With Fink or Fan trusses having an inclination for the rafter not exceeding 30 degrees it is more economical to employ a horizontal chord or tie since it obviates bending of the laterals. Raising the bottom chord, also materially increases the strains in the truss members, hence it increases the cost. A truss whose bottom chord has a rise of two or three feet, as in Fig. 69, presents a better appearance, however, than one with a horizontal chord, and for steep roofs, it will generally be fully as economical to raise the bottom chord because of the shortening of the members. Trusses with raised ties are designated as "Cambered."

Fig. 74.   French Truss with Vertical Struts. Span, 80 ft.

Fig. 74. - French Truss with Vertical Struts. Span, 80 ft.

33. The diagram shown by Fig. 75 represents 1/2 of one of the steel trusses used in roofing a car barn for the North Jersey Rail-

way Co., at Newark, N. J. There were 13 of these trusses, spaced 19' 2 1/4" on centres, each having a span of 98 1/4' between the centres of the supporting columns to which the truss is riveted by splice plates engaging the end connection plate and the end web of the column. The dimensions of the principal members of these trusses are indicated in connection with the illustration. These trusses were shipped in four sections which were assembled in a horizontal plane and riveted up complete at the surface of the ground. The bottom chord was stiffened by lashing a rail on each side of it for its entire length and a sling being attached to the apex of the top chord, the truss was lifted and set on top of the columns by an 8" x 8" gin-pole 50 feet high. The roofing consists of corrugated iron supported by 5" I-beam purlins weighing 10 lbs. to the foot, spanning from truss to truss and bolted to the rafters with two bolts at each end; the general spacing of the purlins being 4' 9 3/4". This may be considered as an example of an extremely light roof, the weight of each truss being only about 4,200 lbs., and the entire weight of the truss, purlins, bracing of the lower chord and corrugated roofing being only 8 lbs. for each horizontal foot of surface covered. The truss shown by Fig. 74 was designed for the roof of a drill hall having a span of 80' and with a spacing, centre to centre, of 20'. The roof was to be constructed with 2x8 rafters supported by purlins at points A, B, C, D, E, and F. Sash were to be placed in the rise C D to light the interior of the building. The joint at X was located with reference to the position of the gallery rod; if there had been no gallery it would have been more economical to space the vertical struts uniformly as in Fig. 70.

Dimensions of Members in Fig. 75.

Rafters..

1 - 2

=

2

-

5"

x

3 1/2"

x

7/16"

L's.

Rafters..

2 - 3

=

2

-

5"

x

3 1/2"

x

3/8"

L's.

Main tie..

1 - 4

=

2

-

5"

x

3 1/2"

x

3/8"

L's.

Main tie..

4 - 5

=

2

-

3 1/2"

x

2 1/2

x

5/16"

L's.

a, a, a.......

=

2

-

2 1/2"

x

2"

x

1/4"

L's.

b, b, b.......

=

1

-

2 1/2"

x

2"

x

1/4"

L.

=

2

-

3"

x

2 1/2"

x

1/4"

L's.

=

2

-

3 1/2"

x

2 /2"

x

5/16"

L's.

Fig. 75.   Diagram of Compound Fan Truss.

Fig. 75. - Diagram of Compound Fan Truss.

Fig. 7G.   Warren Triangular Truss.

Fig. 7G. - Warren Triangular Truss.

32 Cambered Trusses 30081

Fig. 77.

For roofs having a span of 80 to 100 feet, and a rise of from one-fourth to one-third of the span, the Warren triangular truss shown by Fig. 76 is a good type. This truss is best adapted to pin connections.

For a steep roof, the type shown by Fig. 77 is about as economical as any. A complete example of this type of truss with the details of the computations is given in Part II., of Berg's "Safe Building."

