For historic buildings and structures, engineers and architects often rely on current standards and design values to determine adequacy of the wood members to remain in service, but current standards are generally based on lower quality material than was used in many historic buildings. Historic structures built before the establishment of building codes or design values for wood products (or structures which lack grade stamps on individual wood members) present a quandary when determining what design values are appropriate. Frequently an assumed species and grade are assigned, only to show that the wood members are structurally deficient, despite the fact that the structure has stood for decades or centuries without failure. The results of assuming a species and/or grade are often an overly conservative estimate of the design values and unnecessary replacement, repair and retrofit decisions along with associated unnecessary project costs and destruction of historic fabric.
This grading protocol is a review of historical wood testing and standards development, wood condition assessment basics, and a query-based wood grading program. The goal of this protocol is to change the typical decision- making process by giving engineers and architects the means to better understand the grading of wood members in relation to building code requirements.
All work on historic structures should comply with the Secretary of the Interior’s Standards for the Treatment of Historic Properties and, therefore, be compatible both physically and visually with the structure, as well as documented and identifiable (upon close inspection) for future research and preservation efforts. If repairs are necessary, the existing condition should be evaluated to determine the appropriate level of intervention needed. If there are areas of deterioration severe enough to require repair or limited replacement of a member, the deteriorated material should be replaced with the same species and match the original material in composition, design, color, and texture.
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 engineers with additional tools to make more informed decisions regarding these choices.