Вариант 3

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CURTAIN WALLING

The search, in recent years, for new ways of cladding a building, which can take advantage of the modern industrial capacity for prefabrication with a consequent saving of time on the site, has led to the development of curtain walling. This has been made possible by the production of an ever in­creasing range of materials, which have enabled the prefab­rication of units to achieve lightness, simplicity and flex­ibility of operation that make them an economic propo­sition.

In essence, a curtain wall is a row of windows and spand­rel panels in a light containing framework, which is not built up within the main structure of a building, but is suspended right across its face, being merely held back to it at widely spaced points.

The decision to use a curtain wall must rest upon a thorough appreciation of the fundamental requirements of an external wall. The architect must be convinced that the tech­nique to be adopted will meet these requirements, and that in so doing it will give economic advantages which will jus­tify its choice.

The requirements of a non-load-bearing wall are as fol­lows:

1. It must keep the rain and the wind out of the building. A curtain wall presents to the weather a surface made up of impervious materials, chiefly metal or glass; but when these materials are assembled there is a great number of joints which must either be made as impervious as the adjoining materials, or be designed to drain away any water which may penetrate.

2. It should assist materially in maintaining the desired temperature within the building.

3. It must be strong enough to withstand all the forces which normally will act on it, but must achieve the strength without adding excessive load on to the structure of the building and without becoming unreasonably thick.

4. It must conform to the standard of fire-resistance spec­ified by the authority concerned.

5. It should not cost too much to manufacture and erect.

6. It should present an acceptable appearance.

7. It must last.

The strength of a curtain wall lies mainly in the millions, which, on account of the lightness of the wall, may be of remarkably small section, their principal requirement being rigidity to resist wind loading. Lightness brings a whole series of economic blessings. Material is saved; the cost of transport to the site for wall materials drops; charges for mechanical lifting plant and manhandling are reduced. Furthermore, the reduction in dead load can mean a very considerable lightening of the support structure which will be reflected in that costly item, the foundations. A comparison in weights between a curtain wall and a wall of traditional construction can show as wide a difference as from 5 lb per sq ft to 125 lb per sq ft.

The key to a low initial cost in a curtain wall lies in complete prefabrication of component parts, standardization of these parts and the greatest possible degree of off-site assembly within the limits of handling ability. The fewer the joints to be made between members, the lower the labour costs will be, and the speedier will be the erection, with a consequent financial gain through earlier completion.

Weather. One of the chief functions of any external wall being to keep rainwater out of the building, we must ask ourselves how well a curtain wall does this, and how great are the design problems faced in ensuring that it fulfils this function economically and permanently. The problem is fundamentally more serious than that which is met in buildings of masonry and brick. These are porous materials which absorb a large proportion of the water which falls on them. With impervious sheet materials there is no danger of water penetration, and the amount of water which runs down the surface of the wall is very much greater. In a heavy rainstorm a vast amount of water must run off the building without finding one weak spot in the walling. It must also be collected and run to drain at the foot of the wall.

What, then, if this water does penetrate? Obviously the wall has failed in its primary function. But this failure may have effects more serious than damp patches on ceilings and walls: it can cause the structural failure of the curtain wall members themselves.

The joints, therefore, must be watertight¹ so as not to present a capillary path which can draw in the water.

Making the Curtain Wall Architecturally Effective. A long straight 7-storey facade extending, say, 200 ft, consisting above the ground floor of a screen of glass formed into long horizontal windows alternating with blue-grey hands, each storey the same height and the same colour throughout is for many people a rather monotonous effect. And yet that is typical of many curtain wall facades throughout the world today.

The curtain wall is a brilliant product of modern building technique superior functionally in many ways to what it replaces; it admits more light and is speedier and lighter to construct, but the problem that it poses for the architect is to make it interesting and beautiful. There are examples which show promise of good architectural effect and although these depend partially on the particular treatment of the wall they also depend on the relation of one mass to another. In the early stages of any new structural component experiment is essential, and experiment with a wide variety of designs and material is necessary if the curtain wall is to acquire archi­tectural distinction and make its contribution to the beauty of the urban scene. Some degree of variety, consistent of course, with function, is perhaps desirable in the fenestration; while a wide variety of interesting materials are avail­able for the areas between windows with which the archi­tect can experiment. Two colours that complement each other and the broken colour obtained by interesting textures are factors that contribute. Here architecture moves close to painting.