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Water-Supplyand Sewage Disposal
None of you has probably ever thought about what is the most important item in the life of any town. Some think it is the transport, the others electricity, still others may say something else and only very few will mention water-supply and sewage disposal. That is quite understandable, for all the water-supply and sewage systems are hidden underground.
Conduits like arteries and blood vessels in the human organism are located underground together with other communications. Hundreds of thousands of cubic metres of water pass along the conduits (pipe lines).
In towns water is consumed for domestic and sanitary purposes as well as for, fire protection. In industrial enterprises great quantities of water are needed for technological purposes. The amount of water needed for a big consumer like a metallurgical combine is several times greater than that for a big town with a population of 1.5 mln people. Special recirculating systems are to be constructed for such enterprises as their water demand exceeds the debit of the water sources available.
There is not a single branch of the national economy that could do without water of some quality in a certain amount.
The modern water-supply and sewage disposal are a complicated system comprising all kinds of engineering structures used, for industrial and domestic purposes as well as for drainage, purification and disposal of sewage.
If we follow the flow of water through all the water supply and sewage works we shall first come to an area where the intakes, pumping stations, purifying equipment and various reservoirs are located. Even the temporary presence of strangers in this zone is prohibited, nor any construction is allowed that is not directly connected with water supply.
By means of powerful pumps water is taken from the source of supply be it a river, a lake or a well. It is chlorinated to destroy all the disease producing (pathogenic) organisms which may be present in the untreated water.
The water flows to the settling base where all the matter in suspension is removed. From the setting basins water goes to filters for a more thorough purification.
Having reached the town, the water enters house connections and is fed to various household appliances such as sinks, baths, showers, lavatories, wash-stands and so on.
After usage water enters the drainage system, goes to the sewage collector and afterwards is subjected to biological treatment in special sewage treatment works.
Sewage first passes through screens where coarse impurities are detained. Next it is stored in precipitators of horizontal (lateral), vertical or of radial type.
If the degree of purification is sufficient the sewage is disinfected by means of lime and discharged into some water basin.
If further purification is needed the sewage is directed to biological filters for decomposition of organic impurities. Then the effluent is again allowed to settle. And only after that the purified and disinfected sewage may be discharged into some stream or body of water. But we shall not dwell on that here.
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Suffice it to say that the problem of water supply for towns and enterprises and its disposal after utilization (and purification) is one of the most important problems which are of greatest economic significance.
Concrete
These days, a building's framework is as likely to be of reinforced concrete as of structural steel. Concrete is made by mixing together small stones, sand, cement, and water in rotating drums. The mixture is tipped or piped into forms (wooden molds) of the shape required. The coarse stones used in the mix give the concrete its strength, the sand is needed to fill the gaps between the stones; and the cement (mixed with just enough water to make it into a paste) covers the surfaces of all solids and binds the entire mixture into a single mass.
The less water that is used in mixing the concrete, the denser and stronger it is when it has set. The difficulty here is that a dryish mix is not so easy to stir as one that is fairly wet and sloppy. So where a really strong concrete is essential, it is mixed with the necessary minimum of water, placed in the forms, and then vibrated, before it sets, by slowly combing it with electrically vibrated bars. This both drives out any lingering pockets of air and ensures that the mix is thoroughly even.
To make the concrete resistant to bending, engineers reinforce it. They do this by putting bars of steel or miniature steel frameworks into the form before the concrete mixture is poured in just those places where the stress will be greatest. Hence, the name reinforced concrete. With such material an infinite variety of constructional shapes can be produced, including shells and roofs in the shape of hyperbolic paraboloids. For these very modern structural items reinforced concrete is used in thin sheets.
In an ordinary reinforced concrete beam, much of the concrete does little more than hold the steel in place. It can be used more effectively if, before the external load comes on, stresses are put into it. For instance, suppose that a reinforced beam could be bent out of the straight by an inch, either upward or downward, before it developed serious cracks. Then, if we tighten up the reinforcement before any load comes on so as to bend the beam an inch upward, it would take twice as much load as before to bend it an inch downward. In other words, we can, by prestressing it in reverse, prepare the concrete in advance to withstand the pressures and pulls that the external load will cause.
