The truss shown by Fig. 83 is known as a quadrangular truss, although the more common shape for this truss is that shown by Fig. 87. This truss may be considered as two trussed rafters, held in place by the tie T at the centre. The portions of the truss which are in compression are indicated by double lines while the single lines represent the tension members. The dotted lines represent counter-braces or ties, which might be brought into action in case of a heavy snow load on one side only or during a severe gale.
The piece B has no strain when the truss is subjected only to a vertical load, although it is usually put in to brace the post P, which carries all of the load transmitted to the support. This truss is well adapted to steel construction up to spans of 180 feet. When the span exceeds 100 feet, and the truss rests on a brick wall one end of the truss should be supported on rollers to allow for expansion and contraction in the steel. The proportions of the truss shown in Fig. 87 are those of the roof trusses of the Jersey City-station of the Central Railroad of New Jersey. The trusses in this building have a span of 142 feet 4 inches, centre to centre of bearings, with a depth in the centre of 24 feet 4 inches. The distance between trusses is 32 feet 6 inches. The joints are pin-connected, eye-bars being used for the tension members, and one end of the truss is on rollers. Fig. 88 gives the proportions of similar trusses over the amphitheatre of the Madison Square Garden, New York.* In both of these trusses the posts and braces at the ends are made a part of the truss, although they cannot be represented in the strain diagram, and the post P receives the whole load from the truss at its upper end, the diagonal tie transmitting the entire web stress to the top of the post. The brace B should be so constructed as to resist both tension and compression. This truss, like the other, is pin-connected, with eye-bars for the tension members. The principal dimensions of the truss are given in the drawing.
Fig. 87. - Typical Quadrangular Truss.
Fig. 88. - Truss over Amphitheatre; Madison Square Garden.
* Details of these trusses were published in 'Architecture and Building" for April 21, 1890.
Fig. 89 shows another example of this type of truss with intermediate supports for the purlins.
The truss shown by Fig. 90 also belongs to this type, but differs from the trusses shown by Figs. 87-89, in that the diagonals are all in the same direction. This causes a reversal of the stresses in the web members, as indicated by the plus and minus signs, the three diagonals at each side of the centre being in compression and the adjacent uprights in tension. In the trusses shown by Figs. 87-89, all of the diagonals are in tension and all the verticals in compression. The truss proper, is included within the figure A, B, C, E, D.
Fig. 89. - Span, 102 ft.
Fig. 90. - Quadrangular Truss; Span, 80 ft.
It may be noticed that the lower chord of these trusses is segmental in shape, giving a graceful outline, and also the most economical proportions for wide spans.
For shorter spans a full semicircle or semi-ellipse may be obtained by giving a greater curve to the lower chord and continuing it by means of braces. Fig. 91 shows how this was accomplished in the roof trusses over the wings of the Manufacturers' and Liberal Arts Building of the World's Columbian Exposition at Chicago.
The parts included between the joints A, B, C, D form a quadrangular truss, while the portions X, X are merely braces external to the truss.
The trusses supporting the central roof of the Mining Building, represented in Fig. 111, are also of the same type, although in this building the lower chord and braces have the form of a semi-ellipse. In both of these figures the tension members are represented by single lines. All the members in this truss were made of angles, so as to take up either tension or compression, and the joints were riveted.
The stresses in quadrangular trusses due to wind and snow should be determined independently of the dead load, and the members computed for the maximum stress that may be produced by any possible combination of loading.
There are numerous examples in this country of quadrangular trusses of from 100 to 180 feet span.
The truss shown by Fig. 92, which is a diagram of one of the trusses over the Kansas City Auditorium, may be considered as a cross between the Warren truss and the Quadrangular truss, for if the end panels were omitted the truss would be almost identical in shape to that shown by Fig. 90. This truss has pin connections at the joints. The three diagonals each side of the centre are formed by lattice channels, and all of the ties are formed by eye-bars. The plus sign denotes compression and the minus sign tension. The diagram below the truss drawing shows the manner in which the trusses were braced laterally. All measurements are from centre of pins. A description of this truss and of the building may be found in the "Engineering Record" for July 22, 1899.