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Bearing Details
The greatest loads to which normal trusses are subjected are the upward forces/reactions from the support through the bearings. Except for very small
trusses, the line of action of these forces should be close to the centre of a joint, or a structural penalty, in the form of very large timber sizes, will be
incurred owing to large bending moments.
The standard eaves detail (figure 84a) is satisfactory if the shift is not greater than 50mm, or not greater than on third of the scarf length. The 'Alternate' or
'French' heel (figure 84b) is considered in the same way but the key position is where the line of the underside of the rafter intersects the underside of the
ceiling tie.
Another point to note is that as a truss ends at a vertical chord, (figure 84c) there is little scope for tolerance on length or verticality.
Where trusses are to be supported off the face of a wall (figure 84d), placing a nib at the heel of the truss is the most common solution. It is good practice to allow a nominal gap between the vertical chord of the truss and the masonry, for constructional tolerance (figure 84e and 84f). Depending on the reaction, and the grade and size of the timber in the bottom chord, a simple extension of the bottom chord may suffice (figure 84e) to form a 'nib'.

Should the bending or shear stress in the nib be excessive the whole joint can be reinforced. (figure 84f). At greater spans it is possible to use the detail in figure 84g to locate the point of intersection of the principle forces vertically over the bearing.