Imagine this: a towering building swaying under the force of a powerful earthquake. In moments like these, the integrity of its structure is the only thing standing between safety and catastrophe. The problem is, not all buildings are equipped to handle such forces. Choosing the right structural system— moment frame vs. braced frame—isn’t just an engineering decision; it’s a matter of resilience and long-term safety.
But here’s the catch: selecting the wrong system can lead to inefficiencies, higher costs, or even compromised stability when it matters most. That’s why understanding the strengths and limitations of these two systems is critical.
In this post, we’ll break down the key differences between a moment frame vs. braced frame, explain how each performs under various loads, and guide you in choosing the one that ensures your building stands strong against nature’s greatest tests.
Let’s dive in.
Moment Frame vs. Braced Frame – When Are They Used?
Both moment frames and braced frames play a crucial role in structures designed to withstand lateral forces such as wind and seismic activity. Their importance becomes even more pronounced in tall buildings, where lateral loads often dictate the structural design.
Understanding when to use each system ensures optimal performance and cost-efficiency, particularly in high-stakes designs like skyscrapers and industrial facilities.
What Is a Moment Frame?
A moment frame is a structural system in which the connections between beams and columns are designed to resist rotation. These connections, known as moment connections, ensure that the joints remain rigid and prevent rotational movement.
Key Characteristics of Moment Frames:
- Moment Connections: The beam-to-column connections are specifically engineered to eliminate rotation, ensuring the frame can effectively resist bending moments.
- Typical Design: Moment connections often include stiffeners or diagonal reinforcements to enhance rigidity. For example:
- Haunched moment connections.
- Beam flanges are directly welded or bolted to column flanges.
- Bending Moment Distribution: These frames excel at distributing bending moments caused by gravity loads (e.g., dead and live loads) across the structure.
Real-Life Applications of Moment Frames
Architects use moment frames to create open, bracing-free spaces while ensuring stability against lateral forces. Common in reinforced concrete structures, they are used in tall buildings and seismic zones, where rigid connections provide both design flexibility and resilience.
What Is a Braced Frame?
A braced frame is a structural system that relies on diagonal bracing to ensure lateral stability. Unlike moment frames, the joints in a braced frame can be are modeled as pin joints, that can rotate Key Characteristics of Braced Frames:
- Pin Joints: These connections allow rotation at the joint.
- Diagonal Bracing: Strategically placed braces handle lateral loads, such as wind or seismic forces, transferring these forces efficiently to the foundation.
- Bending Moment Distribution: Unlike moment frames, braced frames experience bending moments primarily in the braces and supports, not in the joints.
Within diagonal braced frames, there are four main types:
- Ordinary Concentrically Braced Frames (OCBFs)
- Special Concentrically Braced Frames (SCBFs)
- Eccentrically Braced Frames (EBFs)
- Buckling-Restrained Braced Frame (BRBF)*
*Also check out EVER Brace for an all-steel BRB, that is revolutionizing seismic stability and construction.
Real-Life Applications
Braced frames resist lateral forces like wind and earthquakes, making them ideal for stabilizing structures. Their design minimizes sway and lateral deflection, ensuring the integrity of the building in challenging conditions.
Braced frames are widely used in steel structures, retrofitting projects, tall buildings, and bridges, where their efficiency and cost-effectiveness make them a popular choice for enhancing lateral stability.
Key Differences Between Braced Frames and Moment Resisting Frames
To clearly distinguish between braced frames and moment resisting frames, here’s a side-by-side comparison highlighting their unique characteristics and applications:
| Aspect | Braced Frame | Moment Frame |
| Connection Type | Pin connections form the beam-column joints. | Rigid (=moment) connections are used between beams and columns. |
| Moment Transfer | Moments are not transferred through the connection. | Moments are transferred through the rigid connection. |
| Lateral Load Resistance | Relies on a bracing system (e.g., diagonal braces) to transfer lateral loads. | Relies on rigid connections to transfer lateral loads. |
| Analysis Complexity | Straightforward and simple to analyze. | More complex to analyze due to rigid connection behavior. |
| Labor Intensity | Requires less labor; construction is relatively simple. | Labor-intensive due to the detailed connection work. |
| Cost Efficiency | Cost-effective because of simple connections. | More expensive to construct due to rigid connections. |
| Stabilization | Provides excellent structural stabilization. | Stabilization is less effective compared to braced frames. |
| Common Material Usage | Frequently used in steel and timber structures. | Primarily used in reinforced concrete structures. |
| Load Analysis | Beams and columns handle vertical loads, while bracing supports lateral loads. | Beams and columns are analyzed under axial loads and moments. |
| Bracing Requirements | Requires vertical planes of bracing for stability. | No bracing needed, as all connections are rigid. |
| Deformation Capacity and Stiffness | Higher stiffness with less deformation capacity. | Greater deformation capacity with less stiffness. |
Key Differences in Load Behavior
Moment frames distribute bending moments across both beams and columns, with the highest moments occurring at the mid-span and supports.
In contrast, braced frames channel lateral loads directly through their braces and supports, effectively reducing deflection.
While moment frames can handle lateral loads, they often experience larger deflections and underutilized beam sections, making them less efficient in these scenarios.
When to Use a Moment Frame vs. a Braced Frame
Deciding between a moment frame and a braced frame happens early in a project’s design. Structural engineers must align the stability system with the architect’s vision and the project’s functional needs.
The choice depends on the space requirements, structural demands, and budget.
When to Use a Moment Frame
Moment frames are the go-to solution for projects that demand open, unobstructed spaces. They’re ideal when adding shear walls or braced frames would compromise the design.
However, their complexity and material requirements make them a pricier option, so they’re best suited for projects with flexible budgets.
Moment frames also shine in reinforced concrete structures, seamlessly integrating gravity and lateral load resistance while preserving architectural freedom.
Use a Braced Frame When
Braced frames, on the other hand, are preferred for handling significant lateral forces like wind or earthquakes.
They’re more economical and efficient, avoiding wasted material and providing superior control against lateral forces. These frames work particularly well in steel structures, delivering cost-effective stability without sacrificing performance.
Conclusion – Building Resilience is Key
The choice between moment frames vs. braced frames isn’t just a technical decision—it’s a pivotal step in shaping a structure’s resilience and functionality.
Each system brings its own strengths, whether it’s the flexibility and open space offered by moment frames or the unmatched lateral stability of braced frames. The right choice depends on your project’s unique demands, vision and budget.
At the end of the day, your building’s ability to stand tall, even against the forces of nature, is what matters most.
If you’re ready to build a structure that doesn’t just meet today’s standards but thrives in tomorrow’s challenges, reach out to us. Let’s discuss how we can help your project not just withstand earthquakes but define resilience.
