Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/500

Rh 452 BUILDING [GENERAL PEIMCTPLES tlie height of the structure ; but they ought not to be determined by that condition alone. Chimney-breasts, or other buttress-like projections, built up with a wall, and extending to more than the thickness of the wall, make it in fact stronger in its transverse section, and justify less general thickness in the body of the wall, whilst window and other openings in a wall leave piers which ought to be of greater thickness than the mere height would require. But all returns, indeed, whether as chimney-breasts or as cross walls, built and bonded with a wall, tend to render unnecessary the full thickness which the height might require ; whilst, as just intimated, the omission of portions of a wall for door and window openings should be compensated for by additional substance to the parts which remain. Walls subjected to undue action, such as that arising from slight joists tailed into them, or that occasioned by inclined timbers, as under galleries in churches, chapels, and theatres, require to be of greater thickness than they otherwise would ; whilst it is quite wonderful to what great heights brick walls may be built with safety, if they are well built, and exposed to no other action than direct vertical weight. When, indeed, such walls stand upon a sufficient foundation, direct vertical weight without motion is a means of security to the walls so long as the weight is reasonably within the power of resistance of the materials to crushing pressure. The object to be looked at, therefore the walls being honestly built is, as before remarked, to make the weight to be imposed upon any wall act upon its solids vertically and steadily. Floors. Floors upon girders, or framed to strong trimmers the girders or the trimmer-joists running into and bearing upon the piers or solids of the walls are far preferable to what are termed single floors, of which each joist runs into the wall. Girders as the basis of floors render plates in the walls wholly unnecessary, by depositing the weight in the right places, without requiring plates to carry it on from the weaker to the stronger places ; and being of necessity stout and rigid, they form a fair tie and strut to the walls into which their bearing ends are tailed. Whether girders or trimmer-joists be employed for placing the weight of floors upon the walls of a building in the safest manner, the bearing timbers ought to be placed upon pieces of stone as templets built into the walls, and to be made to take a cog-hold of the templets, so as to enable them to tie and stay the walls by means of the cogs. It is by means of the girder bearing upon the solids of the walls, though with bad carpentry, that the French are able to carry up their soft, coursed-rubble, stone walls to heights that would certainly be unsafe if the walls were seamed with wooden plates, and shaken by floors of single joist ; such, for instance, as a wall of a total height of say 85 feet, with a thickness of 18 English inches on the ground-floor and through six stories, or about 65 feet, and finished by a gable, the basement being vaulted with walls about 20 inches thick. It is by means of the solidity given to the floors by the girders, and the solid bearings which the girders obtain, that the floors are able to carry the dead weight of matter which renders them practically fireproof, as described below, in addition to the moving weights to which the floors of buildings are necessarily exposed in Bad fram- use. We can and do frame floors most effectively by ing in carpentry alone ; whereas the French do the work so France. badly, that no important bearing is, or indeed may be, trusted by them to the framed joint dog-nailed stirrup straps of iron being always brought in aid. But the common practice in England is to use single or unframed floors, which carry the weight and the vibration to which floors are exposed into the walls, over voids as well as over solids ; while the French frame their floors to or upon girders, by means of which the floors are brought to bear upon the solids of the walls. The walls are thus not only less exposed to vibratory action, but are both tied together and strutted apart with better effect by the stout girders stiffened by joists than by joists which themselves require some foreign aid to stiffen them. Moreover, single floors of joists, unless trimmed at frequent intervals, when, indeed, they may be termed half-framed, are supposed to require plates of timber laid along the inside faces of outer walls and upon internal valls. This defect is avoided by French builders, who exclude all timber, except the bearing ends of girders, from their walls, and use framed floors. When the walls of a building have reached their full height, the wall-plate comes into use legitimately to cope the walls, in fact, and to form a curb as a base upon which to place the roof, which should deposit its weight, never theless, by means of its tie-beams upon the plates over the solids of the wall below, and which should, moreover, oversail, so as to cover and effectually shelter from the weather the enclosing walls also. In setting forth the structural advantages derivable from the use of girders as the bases of floors, it may be necessary to repeat the warning already intimated against the use of girders of a material of uncertain strength, and of treacher ous character when exposed to transverse strain. Cast-iron is of uncertain strength, mainly because of the imperfec tions which the most skilful founders, with the best materials and every appliance at command, cannot always avoid, and which are most liable to occur in the production of complex forms in long lengths ; whilst careless founding and rapid cooling are contingencies connected with the production of cast-iron girders which are necessarily long and complex castings. Cast-iron is treacherous, inasmuch as it is brittle and liablo to be startled into fracture by impact trifling when compared with what it may have borne safely as a dead-weight. Proving long metal castings by straining them upon their transverse section docs but aggravate imperfections, and leave the casting weaker ; whilst no dead-weight proof is proof against blows or other action inducing vibration. It is only under circum stances which do not admit of concussive action upon the beam, or which prevent it from vibrating under any shock that may reach it, that cast-iron can be safely used in beams of long lengths to carry heavy weights, without some appliance to mitigate, at least, the imperfections which this substance exhibits. The application of wrought iron tension bars as soles to beams and girders of cast-iron would prevent the most serious consequences from attend ing the failure of the casting, if the beam were also prevented by binders, or by other sufficient means, from turning round when the blow produces an oblique fracture. Wrought iron girders can be and are now extensively used to carry floors, partitions, and even walls with safety. Inasmuch as some soils are liable to change in form, expanding and contracting under meteorological influences, as clays which swell when wetted and shrink when dried, concrete foundations are commonly interposed upon such soils to protect the building from derangement from this cause ; or rather, for that purpose walls of the cheaper material, concrete, instead of the more expensive brick or stone structure, are brought up from a level sufficiently below the ordinary surface of the ground. When concrete is used to obviate the tendency of the soil to yield to pres sure, expanse or extent of base is required ; and the concrete, being widely spread, should therefore be deep or thick as a layer, only with reference to its own power of transmitting to the ground the weight of the wall to be built upon it, without breaking across or being crushed. But when concrete is used as a substitute for a wall, in carrying a wall down to a low level, it is in fact a wall, wide only in proportion to its comparative weakness in the Wall-plat and roof. Cast-iron girders. Tension bars. Founda tions