Potential Structural Quality of Trenching Lumber

The structural quality of lumber used for trenching was examined at sites in the Washington, D.C., Houston, Texas, and Los Angeles, California areas as part of a study undertaken by the National Bureau of Standards for OSHA to develop comprehensive design procedures for trenching. Only stress-graded Douglas-fir was found in the Los Angeles area. In Washington, D.C., nonstressgraded southern pine and ungraded mixed hardwoods were employed. In Houston, only ungraded mixed hardwoods were found. Reuse of trenching lumber was an accepted practice only in the Los Angeles area. Trenching lumber surveyed in distribution yards was visually graded using conventional ASTM procedures. Some characteristics, such as wane, were considered separately from other grading factors in order to better define both grading quality and manufacturing options. When visually graded for strength, but without consideration for wane or decay, approximately 80 percent of the hardwood and southern pine wales, the most critical shoring members, were of No. 2 or better quality as defined by the Southern Pine Inspection Bureau grading rules. However, 37 percent of this same lumber had wane in excess of that permitted for a No. 2 grade of southern pine. It is possible that some of this additional wane could be tolerated for this use. Decay and handling damage were minor contributors to potential strength loss. Many pieces could be upgraded by the process of partial length grading. All the lumber in the Los Angeles area was grade marked Douglas Fir and was equivalent to a No. 2 Southern Pine grade or better. An essential conclusion of the NBS study was that No. 2 level structural quality is an adequate lumber design base for contemporary trenching practice. This may provide the basis for future grading requirements.

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