goes from failure, poor, good, very good and excellent.

Some stuff from reading;

While it is recommended that all loads supported by a tension chord be located at a panel point, this may not always be possible,and bending moments may be introduced into the bottom chords. Such additional loads must be accounted for by designing and sizing the bottom chords as combined bending and axial tension members.

The compression chord usually carries most of the external loads, most often applied as uniformly distributed loads along the panel length. When the chord is continuous over several panels, it should first be evaluated as a continuous beam with the panel points assumed to be nonyielding supports. The reactions thus determined become the panel point loads used in the axial force analysis of the truss. The bending moments induced by the uniform loading are included in the combined stress analysis.

The combined axial and bending stresses at the center of the compression chord must also be evaluated. Consider first an initially straight laterally restrained chord, continuous over several panel points, with uniform transverse load and some axial force. Obviously the panels are in reality a series of interconnected beam columns supported at panel points and free to deflect in the plane of the truss.

At first glance the effective column length appears to be the distance between the points of contraflexure (the inflection points as the rafter curves over each strut then deflects down in span under the load). However, it can be shown that the effective length increases with increasing load, and of course, the stresses are nonlinear with load. The designer wishing to persue this complicated analysis can refer to section 15.4 of the NDS.

Normally, in sawn timber the complexities of the preceeding are avoided by assuming a conservative effective length. (it goes on, but the easy way is to multiply the point to point length x 1.5 to get the safe effective length, happy to see that).