STRUCTURAL ELEMENTS: ARCHES

Throughout 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, which survives today in Mycenean cyclopean construction, 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, of which monumental examples survive in Egypt from the 3^rd millennium ВС, consisted of only two long blocks inclined toward one another as an inverted V-shape. This form was probably constructed even earlier in timber. The third, of which surviving examples are very widespread, was what is commonly known as the false or corbelled arch.

None of these early forms was very efficient. Spans rarely exceeded 6 ft. 6 in. (2 m). The spanning of substantially wider gaps called for true arches constructed on centering from large number of bricks or stone voussoirs. Small true brick arches appeared first m Mesopotamia and Egypt.

From the first century AD the Romans began to use concrete in place of cut stone for all the longer spans.

Later brick and stone arches departed from Roman precedents mainly in adoption of other profiles. Of these, the most important were the pointed profiles of most Islamic and of Gothic arches. The Islamic form appeared first and was preceded by a Sassanian form of roughly parabolic profile. The chief merit of the pointed profile was probably the ease with which it could be used in ribbed vaults of any plain shape and without aesthetic inconsistency throughout structures that vaulted in this way.

Early cast-iron arches of the late 18^th and the early 19^th centuries all closely resembled braced timber arches. Later steel and reinforced-concrete arches have usually been given the necessary stiffness simply by the adoption of 1-shaped, boxlike or tubular cross section.

FLOOR SYSTEMS

The continuous slab constitutes a self-contained floor system, though it may be desirable for non-structural reasons to add a separate top surface and a separate ceiling below. Before the development of the reinforced-concrete slab, the nearest equivalents were the floor composed of beams of timber or stone set immediately alongside one another, and the floor provided by a more or less solid fill above a brick or concrete vault. The first of these involved a very extravagant use of material and hence expenditure of effort, so it usually gave way to a more differentiated form with increasing skill in construction. The second was more efficient, inherently strong, and fireproof, and continued to be used tor these reasons until supplanted by the reinforced-concrete slab. But it had the drawbacks of greater overall depth than alternative forms, and of greater weight plus the generation of outward thrusts, so that stronger walls were called for.

The alternative to these forms was always some composite system, with beams as the principal spanning and load-bearing elements. In the commonest of these systems, still widely used, light timber beams span at short intervals between opposite walls and are covered by boards or twigs and rammed earth.

Today the usual floor system, apart from intermediate floor within single dwellings, is the reinforced-concrete slab with or without projecting beams. For very heavy loadings and wide spans, a grid of beams within a bay may be used to stiffen and strengthen the slab without requiring it to be of great thickness throughout. In all cases, the slab has a great advantage over the earlier systems because it is a good horizontal diaphragm, binding the walls or columns together and distributing any side loads between them.

Structural Systems

The structures of buildings are mostly skeleton frames of various types. New domestic housing in many parts of the world today is commonly made from timber-framed construction. Wood products are becoming a bigger part of the construction industry. They may be used in both residential and commercial buildings as structural and aesthetic materials. In buildings made of other materials, wood is still found as a supporting material, especially in roof construction, in interior doors and their frames, and as exterior cladding.

Laminated veneer lumber functions as beams to provide support over large spans, such as removed support walls and places where dimensional lumber is not sufficient, and also in areas where a heavy load is bearing from a floor, wall or roof above on a short span. Wood I-joists are used for floor joists on upper floors. They are engineered for long spans and are doubled up in places where a wall will be aligned over them. Glued laminated beams are created by glueing the faces together to create beams. By glueing multiple, common sized pieces of lumber together act as one larger piece of lumber. Manufactured trusses are used in home construction as a pre-fabricated replacement for roof rafters and ceiling joists. It is seen as an easier installation and a better solution for supporting roofs as opposed to the use of lumber struts and purlins as bracing.

Steel is one of the major structural materials in buildings. It is a strong and stiff material. It can be quickly fabricated and erected. The lightest and most efficient structural shape is the bar (or open web) joist, a standard truss made with angles for

the top and bottom chords, joined by welding to a web made of a continuous bent rod. It is used almost exclusively to support roofs and can span up to 45 metres. The standard rolled shapes are frequently used as beams and columns, the wide flange, or W-shape, being most common. Where steel beams support concrete floor slabs poured onto a metal deck, they can be made to act compositely with the concrete.

Steel columns are joined to foundations with base plates welded to the columns and held by anchor bolts embedded in the concrete. The erection of steel frames at the building site can proceed very rapidly, because all the pieces can be handled by cranes and all the bolted connections can be made swiftly by workers with hand-held wrenches.

Reinforced concrete is also a major structural material in buildings. In situ concrete is used for foundations and for struc­tural skeleton frames. The oldest framing system is the beam and girder system, whose form was derived from wood and steel construction: slabs rest on beams, beams rest on girders, and girders rest on columns in a regular pattern. This system needs much handmade timber formwork, and in economies where labour is expensive other systems are employed. One is the pan joist system, a standardized beam and girder system of constant depth formed with prefabricated sheet-metal forms. The simplest and most economical floor system is the flat plate where a plain floor slab rests on columns spaced apart. If the span is larger, the increasing load requires a local thickening of the slab around the columns. Concrete columns are of rectangular or circular profile and are cast in plywood or metal forms. The reinforcing steel never exceeds 8 percent of the cross-sectional area to guard against catastrophic brittle failure in case of accidental overloading.

