In the United States, we are blessed with an abundance of quality building materials we may chose to use while fabricating our wood truss products. As long as we are able to confidently predict lumber values and our truss design software has the proper values entered, we can be assured a quality product is produced. Or can we?
Lumber manufacturers strive to evaluate lumber in the most economical, practical ways in order to compete in the market place. Their reputation depends upon sound methods and procedures. In order to stay in business, they must meet demand and meet or exceed quality standards. The lumber producer, however, is not the ultimate decision maker in determining what gets used in the truss. This is the responsibility of the component manufacturer.
In addition to verifying grade stamps, lumber traceability, and qualifications, the component manufacturer must further evaluate the suitability of any given member. Careful inspection and evaluation must happen with each piece. Lumber soundness is sometimes missed or is not apparent during lumber manufacturing. If defects exist, often times the damage occurs while processing the component. Thorough understanding must exist within the workforce to be able to make good decisions.
What are some of the hidden defects that might exist? In this article, let’s address one of the more subtle issues that is sometimes overlooked. While it’s a quality attribute analyzed during grading, grain pattern is very important to sound lumber. Short grain conditions can exist where the lumber while growing makes abrupt changes in grain direction. This can cause difficult to identify issues with strength. The photo illustration is an example of a short grain failure. The problem did not manifest itself until 30 years of service. Could the truss manufacturer have caught this problem before the truss was used? Probably not. Attic conditions exacerbated the strength of the lumber at the defect due to the drying of the lumber in high attic temperatures over time. Fortunately in this case redundancy in adjoining components carry the applied loads as this component shown failed. Repetitive bending, system design protected us from catastrophic failure. Despite careful inspection, sometimes these defects are not identifiable, however, understanding short grain characteristics is an important aspect to keep in mind when determining quality.