Wood Products on the Job - Structural Applications
Floor Systems
Wood products are used extensively for floor framing in North American residential construction. Dimension lumber, including MSR lumber and finger-jointed lumber designed for use in horizontal applications, wood I-joists, and parallel chord trusses or specialty trusses, are the common joist materials in wood construction.

Codes and Spans
The allowable spans for floor joists are set out in the model building codes. Several tools, both published and electronic, are available to help designers and builders quickly select or check floor spans. The floor spans are based on the distributed loads likely to be encountered for different living areas such as bedrooms.

Using anticipated loads, calculations are made to determine how far joists can span and deliver a high level of service. For most typical residential applications, the joists are considered to be load sharing when their spacing does not exceed 24 inches. This system distributes loads so they are shared by several members simultaneously. The load sharing and the performance of the floor are improved by installing cross-bracing between the floor joists, by installing strapping or by gluing and screwing the floor sheathing to the joists.

Span tables are based on code requirements. Floor joists must meet at least three design criteria and the one that governs the allowable span depends on the span arrangement:

1. Shear: horizontal shear is the tendency for short, heavily loaded beams or joists to split and fail in a horizontal direction.

2. Bending: excessive bending forces will cause long and heavily-loaded beams or joists to crack and fail at the center.

3. Deflection: A beam that is strong enough to resist shear and bending failure may nonetheless lack stiffness to remain straight when loaded. Deflection is an appearance deficiency.

When designed to meet the preceding three criteria, joists will usually perform well for the vast majority of floor configurations. In Canada, a fourth floor joist design criteria, vibration control, is used to further ensure floor performance.

Fire and Sound
Other considerations for floor performance are noise transmission and fire resistance. Typical residential flooring systems provide a minimum level of sound transmission and fire resistance rating. All second story floors and many first story floors will have a finished drywall ceiling below. In some instances, the ceiling of the first story floor remains unfinished. The addition of drywall to the ceiling significantly improves the fire and sound performance of the floor assembly.