Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/499

Rh OF CONSTRUCTION.] BUILDING 451 should be composed of particularly well-formed bricks of really good quality, set in cement or in some quick-setting mortar, that there may be no yielding to the pressure which must be immediately thrown upon this part of the construction. Another built beam, of greater depth, because of the absence of any inverted arch to stiffen it, is thrown across over the back of a semicircular arch, with its abutting ends extended in like manner. ^7 Fro. 5. Plan at twice the scale of the Section (fig. 4). aius. To relieve the work from water, a drain being run along over the middle of the inverts, or side-drains being passed by ring culverts through the buttresses, drain-shafts are carried up at the backs of the buttresses against the springings of the counter-arches, to within a few feet of the surface. These shafts, being steened with open joints at intervals to admit drainage water and communicating with the drains below, prevent the possibility of water lodging about the backs of the counter-arches, or even in the ground itself. The drain shafts should be semi-domed with bricks set dry and covered in, and the walls also backed up with good clean gravel, through which the surface water may percolate and pass freely down to the shafts, The constructions are assumed to be of brickwork, for the obvious reason that the cases supposed being clay cuttings, brick is the material which would be most economical. But if masonry be cheaper, it may of course be used with the same effect. Where a cutting intersects loose beds of laminated stone, and particularly strata inclined to the horizon, so as to be unsafe with the ordinary slopes, such constructions are available ; and in cases where the sides of the cutting will stand vertically or nearly so, as in chalk, it may be useful to apply similar construc tions, though of slighter character, to check the separation and fall of masses from the precipitous sides. It is obvious, too, that these constructions present the means of security, when the stratum forming the base of any cutting is too weak to bear the weights of slopes, or of retained sides, without rising between them. Sheet-piling may be driven to any depth along the backs of the counter-arched walls so as to be retained at the head by the walls ; and thus in effect the walls would be carried down to a safe depth, even through the weak stratum ; whereas such piling at the toes of slopes is commonly found to be almost if not wholly useless, for the want of a stay to the head. The ignitibility of timber, and the rapidity with which cstruc- ifc 1jU1-ns when Placed in circumstances so favourable to s gene- tnat c ^^t as by its disposition in an erected building, have y. led to its prohibition for the purposes of the main enclosures of houses and buildings generally, in London, and in many of the larger provincial towns. It is possible, however, so to protect timber employed in the enclosures and for the internal partitions and floors of buildings as to render mere dwelling-houses practically incombustible. Whilst, how ever, the liability of timber to take fire and to burn may in a great measure be counteracted, and notwithstanding that this material combines the advantage of economy with security, stone and brick are undoubtedly better adapted for the main structure of a building. Brick or stone, or. brick and stone together, with a setting material, ought to be employed, but in such manner only as to be free from dependence upon other and less trustworthy materials. The most perfect erections as buildings are those in the composition of which this principle has been understood and fairly practised. If adventitious aid be given to brick or stone walls by foreign materials, the materials ought to be at the least harmless. Iron in bulk is not a proper Iron, substance to incorporate with walls because of its great expansibility by heat ; but iron used in thin laminae, as hoop-iron laid in walls in the bed-joints of the brick or Hoop-iron, stone, cannot be productive of any bad consequences, while it is most beneficial in that form as a tie to the structure. Bricks come ready shaped to the hands of the workman Bricks and in a form the best adapted for the arrangement in the con- stoue - struction of a wall which, under the designation of bond, gives it such a degree of consistency that a weight placed upon the top is carried by the wall in every part throughout its whole thickness, and throughout a greater or less proportion of the length according to the height of the wall. Stone, on the other hand, comes to the workman without regular form; and with skill on his part to dispose and arrange the materials, good erections may be produced of rubble ; for although the thickness of which walls may be built of rubble with safety will depend in a great degree upon the quality of the mortar, much depends also on the skill of the workman in bedding and bonding the stones. Under any circumstances, however, a wall so composed cannot safely be charged with heavy weights, or be exposed to the vibrating action of floors, until the mortar shall have indurated to some extent ; whereas a wall of brickwork is secure by the horizontal bedding of the bricks, and by the effect of the transverse bond which the alterna tion of header and stretcher almost necessarily produces. Stone, again, may be dressed to any shape, and so as to mould it to every variety of construction with the smallest possible quantity of mortar or cement. From blocks with rough hammer-dressed parallel beds, up to the most complete and perfectly wrought parallelepipeds adapted to any arrangement of bond that may be best adapted to the structure,. and with combinations of rudely formed and perfectly formed pieces of stone, walls may be built of stone of greater strength than the best brick can be made to yield, whilst stone walls are liable to be inferior in every respect to brick-built walls of ordinary quality. Some combinations of the two kinds of materials have Comlmia- the effect, however, of making a better wall than could be tions of ^ IC produced by the main constituent in the form employed alone. A stone-rubble or pebble built wall is greatly im proved by one or two bonding courses of brickwork at short intervals; and a brick wall is improved and adapted for a higher purpose by thorough courses, at intervals, of good stone, wrought to bed and joint truly ; whilst on the other hand, a wall sxibstantially of stone-rubble or pebble, and faced with brickwork, is essential^ an unsound wall ; and in like manner a brick wall faced with wrought stone is liable to be weaker than the brickwork would have been without the stone. With regard to the thicknesses of the walls of building*, Thickness it is generally considered that these .should be governed by of walls.