When designing structures, especially tall buildings, one of the most critical factors to consider is lateral load resistance. Forces from wind and seismic activities can impose significant stress on a structure, and ensuring that it can withstand these forces is paramount to its stability and safety. Two commonly used systems for resisting these lateral loads are braced frames and moment-resisting frames. While both provide essential functions in stabilizing a structure, they differ in their mechanisms, construction methods, and performance.
Braced Frames
Description of Braced Frames
Braced frames are a structural system in which the beams and columns are arranged in an orthogonal pattern, both vertically (in elevation) and horizontally (in the plan). These frames typically use pinned connections at the beam-column joints, meaning that the joints do not transfer moments—only axial forces (tension or compression).
Structural System and Function
The primary mechanism of a braced frame involves the use of a bracing system. This can consist of horizontal and vertical braces that transfer lateral loads (wind or seismic forces) from the structure to the foundation. Because the connections do not transfer moments, the analysis of braced frame structures is generally simpler compared to moment-resisting frames.
Advantages of Braced Frames
- Cost-Effective and Labor-Efficient: Due to the simplicity of the pinned connections, braced frames are relatively inexpensive to design and construct. The connections are easy to analyze and fabricate, which reduces both material and labor costs.
- Good Stabilization and Rigidity: Braced frames provide excellent stabilization due to their ability to efficiently transfer lateral loads through the braces. This enhances the overall rigidity of the structure.
Limitations of Braced Frames
- Less Deformation Capacity: Braced frames are typically stiffer than moment-resisting frames, meaning they have less capacity to deform under load. While this can be advantageous in some scenarios, it also limits the flexibility of the structure during extreme loading events.
Applications of Braced Frames
Braced frames are most commonly used in steel and timber structures, where the simplicity of the bracing system aligns with the materials’ properties and cost considerations.
Moment-Resisting Frames
Description of Moment-Resisting Frames
In contrast to braced frames, moment-resisting frames employ rigid connections between the beams and columns. These rigid joints allow for moment transfer, meaning that forces are transferred through both axial forces and bending moments. This type of frame system relies heavily on the joints to transfer lateral loads to the foundation.
Structural System and Function
Moment-resisting frames rely on the rigid connection between beams and columns to resist lateral forces. This means that the frame itself, rather than relying solely on braces, acts as a continuous structure that can transfer both axial and bending forces. Because of the complex nature of these connections, the analysis and design of moment-resisting frames are more intricate than for braced frames.
Advantages of Moment-Resisting Frames
- Greater Deformation Capacity: Moment-resisting frames allow for more deformation under lateral loads. This flexibility is beneficial in earthquake-prone areas where the structure needs to absorb and dissipate energy from seismic events.
- Effective for Concrete Structures: Moment-resisting frames are commonly used in reinforced concrete buildings, where the materials and construction methods naturally lend themselves to rigid joint design.
Limitations of Moment-Resisting Frames
- Higher Cost and Labor-Intensive: Constructing moment-resisting frames is significantly more expensive than braced frames. The rigid connections require more sophisticated materials, detailed design, and precise fabrication, all of which increase the labor and material costs.
- Less Stabilization Efficiency: Moment-resisting frames, while flexible, do not provide as efficient lateral load resistance as braced frames. The rigid connections are designed to resist bending moments, but they are not as effective at stabilizing the structure as bracing systems.
Applications of Moment-Resisting Frames
Moment-resisting frames are predominantly used in reinforced concrete structures, where the properties of concrete allow for the creation of strong, rigid joints capable of transferring moments.
Comparison of Braced Frames and Moment-Resisting Frames
Connection Types
One of the most significant differences between the two systems lies in the connection types. Braced frames use pinned connections, which only transfer axial forces. In contrast, moment-resisting frames use rigid connections that transfer both axial forces and bending moments.
Moment Transfer
In braced frames, the connections do not transfer moments; instead, the bracing system takes on the responsibility of transferring lateral loads. Conversely, in moment-resisting frames, the rigid joints themselves transfer lateral loads and moments to the foundation.
Structural Analysis
The analysis of a braced frame structure is relatively straightforward due to the simple nature of the pinned connections. On the other hand, the analysis of a moment-resisting frame is more complex, as it requires accounting for the bending moments transferred through the rigid joints.
Labor and Cost
Braced frames are more labor-efficient and cost-effective. The pinned connections are easy to fabricate, and the overall system is simpler to construct. Moment-resisting frames, however, are much more expensive to build due to the complex, rigid connections and the labor-intensive process required to construct them.
Stabilization and Performance
While both frame systems serve to stabilize the structure, braced frames are generally more efficient in terms of resisting lateral forces. The bracing system offers superior stabilization, while moment-resisting frames, due to their flexibility, may not provide as robust resistance to lateral loads.
Deformation Capacity and Stiffness
Braced frames are stiffer, providing less deformation capacity compared to moment-resisting frames, which allow more deformation under load. The latter’s increased flexibility makes them more suitable for buildings in seismic zones, where absorption of energy from seismic forces is critical.
Conclusion
In summary, both braced frames and moment-resisting frames are essential in providing lateral load resistance in tall structures, but they operate in fundamentally different ways. Braced frames offer a simpler, more cost-effective solution with excellent stabilization but limited deformation capacity. Moment-resisting frames, though more expensive and labor-intensive to construct, allow for more flexibility and better energy absorption, making them ideal for certain applications like reinforced concrete buildings. Understanding the differences between these two systems is crucial in selecting the appropriate design for a structure based on specific load, material, and performance requirements.