Common Lateral Force Resisting Systems In Heavy Timber Structures Are

Common Lateral Force Resisting Systems in Heavy Timber Structures

Heavy timber construction, characterized by its use of large, dimensioned lumber and glulam members, offers a sustainable and aesthetically pleasing approach to building design. However, the successful implementation of heavy timber structures hinges on a robust understanding and proper design of lateral force resisting systems (LFRS). These systems are crucial for ensuring the stability and safety of the building against wind, seismic, and other lateral loads. This article delves into the common LFRS employed in heavy timber structures, examining their mechanics, advantages, disadvantages, and design considerations.

Understanding Lateral Loads and their Impact

Before diving into specific LFRS, it’s crucial to understand the nature of lateral loads. These forces act horizontally on a structure, causing it to sway or potentially fail. The magnitude and direction of these loads depend on several factors including:

• Wind: Wind pressure varies with height, terrain, and building shape, creating significant horizontal forces.
• Seismic Activity: Earthquakes induce dynamic lateral forces that can be considerably destructive, demanding robust LFRS designs, especially in seismically active regions.
• Snow Load: While primarily a vertical load, heavy snow accumulation on a roof can create lateral forces, especially on sloped roofs.
• Live Loads: Unbalanced live loads, such as crowds concentrated on one side of a floor, can generate lateral forces.

Ignoring these loads can lead to catastrophic consequences, including structural collapse, damage to non-structural elements, and injury or loss of life. Therefore, designing an effective LFRS is paramount.

Common Lateral Force Resisting Systems in Heavy Timber Structures

Several systems can effectively resist lateral forces in heavy timber buildings. The choice depends on factors like building height, geometry, seismic zone, and cost considerations. Here are some of the most prevalent:

1. Shear Walls

Shear walls, perhaps the most common LFRS in heavy timber, are vertical elements designed to resist lateral loads by shear. They are typically constructed using:

• Solid Timber: Thick, large-dimension lumber or glulam panels are used to create a monolithic shear wall. These are exceptionally strong and stiff but can be heavy and require careful detailing to ensure proper connections.
• Timber-Frame Shear Walls: These walls consist of a timber frame (studs and headers) braced with diagonal members (sheathing or cross bracing). The sheathing material (plywood, OSB) provides diaphragm action, distributing the load across the wall. This approach is often more cost-effective and easier to construct than solid timber walls.

Advantages:

• High strength and stiffness: Particularly effective in resisting lateral loads.
• Relatively simple design and construction: Depending on the chosen approach.
• Good integration with the overall timber structure: Provides architectural aesthetic appeal.

Disadvantages:

• May require significant wall thickness: Can reduce usable floor area.
• Solid timber walls can be heavy: Requiring stronger foundations.
• Proper detailing of connections is critical: Failure of connections is a major concern.

2. Braced Frames

Braced frames use diagonal bracing members within a timber frame to resist lateral forces. They can be either:

• Eccentric Braced Frames (EBFs): These frames utilize a flexible connection at the brace-to-column junction allowing for yielding in the brace, thereby dissipating energy during seismic events. They are typically used in higher seismic zones.
• Concentric Braced Frames (CBFs): The brace members are connected concentrically to the columns in this system. These are simpler to design and construct than EBFs but generally provide less energy dissipation capacity.

Advantages:

• Efficient use of materials: Bracing members can be strategically placed to optimize strength and stiffness.
• Suitable for both low and high-rise buildings: Design can be adapted to suit the building’s requirements.
• Can provide architectural interest: Diagonal bracing can be visually appealing.

Disadvantages:

• Bracing members can interfere with usable floor space: Careful planning is required.
• Connections must be carefully designed and detailed: To ensure adequate strength and stability.
• Potential for buckling of braces: Requires proper consideration of member sizes and connection design.

3. Moment Frames

Moment frames rely on the moment capacity of the connections between columns and beams to resist lateral loads. This requires specialized, strong connections capable of transferring significant bending moments. In heavy timber, this often involves the use of:

• Steel connections: Provides excellent strength and ductility, often used in conjunction with timber members for optimal performance.
• High-strength bolted connections: Requires careful design to ensure sufficient capacity to resist moment and shear forces.
• Advanced timber connections: These specialized joints use innovative techniques, like dowel-type fasteners or glued-laminated connections, to improve the moment-resisting capacity.

Advantages:

• High ductility and energy dissipation capacity: Especially beneficial in seismic zones.
• Relatively flexible design possibilities: Allow for more open floor plans than braced frames.
• Can be aesthetically pleasing: Allows for the integration of timber elements.

Disadvantages:

• More complex design and construction: Requires specialized engineering expertise and precise fabrication.
• Connections are critical and expensive: Requires careful attention to detail.
• Potential for brittle failure: Requires robust design and quality control measures to minimize such risks.

4. Diaphragms

Diaphragms are horizontal elements, usually composed of timber sheathing or decking, that act as load distributors. They transfer lateral loads from the walls or other LFRS to the shear walls or braced frames. They play a critical role in ensuring that the lateral loads are distributed efficiently across the structure.

Advantages:

• Efficient distribution of lateral loads: Ensures that the load is evenly distributed amongst supporting systems.
• Relatively simple to construct: Using standard timber sheathing or decking.
• Integrates well with other LFRS: Works effectively in conjunction with shear walls and braced frames.

Disadvantages:

• Requires adequate connection to LFRS: Failure to connect properly can lead to diaphragm collapse.
• Sheathing material needs to be strong and stiff: To withstand lateral loads.
• Potential for uplift at edges: Requires proper anchoring to prevent this.

Design Considerations for LFRS in Heavy Timber Structures

The design of LFRS for heavy timber structures demands careful consideration of several factors:

• Material Properties: Accurate knowledge of timber properties, including strength, stiffness, and durability, is essential.
• Connection Design: Connections are a critical component of LFRS and their design requires specialized expertise. Failure of connections is a major cause of structural collapse.
• Seismic Design: In seismic zones, special attention should be paid to the system's ability to dissipate energy and withstand dynamic loads.
• Wind Load Analysis: Accurate wind load analysis is necessary to determine the required strength and stiffness of the LFRS.
• Code Compliance: The design must comply with relevant building codes and standards, which vary by location.
• Sustainability: Choosing sustainable timber products and designing for durability contribute to environmentally friendly construction practices.

Conclusion

The selection and design of appropriate lateral force resisting systems are crucial for the safety and stability of heavy timber structures. The choice between shear walls, braced frames, moment frames, and the effective use of diaphragms depends on multiple factors including building height, location, seismic activity, and budget. Careful attention to material properties, connection design, and code compliance is crucial to ensure the long-term performance and safety of the building. Employing experienced structural engineers with specialized knowledge in heavy timber design is highly recommended to ensure a successful project. By understanding these systems and their intricacies, architects and engineers can design resilient and aesthetically pleasing heavy timber structures that meet the demands of modern construction.