#054 - StructuralCodes | An Open-Source Python Library for Capacity-Based Design

Building on an Open-Source Framework for Global Design Code Integration

Hello everybody,

Christmas looms ever closer, bringing with it the usual chaos, expense, and domestic obligations. Meanwhile, I’ve been thoroughly derailed by my latest unnecessary acquisition: a VR gaming setup (Meta Quest 3 and a PSVR2).

In a burst of unforeseen enthusiasm, I’ve danced more in the last 48 hours than in the preceding 15 years, mercifully unwatched, save for my wife, who now possesses several incriminating clips of me in action.

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Expanding StructuralCodes: A Practical Approach to Capacity-Based Design in Python

This article marks the beginning of a thought experiment, one I hope will grow with your input and ideas.

My aim is to build upon the incredible work already done in StructuralCodes, an open-source Python library designed to perform capacity-based design checks for structural elements through a modular and flexible framework.

The contributors behind StructuralCodes, led by Morten Engen, have created something remarkable. Their deliberate design and thoughtful framework provide a strong foundation that the engineering community can build upon.

Currently, StructuralCodes is focused on Eurocode design, but its core architecture is designed to be adaptable to any structural design code. It’s an excellent example of what’s possible for open-source tools in structural engineering.

A huge shout out to Morten and the folks who have developed it.

I’ll explore StructuralCodes in greater detail later, but for now, I want to focus on the bigger picture: how can we, as practicing engineers, expand on this foundation to develop tools that work in the real world?

The Goal: Simplicity, Reusability, and Practicality

In consulting, delivering high-impact, practical solutions is the priority. While theoretical exploration has value, the day-to-day realities of engineering demand tools that are simple, reusable, and effective.

My aim is to build on the incredible work already done in StructuralCodes while inviting collaboration from the Flocode community. You, like me, likely understand the practical challenges we face.

• What workflows do we need to optimize?

• What pain points can we address?

• What would make this tool truly useful in your professional practice?

Getting the Framework Right

Getting the framework right from the start is critical. A practical, intuitive, and simple atomic structure lays the foundation for adding complexity later.

This is where I need your input.

• What’s the cleanest way to approach this problem?

• How do we ensure the library remains practical and not just a solo indulgent odyssey?

• What features or workflows matter most to you?

I’ll outline my own thoughts in the sections that follow, but this project will only succeed if we work together to shape and refine it. Consider this an open invitation to share your thoughts, experiences, feedback, and ideas.

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Why Use Python for Structural Design?

Capacity-based design ensures structures perform safely by prioritizing ductile failure and clear failure hierarchies. Yet many engineers still rely on spreadsheets or proprietary tools, which, while functional, lack flexibility and transparency. Python offers a better approach:

• Transparency: Open-source tools allow you to see, validate, and trust the calculations.

• Flexibility: Adapt the code to suit your workflows and design codes.

• Interoperability: Integrate seamlessly with other tools for automation and efficiency.

However, the challenges are real: design codes evolve, interpretations vary, and disagreements persist.

It doesn’t need to be perfect—it just needs to work. Refinement will come naturally through iteration and use.

structuralcodes

https://github.com/fib-international/structuralcodes

StructuralCodes is an opensource Python library developed by a fantastic group of engineers who have taken on a very difficult but important task.

StructuralCodes facilitates capacity-based design checks for structural elements, ranging from simple code compliance verifications to more complex analyses like moment-curvature relationships, shear capacity evaluations, and material behavior modeling, all through a modular framework that integrates different design codes, material properties, geometric definitions, and section analysis.

For now, StructuralCodes primarily supports Eurocode, and my focus here is to expand on these ideas, adapting them for capacity-based design in general—and for ACI 318 in particular. Most of my projects rely on US or Canadian codes, for now I’m focused on the US based ACI 318-19 code.

Early next year, I’ll be hosting Morten Engen, one of the creators of StructuralCodes, on the Flocode podcast. We’ll discuss the complexities and possibilities of the library, digging into the challenges he’s faced and some of the lessons learned.

I’ve already spoken to Morten about some of these topics and he’s keen to collaborate with the Flocode community to expand StructuralCodes.

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How Should We Structure Such a Library?

The strength of StructuralCodes lies in its modular framework, which separates materials, geometry, constitutive laws, and design equations.

