4.2 Read the text and give definitions of different structural elements Text 4 a Structural Elements

From the beginning of recorded time man has designed and erected structures of varrous size and beauty. One of the most important considerations in design of safe, strong, beautiful and economical structures is the determination of the correct shape and dimensions for all of the structural elements. Let's speak about some of them.

Arches. Through most of architectural history, the arch has been the chief means of overcoming the spanning limitations of single blocks of stone or lengths of timber. There were three types of arches in ancient architecture. One consisted of only three rough blocks of stone, the central one somewhat larger than the gap between the other two and wedged between them. A second consisted of only two long blocks inclined toward one another as an inverted V-shape. This form was constructed in timber. The third was known as the false or corbeled arch. Later appeared small true brick arches. From the first century AD the Romans began to use concrete in place of cut stone. Arches of other profiles were made. Of these, the most important in the Middle Ages were-the pointed arches. Early cast-iron arches of the late 18-th and early 19-th centures all closely resembled braced timber arches. Later steel and reinforced concrete arches have usually been given the necessary stiffness by the adoption of an I-shaped, boxlike, or tubular cross section.

Beams ( as well as girders and columns ) are used to support the whole structure. There are different types of beams. A simple beam is a beam that rests upon a support an each end, there being no resistant at the supports. This is the type most commonly in use. A cantilever beam is a beam that is supported at only one end. A beam projecting from a wall is an example. An overhanging beam rests upou two or more supports, one or both ends projecting beyond the supports. A continuous beam is a beam that rests upon more than two supports. A fixed beam is a beam that is restrained against rotation at the end or ends. Beams were first made of stone and timber. Iron beams became structurally important elements only in the late of 18-th century. Reinforced concrete beams are widely used now, since the strength of the concrete, the overall geometry of the element were under the designer's control.

Domes. The dome may be regarded as the three-dimensional counterpart of the arch. In its true circular form, a vertical arch is rotated around a vertical axis and sweeps out, at every level, a continuous circular horizontal ring. Loads can be transmitted both along the meridian lines of the vertical arches and around the horizontal rings.

Masonry domes have been constructed for centuries, but in modern times the masonry has been largely supplanted by steel and concrete.

Shells. The term shell is used to denote a spanning and space -enclosing elements of domed or other vaultlike form, but with a thickness and order of magnitude less than was usual for these masonry and mass-concrete forms. Shells are similar to domes in that they transmit forces in three dimensions. This result in great economy of material; in fact no other system of construction utilizes materials so efficiently. Shell construction for buildings is of concrete, although some experiments have been made with shells of plywood. Shells have been made in a wide variety of forms. The most unlimited latitude in design forms is one of the main advantages of shell construction.