Brick arches are constantly being turned, and of many sorts. An arch consists of a series of wedge shaped blocks, known as voussoirs, arranged in a curve, and so locking one another together that unless the abutments from which the arch springs give way, it will not only carry itself, but sustain a heavy load. It is a constant practice to cut bricks to this shape and build them into an arch, and these are sometimes cut and rubbed; sometimes, when the work is rougher, they are axed. But in order to save the labor of cutting, arches are sometimes turned with the bricks left square, and the joints wedge shaped. In this case the rings should be only half a brick each, so that the wedge need not be so very much wider at back than at face, and they are set in cement, as that material adheres so closely and sets so hard. Arches of two or more half-brick rings in cement are good construction, and are also used for culvert work.
A less satisfactory sort of arch is what is called the flat arch. Here, instead of being cambered as it ought to be, the soffit is straight; but the brickwork being deep, there is room enough for a true arch that does the work, and for useless material to hang from it. These arches are generally rubbed or axed, and are very common at the openings of ordinary windows. But no one who has studied construction can look at them without a kind of wish for at least a slight rise, were it only two inches. Sometimes when these straight arches are to be plastered over they are constructed in a very clumsy manner, which is anything but sound, and from time to time they give way. The weight of brickwork, of course, varies with the weight of the individual bricks. But stock brickwork in mortar weighs just about one hundred weight per cubic foot, or 20 cubic feet to the ton. In cement it is heavier, about 120 lb. to the cubic foot.
The strength of brickwork depends of course on the strength of the weakest material--i.e., the mortar--though when it is in cement the strength of brickwork to withstand a weight probably approaches that of the individual bricks. Some experiments quoted in Rivington's Notes give the following as the crushing weight per foot--that is to say, weight at which crushing began--of piers having a height of less than twelve times their diameter:
Tons per foot. Best stocks, set in Portland cement and sand 1 to 1, and three months old. 40 Ordinary good stocks, three months old. 30 Hard stocks, Roman cement and sand 1 to 1, three months old. 28 Hard stocks, lias lime, and sand 1 to 2, and six months old. 24 Hard stocks, gray chalk lime, and sand, six months old. 12
The rule given in popular handbook, that brickwork in mortar should not have to carry more than three tons per superficial foot, and in cement more than five tons, is probably sound, as in no building ought the load to approach the crushing point, and, indeed, there are many sorts of foundations on which such a load as five tons per foot would be too great to be advisable.
It is a rather interesting inquiry, whenever we are dealing with a building material, if we ask what can we best do with it, and for what is it ill fitted. The purposes for which brick can be best used depend, of course, upon its qualities. Speaking generally, such purposes are very numerous and very various, especially the utilitarian purposes, though rich and varied ornamental work can also be executed in brickwork.
Perhaps the most remarkable quality of brickwork is that it can be thrown into almost any shape. It is in this respect almost like a plastic material, and this peculiarity it owes chiefly to the very small size of each brick as compared with the large masses of the brickwork of most buildings. Stone is far less easily dealt with than brick in this respect. Think for a moment of the great variety of walls, footings, piers, pilasters, openings, recesses, flues, chimney breasts, chimney shafts, vaults, arches, domes, fireproof floors, corbels, strings, cappings, panels, cornices, plinths, and other features met with in constant use, and all formed by the bricklayer with little trouble out of the one material--brickwork! A little consideration will convince you that if the same material furnishes all these, it must be very plastic. As a limitation we ought to note that this almost plastic material cannot be suddenly and violently dealt with--that is to say, with the exception of some sorts of arches, you cannot form any abrupt or startling feature in brickwork, and you are especially limited as to projections.
If you wish to throw out any bold projection, you may support it on a long and sloping corbel of brickwork. But if there is not room for that, you must call in some other material, and form the actual support in stone, or terra cotta, or iron, and when you have gained your projection, you may then go on in brickwork if you like.
Brick cornices should be steep, but cannot be bold, and so with other ornamental and structural features. A noteworthy property of brickwork, and one of immense value, is that it is thoroughly fireproof; in fact, almost the only perfectly fireproof material. There is an interesting account of the great fire of London by one of the eye witnesses, and among the striking phenomena of that awful time he notes that the few brick buildings which existed were the only ones able to withstand the raging fire when it reached them.
In our own day a striking proof of the same thing was given in the great fire in Tooley street, when Braidwood lost his life. I witnessed that conflagration for a time from London Bridge, and its fury was something not to be described. There were vaults under some of the warehouses stored with inflammable materials, the contents of which caught fire and burnt for a fortnight, defying all attempts to put them out. Yet these very vaults, though they were blazing furnaces for all that time, were not materially injured. When the warehouses came to be reinstated, it was only found necessary to repair and repoint them a little, and they were retained in use. The fact is that the bricks have been calcined already, so has the lime in the mortar, and the sand is not affected by heat, so there is nothing in brickwork to burn. Against each of these good qualities, however, we may set a corresponding defect.