Tri Bearing Roof Truss: Design, Benefits, and Applications

The tri bearing roof truss is a structural system designed to span larger openings with three distinct support points, typically two exterior bearing walls and a central interior support. This configuration enables different load paths, potentially reducing shed points and deflection compared to traditional two-point bearing trusses. The concept is widely used in commercial, agricultural, and some residential applications where intermediate support is feasible and cost-effective. This article explains its design, benefits, limitations, and practical considerations for U.S. construction.

What Is A Tri Bearing Roof Truss

A tri bearing roof truss consists of two end bearers and a central bearing that supports the top or bottom chords and web members. The third bearing point creates a three-point support system, altering how live and dead loads are transferred to the supporting structure. Unlike standard king post or queen post trusses, a tri bearing configuration distributes reactions across three supports, which can influence chord depth, spacing, and overall span capability. The central bearing is often located at mid-span or at a location dictated by architectural or structural requirements.

Key Differences From Other Roof Trusses

Compared to common two-bearing trusses, tri bearing designs offer different advantages and trade-offs. Advantages include reduced maximum reaction at any single bearing, potential for longer spans with a lower overall truss height, and flexibility in accommodating interior columns or equipment framing. Limitations include increased complexity in analysis and fabrication, potential need for precise alignment during erection, and adjustments to web member configurations to maintain stability.

In practice, many builders use tri bearing configurations when interior supports are present or when architectural constraints require a central support without compromising roof geometry. Detailed structural analysis is essential to ensure the three-point load path meets code requirements and does not introduce excessive deflection or distortion in the roof plane.

Load Paths And Design Considerations

Understanding load paths is critical for tri bearing trusses. Live loads (snow, wind) and dead loads (roof coverings, sheathing) travel from the top chords through web members to the three bearings. The central bearing draws a portion of the reaction, reducing the moment forces at the outer supports. This redistribution can influence:

• Chord depth and member sizes needed to meet bending and shear requirements
• Web member configuration to prevent buckling and maintain stability
• Connection details at bearing supports to transfer loads safely
• Deflection limits to satisfy serviceability criteria

Code-compliant design requires using appropriate design methods (e.g., LRFD or ASD) and adherence to national and local standards for connections, fasteners, and load combinations. Where interior columns create a tri bearing layout, engineers must verify that the central bearing does not create unintended restraint or differential movement that could damage the roof plane.

Materials, Connections, And Fasteners

Tri bearing trusses commonly use treated or untreated lumber in modern practice, with steel plates and gussets for joints depending on design loads and span. Design considerations include:

• Top and bottom chord material grades suitable for anticipated bending stresses
• Web member sizing to resist shear and buckling under combined loads
• Gusset plates or pinned connections that permit the intended rotation while maintaining stability
• Anchorage and bearing conditions at central and end supports to prevent slip or settlement

Fastener choices—such as bolts, screws, and nails—must meet code requirements for withdrawal resistance and shear capacity. In corrosive environments or exposed conditions, protective coatings or galvanized hardware may be recommended to extend life and maintain connection integrity.

Applications And Code Compliance In The United States

Tri bearing roof trusses are suitable for projects where interior supports are available or where long spans with limited height are desirable. They appear in agricultural buildings, industrial warehouses, and some commercial structures with clear-span needs or limited roof pitch. In the United States, design and fabrication must comply with relevant codes and standards, including:

• International Building Code (IBC) and applicable amendments
• American Wood Council (AWC) National Design Specifications (NDS) for wood, and the Wood Design Standards
• Manufacturer and engineer-stamped truss drawings for approval and field construction

Coordination with the building envelope and mechanical systems is essential to prevent interferences between the central bearing and interior equipment or utilities. Inspectors will verify bearing alignment, bearing sizes, and connection integrity during framing and after roof installation.

Installation And Maintenance Best Practices

Proper installation is crucial for ensuring the performance of a tri bearing roof truss. Key steps include:

• Careful marking and alignment of all three bearing points on the supporting framework
• Temporary bracing during erection to maintain geometry until all connections are secure
• Verification of truss spacing, bearing depth, and fastener torque per manufacturer specifications
• Post-installation inspection to confirm no shifting or misalignment due to settlement or weather effects

Regular maintenance should focus on bearing conditions, corrosion protection for metal hardware, and ensuring attic ventilation to minimize moisture-related issues that could affect wood components. Any modifications after installation should be evaluated by a structural engineer to preserve load paths and safety.