Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/501

Rh OF CONSTRUCTION.] BUILDING 453 of absonce of manipulated bond in its construction, and encased by the soil within winch it is placed. The foundation of a building of ordinary weight is. for the most part, sufficiently provided fur by applying what are technically tunned footings to the walls. The reason for a footing is, that the wall obtains thereby a bearing upon a breadth of ground so much greater than its own width or thickness above the footing, as to compensate for the difference between the power of resisting pressure of the wall and of the ground or ultimate foundation upon which the wall is to rest. It will be clear from this, that if a building is to be erected upon rock as hard as the main constituent of the walls, no expanded footings will be necessary; if upon chalk, upon strong or upon weak gravel, upon sand; or upon clay, the footing must be expanded with reference to the power of resistance of the stratum to be used as a foundation ; whilst in or upon made ground, or other loose and badly combined or imper fectly resisting soil, a solid platform bearing evenly over the ground, and wide enough not to sink into it, becomes necessary under the constructed footing. For this purpose the easiest, the most familiar, and, for most purposes, the most effectual and durable, is a layer of concrete, which may be formed so as to cover a surface large enough to obtain from the most yielding soil the amount of resistance to pressure required to support the weight of the intended building. It will be evident that upon a concrete founda tion a footing or expanded base may or may not be required to a wall, according to the hardness of the concrete and the kind of wall to be built ; but it is perhaps better to give the footing to the wall than to wait for the sufficient induration of the concrete to enable the wall to do without a footing ; and better still, to lay the concrete of such height only with reference to the spread or extent of base beyond the toes of the footing, as the gravel of which the concrete is made would stand at in an uncombined condi tion. Concrete, indeed, is at all times more safely to be regarded as a substance to be placed as a layer, than as a substance to be set up as a wall ; for although excellent erections as walls may be made of concrete as erections iu the same form may be made of tempered clay or of pisd- work neither concrete nor tempered clay is to ba regarded as a proper substance with which to form the lofty walls of buildings in towns. The use of lime concrete involves walls of a considerable thickness; Portland cement concrete, a stronger material and having the property of hardening rapidly, is more commonly used for thin walls. Several patents have been taken for erecting buildings with concrete walls by the construction of troughs formed of frames and movable boards or shutters. But considerable doubt exists as to the extent to which such walls can be built, as many have cracked, proved leaky, and shown other faults ; whilst much economy does not result except where the work is very plain and straightforward, where little is spent on sub sequent finish, and where the materials can be obtained at very little cost either for themselves or for carriage. Construction for Protection against Fire. Houses seldom take fire from common accidents, such as occur to the lighter movable furniture and to drapery ; but, for the most part, from the exposure of timber in or about the structure to the continued action of fire, or of heat capable sooner or later of inducing the combustion of timber ; and as the source is most commonly in defective gas-piping, or in some stove, furnace, flue, pipe, or tube, for generating or for conveying heat, or for removing the products of com bustion, much of the real danger to buildings from fire would be prevented by avoiding that degree of proximity of timber to all such things as can lead to its com- b lstion. With a view to lessen the danger to which buildings with timber in their structure are exposed from fire, it will bo well to consider how far the timber and wooden fittings commonly used may be necessary either to the stability of the buildings, or to the comfort and con venience of the inhabitants. But it is not our intention to describe here the various modes of fire-proof construe- Protective tion, but only to notice the principles upon which ordinary measures buildings may be rendered nearly incombustible. So long ^ auist as danger of fire is brought to buildings through pipes and tubes, the necessity must be admitted of guarding the combustible materials used in buildings from any chance of becoming ignited. When heat is produced and passed through pipes in any manufactory, whether it be to act as power, or for drying or for warming, the fires used may be guarded, and the machinery which regulates the intensity of the heat to be transmitted may be under constant care ; but even in such cases there can be no certainty that the heat shall not at some time arrive at the point of danger. But when heat is diffused throughout dwelling-houses by means of apparatus which is committed to persons unskilled in its use, and unconscious or careless of the danger which may arise from neglect, it seems impossible to lay down inflexible rules for distances from timber which shall render it safe from heated pipes. Twelve or fifteen inches may not be a greater distance than safety requires under some circumstances, whilst there are many cases in which the actual contact of such pipes with timber is hardly inconsistent with safety. When the air about heated bodies is not confined, as it would be within the timbers of an ordinary floor, a distance between the timber and the heated surface equal to the longest diameter of the tube or pipe will be found sufficient if the temperature of the pipe does not exceed that of boiling water. It is to be understood that a piece of wood will bear a powerful dead- heat upon its sides for an indefinite period without ignit ing, unless a transverse section of the fibre, as at or around a live knot, or where a branch had been lopped, present itself to the action. It is by the end that a piece of wood exposed to powerful heat most readily ignites. The gases evolved in the substance of the timber by the action of heat applied to its surface, expanding as they are evolved, are thrown out by the pores among the fibres at the ends, if the ends are near enough to the action to allow of this effect, with less power than may be enough to obtain vent for the inflammable gases laterally. The English Government, when it has legislated upon Buildings such matters, has generally confined itself to making A - cts - provision that the enclosing walls of buildings should be formed of incombustible materials. In provisions regard ing the least thicknesses of such walls, these were generally determined with reference to the height of the building, and to the area to be enclosed, as an indication of the probable lengths of the walls ; and this both for the pur pose of promoting safety of structure, and of checking the spread of fire from building to building. As, however, in most cases greater thickness is required in the side wall of an ordinary dwelling-house in a town to render its structure secure than is necessary to enable it to check the spreading of fire, such walls are frequently made of greater thickness than would be necessary to fulfil the objects which the Legislature has had in view, if the wails were not supposed to extend the whole length of the two longer sides of a parallelogram without intermediate cross or return walls. A solid, well-built brick wall, one brick or 9 inches thick, between two ordinary dwelling-houses of five or six squares in area each, will prevent the communication of fire through it from one to the other. But, in towns, ordinary dwelling- houses, which occupy each an area of five or six squares, are generally disposed in plan as parallelograms, having their opposite sides 18 or 20 feet, and 28 or 30 feet respectively in length, and are seldom carried up to less height than 35