For such roofs, some form of the scissors truss (so named from its resemblance to a pair of scissors) is most often used. When correctly designed with members of the proper size, and with the joints carefully proportioned to the stresses, the scissors truss makes a very good truss for supporting the roof over halls and churches, up to a span of 48 feet, but above that, they should be used with much caution.
If we take a simple king-rod truss, as shown in Fig. 28, and screw up on the centre rod, we will raise the tie in the centre and bring the feet of the rafters toward each other, as shown by the dotted lines; consequently by making the tie and centre rod of the proper length and strength we can have the form of truss shown in Fig. 29, and still keep the feet of the rafters a and b from spreading. To do so, however, the tie c c must offer a much greater resistance than when it is level, and it requires great strength in the centre tie r to keep the tie c c in position.
The pieces d d are simple struts used for bracing the rafters or to assist in supporting the purlins. In practice d and c are generally made in one piece, for convenience of construction, but it should be remembered that the part c is in tension and the part d in compression. If there are two purlins on each side to be supported an additional brace and tie should be used, as shown in Fig. 30. This also increases the tension in the lower part of the tie c. Very often a "collar-beam" is placed across the truss, as shown by the dotted lines at h (Fig. 30) and the ends are made to take the place of the lower brace. If the collar-beam is made of two timbers bolted on each side to the ties c c, it will reduce the stress in the centre rod, but increases the stress in the other two rods. As there is no way of telling just how much stress the collar-beam will appropriate, it is generally better not to put in the collar-beam unless it is necessary to support the ceiling.
Trusses of the type shown in Figs. 29 and 30 are commonly known as "scissors trusses." When using such trusses the inclination of the tie-beams should be made as little as the conditions will permit, and the stresses should be accurately determined.
The lines a and c, Fig. 31, represent the stresses in the rafter and tie-beam, respectively, of the truss shown in Fig. 29, while the lines a1 and c1 represent the stresses in a truss of the same span and pitch, but with a horizontal tie-beam, the external load being the same in both cases. The line r, Fig. 31, represents the stress in the centre vertical tie, Fig. 29, due to the external load only, while in a king-rod truss, such as is shown in Fig. 8, the stress from the same load is represented by the line r1.
As the size of the truss members is governed principally by the stresses, it is evident that for the same span and loads a truss of the shape shown in Figs. 29 and 30 will cost considerably more than one with a horizontal tie-beam.
16. Another type of truss, much resembling the scissors truss.
but being quite different in principle, is shown in Fig. 32. In this truss the pieces c c are in tension their full length, and should pass by each other. The piece B, in this truss is in compression, and must be of pretty large dimensions to resist the compressive stress. This type of truss should not be used for spans exceeding 35 ft. A further illustration of this truss is shown in Chapter III (Layout Of Trussed Roofs - Bracing Of The Roof And Trusses).