This structure is intuitive, scalable, and practical, allowing us to focus on specific components of our design. By isolating the shared mechanics of capacity-based design from code-specific details, the library becomes adaptable to multiple global design codes. This is not something we normally need to think about but it’s a helpful thought exercise!

Adapting to different codes is challenging because of their underlying philosophical differences. While the core principles of structural mechanics are universal, the way each code handles safety factors, material properties, and empirical relationships can vary significantly.

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Key Differences in Global Design Codes

International structural design codes differ in their philosophical underpinnings and practical implementation. While the fundamentals of structural mechanics are universal, codes like Eurocode, ACI, CSA, and others approach design in ways that reflect regional priorities, practices, and conditions.

Here’s a high-level overview of how these codes differ:

• Eurocode (Europe):

  • Focuses on probabilistic safety and explicitly addresses serviceability, cracking, and long-term effects.

  • Safety factors (γc for concrete, γs for steel) are applied separately to materials.

• ACI 318 (United States):

  • Takes a deterministic, practical approach, streamlining calculations with strength reduction factors (φ).

  • Prioritizes strength checks first, with serviceability considered separately.

• CSA A23.3 (Canada):

  • Similar to ACI but emphasizes durability, particularly for cold climates, with stricter crack control provisions.

• AS 3600 / NZS 3101 (Australia/New Zealand):

  • Combines Eurocode and ACI principles, with a strong emphasis on seismic ductility, especially in New Zealand.

• IS 456 (India):

  • Simpler, empirical code with a structure similar to older versions of ACI.

Even small differences, like unit systems (metric vs. imperial) or terminology (fck vs. f'c), reflect deeper variations in design philosophy.

These distinctions are not superficial—they impact how material properties, safety factors, and load-resisting mechanisms are modeled and validated.

Understanding these nuances is vital for adapting tools like StructuralCodes. For instance, a modular structure allows engineers to switch from Eurocode's probabilistic philosophy to ACI's streamlined strength checks without rewriting core mechanics.

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Why the Modular Structure Matters

The modular structure of StructuralCodes is what makes it flexible enough to accommodate these global differences. Here’s how it works:

1. Materials and Constitutive Laws

   • Material definitions (concrete, steel, etc.) form the foundation of any design code. These include parameters like compressive strength, tensile strength, modulus of elasticity, and strain limits.

   • Constitutive laws—like elastic-plastic, parabolic-rectangular, or bilinear models—tie these properties to stress-strain behavior.

   • While the mechanics of material behavior are universal, the specific parameters and limits differ between codes. A modular approach ensures that code-specific material logic can be added without disrupting the shared mechanics.

2. Geometry and Sections

   • Geometry defines the shape and reinforcement layout of structural members. Tools like Shapely (used in StructuralCodes) enable mathematical representation of complex shapes, including concrete sections and reinforcement placement.

   • Section calculators build on this geometry to derive critical properties like bending moment, shear force, and axial force capacities.

   • This separation allows geometry to remain independent of design codes, ensuring reusability and flexibility. For example, the same rectangular section can be analyzed using Eurocode, ACI, or CSA simply by changing the design equations.

3. Design Code Equations

   • Design equations from each code (e.g., Eurocode shear checks, ACI flexural capacity) must be modular and isolated to ensure clarity. Everything else should persist - the structural mechanics are handled separately from the code mandated empirical calculations.

   • The ability to switch between codes—for example, using a function like set_design_code('aci318_19')—ensures the library can grow incrementally without becoming unmanageable.

By modularizing these components—materials, geometry, constitutive laws, and design equations—we can build a framework that is simple, flexible, and scalable to handle different design codes.

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The Vision: Keep It Practical

This library must be:

• Lightweight: No one wants to read a dissertation to use it.

• Flexible: It should work for various codes and workflows.

• Easy to Maintain: Over-complicated systems are doomed to fail when time is limited.

I’ve often fallen into the trap of overengineering my own tools. Months later, I return to a project only to rebuild it from scratch because I can’t remember what I was doing. This project must avoid that fate. A clear API, practical documentation, and example-driven tutorials are non-negotiable.

The StructuralCodes documentation is already excellent. It’s concise, clear, organized and well-written.

StructuralCodes Documentation

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Questions for the Community

I’d love to hear your thoughts on these questions. Below, I’ve added my own perspective to spark discussion. Feel free to challenge, expand, or refine these ideas.

