Wood Design and Construction
Material behavior
Wood is orthotropic: strength and stiffness differ along and across the grain. Design uses reference design values modified by factors for load duration (CD), size, moisture, and temperature. Key concepts are the difference between bending, shear, and compression parallel and perpendicular to grain, and the role of the modulus of elasticity for deflection.
Members and connections
Joists, rafters, and beams are sized for flexure and shear, with deflection often governing. Columns are checked for slenderness and buckling. Connections (nails, bolts, screws, and connectors) are often the weak link. Traps include ignoring the load duration factor for short-term loads, forgetting that wet-service conditions reduce values, and assuming nailed connections have the same capacity in withdrawal as in shear.
Construction
Construction topics include framing, sheathing, bracing, and protection from decay and fire. Candidates should know that moisture control and proper flashing protect wood from rot, and that heavy timber has a fire-resistance advantage from its charring behavior.
Worked rule of thumb
For a wood beam with reference bending value Fb = 1,200 psi and CD = 1.15 for snow, adjusted Fb' = 1,380 psi. If the section modulus S = 50 in³, nominal moment capacity is Fb' × S = 69,000 lb-in ≈ 5,750 lb-ft. Compare to the factored demand from wL²/8, and check deflection separately.