MEASUREMENTS OF DELAYED WARP IN EASTERN SPRUCE STUDS

ABSTRACT

Nearly eighty percent of the lumber manufactured in the United States is softwood dimension lumber used for the manufacture of housing. The principal quality related problem with softwood dimension lumber is that it is subject to warp related degradation. Much of the degradation occurs during the initial drying process, however, residual stresses and moisture content changes that occur after kiln drying can either reduce or increase the amount of warp. The magnitude of post kilning warp change, here termed delayed warp, has not been well quantified. Measurements of twist, bow, crook and moisture content were taken from eastern spruce (Picea spp) studs immediately after drying and planing. The wood was allowed to sit without restraint for a period of two weeks after which the measurements were repeated. Of the 1224 studs measured, over 76 percent experienced changes in twist, over 84 percent experienced changes in bow and over 73 percent experienced changes in crook. Increases in warp generally exceeded decreases, although a surprising percentage of the studs showed decreases in warp. Attention was focused on finding a correlation between moisture content and the changes in warp, but the correlation coefficients were low for both the increases and the decreases. The most likely cause of the warp was the high percentage of juvenile wood present in the raw material. The most likely causes for the changes in warp during the two-week period were stress relaxation aggravated by a change in the moisture gradient.

Keywords and Phrases:

• drying
• drying related degrade
• residual strain
• warp