What is the size factor denoted as in wood design for visually graded timber?

In wood design for visually graded timber, the size factor is denoted as Cf. This factor is essential because it adjusts the strength properties of a piece of lumber based on its size. As the size of timber increases, its load-carrying capacity does not scale linearly due to the presence of defects, biological variability, and changes in the structural response. The size factor accounts for these variations to ensure that designs are both safe and efficient.

Using the size factor allows engineers to apply standardized strength values from tests performed on smaller specimens to larger pieces, facilitating more accurate and reliable structural designs. This factor plays a critical role in ensuring that timber structures can withstand expected loads while accounting for the inherent characteristics of wood as a material.

The other options represent different values or factors in wood design that relate to other parameters, but they do not signify the size factor specifically.