ROOF TRUSSES
Even though it takes loads similarly to a beam, a truss is not a beam but a space frame with several connected members acting in either tension or compression. It is designed to carry heavy loads across long spans. Trusses can be thought of as open-webbed beams, in that the top flanges (chords) are in compression, and the bottom chords are in tension. These chords may also carry moment, depending on how load is transferred to the truss. The individual web pieces are either in tension or compression, depending on how the truss is configured. Connections at truss joints are critical; they should not be compromised through additional carpentry.
Since they are strong in one direction but weak in another, trusses, especially light trusses, must be handled with care. During erection, they can easily tip if not securely braced. When framed for loading, the top chords must be carefully aligned and straightened, uniformly spaced and parallel, and with the truss webs vertical or nearly so. Since the bottom chords will be in tension when fully loaded, less care needs to be given to aligning them. Obviously, the chords must not be damaged.
When placing roofing on trusses, try to keep the roofing operations more or less equal on each side, i.e. do not complete one side then do the other. Asymmetrical loading will introduce un-balanced horizontal thrust in the truss. This is more critical on steeper pitches.
Even though it takes loads similarly to a beam, a truss is not a beam but a space frame with several connected members acting in either tension or compression. It is designed to carry heavy loads across long spans. Trusses can be thought of as open-webbed beams, in that the top flanges (chords) are in compression, and the bottom chords are in tension. These chords may also carry moment, depending on how load is transferred to the truss. The individual web pieces are either in tension or compression, depending on how the truss is configured. Connections at truss joints are critical; they should not be compromised through additional carpentry.
Since they are strong in one direction but weak in another, trusses, especially light trusses, must be handled with care. During erection, they can easily tip if not securely braced. When framed for loading, the top chords must be carefully aligned and straightened, uniformly spaced and parallel, and with the truss webs vertical or nearly so. Since the bottom chords will be in tension when fully loaded, less care needs to be given to aligning them. Obviously, the chords must not be damaged.
When placing roofing on trusses, try to keep the roofing operations more or less equal on each side, i.e. do not complete one side then do the other. Asymmetrical loading will introduce un-balanced horizontal thrust in the truss. This is more critical on steeper pitches.
Very nice illustrations.
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