Bowing in Roof Joists Induced by Moisture Gradients and Slope of Grain.

摘要

During winter months many houses constructed with flat or low-pitched roofs have exhibited upward bowing deflections of 5/8 to 3/4 inch at midspan causing a separation between ceiling and partitions. Where the problems exist, a definite moisture gradient across the depth of the roof joist has been measured in the field. Wood has a very low shrinkage coefficient in the longitudinal direction which is essentially parallel to the axis of the lumber. Its magnitude is far too low to account for the bowing deflections recorded in actual houses. However, shrinkage values in the radial or tangential direction are 50 to 80 times greater than the longitudinal. Any significant slope of grain produces shrinkage components from the tangential and radial direction relative to the axis of the member. Twenty-six specimens of three different species were cut with varying slopes of grain. A moisture gradient was created across the depth of the specimen and bowing deflections were observed. Relationships between bowing deflection and grain angle were established. The magnitudes of observed deflections agreed with those measured in the field. (Author)