WOOD - Materials for Civil and Construction Engineers - page 438

It should be noted that there are many footnotes associated with this table,
and before use of this information, the reader should consult the cited refer-
ence (APA-EWS, 2004). Table 10.12 addresses design values only for bending
members. A similar table is available in the cited reference for design values
for members subjected to axial tension or compression.

■ 10.13.3 Composite Structural Members

As with natural wood products, glued products can be combined in a vari-
ety of ways to produce structural elements. Typical examples of manufac-
tured composite members (Figure 10.21) are

I-beams
Box beams
Stressed skinned panels
Structural insulated panels

For example, I-beams are produced by gluing flange elements to an oriented
strand board web, as shown in Figure 10.21(b). The flange elements may be
either dimensional lumber or, more commonly, laminated veneer lumber.
As manufactured products, the properties of the elements are frequent-
ly specified based on the performance of the member, rather than being
based on the individual material components. For example, I-joists are spec-
ified based on the spacing of the joists and the depth of the member. Stan-
dards for composite structural members may be developed by national
organizations, such as ASTM, or by trade associations concerned with the
proper use of the products they promote.

SUMMARY

Wood is an extremely flexible building material. Historically, natural wood
products were the only option available to the engineer. However, modern
forestry practices limit the size of natural products that are available. The need
to increase the efficiency of using wood products has led to the development
of engineered wood products. These products are frequently more economical
than natural wood, particularly when large dimensions are required. In addi-
tion, by careful control of the manufacturing process, engineered woods can be
produced that have characteristics superior to natural wood. The characteris-
tics of engineered wood products depend on the wood stock used, the quality
of the adhesive, and the manufacturing process. Several factors make wood
unique when compared to the other materials used in civil engineering, in-
cluding anisotropy, moisture sensitivity, creep, and the existence of defects in
wood products. Furthermore, when wood is exposed to the environment, care
must be taken to prevent degradation due to fungi, bacteria, and insects.