Incising Factor Explained:

In order for sawn lumber to be used in outdoor conditions, it requires a preservative treatment to prevent decay in the wood. This process includes permeating the wood with chemical preservatives to prevent decay-causing fungus to enter the wood. In most cases, this is possible by a pressure treatment, but certain species of wood must be incised (small slits into the wood), to receive the chemicals more readily.

From the Forest Products Journal[1], Incising is a pretreatment process in which small incisions, or slits, are punched into the wood surface.This increases the amount of exposed end- and side-grain surface area, which increases chemical preservative penetration and retention.

Note: If the wood is not incised, then this factor can be ignored, or taken as unity (1.0).

Incised Wood Factors [2]

The loss of the area, and section modulus from the small incisions of a wood member will require certain reduction factors to be used. If the wood is incised parallel to the grain a max. depth of 0.4" (10.2mm), a maximum length of 3/8" (9.5mm), and a density of incisions up to but not exceeding 1100 incisions/sq. ft the following table may be used:

Table 1: Sawn Lumber Incising Factors (Ci)
Design Value Ci
E, Emin 0.95
Fb, Ft, Fc, Fv 0.80
Fc-p 1.00

Note: If incising patterns exceed the above limits, then factors should be determined by testing or by calculation using reduced section properties.


References:

  1. Forest Products Journal (Vol 55, No. 9), "Evaluation of a reduced section modulus model for determining effects of incising on bending strength and stiffness of structural lumber", 2005
  2. American Forest and Paper Association, "National Design Specification for Wood Construction", 2005

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