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In calculating effective stiffness for fire-reduced sections, transverse layers that are the final partially charred layer and are thus the outermost layer on the charred side of the panel are not included in the summation of layers.
The calculation of the neutral axis ỹ is given in Equation 8 of Chapter 8 of the CLT Handbook as follows
ỹ = ∑ yi Ei ti / ∑ Ei ti
where yi is the distance from the unexposed side to the centre of the layer.
As Ei in the transverse layers is effectively zero when compared to the longitudinal Ei , this reduces to Equation 9
ỹ = ∑ yi ti / ∑i ti
where the summation is over longitudinal layers only.
Double Outermost Layers
Double outermost layers are treated as a single layer in this calculation, on the assumption that the lamination between these layers is at least as strong in bending as the wood itself, and the two layers act as a unit.