Section 27-619. Use of non-stress grade wood

The use of non-stress grade
  wood  in  structural  elements  shall  be  limited  to   the   following
  conditions:
    (a)  Studs,  joists,  and  rafters  proportioned  on  the basis of the
  empirical provisions of section 27-622 of this article.
    (b) The architect or engineer responsible for the design may assign an
  allowable stress value for the proposed material based on the provisions
  of reference standard RS 10-8 relating to  "other  species  and  grade".
  Under  such  condition, the required species and grade of wood, together
  with the assigned stress value, shall be conspicuously indicated on  the
  plans.