C h u r c h Spire. Fig. 250 shows the method of framing a church spire, or other high tapering tower. The base of the drum N is square and is supported by the posts A, one at each corner, which rest on the sills B. The sills are supported by the roof trusses of the main roof. The corner posts extend the full height of the drum and are strongly braced in all four faces, with intermediate tical studding C between them to form the framework for these faces. The spire itself may rest on top of this square drum or there may be another eight- or twelve-sided drum constructed on the top of the first drum, on which the spire may rest. This depends upon the design of the spire. The hip rafters D do not rest directly on top of the drum, ever, as this arrangement would not give sufficient anchorage for the spire. They are made so as to pass close inside the plate E at the top of the drum and are securely bolted to this plate with strong bolts. This is shown at L, which is a plan of the top of the drum, showing the hip rafters in place. The plate is shown at E, and the hip rafters at D. The rafters extend down into the body of the drum as far as the girts 11 (shown in the elevation) to which they are again securely spiked or bolted, being cut out at the foot so as to fit against the girt. In this way a strong anchorage for the spire is obtained.
Horizontal pieces I are cut in between the hip rafters at inter vals throughout the height of the spire, braces K, halved together at the center where they cross each other, are firmly nailed to the rafters at each end. These braces are needed only in lofty spires, which are likely to be exposed to high winds. At the top the hip rafters bear against a post 31, the same as in the other towers. If a conical spire is called for in the design, the horizontal pieces I must be cut to the shape of segments of circles, and in this case the rafters are no longer hip rafters. The horizontal pieces I will receive the boarding, which will form a smooth conical surface.
The spire above the drum is usually framed on the ground before being raised to its final position. It then may be raised part way and supported by temporary staging while the top is finished and painted, after which it may be placed in position on the top of the drum.
Domes. Timber domes have been built over many famous buildings, among which may be mentioned St. Paul's Cathedral at London, and the Hotel Des lnvalides at Paris. While these structures arc domical in shape they are not, strictly speaking, domes, because they do not depend for support upon the same principle which is implied in the construction of a dome. They are, correctly speak ing, arrangements of trusses of such a shape as to give the required domical form to the exterior of the roof.
Fig. 251 shows such a truss supported at either end on a masonry wall. Fig. 252, which is a plan of the framing of this roof, shows how the sections or bents may be arranged. There are two complete bents, A B and C D, like the one shown in the elevation, Fig. 251,
which intersect each other at the center. A king-post A in the elevation is common to both bents and the tie-beams B are halved together where they cross. These two bents divide the roof surface into four quarters, which are filled in by shorter ribs, as indicated in the plan, Fig. 252. The posts C, in Fig. 251, carry all the weight of the roof to the walls and are braced by means of the pieces D.
The rounded shape is given to the exterior and interior of the bent by pieces of plank bent into position as shown. The whole is covered with boarding which is cut to a special shape so that it can be bent into place. The methods of applying the boarding to domical roofs • will be explained in connection with other rough boarding.
The arrangement of trusses described above is suitable for a plain domical roof without a lantern or cupola on top, but very frequently this feature is present in the design, and the roof must be framed to allow for it. There are several different ways of arrang ing the trusses so as to leave an opening in the center of the roof for the lantern. Fig. 253 shows a very good arrangement. Four trusses A span the entire distance between the walls, and are placed as shown in the figure, so as to leave the opening B in the center. Four half trusses C are inserted between them, as shown, and eight shorter ribs D are employed to fill in the rest of the space.
Fig. 254 shows another arrangement, providing for a lantern at the center. There are a number of ribs A, twelve in number, in the figure, all radiating from the center where there is a circular opening for the lantern or cupola. In Fig. 255 is shown a section through a domical roof framed in this way, showing an elevation of one of the ribs. The rib is so constructed as to be entirely contained in the restricted space between the lines of the exterior and interior of the roof.
Pendentires. In the preceding paragraphs we have considered the subject of domical roofs covering buildings of circular plan, which is the simplest possible case, but unfortunately not the most usual one. It very often happens that a domical roof must be erected over a building which is square or rectangular in plan, in which case a new and difficult problem must be considered, namely, that of the pendentives. A horizontal section taken through a dome must in every case show a circle or possibly an ellipse. If, then, we con sidcr the horizontal section cut from a domical roof by the plane of the top of the wall, it must usually be a circle and can not exactly coincide with the section cut from the wall of the building by the same plane, unless the building is circular in plan. This is shown in Fig. 256 in which A B C D represents the section cut from the wall of the building by a horizontal plane, and the circle E F G H represents the section which would be cut from a domical roof cover ing the building if the framing , for the dome were carried down to meet this plane all the way around.