- Q. A design we’re bidding on has several peeled cedar logs that function as both architectural and structural elements. Are there any rules of thumb for determining their strength so that they can be safely used without redundant grade-stamp framing — and so they meet the approval of a building inspector?
A. Peggi Clouston, associate professor of Building Materials and Wood Technology at the University of Massachusetts Amherst and a specialist in timber design, responds: Certainly, natural whole elements of a tree can be used safely as structural members in a building. Trees are, after all, innately structural. But instead of rules of thumb, the IBC requires inspection by a certified grading agency or the engineer of record to estimate just how strong a log actually is and whether it is suitable for a particular structural application (see Section 2303.1.10, 2006 IBC).
Complex grading rules establish limits on the size and number of strength-reducing growth characteristics — knots, checks, splits, holes, and the like — allowed for any particular species and for a number of anticipated end-uses. Stress grades — as shown on grade stamps — are a direct result of this grading process, and strength values derive from the stress grades. Peeled cedar logs used as round timber piles or in log buildings are covered by regulatory standards ASTM D25 and ASTM D3957.
In addition to grading issues, connections of round and nonstandard cross-section geometries are of particular concern to building inspectors. Joints of wood structural members with a nonrectangular shape are generally custom made, with design properties based on limited experimental test data. So the best approach to incorporating logs into a building is to enlist the expertise of a registered structural engineer, who can attest to the structural design and the strength of the wood element for each specific application.
