When the ceiling consists of intersecting vaults the longitudinal system of trussing cannot, of course, be used, as the trusses would cut through the side vaults.

78 Ceilings With Cross Or Groined Vaults 300185Fig. 182.   Interior, Asbury M. B. Church, Denver.

Fig. 182. - Interior, Asbury M. B. Church, Denver.

A ceiling like that shown in Fig. 182 is occasionally used in Romanesque churches, and the manner in which the roof and ceiling of this church are supported may be of interest.

The audience room is formed on the plan of a Greek cross, the arms being of the same width and of nearly equal length. The two front corners of the crossing are formed by large wooden posts, furred and plastered, being located so that they do not obstruct the view from any of the seats.

78 Ceilings With Cross Or Groined Vaults 300187

As the length of the arms is only about 20 feet, the roof and ceiling are supported entirely by four double trusses, one on each side of the crossing, the purlins from these trusses to the gable walls being trussed.

Fig. 183 shows a section through the roof and ceiling, and Fig. 184 a detail of one of the trusses. The truss is a combination of the king-post and queen rod truss, the king-post being used principally to give the idea of a truss, as all the other members, except a portion of the tie-beam, are concealed. Circular ribs were placed between the tie-beam and king-post, projecting about 6 inches below the ceiling, but these were used merely to further the appearance of a truss, although they add slightly to the lateral stiffness. But one purlin on each side of the roof was used, as the upper length of the rafters is braced by the collar beams, which were necessary to support the furring for the central vault.

An unusual feature of this truss, and as a rule an undesirable one, is that the tie-beam is raised 3 feet 8 inches above the bottom of the principals, so that the portion of the principals below the tie-beam has to support the full weight of the truss. This brings a transverse strain at the juncture of the tie-beam and principal, and to resist it a steel plate was bolted in the centre of each truss as shown.

78 Ceilings With Cross Or Groined Vaults 300188

Fig. 185.

In this particular instance there was very little opportunity for the principals to spread at the bottom, as there is 20 feet of wall on each side which must be moved before the iron shoes can slide outwards, and as long as they cannot move no transverse strain can come upon the principals. The steel plates were inserted, however, as an extra precaution. Of course the walls could have been carried up to the tie-beam, but this would have added considerably to the expense of the building, as they are of stone, and would also have detracted from the external appearance. Two trusses like that shown were placed side by side and 6 inches apart. The crossing is supported by heavy valley timbers, and by extending the trussed purlins, to take the pendentive angles of the vaulting.

Fig. 186.   Interior of Trinity Church, Boston.

Fig. 186. - Interior of Trinity Church, Boston.

Fig. 185 shows a detail of the inner end of the trussed purlins, one coming in from each side of the crossing, and each pair being bolted together and to the valley timber at the top.

The ribs of the vault, at the intersection, are supported at their lower end by a built up diagonal rib, corresponding to the hip rafter of a roof, and this is supported by the lower chords of the trussed purlins, which are hung by 7/8-inch rods from the top, the purlin being trussed to act as a cantilever.

This method of supporting the crossing is often applicable when the ceiling has a different shape from that shown.

The shape of the ceiling shown in Fig. 182 is very similar to that in Trinity Church, Boston, but in Trinity Church the crossing is covered by a tower with a flat paneled ceiling.

The author does not know how the roof and ceiling of Trinity Church are supported, but from the appearance of the building he judges that the trussing must be similar to that shown in Fig. 184, except that the crossing is covered by the tower.