Support The Load

The parts of a woodworking project are elements of what engineers call a "stress system." Each joint must withstand a certain amount of stress pushing or pulling at the members of the joint. This stress comes from many different sources. It could be external (coming from outside the structure); for example, when you sit on a chair, your weight stresses the chair joints. If you scoot the chair across the floor, the friction between the floor and the chair legs creates stress. Or the stress could be an internal factor, inherent to the structure. The tension in a woven seat, for example, stresses the joints between the rails and the legs. Even the weight of the individual chair parts, no matter how small or light they may be, is an internal stress to be reckoned with.

There are four types of stress, categorized by the direction of the force relative to the joint (See Figure 1-3):

■ Tension pulls the members of a joint apart.

■ Compression squeezes the members together.

■ Shear pushes the members in opposite directions. The lines of force are parallel, but not aligned as they are with tension and compression.

■ Racking (or bending) rotates the members around one another.

I -2 Although there are hundreds of fitted joints, they can all be organized into four categories — simple joints (1) such as the rabbet-and-dado joint, reinforced joints (2)

such as the dowel joint, mortise-and-tenon joints (3) such as the haunched mortise and tenon, and interlocking joints (4) such as the through-dovetail joint.

I -2 Although there are hundreds of fitted joints, they can all be organized into four categories — simple joints (1) such as the rabbet-and-dado joint, reinforced joints (2)

such as the dowel joint, mortise-and-tenon joints (3) such as the haunched mortise and tenon, and interlocking joints (4) such as the through-dovetail joint.

Even before they've been glued or fastened, fitted joints resist one or more types of stress. (See Figure 1-4.) After they're assembled, they resist all types to a greater or lesser degree. When choosing a joint, try to pick one that wall withstand the anticipated stress without glue or fasteners. That way, if the glue bond or the hardware fails, the joint will stay together.

■ Use larger joints and structural members. This distributes the load over a larger area and larger mass. (See Figure 1-5.)

■ Use smaller members, but more of them. This too increases the area and mass that supports the load. (See Figure 1-6.)

■ Triangulate the members. Rearrange the structural members or add new members, braces, glue joints, or fasteners to create structural triangles. When a triangle is fastened at all three corners, it's very rigid. This is why engineers triangulate bridges and roof trusses. (See Figure 1-7.)

■ Orient the wood grain properly; wood is always strongest parallel to the grain. A tenon or dovetail cut across the grain will soon break.

■ Increase the glue surface in a joint by making the fitted surfaces more intricate. (See Figure 1-8.)

■ Increase the size or the number of fasteners.

■ Use both glue and fasteners.

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