Structure. The basic elements of any ordinary structure are the floors and roof (including horizontal supporting members)

The basic elements of any ordinary structure are the floors and roof (including horizontal supporting members), columns and walls (vertical members), and bracing (diagonal members) or rigid connections used to give the structure stability.

One- to Three-Story Buildings

With low buildings the variety of possible shapes is much greater than with taller buildings. In addition to the familiar box shape, which is also used in very tall buildings, low buildings may use cathedral-like forms, vaults, or domes. A simple single-story structure might consist of a reinforced-concrete slab laid directly on the ground, exterior masonry walls supported by the slab (or by a spread footing cast continuously around the perimeter of the building), and a roof. For low buildings, the use of interior columns between masonry load-bearing walls is still the most common construction method. Spaced columns supported by the slab or by individual spread footings may be used, however; in that case the exterior walls can be supported by or hung between the columns. If the roof span is short, abutting planking made of wood, steel, concrete, or other material can be used to form the roof structure.

Each structural material has a particular weight-to-strength ratio, cost, and durability. As a general rule, the greater the roof span, the more complicated the structure supporting the roof becomes and the narrower the range of suitable materials. Depending on the length of the span, the roof may have one-way framing beams (Figure 2a and 2b) or two-way framing (beams supported on larger girders spanning the longest dimension). Trusses can be substituted for either method. Trusses, which can be less than 30 cm or more than 9 m deep, are formed by assembling tension and compression members in various triangular patterns. They are usually made of timber or steel, but reinforced concrete may be used.

The structure of a simple one-story building may also consist of the wall and roof framing combined by being either fastened together or shaped in one piece. The possible structural shapes are almost infinite and include the three sides of a rectangle fastened together into a unit called a bent (Figure 2c), the familiar church form of vertical sides and sloping roof (Figure 2d), the parabola (Figure 2e), and the semicircle or dome.

The supporting structure and exterior walls, floor, and roof may also be made as a unified whole, much like a rectangular pipe with closed or open ends. These forms may be cast in reinforced plastic.

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