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Technically, any horizontal load-bearing structural member that spans a space and is supported at both ends is considered a beam. A member fixed at one end only is called a cantilever. Steel members that consist of solid pieces of regular structural steel are referred to as "structural shapes." A girder (shown in figure 2-2) is a structural shape. Other prefabricated, open-web, structural-steel shapes are called bar joists (also shown in figure 2-2).

Horizontal structural members that support the ends of floor beams or joists in wood-frame construction are called sills or girders see figures 2-1 and 2-3). The name used depends on the type of framing and the location of the member in the structure. Horizontal members that support studs are called soleplates, depending on the type of framing. Horizontal members that support the wall ends of rafters are called rafter plates. Horizontal members that assume the weight of concrete or masonry walls above door and window openings are called lintels (figure 2-2).

The horizontal or inclined members that provide support to a roof are called rafters (figure 2-1). The lengthwise (right angle to the rafters) member, which supports the peak ends of the rafters in a roof, is called the ridge. The ridge may be called a ridge board, the ridge piece, or the ridge pole. Lengthwise members other than ridges are called purlins. In wood-frame construction, the wall ends of rafters are supported on horizontal members called rafter plates, which are, in turn, supported by the outside wall studs. In concrete or masonry wall construction, the wall ends of rafters may be anchored directly on the walls or on plates bolted to the walls.

A beam of given strength, without intermediate supports below, can support a given load over only a specific maximum span. When the span is wider than this maximum space, intermediate supports, such as columns, must be provided for the beam. Sometimes it is either not feasible or impossible to increase the beam size or to install intermediate supports. In such cases, a truss is used. A truss is a combination of members, such as beams, bars, and ties, usually arranged in triangular units to form a rigid framework for supporting loads over a span.

The basic components of a roof truss are the top and bottom chords and the web members. The top chords serve as roof rafters. The bottom chords act as ceiling joists. The web members run between the top and bottom chords. The truss parts are usually made of 2- by 4-inch or 2- by 6-inch material and are tied together with metal or plywood gusset plates, as shown in figure 2-4.

Roof trusses come in a variety of shapes and sizes. The most commonly used roof trusses, shown in figure 2-5, for light-frame construction are the king-post, the W-type, and the scissors trusses. The simplest type of truss used in frame construction is the king-post truss. It is mainly used for spans up to 22 feet. The most widely used truss in light-frame construction is the W-type truss. The W-type truss can be placed over spans up to 50 feet. The scissors truss is used for buildings with sloping ceilings. Generally, the slope of the bottom chord equals one-half the slope of the top chord. It can be placed over spans up to 50 feet.


LEARNING OBJECTIVE. Upon completing this section, you should be able to recognize the different types of drawings and their uses.

The building of any structure is described by a set of related drawings that give the Builder a complete, sequential, graphic description of each phase of the construction process. In most cases, a set of drawings begins by showing the location, boundaries, contours, and outstanding physical features of the construction site and its adjoining areas. Succeeding drawings give instructions for the excavation and disposition of existing ground; construction of the foundations and superstructure; installation of utilities, such as plumbing, heating, lighting, air conditioning, interior and exterior finishes; and whatever else is required to complete the structure.

Figure 2-4.-A truss rafter.

The engineer works with the architect to decide what materials to use in the structure and the construction methods to follow. The engineer determines the loads that supporting members will carry and the strength qualities the members must have to bear the loads. The engineer also designs the mechanical systems of the structure, such as the lighting, heating, and plumbing systems. The end result is the architectural and engineering design sketches. These sketches guide draftsmen in preparing the construction drawings.

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