Text B. Elements of a Building. Load-Carrying Frame

The frame is the skeleton around which the rest of the house is built. Until the late 19th century, the exterior walls of a building were used as bearing walls to support the floors. This construction is essentially a post and lintel type, and it is still used in frame construction for houses. Bearing-wall construction limited the height of buildings because of the enormous wall thicknesses. In bearing-wall construction, the walls transmit the load to the foundation. Bearing-wall construction is usually most economical for buildings less than four stories high.

In skeleton construction, all loads are transmitted to the foundation by a rigidly constructed framework made up of beams, girders, and columns. This skeleton carries the roof, walls, and floors, together with their loads. Beams and girders run horizontally. Girders are usually larger than beams. Closely spaced beams are called joists, especially in wooden buildings. Purlins are small beams that brace rafters or girders and help provide the structure to support roofs. Beams above window and door openings are called lintels. Slabs are beams whose width is greater than their depth. Columns are heavy vertical supports that carry the load of beams and girders. Trusses consist of wood or steel supports that are connected in triangular patterns. The trusses provide the strength and rigidity to span large distances with relatively small amounts of material. Arches are curved supports that usually extend over openings.

Skeleton construction is better for taller buildings. All buildings in the skyscraper class are of skeleton construction. The 10-story Home Insurance Building in Chicago was one of the first buildings to have a skeleton construction. Completed in 1885, this building is often considered the world's first metal-framed skyscraper. Skeleton construction, consisting of steel beams and columns, was first used in 1889.

The three major types of structural frames are wood, steel, and concrete. Wood frames are light, cheap, and simple, consisting of interlocking arrays of vertical and horizontal beams and studs. Steel structural frames consist of vertical members (columns) and horizontal members (girders and beams) that are riveted, bolted, or welded together. Concrete structural frames have the advantage of costing less than steel ones, but they must usually be reinforced by steel to carry heavy loads. Steel rods are positioned in an interconnected framework surrounded by a wooden or steel form, and then concrete is poured into the form. The form is withdrawn once the concrete has set. In a method known as prestressing, high-strength wires are stretched and held tight while concrete is set around them. In this way a concrete span as long as 100 feet (30 m) can be attained.