92. For drill rooms, skating rinks, etc., it is important that there shall be no posts or other obstructions to interfere with the movements of the men or skaters, while in the case of Coliseums or auditoriums it is equally important that the view from all portions of the building shall be unobstructed.
This necessitates a single span for the roof, and as buildings of this class are generally very large, the roof is usually the most costly portion of the building, and requires considerable engineering skill to properly design it.
Wooden trusses could be used for spans up to 100 feet, but as they would require a greater height for the walls, to give the same clear height at the center, there is very little economy in using them, except in buildings where high walls are required on account of several galleries.
When a wooden truss must be used, a truss of the type shown by Figs. 58 and 59, will generally be most economical, where there are galleries, while for buildings without galleries a truss such as is shown by Fig. 61, may be used. The latter truss gives a very pleasing effect.
The roof of the Mormon Tabernacle, at Salt Lake City, is a very interesting example of an arched wooden roof. Timber was the only material available for roofing this building, because iron was very expensive in those days (1866-7), having to be freighted from the Missouri River. Nails cost about a dollar a pound and were used very sparingly.
The supports or walls of the building are 44 buttresses or pillars of sandstone masonry, each 3' x 9', and ranging in height from 14 feet at the east end to 20 feet at the west end. Each buttress has the foot of an arch to support, these arches forming the roof of the building. These buttresses are set with their axes normal to the sides of the structure, those at the ends being on radial lines. The arches are lattice trusses, the kind so often used during the early days of bridge construction, built of Utah red pine cut from the mountains near Salt Lake City.
Each arch consists of four curved chords or courses of timber braced together as shown. Each chord is composed of four pieces of 2 1/2 x 12-inch plank, two on each side of the lattice bars that pass between them. The lattice bars are also 2 1/2 x 12 inches. The two lines of plank forming each side of each chord are continuous. The joints are alternating, and as the strains are compressive, they are not spliced, but are simply butt joints, with an even bearing. The lattice bars and the chords intersect at the same points, and they are connected together by wooden pins about \\ inches in diameter. Each chord is thus 15 inches thick, and the depth of the arch is 10 feet.
Fig. 219. - Plan of Tabernacle at Salt Lake City.
As the arch has no horizontal chord or tie rod nor any abutment to thrust against, its strength depends entirely on its stiffness, and this is increased in the following manner: An extra stiffening brace, shown by dotted lines, starts from the outside chord at its foot and crossing the arch in a vertical plane it intersects the inside chord (being tangent to the inner curve line), and passing on it crosses the arch again and rejoins the top chord, well up on the flatter part of the arch. There are two braces, each of two 2 1/2 x 12-inch plank, one brace on each side of the arch, and they are fastened to each chord by a large iron bolt passing clear through the ten thicknesses of plank at these intersections. Very little iron besides these bolts was used. There is a full system of cross-bracing between the arches.
The half arches for forming the semi-circular ends of the building are similar in construction to the main arches. The tops of the half arches are connected to a small, nearly semi-circular construction of timbers built horizontally into the adjoining main arch. This main arch is well braced against the next one to it, but beyond this the regular cross-bracing is relied on to transmit the thrust from the half arches along the axis of the building.
Fig. 220. - Arched Tru99. Tabernacle at Salt Lake City.
The roof was originally of wooden shingles, but about the year 1892 these were replaced by a patent tin roof. This tin roof does not give as good satisfaction as the shingles, there being many leaks. The flatter part of the roof, over the center is of sheet iron.* The design of the tabernacle is said to have originated with Brig-ham Young, the necessary drawings being made by Architect W. H. Folsom. who also worked out the details.
93. Figs. 221, 222, and 223 show the ingenious method adopted by Mr. Carl Pfeiffer, a former architect of New York City, for roofing a riding school on Fifth Avenue, in that city. The riding room is 106' 6" long and 73 feet wide. This space is kept entirely clear of posts or columns, and the entire roof is supported by two arched trusses, one of which is shown in Fig. 222. The location of the trusses with reference to the plan is shown by Fig. 221. The roof between the trusses and on either side is supported by smaller trusses resting on these large trusses,but each of the arched trusses eventually supports a roof area of about 2,930 square feet and a great amount of extra frame work. It was desired to provide for the thrust of the main trusses without having rods exposed in the room, and the method adopted for taking up the thrust is rather unusual. Opposite the upper ends of the iron posts which receive the arched ribs are oak struts which are held in place by iron tie bars and heavy iron beams, which together form a horizontal truss at each end. These horizontal trusses are prevented from being pushed out by two 3-inch by 1-inch tie bars in each side wall, shown in the plan, Fig. 221. The bottoms of the two iron posts are also tied together by iron rods placed under the floor of the room. Altogether this gives for the tie rods of each truss two bars 3" x1" and one rod 1 1/2 inches in diameter.
*W. P. Hardesty, C. E., Engineering Record, Jan. 27, 1900.