Log and Timber Checking
The Timber Products Inspection, Inc. (TP) Log Program is an internationally accredited program that assigns stress grades to logs and timbers. The program is based upon ASTM Standards where the strength of a log is derived from visual observations of lumber defects such as knots, slope of grain, decay, split, checks, etc…. This document provides a brief overview of how “checks” are addressed within the program.
What is a Check?
A check is a lengthwise separation of the wood which extends across the rings of annual growth. Checks are typically formed during the drying process when the log materials near the surface loses moisture and shrinks before the wood at the inner core. Checking is routinely encountered within the log and timber industry where it is a natural occurrence in wood components that contain the pith, or center of the tree.
How Does Checking affect the Structural Capacity?
Shakes, checks and splits in lumber reduce the shear strength capacity of bending members. To account for this strength reduction; the TP Log Program stress values are derived using ratios of safety factors in accordance with ASTM D245 which states:
4.2.3 For all lumber thicknesses, a strength ratio of 50% shall be used for all sizes of shakes, checks and splits. A 50% strength ratio is the maximum effect a shake, check or split can have on the load-carrying capacity of a bending member. Laminations in grading rules placed on the characteristics at time of manufacture are for appearance and general utility purposes, and these characteristics shall not be used as a basis for increasing lumber shear design values.
Base on the above statement, the practices set forth within ASTM standards and utilized within TP’s Log Program account for the maximum effect a check can have on the structural capacity of a log or timber.
Summary
Checks are a natural occurrence and commonly encountered within the log and timber industry. The TP Log Grading Program accounts for checking as it relates to the structural capacity by applying safety factors within stress values. These values are derived in accordance with ASTM guidelines and are readily available for building designers from Timber Products Inspection, Inc. upon request.
