In simple roofs, with ceilings formed of plane surfaces, the ceiling joists are generally supported by resting at their ends on the tie-beams of the trusses, as shown in Fig. 159, the joists extending from truss to truss. With such construction the tie-beams usually project beneath the ceiling and are either cased or furred and plastered.
When the span is quite wide, however, so that the strains in the trusses become considerable, thereby necessitating large timbers for the tie-beams, it is more economical and better construction to support the ceiling joists by beams or girders, spanning from truss to truss and corresponding to the purlins in the roof. Such construction is shown in Fig. 160 and in plan in Fig. 162. The ends of the supporting beams B should come opposite or close to a joint of the truss (which should be supported by a rod), so that no bending moment will be produced in the tie-beam.
The advantages of this method are that the tie-beams, being relieved of all transverse strain (except that produced by their own weight) need only be made strong enough to resist tension, and the load on the truss is also lightened by one-half of the weight on the outer ceiling joists, as this is carried directly by the wall, while in the other method the entire weight of the ceiling comes on the trusses.
When the truss span is less than 36 feet there will be no saving by this method, as it is always necessary to make wooden tie-beams considerably larger than the calculated size, on account of the cutting for the rods at the joints, and frequently, also, on account of splicing.
The ceiling shown in Figs. 153 and 154 is supported by ceiling beams or purlins, principally to obtain the paneled effect and that the ceiling boards may run at right angles to the trusses.
By dropping the truss beams and purlins beneath the ceiling, as in Fig. 160, the ceiling is divided into rectangular panels, adapting it to a more effective ceiling decoration than is practicable with perfectly plane ceilings.
When there is a large gable on each side of the main roof, so that the ceiling has the shape of a cross, and the ceiling is suspended, it is usually necessary to support the roof and ceiling over the crossing by diagonal trusses. Fig. 162 shows the framing plan of the roof and ceiling of the church in which the truss shown in Fig. 160 is used. In this church one corner of the crossing is supported by the tower, the other three corners being supported by wooden posts, two of which come in a partition. The four gables are of the same width, the distance between the posts being 37 feet 10 inches. The beams B B (Fig. 162) are the ceiling purlins, of which two are shown in Fig. 160. The beams P are the roof purlins, which were supported as shown in the same figure, and the three trusses, C, C, C, are of the shape therein shown. All of the ceiling purlins are supported by double stirrups, which straddle the rods. The truss A is a full truss, having the same rise as the trusses C, but, of course, a greater span.
The other diagonal is made of half trusses, supported from the centre of the truss A by large stirrups, as shown in Fig. 161. These diagonal trusses not only serve to support the inner ends of the ceiling and roof purlins, but they also support the valley ends of the common rafters. The ridges are also supported by purlins, marked R P on the plan. These purlins have a span of nearly 19 feet, and hence it was necessary to brace them from the lower joint of the diagonal trusses, as shown by the dotted lines. There is a practical difficulty in using two full diagonal trusses of this shape (with wooden tie-beams), in that it is a very difficult matter to make a sufficiently strong joint where the tie-beams intersect; the strain at this point being very great and it being quite impracticable to connect the parts by iron tie-plates.
It was for this reason that the author adopted the method shown in Fig. 161, of one complete truss and two minor trusses. By extending the main ties of the truss A to the rafters a strong joint is made at the centre, and the weight of the half trusses being supported by the stirrup from the top of truss A, does not come upon the centre rod. The end of the half truss is prevented from slipping out of the stirrup by iron ties bolted on each side of the half truss. Only one half truss is shown in Fig. 161, but another is carried on the other side of truss A. As truss A supports the inner ends of the half trusses it must be calculated to support not only the portion of roof and ceiling that is carried directly by it, but also half of the loads supported by the two half trusses. The half trusses, having only half the span, may have much lighter timbers than truss A.
Although the trusses B have been referred to as "half trusses," each is in reality a full truss, the brace B forming one principal and the lower portion of the main rafter the other. The portion R of the rafter, above the joint, is not a part of the true truss, but merely a beam to support the upper purlin and to brace truss A sideways. In roofing the church shown by the floor plan Fig. 163, the author used the same method of construction as shown by Figs. 160-162, except in the manner of supporting the half diagonals B B. The points a, b, c and d form a perfect square with a gable on each side. The ceiling is of lath and plaster, level in the centre and sloping up from the side walls. The span of this roof being a little less than that of the roof shown by Fig. 160, it was practicable to throw the weight of the half trusses on the centre rods of truss A.
Instead of using a single rod at the centre of the through truss, two rods were used, one on each side of the truss, and the washer at the bottom extended 4 ins. beyond the sides of the full truss, as shown in Fig. 164 to form brackets for supporting the tie-beams of the half trusses. The tie-beams of the half trusses were also tied together by two bent iron straps, s, spiked to each side of each tie-beam and passing over the tie-beams of the through truss.
The tops of the half trusses were secured to the top of the through truss by means of four 6"x6" angles bolted to the principals, as shown in Fig. 164. This construction proved very satisfactory both for strength and facility in erecting It is also an economical construction.
*Figs. 165 and 166 are reduced from the working drawings.
Fig. 163. - Dimensions of Diagonal Truss.
When the four angles of the crossing are formed by solid walls 10 feet long or more from the angle the walls will have sufficient stability to resist any thrust that can come upon them from the diagonals, and in such cases four braced valley beams may be used, abutting against each other at the top without provision in the diagonals themselves for taking up the thrust; but when the roof is supported by posts, as in this case, the diagonals should give no horizontal thrust. A further precaution against spreading in the roof illustrated by Figs. 160-162 was taken by bolting the trusses C together over the supports, as shown by the dotted lines in the plan, the bolts also passing through the diagonals.
Fig. 106 - Dimensions of Scissors Truss of 34' Span.
Where steel trusses are used both diagonals may be made exactly alike, as the intersection joint can easily be made in steel by means of plates and rivets. The centre member in such a case should, of course, have a sectional area equal to the sum of that required for the trusses acting separately.