These involve pieces of Douglas Fir a. The tensile strength parallel to the grai

Question

These involve pieces of Douglas Fir

a. The tensile strength parallel to the grain is 19,500 psi at 12% moisture content. Estimate the compressive strength parallel to the grain, the compressive strength perpendicular to the grain, and the tensile strength if the grain is 25° from parallel to the face of the lumber in the longitudinal direction.

b. Estimate the maximum short term tensile load for a sawn 2x8x16 piece (1 ¾” by 7¼” by 16’) from this Douglas Fir at 12% moisture with the grain parallel to the longitudinal direction of loading with and without a 2” diameter knot through the 2” thickness perpendicular to the 8” direction. If the maximum load is applied for more than 10 years, will the load decrease or increase? Why?

Explanation / Answer

The compressive strength of lumber parallel to the grain is much higher than that perpendicular to the grain. Columns, posts and members of a truss are subjected to axial loads parallel to the grain of the wood. When a column rests on a beam, the load from the column creates compressive (bearing) stress on the beam that is perpendicular to the grain of the wood.

The compressive strength of most softwood lumber parallel to the grain is in the range of 2000-4000 psi. The stress at the proportional limit is about 80% of this strength. The compressive strength perpendicular to the grain varies between 12-18% of that parallel to the grain. The compressive stress at the proportional limit, for loading perpendicular to the grain, is about 12-25% of that for loading parallel to the grain. In dry condition, lumber can possess significantly higher compressive strength. For example, the compressive strength (parallel to the grain) of Douglas fir in green condition is about 3700 psi, while that at 12% moisture content is about 7000 psi. Seasoning increases the compressive strength. Typically, the compressive strength (parallel and perpendicular to the grain) increases an average 4-6% for every 1% decrease in moisture content.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.