Sub-Models in CSI Software: Flexibility in Modeling and Design
Throughout the life cycle of a structural project, it is common to encounter questions that require analyzing multiple scenarios or construction stages, while still maintaining the global consistency of the model. Determining different interactions with the foundations, modifying element stiffness, or simulating alternate seismic scenarios are examples of situations in which working with multiple independent files becomes time-consuming and prone to mistakes.
The solution proposed in this article involves creating sub-models within the same environment, thus eliminating the need to split the project into multiple files. Each sub-model preserves the main geometry and loads but adjusts key parameters to explore different behaviors or satisfy specific code requirements. The following sections explain how this strategy works effectively in CSI software (SAP2000, ETABS, and CSiBridge).
What are Sub-Models?
A sub-model is a variation of a main structural model, where certain conditions or properties are adjusted to analyze alternative scenarios. Unlike traditional methods, which involve creating multiple independent models, CSI’s SAP2000, ETABS, and CSiBridge allow these variations to coexist within a single model. This is accomplished by sharing geometries, loads, and support conditions, but adjusting critical parameters, such as stiffness, boundary conditions, cross sections, and the addition/removal of structural elements.
This method helps avoid modeling errors, addresses uncertainties in structural behavior, and enables efficient design across multiple scenarios. The present article shows how this technique applies in practice, offering examples of structural analysis and design.
How do Sub-Models work?
1. Sharing the Base Model
- The structure’s geometry, applied loads, and general definitions remain consistent for all versions, preventing data duplication and boosting efficiency.
2. Individual Parameterization
- Specific variations such as element stiffness, supports, or connection types are configured separately for each sub-model.
3. Isolated and Combined Results
- They allow evaluating the structural behavior of each variation on its own, as well as combining the respective results as needed.
Scope and needs: Why use Sub-Models?
Sub-models are a fundamental tool in uncertain contexts or when multiple functional and code scenarios must be evaluated.
1. Managing Uncertainties in Modeling
In situations where structural response depends on uncertain parameters, sub-models allow changing those parameters and assessing how structural behavior is influenced.
Example: Foundations in inconsistent soils
- Sub-Model A: Considers supports as rigid.
- Sub-Model B: Introduces elastic springs that simulate soil stiffness.
The engineer can compare displacements and internal forces for each case, ensuring that the chosen design solution is safe under any scenario.
2. Design for Multiple Code Scenarios
Each sub-model can be configured to meet different regulatory or functional requirements. This approach eliminates the errors and time investment of creating independent files for each scenario.
Example: Seismic analyses according to Eurocode 8
- Sub-model of the primary seismic structure: disregards the stiffness of secondary seismic elements; bending and shear stiffness is set to half of the elastic or secant stiffness at yielding.
- Sub-model for global seismic analysis: considers the stiffness of both primary and secondary seismic elements, with bending and shear stiffness set to half of the elastic or secant stiffness at yielding.
Both sub-models are essential for the seismic design of primary and secondary elements, as well as for checking the contribution of secondary seismic elements. (See article “Creating sub-models for seismic analyses: 3 distinct models” by CSI Portugal).
3. Simplified and Integrated Linear Analyses
Sub-models can also be used to deactivate or change the stiffness of certain elements in order to account for certain nonlinear behaviors in a simplified way.
Example: Considering braces that only work in tension in response-spectrum analyses
Braced frames that only resist tension require nonlinear analyses, which conflicts with modal response-spectrum analysis. However, multiple sub-models can be created for different active brace scenarios and a modal analysis can be performed for each one. For more details on this solution, see the second example in the article “The Use of Sub-Models: What Are They For?” by CSI Portugal.
4. Linear Analyses of Partial Models
It may be necessary to run modal or buckling analyses, or simply static linear analyses, on a partial model corresponding to a particular construction stage. For example, it might be crucial to determine the critical loads of a steel girder in a bridge before pouring the deck slab.
5. Reducing Errors and Increasing Efficiency
Creating sub-models in a single environment minimizes the risk of inconsistencies between versions (e.g., divergent geometries) and promotes centralized updates. Any modifications to the base structure are automatically reflected in all sub-models.
Practical Example: Using Sub-Models for Integrated Design
The video below details the steps to create two sub-models for analysis and design, considering different interactions between the foundation and a steel structure:
- Sub-Model with fixed supports: Ignores the deformability of the foundations.
- Sub-Model with foundation slab: Considers soil-structure interaction by modeling a flexible slab on springs.
Conclusion
Sub-models, or multi-conditional models, are an essential tool for engineers facing multiple project scenarios. This flexibility - and efficiency - focused approach eliminates the risks tied to using separate files and delivers reliable results in less time.
By enabling changes in stiffness, boundary conditions, and structural elements within the same environment, CSI software users gain a robust and versatile methodology for creating safe, optimized solutions that suit a wide array of engineering challenges.
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