A braced frame uses diagonal bracing to resist lateral loads, while a rigid frame relies on the bending resistance of its beams and columns. This leads to significant differences in cost, efficiency, and application.
Here's a breakdown of the key differences:
Structural Behavior
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Braced Frame: Resists lateral loads primarily through axial tension and compression in the bracing members. Think of it like a truss lying on its side. The braces create stiff, triangulated geometries that prevent deformation.
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Rigid Frame: Resists lateral loads through the flexural action (bending) of the beams and columns. Strong moment connections between the beams and columns are crucial to transfer these bending forces. The frame deforms in a "sway" pattern.
Efficiency
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Braced Frame: More efficient at resisting lateral loads. Axial forces are generally more efficiently resisted than bending forces. This translates to smaller member sizes for the same load capacity.
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Rigid Frame: Less efficient at resisting lateral loads. Requires larger beam and column sizes to resist bending moments effectively.
Cost
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Braced Frame: Generally less expensive due to the smaller member sizes and simpler connections (although the braces themselves require connections).
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Rigid Frame: Generally more expensive. Requires heavier members and more complex, moment-resisting connections.
Architectural Implications
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Braced Frame: The diagonal braces can obstruct openings and limit architectural flexibility. They might not be aesthetically desirable in all cases.
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Rigid Frame: Offers greater architectural flexibility as there are no diagonal braces to obstruct the space. Allows for larger, uninterrupted openings.
Drift
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Braced Frame: Significantly reduces lateral drift (sidesway) due to its stiffness.
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Rigid Frame: Tends to exhibit more lateral drift under lateral loads. Drift control can be a major design consideration.
Summary Table
| Feature | Braced Frame | Rigid Frame |
|---|---|---|
| Load Resistance | Axial forces in braces | Bending in beams and columns |
| Efficiency | High | Low |
| Cost | Lower | Higher |
| Architectural Impact | Obstructs openings, less flexibility | More flexible, open spaces |
| Drift | Lower | Higher |
Applications
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Braced Frame: Commonly used in high-rise buildings and industrial structures where lateral loads are significant and architectural constraints are less stringent.
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Rigid Frame: Used in low-rise buildings, particularly where large open spaces are required and the bays cannot accommodate diagonal braces. Can also be used in conjunction with other lateral resisting systems like shear walls.
In conclusion, the choice between a braced frame and a rigid frame depends on factors such as cost, architectural requirements, and the magnitude of lateral loads. Braced frames are generally more economical and efficient for resisting lateral loads, while rigid frames offer greater architectural flexibility at a higher cost.
