Beams, Joists, and Wall Studs Information
Rafters, beams, and joists are structural profiles that are used to frame buildings.
When selecting products, builders must account for spans—the clear distances between supports. Spans are based on uniform gravity loads and may be evaluated for wind. Typically, the design of continuous spans is based on the longest span so that, for example, the shortest span is not less than 50% of the longest span. Other specifications to consider when selecting rafters, beams, and joists are:
- moment, shear, and reaction capacity for normal load duration
- deflection, both in terms of bending and shear deformation
- live load and dead load
Live load is that amount of weight that a structure is designed to support. Dead load refers to the weight of the structure itself.
Selecting rafters, beams, and joists requires an understanding of product features. Rafters, or roof beams, are sloped, structural members that extend from the ridge of a roof to the down-slope perimeter or eave. Rafters are designed to support the roof sheathing and usually follow the angle of the roof. With a flat roof, rafters are sometimes called roof joists. Beams are structural elements that are designed to carry vertical gravitational forces or horizontal loads. They are subject to bending loads and are usually made of steel, wood, or concrete.
There are three types of beams: simple, continuous, and cantilever. Simple beams rest on two supports and have their ends free. By contrast, continuous beams rest on three of more supports. Cantilever beams, or cantilevers, are supported on only one end. With these rafters, beams, and joists, the beam carries the load to the support where it is resisted by moment and shear stress.
Rafters, beams, and joists are used in several different applications. Joists are horizontal or near-horizontal structural members that support a floor or ceiling. The weight of the floor or ceiling covering, along with the size of the span, form the basis for calculating the size joists that are needed. Joists are laid out in parallel and may be supported by larger beams, bearing walls, or steel girders. There are two basic types of joists: floor joists and roof joists. Floor joists are the principal elements in a wooden floor. The flooring material attaches to the top of the joists. If the room below the floor has a finished ceiling, then the ceiling material attaches to the bottom of the joists. Like rafters or roof beams, roof joists are used in roofing projects. Before the joists are fitted, wall plates are installed. In residential construction, these rafters, beams, and joists are mounted and then fitted with firings, lengths of timber that are cut from corner to corner to form very long triangles.
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