Alternatives to a shear wall are structural systems designed to resist lateral forces, such as wind or seismic loads, thereby preventing a structure from deforming sideways. Common alternatives include braced frames and moment frames.
Why are Alternatives Needed?
Structures need lateral resistance to maintain their shape and stability when subjected to horizontal forces. As highlighted in the provided reference regarding a lack of diagonal forces, without adequate lateral bracing, a simple rectangular frame subjected to sideways pressure would deform significantly, going "from being a square to being a rhombus... makes it a parallelogram." This deformation indicates structural instability and potential failure. Shear walls are one way to prevent this, but other systems can achieve the same goal.
Key Alternatives to Shear Walls
When architectural constraints, material preferences, or structural requirements make shear walls unsuitable, engineers and architects can utilize other structural systems:
Braced Frames
Braced frames incorporate diagonal elements (braces) within the rectangular bays of a frame. These diagonal braces resist the parallelogram deformation described in the reference by creating stable triangular shapes. When lateral forces push on the frame, the braces are put into tension or compression, effectively transferring the load down to the foundation.
- Types of Braced Frames:
- Concentrically Braced Frames (CBF): Braces connect directly to beams and columns, with their centerlines intersecting at the joints.
- Eccentrically Braced Frames (EBF): Braces are offset from the beam-column connections, incorporating specific link beams that deform under seismic events, dissipating energy.
Moment Frames
Moment frames, also known as rigid frames, rely on the rigidity of the connections between beams and columns to resist lateral loads. Unlike simple frames or braced frames, the beam-column joints in a moment frame are designed to transfer bending moments as well as shear and axial forces. This creates a stable system without the need for diagonal bracing or solid shear walls in every bay.
- Characteristics of Moment Frames:
- Provide open bays, offering greater architectural flexibility compared to shear walls or braced frames.
- Lateral stiffness is achieved through strong, rigid connections and larger beam/column sizes that can handle the significant bending moments induced by lateral loads.
Choosing the Right System
The choice between shear walls, braced frames, or moment frames depends on various factors, including:
- Building height and function
- Seismic and wind load requirements
- Architectural layout and desired open spaces
- Construction cost and complexity
- Material availability (steel, concrete, wood)
Each alternative offers distinct advantages and is selected based on a holistic evaluation of project needs.
