Trusses

Applications, History, Design and Manufacturing

Wood trusses are widely used in single- and multi-family residential, institutional, agricultural and commercial construction. A truss is a structural frame relying on a triangular arrangement of webs and chords to transfer loads to reaction points. This arrangement gives them high strength- to-weight ratios, which permit longer spans than conventional framing, and offers greater flexibility in floor plan layouts. They can be designed in almost any shape or size, restricted only by manufacturing capabilities, shipping limitations and handling considerations. Light frame wood trusses are prefabricated by pressing galvanized steel truss plates into wood members that are pre-cut and assembled in a jig… Applications History Design and Manufacturing.pdf (88 KB)

Storage Handling and Bracing

Trusses are slender elements. They are very strong when placed in the vertical position, but can be easily damaged or broken if racked or bent in the lateral direction. Damage or failure can occur at the joints (connector plates) or within the lumber members.

When trusses arrive at the job site they should be checked for any permanent damage such as cross breaks in the lumber, missing or damaged metal connector plates, excessive splits in the lumber, or any damage that could impair the structural integrity of the truss… Storage Handling and Bracing.pdf (440 KB)

 The Role of the Designer

The design of light frame trusses in general and truss plate connections in particular does not follow the traditional design procedure used for the design and construction of many wood structures. In the traditional design role, the designer provides details of all wood member sizes and wood connections to be used in the structure. With light frame trusses, the design of the members and connections is generally the responsibility of the truss plate manufacturer.

Most manufacturers are equipped with computer software that can rapidly design trusses of any configuration. In order to do the design, they must be provided with the following information:

  • the type of structure
  • design loads: special loading such as snow drift loads must be specified
  • truss spans
  • truss shape required including depth and clearance limitations
  • bearing details
  • overhang details

Truss plate manufacturer

The truss plate manufacturer provides structural drawings that include such information as:

  • the loads on which the design is based
  • truss spacing
  • points of support
  • species, grade and size of lumber required for the chords and webs
  • type, size and location and orientation of truss plates
  • lateral bracing requirements
  • bearing size
  • general notes.

Inspecting trusses

When inspecting trusses in situ, it is important to carefully examine the truss plate connections. The plates must be properly installed and of the size specified in the drawings. A minor shift in the location of the truss plate can severely weaken the connection, so it is important to ensure that the plates are installed in the locations specified on the truss plate manufacturer’s drawing.

The Building Designer

The building designer is typically responsible for the structure that supports the trusses including the support of the truss bracing system. Bearing walls and lintels must be designed for the truss loads, including the point loads from girders. The building designer should also ensure that all connections between the trusses and support framing, such as hangers and uplift anchors are adequate. Truss drawings often call for bracing of long web members loaded in compression. It is typically the responsibility of the building designer to design the system for supporting the web bracing and to ensure that the bracing is properly installed.

Additional information

Introduction to Wood Design

Wood Trusses – Strength, Economy, Versatility (International Building Series)