the pine core. The joints are all made by locking and tacking the tin; soldering is prohibited, since the heat of a fire would melt it at once. Skylights. Another use of metal for exterior work is found in the framework of skylights. These, if large, are made of light steel Fig. 198. Hip Skylight Fig. 199. Gable Skylight angles or T-irons, and covered with sheet metal, sometimes with a lining in parts of terra cotta blocks. If the frames are of ordinary size, they are formed of heavy sheet metal, usually galvanized iron or copper, and they come in a variety of designs suited to different uses. The two principal forms of skylights are the hipped skylight, Fig. 198, and the gabled skylight, Fig. 199. All skylights should be provided with ventilators of some sort; often a greater circulation of air is obtained by building a vertical wall containing sashes or louver blinds. Roof Houses. Roof houses, often necessary to give access to roofs or to house machinery, are constructed similarly to large skylights, but without the glazing or louvers in the walls. If more than 40 feet above the sidewalk, they must be, according to the building laws of many cities, of fireproof construction, and they are usually made of T-irons with a terra cotta filling covered with sheet metal, Fig. 200. It is essential that the T-irons be set with their PLASTER METAL flange inside or on the opposite side from the metal covering; otherwise the pounding-down of the seams of the metal will be likely to start the terra Fig. 200. Typical Hollow Tile Wall cotta out of place. On account of this pounding-down of the seams, it is often better that the blocks should be of solid terra cotta and not hollow tiles. The connection between these houses and the main roof must be taken care of as already described for roof and wall flashings, due allowance being made for expansion of the metals. It may be well to note here that zinc for flashings should never be used where it will come in contact with iron, lead, or copper, as this contact will produce voltaic action which would finally destroy the zinc. FLOOR FRAMING General Features for Various Buildings. The exterior wood framing of city buildings is in most cases confined to roofs, since no exterior walls within building limits of modern cities may be constructed of wood. Wooden roofs covered with slates or other fireresisting materials are allowed, however, in many cities, up to a certain height above the sidewalk. The only difference which we shall find in this wooden construction from the suburban house considered in Part I is that usually the spaces to be covered are larger and the roofs, consequently, must be heavier. Floors also must be stronger, and because of the variety of city structuresfrom comparatively light buildings to heavy stores and warehouses a great range in floor construction is required in their erection. CITY DWELLINGS For brick dwellings the floor construction differs little from the wooden houses already described, the principal differences being that the spans are greater and, consequently, the timbers are heavier, and also the fact that the outside bearings are taken by the brick walls instead of by wooden girts. The joists in this case should run on to the wall at least 4 inches and should be beveled at the end, so that in case of fire the floors may fall without destroying the wall, Fig. 201. The joists must be anchored to the walls about every five feet with iron ties secured to the joist at the side, and low down, to allow the joist to fall out if burned. These ties should be continued across the building by tying the inner ends of the joists together and putting an anchor in the opposite wall, in as nearly a direct line across the house as possible. All large timbers, such as girders, should have anchors and should rest on cast-iron wall plates. Partitions carrying floors should have a stud set close against the brick wall and bolted to it, and all large openings, such as stair wells or skylights, should have headers hung to the trimmers by stirrup irons or patent hangers, shown in Fig. 202. If there are openings in the brick walls which come so near to the bottom of the joists that an arch cannot be turned, a header should be cut in, or a steel beam inserted in the wall. Joists are sometimes fastened to the walls by hangers, and do not run into the wall at all; but while this preserves the full strength of the wall, it does not make so good a tie and is not generally done. The wall plate of a brick dwelling is usually made of a plank the thickness of the wall, secured by 3-inch bolts which are built into the wall, as in Fig. 203. These bolts should run at least 20 inches down into the wall, and have a large washer plate at their lower end. When the wall has been brought to the required height, the plate is bored with holes to fit the bolts, and a nut and washer screwed on. Over the plate the rafters are notched, and the roof constructed as for a wooden house. Fig. 203. Plate Bolt STORES AND OFFICE BUILDINGS In the construction of stores, warehouses, or office buildings, with wooden floors, the use of partitions for carrying the floor should be avoided, and columns and girders substituted. The reason for this is that under heavy loads the studding will often spring enough to crack the plastering, and, besides, the occupancy of this class of buildings by different tenants will require numerous changes in the partitions from time to time. The large girders and posts also offer greater resistance to the action of fire, and permit fewer concealed spaces. Many city laws require the use of brick walls, trusses, or columns and girders, for support, if floor spans exceed 30 feet, and this is a good rule to observe. Wooden Columns. In establishing a line of columns and girders, the columns should be spaced about 12 or 14 feet apart for wooden girders, a spacing that can be increased to 25 feet if steel girders are used. Solid wooden posts will last better, if bored from end to end through the center with a hole about 1 inches in diameter, and with a -inch hole bored into the center at the top and bottom. This allows a circulation of air through the center of the post, and guards against dry rot, especially if the post is not thoroughly dry when set. The central boring should be done from one end, and if it comes more than an inch out of center at either end, the post will be weakened and should be rejected. Fig. 204. Steel Post Cap All wooden posts carrying girders and posts above, should have an iron cap with side plates to receive them, Fig. 204, so that the post above will be supported directly by the post below, and will not stand on the girder. This is necessary for two reasons. One is, that if the girder ran over the top of the post and the post above were set upon it, the shrinkage of each girder would be multiplied by each succeeding floor, and in a building of four or five stories this might amount to 2 or 3 inches; the upper floor beams at their inner end would then be that much lower than the outer ends which are supported by the rigid masonry. Another reason is, that the crushing strength of the girder in its longitudinal position is not so great as the post standing on end, and it might be unable to support the accumulated weight of several stories. This support was formerly obtained entirely by the use of cast-iron pintles, Fig. 205, which Fig. 205. Elevation and Plan of Cast-Iron Pintle were molded with top and bottom plates to fit the posts, the weight being transmitted by the cross-shaped metal. This is an effective method, but has been superseded by the more modern steel caps. The bearing of the girders should be at least 5 inches on either end, and a box anchor of some kind should be used to support the wall end of the girder, as shown in Fig. 206. Fig. 206. Box Anchor on Wall Iron and Steel Columns. For large spans and heavy weights it is often necessary to use iron or steel columns with wooden girders. If cast-iron columns are used, they should be made with a shell not less than inch thick. This is necessary on account of the danger of an unequal thickness in the shell. To obtain the hollow column, the casting must be made about a core, and although this core is accurately centered, there is danger of its being displaced by the pouring-in of the molten iron, since being of a lighter composition, it will have a tendency to "float". This may result in an added thickness to one side of the shell of the column and a corresponding lack of metal on the opposite side. To guard against this defect, cast-iron columns should always be tested by boring small holes on opposite sides, and if more than one-fourth of the thickness of shell is wanting in any column it should be rejected. The outer surface of cast iron should be smooth and clean, with sharp angles; and all projections, such as lugs, caps, or bases, should be closely examined to detect the presence of cracks. T Cap and Base. The top and bottom of all cast-iron columns should be turned off in a lathe to insure a bearing at right angles to the axis, and plates should be used to increase the bearing. These plates are cast with a ring or with projections to hold the column in place against movement, and they should be planed to a perfectly even bearing. The cap must never be spread out as a casting, Fig. 207, but should carry up the line of the column itself, all ornamentation requiring a great projection being cast separately and fastened on, Fig. 208. This allows the shaft of the column to run Fig. 207. Improperly |