The specification of the categories and the design values for both dimension lumber (width < 5") and heavy timber (width >= 5") are in the National Design Specification (NDS) for Wood Construction supplement: Design Values for Wood Construction. It is published by the American Wood Council of the AF&PA. http://www.awc.org/
Click on Design Standards.
[This message has been edited by Dick Schmidt (edited 02-28-2002).]