This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
(1) Triangular roof trusses;
(2) Crescent roof trusses;
(3) Roof trusses other than these.
Fig. 1 shows various forms of triangular roof trusses. The Pratt and Howe trusses are shown respectively by a and b. These trusses obtain their name on account of their web bracing being of the Pratt or Howe type. The triangular truss in most common use is the Fink, next to which is the Saw-tooth. The Fink truss is built in a variety of forms, as shown in Fig. 1 (c, d, e, and f), c being for spans up to 60 feet; e for spans up to 70 feet, and d and / for spans up to 80 feet and over. The great advantage of this style of truss is that many of its members have the same stress, and therefore it can be constructed more cheaply on account of the fact that a large amount of the same sized material can be purchased at once.
When the top chord of a roof truss becomes bent as shown in Fig. 2, the truss is called a crescent roof truss. The bracing in the crescent roof trusses is not of any particular form, being as a usual thing built of members which can take either tension or compression. This is made necessary by the fact that the curved upper chord may cause either tension or compression in the webbing, according to the angle of its inclination with the horizontal.
Roof trusses which do not come under either of the above classes may be regarded in a class of their own. To this class belong those trusses which are somewhat like a bridge truss in that the two chords are horizontal or nearly so. The ends of these trusses may be rectangular or not. For various types of this class of truss, see Fig. 3.
In addition to the above classification, which is based on the form of the chords, roof trusses may be divided according to the manner in which their members are connected. This classification is that of pin-connected and riveted. For a definition of this, and for figures showing such joints, see "Statics," pp. 22 and 23.
Fig. 1. Triangular Roof Trusses.
Fig. 2. Crescent Roof Trusses.
Fig. 3. Trusses with Chords Almost Parallel.
Trusses are seldom built as pin-connected unless they are of long span, since roof trusses are comparatively light, and pin-connected trusses, unless of considerable weight, do not give very great stiffness.
Riveted roof trusses are used for nearly all practical purposes, since they give great rigidity under the action of wind and of moving loads, such as cranes, which may be attached to them.
2. Physical Analysis of Roof Trusses. In pin-connected roof trusses, the tension members consist of I-bars or rods; and the compression members are made of channels or angles and plates, either plain or latticed. In riveted trusses, both tension and compression members are made up of angles and plates or a combination of the two. The top chords of roof trusses of medium span usually consist of two angles placed back to back. If the stress becomes too great to be taken up by two angles larger than 5 by 3½ inches, then two angles and a plate are used (see Fig. 4). In case the roof truss is of great size and the stresses are exceedingly large, the chord member may be built up in a manner somewhat similar to a bridge truss, being constructed of two channels and a plate, or four angles and three plates. Figs. 5 and 6 show cross-sections of chords for long-span riveted trusses. These cross-sections may also be used for pin-connected trusses.
Fig. 4. Chord Section for Heavy Stresses.
Fig. 6. Chord Sections for Trusses of Long Span.
The web members of a truss usually consist of one angle; and if this is insufficient, two angles back to back are used. Fig. 80, page 65, gives a diagram of a roof, and shows not only the roof trusses but also various other parts which will be referred to in the succeeding articles.
3. Wind Pressure and Snow Loads. The wind pressure on a flat surface varies, of course, with the velocity of the wind, and is very closely given by the formula:
P = 0.004V2
Velocity (Miles per hour)
Pressure (Lbs. per square foot)
The pressures indicated in Table I are perpendicular to the direction of the wind. When the wind blows on an inclined surface, the wind is assumed to be acting horizontally, and the normal component on the inclined surface is determined. This component is not equal to the horizontal pressure times the sine of the angle of inclination, as one would suppose (see Fig. 7), but is greater by a small amount. Roofs are usually figured on a basis of 40 pounds pressure on a vertical surface. The value of the normal component for a horizontal wind pressure of 40 pounds per square foot, is given on page 24 of "Statics," and is here, for convenience, reduced to the normal pressure for any given pitch.