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Design Method
 
Bending Moments 
Bending moments are generally induced in the Chord members due to the loadings (tiles, ceiling, snow etc) placed directly onto them. It is unusual for Web members to be subject to bending moments.
The magnitude of the bending moment in a particular chord is largely due to the Panel Length (the distance between the joints at each end of the member, usually measured horizontally, also known as the Bay Length).
The general rule is, the longer panel length the greater the bending moment and hence the larger the section of timber required to safely resist the
bending moment.
 
Further, BS. 5268-3 defines the maximum bay lengths permitted in Table 3, a copy of which is given below:
 
The choice of a different truss type, with a smaller panel length (and hence more webs), will usually yield a smaller section of timber required. 
 
The method of calculation relating to bending moment is as follows:
 
The applied bending stress (calculated from the bending moment divided by the section modules of the timber being considered) is compared with the permitted bending stres s for the particular timber grade or strength class.
 
The resulting ratio
 
This ensures that the actual bending stress in the timber cannot exceed the permitted stress, causing the timber to fail.
Axial Force
 
Axial forces within the trussed rafter are calculated by analysing the whole frame. The greater the number of panels (webs) the greater the axial forces can be. Also, the lower the pitch of the top chord the greater can be the axial force.
 
As mentioned previously, axial force can be either tensile or compressive and, if compressive, can lead to problems with out-of-plane buckling.

In a similar way to bending moment, the actual axial stress in the timber (calculated from the axial force divided by the area of the timber section), is compared with the permitted axial stress of the timber grade or strength class being used.
 
 
This ensures that the timber never exceeds its permitted axial stress limit.
Generally, web members will be subjected only to axial force, whereas chord members will be subject to a combination of bending and axial stresses.

 
For chord members therefore, the calculation becomes:

 
 
To ensure that the timber section is within its defined limits for both bending and axial stress.  This ratio is known as the combined stress index (CSI) or stress summation.