Figure 2 (fii. 4) shows a system in which hook-shaped iron rods about X inch thick are hung at distances of about 2X feet between the joists, which lie from 2Y? to feet from one another. This is seen in perspective in Figure 4 (pi. 4). Here again we have bars parallel to the joists, and upon the network thus formed is a layer of mortar about 2Y, inches thick, which is supported until it has set. When a wooden floor is to be laid, wooden sleepers are notched upon the beams, as is seen in the illustration. The system shown in Figures 3 and 5 resembles the preceding-, except that here the iron cross-bars are run through and bolted inside the iron bands encir cling the joists.
Vaulted iron girders are used to replace the transverse arch in vaulted ceilings, the vaults, generally made of hollow brick, are supported upon the lower flanges of the girders. The cavities thus formed are then bricked up to the upper edge of the girder. The superior advan tage offered by iron girders over the transverse arch consists in there being no thrust upon the walls, and, moreover, there is a gain in height. Con structions of this kind on a g-reat scale are often seen in large factories. The example given in Figure 6 is the interior of the Joint-stock Spinning works at Chemnitz, in which the girders, made of plates riveted together and braced by horizontal round iron tie rods, are intended to support the vaults.
Iron Columns are hollow castings. The diameter and thickness of the metal must depend upon the height of the columns and the weight to be supported. As a rule, the inside diameter is four-fifths of the outside. Particular attention must be paid to the manner in which the lower ends of the columns are secured. It is advisable to screw down to the stonework an iron plate cast for the purpose, and upon this to fasten the foot of the column either with screws or, at least, by rebating-. The manner of secur ing the upper end will depend upon whether another column is to be set over it. In this case, which is also seen in the fonn of construction exhib ited in Figure 6, a connection will be necessary between the columns that are set one over another, and it must be made either directly or by connect ing pieces. If necessary, the broad bottom plate of the upper coluinn may be set upon the broad top plate of the lower. It need hardly be said that the girders supported by the columns are to be secured to them by flanges, bolts, etc.
Iron _Roof modern times iron has been much used for roofs and roof-trusses, particularly where safety from fire and great solid ity, combined with light appearance, are desired. For very wide spans without intermediate supports, iron is exclusively employed, as in such cases woodwork is not only very cluinsy and complicated, but is also always untrustworthy, and generally more expensive. Iron roof construc
tion may be classified in two divisions, as follows: (1) That with complete attic framing, where an attic space is to be obtained; (2) That without attic framing-, where no attic room is required. Roofs of both these varie ties are constructed either of iron alone or of iron and wood combined; also of cast or wrought iron in the iron roof proper, and of the two com bined. The system of construction here, as well as when wood is employed, will be much influenced by the form of the roof. Reference is made here more particularly to the saddle roof.
regards roof construction of the first kind—that is, with the attic and the iron framework necessary in this system—the best plan, but suitable only for small spans, is to fit cast-iron shoes on the ends of the I-beams, or to screw them on, and into these to set iron rafters united at their upper ends by bolted or riveted plates. The beam in this case takes up the entire horizontal thrust of the roof. When the span is greater, additional supports are placed at various points throughout their length under the rafters, so as to avoid too heavy cross-sections; they should meet the joists only when they are directly supported from below, if the ceil ing joists are not to suffer from an improper distribution of the weight.
Chem/11'1z example of attic construction on a large seale is given in Figure 7 (pi. 4), which represents the spinning works at Chemnitz. Reference has already been made to the construction of the girders and columns in this building. Each rafter in this case has a direct double support from below; besides, there is a sort of iron collar beam introduced higher up which contributes to the stiffening of the rafter in the same manner as each of its supports. Round iron rods are used to connect the rafters along the length of the roof; these are arranged in pairs, one above the supports and one in the crown of the roof. To prevent dis placements, crosses of ronnd iron are 1a'id along the length of the roof be tween each pair of supports, as seen in the Figure. The iron ceiling joists, each composed of four parts spliced together, are supported in their length on three points of the joist, partly in a direct manner by iron col umns and partly by " reverse truss-frames," which proceed lengthwise and rest upon the columns by their ends, and consist, each, of a horizontal tie beam, two small posts, and three tension rods. The Figure also shows these accessory constructions, which are largely employed in other works.