It is often desirable to place a vaulted ceiling above the nave or auditorium of a church or hall, while maintaining a simple pitch roof on the outside. This may be accomplished in several ways; the most desirable construction for any particular building depending largely upon the width of the room, the character of the supports and whether the ceiling is to be a simple or groined vault.
Fig. 175 shows a section of the vaulted ceiling and the truss supporting it as contemplated by Mr. H. H. Richardson in his design for the Cathedral of All Saints, Albany, N. Y. In this case the short hammer beam is supported by an engaged column and corbel instead of a bracket. The ceiling also terminates in a series of corbels and arches which throw it out from the wall line. The span of the truss is about 32 feet. Between the trusses the ceiling was to be supported by cradling spiked to collar beams and to the rafters, as shown in the right-hand side of the figure, the duty of the trusses being principally to support the rafters.
This form of construction was frequently used in mediaeval buildings. It requires very heavy walls, and large timbers in the trusses to keep the roof from spreading.
For light walls, it would be better and safer to use ornamental tie-beams in place of the hammer beams, and an ornamental king post, as in the roof of Trinity Church, Boston [see Fig. 182].
A flat vaulted ceiling, with a contour about as indicated by the dotted line in Fig. 156, can readily be suspended from scissors trusses, but such trusses cannot well be used for semi-circular vaults, unless the top of the wall is considerably above the springing of the vault.
76. Fig. 176 shows a scissors truss designed by the author for supporting a ceiling that was very nearly semi-circular in section, and Fig. 177 shows a detail of the joints at 5 and 7. The ceiling was formed by curved ribs cut from 2-inch plank and spiked to the longitudinal beams shown by the etched sections in Fig. 176. The beams at A and B are held in place by the rods R R.
Fig. 178 shows the construction of a double vaulted ceiling over a Catholic church* that is quite interesting. The shape of the ceiling is roughly shown by Fig. 179.
Fig. 179. - Showing Vaulted Ceiling.
Although the ceiling is quite elaborate, the manner of supporting the roof and ceiling is really quite simple. Fig. 178 shows one of the supporting trusses, which are ordinary scissors trusses. The 6"x6" beam at A, extends the full length of the church and is suspended from the top of the truss near the purlin B, by a 1 1/4" rod. The beam is also rigidly held in place by means of the horizontal beam D, and by a 6x6 brace. The cross beam D is tied to the longitudinal beam by a bolt strap passing through the latter. The longitudinal beam at A supports the bottom of the diagonal ribs of the upper vault and the top of the ribs of the lower vault. The double lines at P indicate the studding which forms the end of the upper cross vaults, P, P, Fig. 179. This roof could probably have been more economically constructed, by using longitudinal Howe trusses at P, letting the tie-beam answer for the longitudinal beam at A, and supporting the centre of the roof by means of king rod trusses, resting on the Howe trusses.
•Designed by Andrew Roth, Architect.