Fig. 213.-Ordinary roof of a gas-house. A, B, is the main rafter.

a, a' a" are vertical tie-rods.

b, b' b' are braces,

C, D, is the main tie.

E, F, is the ventilator.

Fig. 212-Represents the ar
rangements of the parts at
the joint c in Fig. 210.
A, side view of the pieces
and joint.

a, principal rafter of the
cross section B.

b, common rafter of the cross section C.

c, cross section of purlins and joint for fastening the common rafters to the purlins. d, cast-iron socket arranged to confine the pieces a, b, c, e.

B

The

697. Fig. 214 shows a mode of secondary trussing. A is a strut for supporting the middle of the main rafter. lower end of A is secured to a block which is supported by the tie-rods B and D. The tie-rods C and D serve the office of a single tie for supporting the lower end of E. In this

way the rod D performs a double office. It may be questionable whether this arrangement is as good as it would be to have one continuous rod pass from E to F, and another rod (D) to act with B.

E

Fig. 214-A is a strut, the lower end of which is supported by the ties B and D. C and D serve the office of a continuous tie for supporting the lower end of the strut E

It may be observed that in this Fig. the tie-rods are inclined and much longer than the struts, which is the reverse of the condition in Fig. 213. If iron only is used the arrangement of Fig. 214 will generally be the most economical, but if wooden struts are used the plan of Fig. 213 may be preferable.

Fig. 215.

698. Depot Roof Truss. Fig. 215 shows a truss which has been used in many cases for supporting the roofs of depots and of other large buildings. The passenger depot of the

Michigan Central Railroad at Chicago was built after this plan. It was destroyed by the great fire in 1871. The plan of the arch is a Howe truss, having curved wooden chords, wooden braces and iron ties to connect the two chords. The truss formed an arch, the thrust of which was resisted by a long horizontal tie-rod.

The same style was adopted in the new roof over the depot at Troy, New York; and the Grand Central Depot in New York City.

699. A novel plan was used in making the roof over the rolling-mills at Milwaukee, Wis. An arch was made of boards so placed as to break joints and form a rib about a foot wide and eighteen inches deep, and one hundred and eighty feet span. The boards were bolted together so as to make the rib continuous, and then the upper part of the arch was trussed after the Howe plan. The main objects of this plan were cheapness and to secure the whole inclosed area free from posts or other similar obstructions. But it was found that the arch was too weak, especially when required to carry the large ventilator which was placed over it, and posts were afterwards added.

700. Roofs and Domes. In some cases-especially in state buildings-domes are placed upon roofs for architectural effect.

Figs. 216 and 217-Are two trusses, which are made in pairs, and are placed fourteen inches apart, for supporting part of the dome (octagonal) of the State capitol at Montpelier, Vt.

a a a are the short timbers for connecting the two trusses.

A is a timber resting upon the cross pieces a a a.

C is a post of the dome resting upon the piece A.

Span, sixty-seven feet four inches.

The dome of the State capitol, Vermont, rests upon wooden trusses (Figs. 216 and 217), having a span of sixty-seven feet four inches. The trusses are supported at the ends only. They are placed in pairs, fourteen inches apart. The Fig. shows two pairs. They are connected by short cross beams, a a; upon which rest other timbers, A, for receiving the posts, C, of the dome. It is profitable for the student to make a careful study of the details of this structure.

Where the thrust is severe especial care should be taken to secure a good bearing for the ends of the timbers. The lower ends of the main rafters tend to shear the main tie at its ends, and to prevent this action they should enter the tie at a reasonable distance from its ends. The bearing pieces are of white oak, and the rest of the timber is spruce. The trusses are constructed differently, because the posts of the dome bear upon them in different places.

701. Roof over the large hall of the University of Michigan. This truss and dome presents a very novel feature (Fig. 218), inasmuch as a part of the dome rests directly, or nearly so, upon the posts which support the roof, while the other part rests directly upon the trusses which support the roof. The span is eighty feet in the clear, and the depth of the trusses is sixteen feet. The main rafters are pieces of solid pine fourteen inches wide by sixteen inches deep. They are not of equal length, the longer ones having a horizontal run of forty-seven feet, and the shorter ones thirty-three feet. The secondary trussing is distributed according to the strains. The dome is thirty feet in diameter at the base.

The ceiling of the large hall being attached directly to trusses, it was necessary to make very strong trusses, so that the action of the wind upon the dome, and also the effect of the changes of temperature might not so disturb the trusses by causing them to deflect, as to destroy the ceiling. (For a computation of the parts, see Wood's Bridges and Roofs, pp.

194-211.