Engineering and Construction

The strength of a piece of wood depends chießy on its species, its grade, and the direction in which the load acts with respect to its grain.

For example, in both tension and compression, wood is typically several times stronger parallel to grain  than perpendicular to grain. With its usual assortment of defects, it is  stronger in compression than in tension. Allowable strengths (structural  stresses that include factors of safety) vary tremendously with species and  grade. For example, allowable com- pressive strength parallel to grain  for commercially available grades  and species of framing lumber varies  from 325 to 1700 psi (2.24 to 11.71 MPa), a difference of more than five times. Figure 3.21 compares the average structural properties of framing lumber to those of some other  common structural materialsÑbrick  masonry, steel, and concrete. Of the  four materials, only wood and steel  have useful tensile strength. Defect- free wood is comparable to steel on  a strength-per-unit weight basis, but  with the ordinary run of defects, an  average piece of lumber is somewhat  inferior to steel by this yardstick.

Figure 3.21 Comparative physical properties of four common structural materials: wood (shaded row), brick masonry, steel, and concrete. Wood has signifi cant strength in both tension and compression. The ranges of values of wood strength and stiffness refl ect differences among species and grades of lumber.

When designing a wooden struc- ture, the architect or engineer determines the maximum stresses that are  likely to occur in each of the structural  members and selects an appropriate  species and grade of lumber for each.

In a given locale, a limited number of  species and grades are usually avail- able in retail lumberyards, and it is  from these that the selection is made.

It is common practice to use a stronger  but more expensive species (Douglas  for or Southern pine, for example) for  highly stressed major members and  to use a weaker, less expensive species  (such as Eastern hemlock) or species  group for the remainder of the structure.  Within each species, the designer se- lects grades based on published tables of allowable stresses. The higher the  structural grade, the higher the allowable stress. But the lower the structural  grade, the less costly is the lumber.

There are many factors other  than species and grade that inß  uence  the useful strength of wood. These include the length of time the wood will  be subjected to its maximum load, the  temperature and moisture conditions  under which it serves, and the size  and shape of the piece. Certain fire retardant treatments also reduce the  strength of wood slightly. All these factors are taken into account when engineering a building structure of wood.