This truss differs from all the other types of trusses that we have considered in that it consists essentially of two separate parts, each acting as a single piece and depending upon the opposing force of its mate to keep it in position. As usually built, each part is a semi-braced arch, the upper and lower members being so connected by bracing as to form a stiff frame or curved rafter.
The first use of the braced arch appears to have been in building railway bridges for French railroads, the earlier forms being rigidly connected at the top.
The first suggestion for hinging the ribs at the crown was made by M. Manton, a French engineer, who, in 1861, suggested the type of arched truss shown in Fig. 99.
It is evident that the fundamental principle of this truss is the same as that of the roof trusses shown in Figs. 100 to 102.
The first application of this principle to roof trusses, at least on a large scale, the author believes to have been in the train sheds of a Union Railway station at Frankfort-on-the Main, Germany, which was completed in the year 1888. These trusses have a span of about 184 feet. The large roof of Machinery Hall, of the Paris Exposition of 1889, was supported by this type of truss, the span in this case being 368 feet, exceeding anything hitherto attempted in a roof truss. The use of this truss in the Manufacturers and Liberal Arts Building of the Columbian Exposition made the type familiar to most of the architects of this country, and it has since been extensively used for roofing exposition buildings, armories, train sheds, etc.
In all of these trusses the arched ribs are supported at the bottom on a turned steel pin, and with the exception of the Frankfort truss all abut against a pin at the top, thus giving the name - Three-Hinged Arch.
Usually the trusses are surmounted by a lantern, as shown in Figs. 101 and 102, which are bolted or riveted together at the centre.
With the shape of ribs generally used there is always a slight outward thrust at the bottom, which must be resisted either by the abutments, as in Fig. 99, or by tie-bars connecting the lower pins. Most of the trusses erected in this country have tie-bars placed just beneath the floor, but the trusses in the Frankfort Depot and Machinery Hall, Paris, had no ties, and Messrs. D. H. Burnham & Co. used three-hinged arches of 160 feet span in the First Regiment Armory, Chicago, which support two floors and a gallery, besides the roof, without ties.
Fig. 100. - Half-Truss, Machinery Hall, Paris Exposition, 1889.
The special advantages of this type of truss for the class of buildings above mentioned are economy, maximum clear space beneath the truss and provision for expansion and contraction. Much of the economy of the truss lies in the fact that it requires no columns to support it, and, the base of the truss being very near the ground level, it is well proportioned to resist wind pressure. A great advantage of this truss is the free movement allowed under temperature changes without strain to the structure, the centre rising or falling freely with a slight rotation of the semi-arches about the pivots. In the case of the trusses of the Paris Exposition it was estimated that a range of temperature of 100° Fahr. would produce a change in level of 2 7/8 inches at the centre pivot.
Fig. 101. - Half-Truss, Manufacturers' and Liberal Arts Building, Chicago, 1893.
The arched ribs are always built of plates and angles with riveted connections, and frequently with a solid plate web at the bottom.
The determining of the stresses and detailing of the members and joints will require the service of a competent structural engineer, but the illustrations given will enable the architect to decide on the general shape of the truss for the purpose of making preliminary drawings and the computations and detail drawings can be made later.
Fig. 102. - From Machinery Hall, Chicago, 1893.
41. The trusses in the Liberal Arts Building and in Machinery Hall at Chicago were spaced about 50 feet apart from centres.
In the Liberal Arts truss the inner rib is bent to a curve up to the joint A, and above that it is made up of straight pieces.*
Similar trusses are also used over the train sheds of the Pennsylvania Railroad station and the Reading Railroad station in Philadelphia.
The trusses of the former building have a span between pins of 300 feet 8 inches, a rise of 108.49 feet, with a depth at the springing of 5 feet 3 inches. The lower pins are 5 1/2 inches in diameter.† The trusses in the Reading Railroad Depot have a span of 260 feet.
*Complete drawings and details of the trusses of the Liberal Arts Building are contained in Vol. XXVI. of the "Engineering Record," and of the Machinery Hall trusses in Vol. XXVII.
†Details of these trusses were published in the "Engineering Record" of June 10. 1893.
The roof of the Central Armory, Cleveland, O., Messrs. Lehman & Schmitt, architects, is supported by six three-hinged plate-girder arches, that is, with a solid web instead of bracing. This is the only instance of a solid web in large arches with which the writer is acquainted, and it would appear as though it might be advantageously employed in many instances.
The trusses in the Cleveland Armory have a span of 120 feet and a rise of 52 feet 6 inches, centre to centre of pins. The two end arches also support a gallery as well as the roof. Complete details of these trusses are published in the "Engineering Record" of December 26, 1896.
Other examples of three-hinged arch trusses are given in Chapters VI.