DESIGN OF STRUCTURAL MEMBERS
From the builders standpoint, building designs and construction methods depend on many factors. No two building projects can be treated alike. However, the factors usually considered before a structure is designed are its geographical location and the availability of construction materials.
It is easy to see why geographical location is important to the design of a structure, especially its main parts. When located in a temperate zone, for example, the roof of a structure must be sturdy enough not to collapse under the weight of snow and ice. Also, the foundation walls have to extend below the frost line to guard against the effects of freezing and thawing. In the tropics, a structure should have a low-pitch roof and be built on a concrete slab or have shallow foundation walls.
Likewise, the availability of construction materials can influence the design of a structure. This happens when certain building materials are scarce in a geographical location and the cost of shipping them is prohibitive. In such a case, particularly overseas, the structure is likely to be built with materials purchased locally. In turn, this can affect the way construction materials are usedit means working
DEAD AND LIVE LOADS
The main parts of a structure are the load-bearing members. These support and transfer the loads on the structure while remaining equal to each other. The places where members are connected to other members are called joints. The sum total of the load supported by the structural members at a particular instant is equal to the total dead load plus the total live load.
The total dead load is the total weight of the structure, which gradually increases as the structure rises and remains constant once it is completed. The total live load is the total weight of movable objects (such as people, furniture, and bridge traffic) the structure happens to be supporting at a particular instant.
The live loads in a structure are transmitted through the various load-bearing structural members to the ultimate support of the earth. Immediate or direct support for the live loads is first provided by horizontal members. The horizontal members are, in turn, supported by vertical members. Finally, the vertical members are supported by foundations or footings, which are supported by the earth. Look at figure 2-1, which illustrates both horizontal and vertical members of a typical light-frame structure. The weight of the roof material is distributed over the top supporting members and transferred through all joining members to the soil.
Figure 2-1.Typical light-frame construction.
The ability of the earth to support a load is called its soil-bearing capacity. This varies considerably with different types of soil. A soil of a given bearing capacity bears a heavier load on a wide foundation or footing than on a narrow one.
VERTICAL STRUCTURAL MEMBERS
In heavy construction, vertical structural members are high-strength columns. (In large buildings, these arc called pillars.) Outside wall columns and inside bottom-floor columns usually rest
directly on footings. Outside wall columns usually extend from the footing or foundation to the roof line. Inside bottom-floor columns extend upward from footings or foundations to the horizontal members, which, in turn, support the first floor or roof, as shown in figure 2-2. Upper floor columns are usually located directly over lower floor columns.
Figure 2-2.Typical concrete masonry and steel structure.
In building construction, a pier, sometimes called a short column, rests either directly on a footing, as shown in the lower center of figure 2-3, or is simply set or driven into the ground. Building piers usually support the lowermost horizontal structural members.
Figure 2-3.Exploded view of a typical light-frame house.
In bridge construction, a pier is a vertical member that provides intermediate support for the bridge superstructure.
The chief vertical structural members in light-frame construction are called studs (see figures 2-1 and 2-3). They are supported by horizontal members called sills or soleplates, as shown in figure 2-3. Corner posts are enlarged studs located at the building corners. Formerly, in full-frame construction, a corner post was usually a solid piece of larger timber. In most modern construction, though, built-up corner posts are used. These consist of various members of ordinary studs nailed together in various ways.
HORIZONTAL STRUCTURAL MEMBERS
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.
Figure 2-4.A truss rafter.
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.
Figure 2-5.The most commonly used roof trusses.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015