In geotechnical engineering, soil presents a complex challenge due to its nonlinear, anisotropic, and path-dependent behavior. When applying Finite Element Analysis (FEA) to model soil-structure interaction, incorporating construction stages into the analysis is essential for achieving accurate simulations, realistic predictions, and safer designs. Neglecting this critical aspect can result in unrealistic stress distributions, underestimated settlements, and compromised structural stability. This article explores the key reasons for adopting this approach and highlights its significance in geotechnical analysis.
- Nonlinearity and Stress History in Soil Behavior
Soil does not behave linearly under load; its response depends on the history of applied stresses. The stress-strain relationship evolves as loads are incrementally introduced during construction. Capturing this progression is essential for realistic stress distribution since construction stages alter the stress distribution incrementally, affecting settlement, pore pressures, and strength.
- Sequential Load Application
In construction, loads are applied incrementally as layers or structures are added. FEM analysis that incorporates construction stages and simulates realistic load applications allows for an accurate prediction of settlements. In this way, overestimation or underestimation of soil response, which can affect structural stability, is prevented.
- Consolidation Effects
Consolidation, particularly in saturated soils, occurs over time as pore water dissipates under loading. Construction stage analysis allows engineers to capture time-dependent consolidation, leading to accurate predictions of settlement and pore pressure evolution.
- Interaction with Existing Structures
For projects involving adjacent or existing structures, the impact of staged construction on nearby buildings or infrastructure can be significant. Staged analysis helps assess risks like differential settlement or structural damage.
- Construction-Specific Challenges
Stage-wise analysis helps address temporary conditions during construction, such as dewatering, excessive settlement, etc. By simulating each stage, engineers can identify potential failure mechanisms early and design mitigation measures to maintain stability during construction.
- Compliance with Design Standards
Modern geotechnical design standards, such as Eurocode 7, emphasize the importance of considering construction stages in analyses. These standards require engineers to evaluate intermediate stability during construction and the long-term performance of the soil-structure system after construction, requiring a stage-wise approach to the design process.
Geotechnical Analysis with Construction Stages in RFEM 6
Having established the importance of incorporating construction stages in geotechnical analysis, let us now examine how to apply this approach using the model outlined below. The model features a reinforced concrete building situated on a soil massif, modeled in RFEM 6. The structure comprises a reinforced concrete slab for each floor, a foundation slab, columns, and vertical walls. The applied loads include the self-weight of the soil, the self-weight of the structure, dead loads, and live loads.
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For those new to the workflow for defining construction stages in RFEM 6, we recommend consulting the Knowledge Base articles listed below. It is important to note that construction stages in the software are defined based on two primary factors: the structural elements active during a given stage, and the loads applied at that stage. To ensure clarity and conciseness, the process will be demonstrated for the first construction stage, accompanied by a table outlining how the same workflow can be extended to define subsequent stages.
- KB 1737 | Defining Construction Stages in Terms of Modeling
- KB 1724 | Consideration of Construction Stages in RFEM 6
In the initial construction stage, the focus is exclusively on the soil. To configure this stage, access the "Construction Stages" window and select the “Solids” and “Surfaces” tabs, as illustrated in Image 1. This step ensures the inclusion of the soil solid as well as the surfaces with predefined boundary conditions. Navigate through the associated tabs to adjust the status of the solids and surfaces. For the solids, select “All”, as no other solids are present in the model at this stage. Similarly, in the "Surfaces" tab, include surfaces with predefined boundary conditions, specifically surfaces numbered 31–47 and 54–57, as depicted in Image 2. To streamline this process, pre-defined object selections can be utilized, enabling you to add all relevant elements simultaneously. This approach not only saves time but also enhances accuracy in setting up the construction stage.
Once the structural changes for the construction stage have been defined, the next step is to specify the load cases active during this stage. This can be done in the "Construction Stages" tab of the "Load Cases & Combinations" window, as illustrated in Image 3. For the initial stage, only the self-weight of the soil is considered, and the corresponding load case is assigned accordingly.
In this step, you can introduce additional options, such as modifying the structure, as required for the initial phase under consideration (see Image 4). This is essential because, in analyses utilizing the hardening soil material, the material must be linearized in the first stage. To achieve this, open the relevant window and deactivate the material nonlinearity, as shown in Image 5.
