Timber Trusses for Big Spans

Answers:

1. Glulams are made from select two-by boards of Southern yellow pine or Douglas fir. For their size, glulams are stronger than solid lumber because of the arrangement of the layers of wood and the added strength of the glue. Parallel strand lumber consists of slender strips of wood that are pressed together to form a loaf, then sawn to size. Both of these engineered wood forms are less wasteful and destructive to the environment than solid timber. Low-grade raw materials are used to create a homogeneous end product. Both types of engineered lumber are also more dimensionally stable than sawn timbers of equivalent size, and, as a result, are less likely to twist or check. They are also more expensive.

2. Timber trusses have many advantages over other types of trusses. They can be used outdoors if sheltered or treated with preservatives. They outperform light-wood and steel trusses when fire codes are strict, offering a fire resistance of up to an hour. Timber trusses can be made of recycled wood, often gathered from old buildings. Recycled wood offers an aged patina, is more dimensionally stable than new timber, and doesn't require cutting a new tree.

3. Trusses are different from beams because they are made up of linear elements in triangular formations to carry loads efficiently. The strength of a truss is in its geometry, connections, and members. The structural behavior of each member in a timber truss is different from that of a solid-timber beam. A downward load applied along the length of a beam will cause it to bend, but such a load applied to a truss generates tension or compression that is shared by each of its members in concert. Even if one truss member acts as a column, the overall truss still acts as a unit. For that reason, a truss can be lighter than a beam for a given span and load. Also, the rate at which member weight rises with increasing span is generally lower with a truss than with a beam.

4. Fasteners require holes, grooves, or recesses. These voids reduce the structural capacity of the member. The fasteners with the greatest structural capacity require the largest holes. As a result, such connections often control the design of the truss, necessitating the use of larger timbers if there are multiple voids required in the wood. Fasteners that require fewer holes are recommended wherever the objective is to minimize the size of the member.

5. The shapes of trusses are a function of the shape of the roof. The bottom chord of a truss becomes the ceiling of a room. The arrangement, size, and quantity of web members also affect the look. Whether the members are curved or straight, painted or stained, or treated with preservatives or fire retardants also determines the style of the truss.