is very useful for avoiding future copy-paste operations in case of model changes. Navigate to the External References Workflow → Project Analysis Results. Select all six forces, open the three-dot menu, and click “Copy Parameter”. Then, navigate to the original workflow: Loading → Loads → Element Loads → Footing Point Load. Select all six forces, open the three-dot menu, and click “Paste”.

4. Load Factors and Depth of Fixity

Follow the steps below to learn about Result Combination and Depth of Fixity in Soil-Structure Interaction (SSI) analysis in OpenBrIM.

4.1. Load Factors

Generally, it is practical to extract the Bottom of Column Forces using a Load Combination Table. When this force is set in another model, other created loads remain unfactored. To illustrate these loads, consider the dead load from the footing and piles, water load, and earth pressure (for buried footing). Engineers need to be aware of these factors when post-processing the results.

As a practical approach, the engineer can apply self-weight factors and input factored loads.

4.2. Depth of Fixity

In the Parent Model, to model pile behavior linearly for pile foundations where soil-structure interaction is not a governing factor, the depth of fixity method can be used for simplified analysis.

The effective length of the pile should be identified. This effective length represents the depth of fixity, where the pile's bottom node is located with a fully fixed support. The pile FEA model is created as beam-columns with no support provided along the length. In OpenBrIM, this is achieved by selecting the Soil Spring Type as Linear Spring in the Parent Model. The engineer then needs to identify the node locations and set all of them free except for the bottom one.

To identify the bottom node location, in the Soil-Structure Interaction model, the second inflection point of the bending diagram needs to be selected. An example is provided below.

Which pile should be selected to read the results?

Typically, moment graphs of all piles are similar to each other, so selecting any pile is sufficient to determine the Depth of Fixity.

How can the bottom node location be determined?

There are multiple ways to achieve this; two of these methods are displayed here. For more details about reading results in OpenBrIM, please refer to following page FEA Results [QG-3].

1

Refer to Composite Element Force → Stage → Composite Object, click the three-dot menu, and select Filter. Filter any one of the piles.
After filtering the piles, the second inflection point can be read from the table by interpolating between the moment values and global Z values.

2

Hover over the nodes above and below the second inflection point and read the global (X, Y, Z) locations. Interpolate between the Z values.

4.3. Determine TZ Stiffness for the Parent Model

Instead of using a fully fixed support condition at the pile's bottom node, the engineer can determine a TZ value from the Child Model to assign to the bottom node of the piles in the Parent Model.

To find the TZ stiffness, calculate

at the Depth of Fixity. The steps to determine this value are explained below.
Determine the Depth of Fixity from Section 4.2. The axial force and displacement of the pile at the Depth of Fixity elevation need to be identified.

1	Refer to Node Displacements (Local) → Stage.
2	To filter the nodes in the spreadsheet, in the FEA view, right-click the nodes above and below the second inflection point. The right-clicked elements will be filtered, and two rows should be displayed. Please note that to display these two nodes, you can also filter node names by clicking the three-dot menu. Read the Z displacement values and interpolate for the Depth of Fixity elevation.

1	FELine Force (Internal) → Stage
2	To filter the line element in the spreadsheet, in the FEA view, right-click the line element at the second inflection point. The right-clicked element will be filtered, and two rows should be displayed. Please note that to display these two nodes, you can also filter node names by clicking the three-dot menu. Read the Fx force values and interpolate for the Depth of Fixity elevation.

TZ stiffness,

, can be calculated the obtained values.