2X 14 X X 7.3S= 64.6 2X 1.93X 1.44 = 5.5 14X X .19 = 1.0 71.1 Fig. 276.
÷ 17.86 = 4.00 = Distance of center of gravity from bottom of plate.
MomENT OF INERTIA OF BOTTOM CHORD.
As the bottom chord is subject only to tension, it is not necessary to calculate the radius of gyration or moment of inertia about axis c d.
Diagonals are designed by using 15,000 lbs. tension, and choosing angles whose net section, taking one rivet hole out, will be sufficient for the stress in the member.
Verticals are designed by assuming an allowable fiber stress based on the reduction of 12,000 lbs. for ratio of length to radius of gyration. After the section is determined, using this assumed fiber stress, it is necessary to see that this fiber stress is within the actual allowable stress for the radius of gyration of the member.
Where two angles are used, spread the thickness of gusset plate, the least radius is employed, either parallel with the outstanding legs or through the axis of the gusset. Where side plates are used, as in this case, the radius employed should be that parallel to the outstand ing legs. These angles being spread and either laced or tied with plates, are weakest in the direction of the axis of the truss. The student should follow through the different sizes given for verticals and diagonals, fully understanding the above explanations.
Fig. 27S shows a detail of the connections at one top chord panel point; and Fig. 279, of one bottom chord panel point. It should be noted that the rivets are in single shear, and that the side plates are deep enough to allow connections to be made without the use of gus sets.
In Fig. 267, a detail is shown of a connection suitable for a rod hanging, a balcony, or other member to the truss. Note that the cen ter of rod comes at the intersection of the strain lines at the panel point. This should always be the case unless the chord is made speci ally strong to resist the bending due to a connection between panel points. Note also that the connection is applied directly to the gusset plate by a pin through the clevis nut. This brings only shearing and
bearing strains on the connection, and avoids any direct pull on the heads of rivets or of bolts, which should be divided wherever possible in such cases.
The open holes in top chord are for securing the roof purlins to the truss. These purlins run directly across the top chord.
Fig. 280 shows the detail of a truss for a boiler-house roof. This roof has a high monitor running clown the center, which is also framed with steel; the detail of this frame is shown in Fig. 2S4.
Fig. 2S4 shows a general view of the truss and monitor frame in position, and the roof beams framing to them. This truss was short enough to be riveted up at the shop and shipped whole. The monitor frame, however, was shipped separate from the truss, as indicated by the open holes for connection to truss. As this monitor frame, if shipped whole, would be likely to become bent and distorted, it had to be shipped in two parts, as indicated by the details.
Figs. 281 and 282 show the top and bottom chord splices in the center panel of the truss shown in Fig. 272. Note that the point in top chord is specified to be planed, and therefore the rivets provided are sufficient for only a portion of the stress, the balance being trans ferred by direct compression on the planed surfaces.
Determine all the stresses and suitable sizes to use for a truss loaded as shown in Fig. 2S3, and resting on a brick wall at each end. The load consists of floor joists resting directly on the top chord; and a 6 x 4 x i-inch angle should be provided near every other panel point, punched for lag screws to secure to wood joists for forming a lateral support to truss.
_Bake a complete shop detail of the above truss.
Trussed Stringers. Figs. 2S5 and 2S6 show the two common forms of trussed wooden stringers. These consist of a wooden beam, composed of one or more timbers, stiffened by one or two struts bearing on steel rods, as shown. They are used in timber-framed structures where it is impracticable to obtain timbers sufficiently strong to sup port the loads.