Resistance of Building Materials
Framing materials resist loads according to their strength. Wood, steel, and concrete are our primary structural materials and their strength characteristics are well known. The load carrying capacity of individual elements is a function of the size, shape, and inherent strength of the material, usually stated as allowable stress, and measured in pounds per square inch (psi), or, for some manufactured elements (e.g. nails, hangers), as pounds.
For rough estimating, here are some typical conservative values of allowable stresses:
Steel: 18,000 psi in tension, compression, and bending.
Concrete (plain): 1350 psi in compression and zero in tension and bending.
Structural Lumber: 1200 to 2000 psi in bending, tension and compression parallel with the grain, and 400 psi in compression perpendicular (normal) to the grain, values depending on the species and grade.
Nails: Allowable lateral load (shear) for a 16-penny nail completely driven into side grain of sound seasoned wood is approximately 80 to 100 pounds; its withdrawal load from side grain is about 30 pounds per inch penetration, and next to nothing in withdrawal from end grain.
Actual yield (total) strength is much higher but held in reserve to provide a factor of safety for un-anticipated over-loads, defective material or fabrication. Technical handbooks provide more detailed information.
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