MEMBERS

The main parts of a structure are the load-bearing structural members that support and transfer the loads on the structure while remaining in equilibrium with each other. The places where members are connected to other members are called joints. The total 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, that the

Figure 7-2.-Projecting structural shapes. A. I- or H-beam. B. Channel. C. Tee.

structure is 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.

Horizontal members provide immediate or direct support for the loads. These in turn are supported by vertical members, which in turn are supported by foundations and/or footings, which are finally supported by the earth.

The ability of the earth to support a load is called the soil-bearing capacity. It is determined by test and measured in pounds per square foot. Soil-bearing capacity varies considerably with different types of soil, and a soil with a given bearing capacity will bear a heavier load on a wide foundation or footing than it will a narrow one.

Vertical Members

Columns are high-strength vertical structural members; in buildings they are sometimes called pillars. Outside-wall columns and bottom-floor inside columns usually rest directly on footings. Outside-wall columns usually extend from the footing or foundation to the roof line. Bottom-floor inside columns extend upward from footings or foundations to horizontal members that support the first floor. Upper floor columns usually are located directly over lower-floor columns.

A pier in building construction might be called a short column. It may rest directly on a footing, or it may be simply set or driven in the ground. Building piers usually support the lowermost horizontal structural members. 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. They are supported on horizontal members called sills or sole plates, and are topped by horizontal members called top plates or stud caps. Corner posts are enlarged studs located at the building corners. In early full-frame construction, a corner post was usually a solid piece of larger timber. Built-up corner posts are used in most modern construction. They consist of two or more ordinary studs nailed together in various ways.

Horizontal Members

In technical terminology, a horizontal load-bearing structural member that spans a space and is supported at both ends is called a beam. A member that is fixed at one end is called a cantilever. Steel members that consist of solid pieces of regular structural steel shapes are called beams. However, one type of steel member is actually a light truss (discussed later) and is called an open-web steel joist or a bar-steel joist.

Horizontal structural members that support the ends of floor beams or joists in wood-frame construction are called sills, girts, or girders. The choice of terms depends on the type of framing being done and the location of the member in the structure. Horizontal members that support studs are called sills or sole plates. Horizontal members that support the wall ends of rafters are called rafter plates or top plates, depending on the type of framing. Horizontal members that support the weight of concrete or masonry walls above door and window openings are called lintels.

Trusses

A beam of given strength, without intermediate supports below, can support a given load over only a certain maximum span. If the span is wider than this maximum, the beam must have intermediate supports, such as columns. Sometimes it is not feasible to install intermediate supports. In these cases, a truss may be used instead of a beam.

A truss is a framework consisting of two horizontal (or nearly horizontal) members joined together by a number of vertical and/or inclined members to form a series of triangles. The loads are applied at the joints. The horizontal members are called the upper or top chords and lower or bottom chords. The vertical and/or inclined members that connect the top and bottom chords are called web members.

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