Structural engineering plays a critical role in bringing any building design to life. However, for architects and designers who don’t work with structural systems on a daily basis, some aspects can feel overly complex.
There are three foundational principles that guide smart structural design: Economy, Load Path, and Compatibility.
Understanding core concepts, such as how loads travel through a building, and making informed material choices that balance cost, performance, and constructability helps take the guesswork out of structural decisions, providing both structural engineers and architects with practical tools they can apply to their work and collaboration.
Based on a talk for AIA Cincinnati given by Kip Ping, Principal and Founder of Pinnacle Engineering, this blog explores this.
Economy in Structural Design
“It’s not just about cutting costs; it’s about smart investments.”
Why Economy in Structural Design Matters
Economy isn’t just about cost; it’s an important factor that determines if a project even begins. Though important to a building’s safety, structural elements aren’t usually what clients want to spend money on.
Keeping factors such as material quantity, availability, and design complexity in mind during the early design process can help optimize costs without compromising quality or safety.
Key Considerations in Economic Design
- Material Volume: Cost generally increases proportionally with material use. Reducing excess without weakening the structure is key to maintaining efficiency.
- Material Availability: Uncommon or limited supply materials can drive up expenses.
- Labor Costs: Complicated construction techniques can increase project length and budgets.
How Does This Apply to Structural Design?
Achieving economic design is about finding balance. For example:
- Standardization: Keep wood species and grades consistent to minimize site confusion and inspection delays.
- Optimized Spacing: Adjust wood joist spacing rather than altering member dimensions to save material costs.
- Efficient Design: Use consistent steel members or concrete forms to reduce waste and labor needs.
Think about the bottom line, not just line items. A holistic approach ensures an optimized structural design that aligns with both functionality and budget.
Real-World Example of Economy in Structural Design
Two beams are needed with one being a W8x15 that is eight feet long and the other a W8x18 that is twelve feet long. Another option would be to just use W8x18 for both. The first option would result in about 336 pounds of steel being required whereas the second would require 360 pounds.
In reality, the steel fabricator likely has to purchase at least 20 feet of material, so the first option is really 660 pounds of steel with 324 pounds of scrap after cutting the beams to length. The second option would have no waste and would be the better option from an economy standpoint. Similarly, laying out bearing elements for wood construction in consideration of lumber coming in two-foot length increments can avoid waste there as well.
Load Path In Structural Design
“Even the most advanced calculations mean nothing without a proper load path.“
What is a Load Path?
Load path refers to how forces travel through a structure, from the point where they are applied to the foundation or ground. Whether it’s snow on a roof, a seismic tremor, or wind pressure, establishing a direct and continuous load path is important for safety and structural stability.
Fundamentals of a Direct Load Path
- Direct Loads Are (Usually) Best: Axial loads (those compressed or stretched along straight lines) are preferable to bending loads. Less disruption in load paths typically equates to better economy and design simplicity.
- Avoid Unnecessary Eccentricities: Off-center forces or openings disrupt the efficiency of load transfer, requiring additional design considerations.
- Consider Connections: Structural failures most often occur at connection points, not within the members themselves.
3 Tips for Designing Optimal Load Paths:
- Break the structure into “chunks” to focus on each component separately.
- Use Free Body Diagrams to visualize forces acting on each part of the structure.
- Frequently ask yourself questions such as:
- What are the loads to be resisted? (e.g., snow, wind, seismic activity).
- What path does this load follow through the building?
- Can each connection handle crossing loads effectively?
Real-World Example of Load Path in Structural Design
Consider a tall frame building exposed to high wind pressure. A failure in the load path might not involve a collapsing beam, but rather an unreinforced masonry element that gives way during a catastrophic event like a tornado. Ensuring every load follows the most direct path to the ground helps prevent these scenarios and supports a stable design.
Compatibility in Structural Design
“A structure must perform cohesively—not just be strong.”
What Does Compatibility in Structural Design Mean?
Compatibility in structural design refers to how well a structure’s components work together once built. It involves predicting how materials and elements respond to real-world forces, ensuring they deform and move in together.
Factors Influencing Compatibility
- Stiffness: Stiffness is central to compatibility. When walls, beams, or other components have significantly different stiffness levels, it can lead to uneven deformations.
- Material Properties: Materials with varying thermal coefficients and modulus of elasticity can behave differently under load.
- Geometry: Avoid mixing drastically different span lengths or combining spanning and non-spanning elements.
Real-World Example of Compatibility in Structural Design
Take geographical differences as an example. A building in Colorado might require additional stiffness considerations due to heavy annual snow loads, whereas a structure in Ohio would not face the same challenges. In addition:
- Buildings with tiled floors may demand stiffer beams to prevent cracking compared to those with softer finishes.
- Steel is often chosen over wood in long spans due to its greater stiffness and dimensional stability.
When compatibility is not considered early in the design process, the structure may still meet safety codes but fail to satisfy serviceability expectations, such as cracking under heavy loads or sagging over time.
About Pinnacle Engineering
Pinnacle Engineering’s wide-ranging expertise and commitment to challenging the norm in structural engineering sets us apart. Our clients have been coming back since 1996 for a reason.
Our engineers specialize in commercial, custom residential, and specialized design projects.
If you’re interested in working with us, reach out here.