Concrete can be prestressed in two ways. In the first method, the concrete is cast around stretched steel wires. When the concrete has set, the wires are released and compress the concrete as they contract. Such a method of prestressing produces pretensioned concrete.
The other method is called post-tensioning. In the case of a beam, for example, the concrete is cast around polythene tubes through which, after the concrete has set, steel cables are drawn. These cables are anchored at one end of the beam, stretched by jacks, and then fixed at the other end of the beam. In their stretched position they give a built-in stress to the beam; and this too will be cancelled out when a load is applied.
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In constructing a building, it is possible to cast the floors and walls as well as the framework directly on the spot where they are to stand. The building then forms a monolith-one large artificial stone composed entirely of concrete that has been shaped within wooden molds that fit together perfectly. Thus, no sections have to be joined together later on. To cast all the parts in place, the builder has, of course, to use a great many forms; these are removed as soon as the concrete has set. Arid the concrete of each story must be given plenty of time to harden before work on the next story can begin.
In order to save time, the builder may prefer to use a number of standardized concrete units. These can then be made in advance-that is, either the individual members can be precast (and possibly prestressed), or whole sections of the building can be prefabricated.
Precasting and prefabrication have made possible the speedy erection of buildings designed to use a great many standardized parts (such as window frames).
Types of Buildings
Types of buildings depend upon social formations and may be classified according to the role in the community. The types of buildings may be domestic, educational, office, industrial, recreational, etc. The type and the function of a building govern its design, building materials and techniques. But the common and necessary conditions are: (1) its suitability to use by human beings in general and its adaptability to particular human activities (2) the stability and permanence of its construction.
Speaking of residential construction we must say that the apartment houses are mostly built to suit urban conditions. Group housing provides home for many families and is at once public and private. The techniques of construction or the methods by which structures are formed from particular materials are influenced not only by the availability and character of materials but also by the total technological development of society.
The evolution of techniques is conditioned by two factors: one is economicthe search for a maximum of stability and durability in building with a minimum of materials, labour and time; the other is expressive the desire to produce meaningful form. Techniques evolve rapidly when economic requirements suggest new expressive forms or when the conception of new forms demands new procedures.
Large housing programs have tended to stimulate technological change in the building industry. Craft operations at the building site are being replaced by mechanized operations at the factory and houses are increasingly becoming assemblages of factory-made elements. Windows and doors, once made and fitted by carpenters at the site now arrive from a factory fitted and finished with hardware and glass, ready to be set in place. Modular design (i.e. design in which the elements are dimensioned in combinations of a fixed unit) has led to standardization of elements, interchangeability of parts and increased possibilities for mass production, with resultant economies. Awide variety of mass-produced elements from which substantial portions of the house can be assembled are now available. Examples are kitchen cabinets and mechanical equipment and window and door units. Entire apartment assemblages are available and are being used to an increasing extent. These techniques aim at a higher output of better structures at lower cost.
The high degree of mechanization and standardization is successfully achieved by reinforced concrete blocks and units. Reinforced concrete homes are produced by a variety of construction methods. Various methods of constructing reinforced concrete houses involve extensive use of large sections manufactured in heavily mechanized factories and erected at the site.
The built-in space of an apartment should be carefully thought of as well. The contemporary trend is expressed by joining the living and dining areas into a single space or by relating the kitchen and dining areas. It has become increasingly important as rooms that have become smaller should appear as spacious as possible. Therefore, there is a considerable trend toward built-in furniture. Rooms should be both efficient and visually satisfying. The extent of built-in cabinets must be determined. Drawers and shelves can often be concealed behind walls, freeing valuable floor space.
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The windows and doors must look well from the interior us well as from the exterior. Satisfactory functioning is also involved; windows must be sized and located for the best possible lighting and ventilation; as for electricity it should be mentioned that the electric load of most houses has increased enormously as standards of lighting rose and mechanical and household equipment multiplied. Great technological advances have been made in plumbing. Much progress has been made with respect to standardization and production of the elements of kitchen equipment.