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1. What’s the most pragmatic way to handle different design codes?
My view: The team behind StructuralCodes has already done an excellent job structuring the library. By separating geometry, materials, design code equations, and constitutive laws, they’ve created a scalable framework. The challenge lies in how design codes handle these aspects differently—some parts can be reused across codes, while others need to be built from scratch. I think we stick with this framework and focus on minimal, iterative expansions.

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2. How should we prioritize features?
My view: Start with the basics. Build a simple framework that establishes a scalable pattern. Flexural and shear capacity checks are the obvious starting points. Once we have those, we can expand gradually into more complex checks.

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3. What’s your preferred approach to managing units?
My view: Ignore unit functionality as much as possible. Engineers can manage their own units, as long as the library provides clarity where empirical equations or constitutive laws require specific unit conventions. Keeping unit handling outside the library avoids unnecessary complexity. Am I wrong? Let me know.

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4. How important is support for exotic constitutive laws?
My view: It’s not important for now. This is a classic 80/20 situation—most applications will only need the basics. Advanced models can be added later, but the current priority should be simplicity and broad applicability.

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5. What level of abstraction is most useful?
My view: Avoid over-abstraction. Each function should clearly reference the same nomenclature used in the design codes. Inputs and outputs need to be well-documented, with clear examples in the docstrings. For example, a flexural capacity function should list all required inputs like section size, moment, and rebar spacing, along with their data types.

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6. What challenges do you encounter with capacity-based design tools?
My view: A major challenge is how different engineers interpret code clauses. This won’t change, and this library isn’t going to solve that. Some empirical equations, like those requiring chart read-offs, will always be tricky, but we can address that by documenting assumptions in the relevant docstrings.

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7. How can we ensure the library remains practical for consulting engineers?
My view: Build what we need and use what we need. The library will naturally evolve based on practical use cases. For me, the driving force is solving my own problems—if it works for me, it’s likely useful for others in the field.

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8. What’s the best way to handle updates and code changes?
My view: Updates will come as needed. Design codes rarely undergo major overhauls—most changes are incremental. The easiest approach is to tweak functions or add new ones as the need arises. I envision updating the library when I personally need a change, proposing a pull request for the community, or maintaining my own fork. The success of this approach will depend on collective goodwill and the tool’s usefulness.

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9. What’s missing?
My view: This is the big one. What workflows, checks, or challenges do you think the library should address? Personally, I see the basics—flexure, shear, buckling, deflection—as the most critical. But I’m curious about your most common pain points. Where would this library provide the most value to you?

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A Call for Collaboration
This project is a fantastic opportunity for students, academics, and practitioners alike. It’s a chance to work on something that could have a tangible, positive impact on the structural engineering community. Whether it’s sharing insights, proposing features, or contributing code, your input is invaluable.

Let me know your thoughts—agree, disagree, or have an entirely new perspective? Let’s figure this out together.

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What’s Next?

This is the first in a series of articles where I’ll think out loud about how to approach this challenge. I’m hoping to attract contributions from academics, students, and professionals who share an interest in advancing open-source tools for structural design. Your insights and critiques will help refine this idea into something truly useful.

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Final Thoughts

This initiative is about collaboration. While I have my own thoughts on how to implement capacity-based design in Python, I recognize that there’s no one-size-fits-all solution. The strength of this project will come from the collective input and insights of the community.

I want to take a moment to acknowledge Morten and the contributors behind StructuralCodes. What they’ve built is truly impressive and an incredible service to the industry. If you’re working with Eurocodes and haven’t checked it out, you should.

Their work to date demonstrates a level of careful thought and deliberate architecture that becomes more apparent the deeper you explore. Nothing like this exists elsewhere in structural engineering software today. There are some commercial tools like Tedds, Strucalc, Enercalc and others but these are expensive and inflexible. That’s a topic for another day.

It’s frustrating to see that major design codes from CSA, ACI, AISC and CISC still lack official Python libraries. For a field that relies so heavily on precision and consistency, the absence of such tools is, frankly, surprising. Projects like StructuralCodes fill this gap, and they set a standard for what open-source engineering tools can achieve.

Together, we can build something impactful—something practical, flexible, and open-source that bridges the gap between research and consulting. Whether it’s through sharing ideas, offering feedback, or contributing code, I welcome your collaboration.

Looking forward to hearing your thoughts.

See you in the next one.

James 🌊