Grading of Lumber and Timbers in Historic Structures
Some of the issues associated with assigning structural values to old lumber were first published by Wood (1954). Rather than simply address design stresses, he focused on what many of us do today – he commented on the significance of wood deterioration, moisture and connections. He also addressed re-use of lumber from demolished buildings, a topic that is currently being researched by the U.S. Forest Products Laboratory (Lantz and Falk, 1996; Falk, personal communication). Understanding how recycled lumber behaves is important but that does not address how wood members can be graded in an existing building where the elements have come to behave as a system.
The question of how to properly assign design values to older wood members that did not have a grade stamp (and, therefore, were not within the traditional guidelines of the building codes) became more prominent as historic structures were lost due to poor decisions. Condition assessments of existing buildings, and the decisions that resulted from the assessments, became major factors in preservation and adaptive re-use projects.
The means to grade wood members is well defined but somewhat obscure. In addition to the information given above, several summary reports exist to help the engineer or architect. Shelley (1992) provided some background on the evolution of standards development in the U.S. that focused on how grading standards changed from the early 20th century to the late 20th century. Following procedures spelled out by ASTM, Loferski et al. (1996) described how the mechanical properties of wood members in existing structures could be determined. Similarly, Keenan and Quaile (1982) described how to estimate the load-carrying capacity of structural wood members within extant structures.
Table 1. Characteristics and limitations for lumber meeting the requirements for No. 1 Timbers (SPIB, 2002).
[No. 1 Timbers]
Compression wood – prohibited if in readily identifiable and damaging
Decay – in knots only
Firm red heart – not limited
Slope of grain – 1 in 11
Holes – medium if well scattered
Knots – sound, firm, encased, and pith knots are permitted in sizes not to
exceed the following or equivalent displacement:
|Nominal Width of Face||Narrow Face At Edge of Wide Face (2)||Centerline Wide Face||Unsound Knots (1)|
(1) In unsound knots as allowed, the decay must be confined to the knot itself and not be in surrounding wood and not penetrate deeper than 1-1/2”.
(2) In timbers of equal faces, knots are permitted throughout as specified for narrow faces regardless of location.
Manufacture – standard E
Pitch, pitch pockets, and pitch streaks – not limited
Pith – not limited
Shakes, checks, and splits – splits not longer than the thickness of the piece; shakes and surface checks not deeper than 1/3 thickness if not dry and 3/8 thickness if dry
Skips – hit or miss dressing
Stain – medium if dry; not limited if ordered green
Wane – to occupy not more than 1/6 width of face and 1/3 length
Warp – very light
Despite these efforts, however, the methodology for determining grades of timbers in historic structures has remained obscure. This problem of assigning grade values to timbers in historic structures so that they comply with existing building code requirements is not unique to the U.S. and can be viewed as an international preservation issue; for example, Yeomans (1999) stated the difficulties of addressing the same problem in the U.K.