Precast concrete structural members are fabricated under controlled conditions in a factory. Members that span floors and roofs are usually pretensioned, another prestressing technique, which is similar in principle to post-tensioning. Precast prestressed floor elements are made in a number of configurations. These include beams of rectangular cross section, hollow floor slabs, and single- and double-stem T shapes. Precast concrete columns are not usually prestressed and have projecting shelves to receive floor members. At the building site, precast members are joined together by a number of methods, including welding together metal connectors cast into them or pouring a layer of in situ concrete on top of floor members, bonding them together. Precast prestressed construction is widely used, and it is the dominant form of construction in Russia and Eastern Europe.

Roman Architecture

Many seeds, derived from early Grecian and Etruscan designs, had already been sown in Rome. Modern knowledge of Roman architecture derives primary from the remains scattered throughout the area of the empire. Another source of information is a vast store of records. Especially important is a book on architecture by the architect Vitruvius “De Architectura” (27 B.C.). It consists of ten books and covers almost ever aspect on architecture.

Whereas Greek architecture is tectonic, built up from logical series of horizontal and verticals, Roman architecture is plastic with much use of rounded forms (arch vault, and dome). The Romans were more experimental than the Greeks in their construction. Modifying and expanding earlier form to suit their purpose, the Roman often used supporting columns for decorative effects, while the walls became the essential elements, and they also made extensive use of coloured marble. But the true greatness of the Romans lay in the creation of interior space.

In Roman architecture there were three types of houses: the domus or town house; the insula or multi – storey apartment house, and the villa or country house. The domus was usually of one storey only and inward – looking, consisted of suites or rooms grouped around a central hall, or atrium (a quadrangular court) and one or more peristyle courts. The insula had several identical but separate floors and was often vaulted throughout with concrete construction. Independent apartments had separate entrances with direct access to the street. The villa was derived from the traditional far – house and was more casual in plan than the domus.

The Romans were great builders and engineers famous for their factories, roads aqueducts and bridges, grand thermae and amphitheaters, theatres, and temples. The greatest surviving circular temple of antiquity and in many respects the most important Roman building, is the Panthenon in Rome. In consists of rotunda about 14 feet in diameter surrounded by concrete walls 20 feet thick, in which there are alternate circular and rectangular niches. Light is admitted through a central opening, or oculus at the crown of the dome. The rotunda and the dome are among the finest examples of Roman concrete work. The interior was lined with marble.

The Romans also developed the Tuscan and Composite orders. The Tuscan order is a simplified version of the Roman Doric, having a plan frieze and no mutules in the cornice. The Composite order is a late Roman combination of elements from the Ionic and Corinthian orders.

The evolution of new constructive elements was aided by important technical discoveries. A part from stone Roman architecture soon began to make use of bricks which at the peak of Roman architecture became the main building material because of their adaptability. The invention of concrete provided a material that encouraged the monumental tendencies of Roman architecture.

Civil Engineering

1. The term "engineering" is a modern one. The New Marriam-Webster Dictionary gives the explanation of the word "engineering" as the practical application of scientific and mathematical principles. Nowadays the term "engineering" means, as a rule, the art of designing, constructing, or using engines. But this word is now applied in a more extended sense. It is applied also to the art of executing such works as the objects of civil and military architecture, in which engines or other mechanical appliances are used. Engineering is divided into many branches. The most important of them are: civil, mechanical, electrical, nuclear, mining, military, marine, and sanitary engineering.

2. While the definition "civil engineering" dates back only two centuries, the profession of civil engineer is as old as civilized life. It started developing with the rise of ancient Rome. In order to understand clearly what civil engineering constitutes nowadays, let us consider briefly the development of different branches of engineering. Some form of building and utilization of the materials and forces of nature have always been necessary for the people from the prehistoric times. The people had to protect themselves against the elements and sustain themselves in the conflict with nature.

3. First the word "civil engineering" was used to distinguish the work of the engineer with a non-military purpose from that of a military engineer. And up to about the middle of the 18 century there were two main branches of engineering – civil and military. The former included all those branches of the constructive art not directly connected with military operations and the constructions of fortifications, while the latter , military engineering, concerned itself with the applications of science and the utilization of building materials in the art of war.

4. But as time went on, the art of civil engineering was enriched with new achievements of science. With the beginning of the Industrial Revolution and later there came a remarkable series of mechanical inventions, great discoveries in electrical science and atomic energy. It led to differentiation of mechanical, electrical, nuclear engineering, etc.

It is a well-known fact that with the invention of the steam engine and the growth of factories a number of civil engineers became interested in the practical application of the science of mechanics and thermodynamics to the design of machines. They separated themselves from civil engineering, and were called "mechanical engineers".

5. With the development of the science of electricity, there appeared another branch of the engineering — electrical engineering. It is divided now into two main branches: communications engineering and power engineering.

In the middle of the 20^th century there appeared some other new branches of engineering – nuclear engineering and space engineering. The former is based on atomic physics, the latter – on the achievements of modern science and engineering.

6. At present there are hundreds of subdivisions of engineering, but they all, at one time or another, branched off from civil engineering. The term "civil engineering" has two distinct meanings. In the widest and oldest sense it includes all non-military branches of engineering as it did two centuries ago. But in its narrower, and at the present day more correct sense, civil engineering includes mechanical engineering, electrical engineering, metallurgical, and mining engineering:

Here are some fields of civil engineering:

1. Housing, industrial, and agricultural construction.

2. Structural engineering comprises the construction of all fixed structures with their foundations.

3. The construction of highways and city streets and pavements.

4. The construction of railroads.

5. The construction of harbours and canals.

6. Hydraulic engineering which includes the construction of dams and power plants.

The above enumeration will make clear the vast extent of the field of civil engineering.

Date: 2015-12-11; view: 3925