The Factor of Safety in Design of Timber Structures

Abstract

Much of the reduction in the average strength of clear wood that is made to obtain a design stress is made necessary by the conditions of structural use; it does not in itself produce a margin for safety. A simple way to estimate safety is to use near-minimum values for these conversion factors and then to make a further reduction for unforeseen conditions. This method may indicate a near-minimum factor of safety of 1-1/4 to 1-1/2. A better evaluation is made by recognizing that there are many factors in strength and use that affect safety, and that each factor is multivalued and expressible by a frequency distribution. By suitable mathematical operations, these can be combined into one frequency distribution that shows the range of the true factor of safety. An example of this kind of evaluation indicates that the most probable values of the factor of safety in timber design are in the range of 2 to 2-1/2, and that nearly all values are in the range of 1 to 4. The same distribution can be used to indicate the probability of failure and to show how that probability is related to the working stress level.