EXAMPLE $17\mathrm{.}1$

The required structural dimension of a Douglas fir timber beam is $612$ in$.$ nominal. If the beam is left exposed and unprotected in the building, what is the additional thickness required for a $1$ hour fire resistance?

Char rate $\frac{1}{40}$ in$.\frac{?}{\text{m}\text{i}\text{n}}=0\mathrm{.}025in\frac{.}{m}in$

Charred thickness after $1$ hour in$.$