the steel rods or wires are stretched; after the concrete has been poured and after enough time
has elapsed, the tension in the rods or wires is released. This process, in turn, compresses the
concrete. The prestressed concrete then acts as a compressed spring, which will become uncom-
pressed under the action of tensile loading. Therefore, the prestressed concrete section will not
experience any tensile stress until the section has been completely uncompressed. It is impor-
tant to note once again the reason for this practice is that concrete is weak under tension.Wood
Throughout history, wood, because of its abundance in many parts of the world, has been a
material of choice for many applications. Wood is a renewable source, and because of its ease
of workability and its strength, it has been used to make many products. Wood also has been
used as fuel in stoves and fireplaces. Today, wood is used in a variety of products ranging from
telephone poles to toothpicks. Common examples of wood products include hardwood
flooring, roof trusses, furniture frames, wall supports, doors, decorative items, window frames,
trimming in luxury cars, tongue depressors, clothespins, baseball bats, bowling pins, fishing
rods, and wine barrels (see Figure 17.1). Wood is also the main ingredient that is used to make
various paper products. Whereas a steel structural member is susceptible to rust, wood, on the
other hand, is prone to fire, termites, and rotting. Wood is anisotropic material, meaning that
its properties are direction-dependent. For example, as you may already know, under axial
loading (when pulled), wood is stronger in a direction parallel to a grain than it is in a direc-
tion across the grain. However, wood is stronger in a direction normal to the grain when it is
bent. The properties of wood also depend on its moisture content; the lower the moisture con-
tent, the stronger the wood is. Density of wood is generally a good indication of its strength.
As a rule of thumb, the higher the density of wood, the higher its strength. Moreover, any
defects, such as knots, would affect the load-carrying capacity of wood. Of course, the location
of the knot and the extent of the defect will directly affect its strength.
Timber is commonly classified assoftwoodandhardwood. Softwood timber is made from
trees that have cones (coniferous), such as pine, spruce, and Douglas fir. On the other hand,
hardwood timber is made from trees that have broad leaves or have flowers. Examples of hard-
woods include walnut, maple, oak, and beech. This classification of wood into softwood and566 Chapter 17 Engineering Materials
■Figure 17.1
Examples of wood products.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