Grading should not be a mystery. Nor should every architect or engineer be expected to be fluent in all of the characteristics of lumber grading in order to make an informed decision. The purpose of this wood grading protocol is not to make every architect or engineer a certified lumber grader but rather to give them a simple tool with sufficient supporting documentation to know whether wood members in a historic building may be sufficient or are structurally deficient.
The need for a simple protocol cannot be overstated. It is likely that nearly all engineers involved in assessment of and modifications to existing structures have encountered a situation similar to the two following examples: A late 19th century cotton mill in South Carolina in the process of being rehabilitated had long-span timber beams. Based on a grade assumption of No. 2 southern pine timbers and existing loads, the beams were found to be in need of reinforcement. Simple visual grading of the beams in place found them to be free of knots and other grade-limiting defects. By today’s standards, these beams could be classified as Select Structural, but in fact, they were better than Select Structural because of the lack of defects (since Select Structural allows for knots of a certain size). Using Select Structural design values instead of No. 2 resulted in a 76 percent increase in allowable bending stress, which was sufficient to carry the loads without any retrofit. The cost savings was approximately $1,000,000 to the project.
Similarly, a five-story industrial building in Colorado was found to have a few failed joists. The assumed grade and species dictated that the joists did not have sufficient capacity to carry residential loads, even though the building had served as a warehouse for nearly a century. Grading in place showed that approximately six percent of the joists had large knots or other defects that resulted in grade assignment of No. 3 Douglas-fir to the joists. However, replacement of the few joists with significant knots, splits or slope of grain with higher grade material allowed for use of Select Structural as the grade because the remaining material was primarily defect free. Grading the joists and doing selective replacement saved approximately $900,000 in project costs.
Ultimately, the choice to retain historic fabric and reduce costs for historic preservation projects lies in the hands of the engineer who determines the structural capacity and safety requirements for current or future structure use. The grading protocol is intended to provide architects and engineers with additional tools to make more informed decisions regarding these choices.