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Design Method
A trussed rafter is an engineered framework consisting of structural members forming triangles. The framework derives its inherent strength from this
The members around the perimeter of the trussed rafter are known as chords (top and bottom, also called rafters and ceiling ties), and the internal members providing the internal triangles are known as webs (sometimes also called struts and ties).
A true trussed rafter is formed only when the webs form triangles between the top and bottom chords. Attic frames and Raised-Tie trusses (see section 1.7
and 3.16) do not provide this triangulation and are therefore technically not trussed rafters.
When designing non-standard trussed rafters, it is beneficial to ensure the full triangulation as above, please refer to MiTek's System Design Office if in doubt.
Principles of Design
When loading is applied to a trussed rafter (from tiles, ceiling construction snow etc), two main kinds of force are generated in the members:
1.Bending Moment
2.Axial Force
Bending moment causes neighbouring sections of timber to tend to rotate relative to each other (see figure 21a).
Axial force may be either tensile, i.e. pulling adjoining sections of timber away from each other, or compressive, i.e. crushing adjoining sections of timber into each other (see figure 21b and 21c).
A compressive force may cause the member to buckle (bending sideways out of the plane of the trussed rafter) and this may need to be counteracted by bracing (see sections 2.5 and 3.7) or by increasing the section of timber required for the affected member.
Within a trussed rafter, members will be subject to either axial force alone or a combination of axial force and bending moment. The design of a trussed rafter must allow for these effects, together with the differing forces produced by different types of load (see section 2.7 on Loading and Load cases.)