Several characteristics of wood are also relevant to overall connection behavior:

First, wood is anisotropic, meaning it has different strength properties in different directions: longitudinal (strong), tangential (weaker) and radial (weakest).

▶ Wood is stronger in compression and tension parallel to grain versus perpendicular to grain; this is the preferable way of designing wood connections. However, this is not always practical or possible.

▶ While wood is weaker in compression perpendicular to grain, this is the most common way to transfer loads in construction. The most common loads are gravity loads and the typical way to transfer them is in bearing—thus reliance on the compression perpendicular to grain properties of wood. Fasteners are required to transfer other loads, such as wind uplift and the lateral loads caused by occasional wind and seismic forces, into the foundation.

While capacities are more limited when wood is loaded in compression perpendicular to grain (versus parallel to grain), the limits for bearing conditions on the surface of wood members are deformation-based, not strength-based, and published design values can be increased for smaller bearing areas. Accordingly, dowel bearing strengths are higher relative to compression parallel or compression perpendicular to grain values. However, dowel bearing strengths perpendicular to grain are lower relative to dowel bearing strengths parallel to grain for larger diameter (>1/4-in.) fasteners (see Table 1).

▶ Wood is weakest when tension is applied perpendicular to grain. This should be avoided—which is why there are no published design values for tension perpendicular to grain.

Second, wood connections are stronger when the load is spread over a number of fasteners. Large concentrated loads should be avoided unless designed not to exceed the wood's strength capabilities (e.g., net tension and shear). Spreading the load also builds in a degree of redundancy, which is useful in high wind or seismic events. To accomplish this, designers are advised to:

▶ Use small fasteners.

▶ Use multiple fasteners when possible.

▶ Keep the scale of fasteners relative to the size of wood members being connected.

Third, as with other building materials, wood moves in response to environmental conditions. The main driver for this movement in wood is moisture. Allowances must be made to accommodate potential shrinkage and swelling, particularly in connections. (See Moisture Effects on page 4.)

Connection Types

There are two major types of connections used in wood buildings: mechanical and joinery (sometimes referred to as carpentry). Within these categories are many variations, and thus a wide range of options to accommodate virtually any building design.

Mechanical Connections

Mechanical connections used in wood buildings can be classified into three major categories: dowel, metal connector plates with integral teeth, and shear. There are also a number of proprietary connections that combine characteristics of each of these types.

Dowel-type fasteners. Wood members connected with dowel-type fasteners are probably the most common mechanical connection type because they are effective at transferring loads while also being relatively simple and efficient to install. They come in many forms and their strength properties can be calculated using the National Design Specification® (NDS®) for Wood Construction. Nails are generally used when loads are relatively light, as in multi-family and light commercial buildings. Staples can be used in place of nails, but equivalent capacities need to be determined as the NDS doesn't publish design values for staples. Screws may be more satisfactory than nails under certain conditions (such as exposure to moisture) since they have less tendency to work loose and generally have high wind withdrawal resistance under severe wind events. Timber rivets are a high-strength dowel-type fastener used in conjunction with specially fabricated metal plates.

Dowelled connections transfer the force between members through a combination of dowel bearing and bending of the dowel fastener.