The plus sign adjacent to a member, in all the trusses illustrated, denotes that the member is in compression, while the minus sign denotes tension. In Figs. 76 and 77 the members represented by single lines are in tension. The members above the main rafter as C D, D E, and E F, in Fig. 74, and a and b in Fig. 75 do not form a part of the truss proper but are merely a frame work to support the elevated roof and in drawing the stress diagram, they should be omitted 34. FINK TRUSSES WITH PIN JOINTS. - Fig. 78 shows one-half of a Fink truss designed for pin connections. This truss has a span of 55' 4" between centres of end pins and the distance between the centres of trusses is 6'. The roof is covered with 12" x 20" slate, secured to 1 1/2 x 2 1/4" angle purlins weighing 3 lbs. to the foot and spaced 8 1/2" on centres. The angles span from truss to truss and are bolted to the deck beam with 1/2" bolts. A 1 1/2" by 2 1/4" nailing strip is fastened to every third purlin for securing matched ceiling placed on the under side of the roof. Complete details of this truss were published in "Architecture and Building" for January 18, 1890.

Fig. 79 shows details of the cast-iron struts.

This truss, being put together entirely with bolts and pins, could easily be erected with unskilled labor. 35. TRUSSES FOR FLAT ROOFS.

- For supporting flat roofs or roofs having a fall not exceeding 1" to the foot, one of the types shown by Figs.

80 to 84 will generally be found eco nomical. The choice of the particular type depending somewhat on the span and whether the truss is supported by columns or by brick or stone walls. For spans up to 50' either of the forms shown by Figs.

Fig. 78.   Fink Truss with Pin Connections,

Fig. 78. - Fink Truss with Pin Connections, lower chord between the end joint and the wall, Fig. 80, has no stress from the roof load but is put in to brace the wall, and to stay the truss. In trusses supported by brick walls this type is preferable to that shown by Fig. 81, while the latter is more suitable when the roof is supported by columns. The vertical A, Fig. 81, is in- 80 or 81 will answer all practical requirements. The truss shown by Fig. 80 is intended to be used where the fall of the roof is at right-angles to the truss; this truss can be built, however, with an inclination to the top chord as in Fig. 81. The end brace in Fig. 80 is in tension while in Fig. 81 it is in compression. The portion of the serted to receive the tension or compression from the brace B, and would have no stress from the roof load. The truss shown by Fig. 82, which represents an actual truss is known as a "Double Warren Truss" and is desirable where it is important to make the trusses as shallow as practicable; it can be built with light members and makes a very stiff roof, being especially suitable for roofs supported by steel columns. The strength of this truss under unsym-metrical loads, as, for example, when there is more snow on one side than on the other, would be materially increased by putting a vertical tie in the center as shown by the dotted line; without this member the braces AA, if subject to any stress whatever, would produce a bending in the bottom chord at the center. Fig. 83 represents an actual roof truss with a span of 57' supported by steel columns. The entire load on the truss is transmitted to the columns by the braces B, B, which are in tension. Fig. 84 shows a Warren truss of 96' span over a pier shed, New York City.

Fig. 79. Details of Struts in Fig. 78.

Fig. 79. Details of Struts in Fig. 78.

Fig. 80.   Span, 56 ft.

Fig. 80. - Span, 56 ft.

Fig. 81.   Span, 30 to 36 ft

Fig. 81. - Span, 30 to 36 ft.

Fig. 82. Span. 50 ft.

Fig. 82.-Span. 50 ft.

Fig. 83.   Span. 57 ft.

Fig. 83. - Span. 57 ft.

Fig. 84.   Span, 96 ft.

Fig. 84. - Span, 96 ft.

The plus and minus signs in these illustrations indicate compression and tension, respectively, under uniform dead load. The plus and minus sign together indicate that the member may be subject to either tension or compression, according to the direction of the wind or to an uneven distribution of snow. In most of these trusses an unsymmetrical load may change the stress in the diagonals neat the centre of the truss. Trusses like those shown by Figs. 80-84 are almost invariably built with riveted connections and with angle or channel shapes for all members.

For Horizontal Steel Trusses Intended to Support Floor Loads, the Pratt truss shown by Figs. 85 and 86 is best adapted, the members indicated by double lines being in compression, and those indicated by single lines in tension. When supporting floors subject to moving loads, counter ties should be inserted as indicated by dotted lines. For this truss, pin connections are generally employed and are preferable to riveted connections. When properly proportioned this truss is capable of sustaining almost any load.

Fig. 85.   Pratt Truss; Span, 60 to 150 ft.

Fig. 85. - Pratt Truss; Span, 60 to 150 ft.

Fig. 86.   Suspended Pratt Truss; Span, 40 to 80 ft.

Fig. 86. - Suspended Pratt Truss; Span, 40 to 80 ft.