This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Timber domes have been built over many famous buildings, among which may be mentioned St. Paul's Cathedral at London, and the Hotel Des Invalides at Paris. While these structures are 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 speaking, 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.
Fig. 252. Plan of Framing for Dome Shown in Section in Fig. 251.
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 arranging 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. 253. Framing Plan for Dome Having Cupola.
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.
Fig. 254. Framing Plan for Dome Having Lantern at Center.