Iron and steel

Iron and steel are the world's cheapest and most useful metals. These hard, durable metals are used in making thousands of products, from paper clips to automobiles.

The word iron can refer to both an element and a number of alloys (mixtures) of iron and other metallic elements.

Steel is produced by refining (purifying) iron and alloying it with other metals. Iron may be thought of as a basic material of steel, and steel may be considered the refined product of iron.

The properties of any kind of iron or steel depend largely on the chemical composition of the alloy.

Cast iron is any iron alloy that contains from 2 to 4 per cent carbon and from 1 to 3 per cent silicon, because of its high carbon content, solid cast iron cannot be shaped, no matter how hot it is heated. This kind of iron is made into useful objects by pouring the liquid metal into molds and letting it harden. Cast iron's hardness, low cost, and ability to absorb shocks make it an important construction material.

Steel finds its use in corrugated sheets for roofing, for girders, frames, etc. Various shapes are employed in construction.

Types of construction

In bearing-wall construction, the walls transmit the load to the foundation. 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. Bearing-wall construction is usually most economical for buildings less than four stories high, but 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.

Many parts of a building have no structural function. Nonbearing walls and curtain walls carry only their own weight and serve to divide the interior of a building or to keep out the elements. Such walls are called non-bearing partitions. Other nonbearing parts include windows, doors, stairs, elevators, and other equipment.

In one method of construction, called tilt-up construction, concrete wall panels are formed at ground level. Cranes or derricks then lift them into position.

Classification of building

Buildings are classified by construction as fire-resistive, noncombustible, ordinary, or frame. The most important factor in this classification is resistance to fire. A standard fire testis used in testing samples of materials. In this test, the samples are exposed to a fire of specified intensity. The materials are graded for their ability to withstand the fire. For example, a two-hour rating is given a material that withstands the test for two hours.

Fire-resistive construction is that in which the walls of a building are of masonry or reinforced concrete, and the major structural parts are steel or reinforced concrete and so insulated as to have a four-hour rating. Other parts of the building must have a three-hour rating. Large cities require most buildings in the business districts to be fire-resistive.

Noncombustible construction is similar to fire-resistive construction. However, the major structural parts have a two-hour rating.

Ordinary construction is that in which the exterior walls are masonry or reinforced concrete, and in which the interior structural members are partly or entirely of wood in smaller dimensions than required for heavy timber or steel construction.

Frame construction is that in which exterior walls are wholly or partly of wood. It includes brick or stone veneer, stucco, or sheet metal over wood.