Next, you can adjust the static analysis settings for the individual stages by opening the “Static Analysis Settings” window using the Main tab shown in Image 4. While no changes are necessary for the initial stage where material nonlinearity is deactivated, it becomes important for subsequent stages, such as CS2, where you can enable the "Equilibrium for undeformed structure" option (see Image 6). This ensures that deformations remain zero, allowing you to retain the stresses from the soil's self-weight. This step is crucial to establish the correct stress state, ensuring that the material model provides the appropriate stiffness.
When looking at the settings in Image 6, you can see that the static analysis settings for the initial phase are configured with a single load increment. However, since loads will be applied in successive phases, additional considerations are necessary, such as adjusting the number of load increments to reflect the evolving load conditions. To address this, a new static analysis setting can be created with an increased number of load increments and assigned to subsequent phases. The specific number of load increments for each phase in this model is detailed in the table provided below, ensuring precise alignment with the construction sequence.
| Construction Stage: | Following: | Objects Added | Load Case Active | Structure Modification: | Number of Load Increments: | Additional Considerations: |
|---|---|---|---|---|---|---|
| CS1 | / | Soil Solid, Surfaces with Boundary Conditions | LC4 | Material Nonlinearity Models Deactivated | 1 | / |
| CS2 | CS1 | / | LC4 | / | 1 | Equilibrium for Undeformed Structure (u=0) |
| CS3 | CS2 | Foundation Elements | LC1, LC4 | / | 2 | / |
| CS4 | CS3 | Ground Floor: Walls & Columns | LC1, LC4 | / | 2 | / |
| CS5 | CS4 | Ground Floor: Ceiling | LC1, LC4 | / | 2 | / |
| CS6 | CS5 | 1st Floor: Walls & Columns | LC1, LC4 | / | 6 | / |
| CS7 | CS6 | Roof Ceiling | LC1, LC4 | / | 6 | / |
| CS8 | CS7 | Dead Load | LC1, LC2, LC4 | / | 10 | / |
| CS9 | CS8 | Live Load | LC1, LC2, LC3, LC4 | / | 10 | / |
| Load Case | Load Applied |
|---|---|
| LC1 | Structure Self-Weight |
| LC2 | Dead Load |
| LC3 | Live Load |
| LC4 | Soil Self-Weight |
As highlighted in the introduction, it is crucial to define subsequent construction stages in a manner that accurately reflects the construction process and loading while realistically capturing soil behavior. By following the workflow outlined for the initial construction stage (CS1 Start) and applying the provided notes for other stages, Tables 1 and 2 can serve as a guide for defining the subsequent stages. Note that all construction stages are analyzed using a geometrically linear approach, with the Newton-Raphson method employed for nonlinear analysis. Adjustments to other analysis settings, such as the number of load increments, are detailed in the table below.
Once the construction stages are defined, the next step is to adjust the settings for generating load combinations before initiating the calculations. This can be accomplished in the Combination Wizard for the relevant design situation. Here, you can configure the analysis settings and enable the option to consider an initial state, allowing the assignment of the defined construction stages, as shown in Image 7. This method ensures that load combinations are generated for each stage while incorporating the initial state from the preceding stage, providing a seamless and accurate transition between construction phases.
You now have all the necessary information to initiate the calculation and analyze the results. For instance, you can view displacements at each construction stage, as well as the final displacements corresponding to the completed CS9 stage, where the structure is fully built and all loads are applied. Additionally, you can choose to display results as differences within a load increment in a stage or relative to the preceding stage (Image 8). This enables you to observe the deformations caused by the construction of specific structural elements or the application of loads. In Image 8, for example, the settlements caused by the construction of the foundation elements can be seen.
Conclusion
Incorporating construction stages into geotechnical finite element analysis is essential for ensuring the safety, stability, and durability of structures. By simulating the step-by-step progression of construction, engineers can accurately assess soil behavior, optimize designs, and proactively address potential risks. As projects grow in scale and intricacy, stage-wise analysis becomes increasingly indispensable. With its advanced features and intuitive tools, RFEM 6 provides engineers with a powerful platform to perform detailed stage analyses, enhancing both efficiency and reliability in geotechnical engineering.
