The exact size and the cuts of the gusset plates are generally left to the templet maker; they can be given, however, if it is desirable to do so, by adding the necessary measurements, which should be obtained from the full-sized layout of the joint.
Sometimes, in long trusses, it becomes necessary to draw the elevation of the truss as outlined above, and to supplement this by a larger-scale drawing of each joint, this larger drawing giving all the measurements of the connections as related to the panel point, and the smaller-scale elevation giving the general measurements.
Where it is not essential for appearance or for compactness of details to cut the angles on a bevel parallel to the abutting members, as is shown by some of the drawings, a square cut can be used and will somewha't simplify the shopwork.
Gussets should always be cut as closely as possible, both for neatness in appearance and for saving in weight.
In detailing, always show gussets, where possible, of such shape that they can be cut from a rectangular plate, using one or more of the sides of the original plate, and shearing off only where necessary for compactness of detail.
Compression members made of two angles should always be riveted together through a washer at intervals of two or three feet. In general, it is good practice to follow this for all members' tension as well as compression, as it stiffens the truss against strains in shipment and against possible loading not considered in calculations, and the extra cost is inconsiderable Illustrations of Shop Details. Fig. 268 shows a parallel chord truss carrying a floor, roof, and monitor load. Figs. 269, 270, and 271 show the connection of wood purlin under mon itor girder to steel truss. The floor in this case rested directly on the top chord, which therefore brought bending strains as well as direct com pression; for this reason the channel section was necessary. Note that for determining number of rivets in each member, one-half the stress would be considered, and the rivets taken at their single-shear value. Tie plates are used at intervals to stiffen the lower flanges of the channels forming the top chord.
Fig. 272 shows the strain sheet for another parallel-chord truss 74 feet long, center to center of bearings. This truss carries a roof load assumed as 40 pounds live and 25 pounds dead per square foot, and also carries in the bottom chord a ceiling load of 15 pounds per square foot.
The roof beams span from truss to wall, which is 26 feet. On account of the construction and the long span, the wood framing is not considered as bracing the truss, which is therefore unsupported later ally except at the center where a steel strut is provided.
The manner of working out the stresses of such trusses by the analytical method, will be given below.
In all statically determined structures, there are three equa tions which must be true in order that the structure shall remain in equilibrium: 1. The algebraic sum of the moments, about any point, of all the external forces acting on the structure, must be zero. If this is not the case, there will be a ro tation of the structure about this point.
2. The algebraic sum of all the external vertical forces must be zero.
3. The algebraic sum of all the external horizontal f or c es must be zero.
Both these latter conditions are evidently essential f o r the equilibrium of the structure.
In a truss loaded solely with vertical forces, the first two con ditions are the only ones which would be used. If the truss is acted on by a wind load which has a v e r t i c al and horizontal component, then the third con dition needs to be considered.
In the strain sheet given in Fig. 272, the first thing to deter mine is the panel load. The load at each top panel is 26.25 X 65 X 6.17 = 10,500; the bottom panel load is 26.25 X 15 X 6.17 = 2,400. Having determined these, and noted them as indicated on the diagram. the only other external force to determine is the reaction. As the truss is symmetrically loaded, the reactions are equal, and each equal to half the total load, or 77,400 pounds.
Suppose the top and bottom chords and the diagonal of this truss were to be cut through on the line AB, as shown in Fig. 272. It is evident that, if the truss were then loaded as shown by the diagram, the portions of the top chord on each side of this cut would push against each other, and the portions of the bottom chord on either side would tend to pull apart, and the portions of the diagonal on either side would tend to pull apart. Unless there were some way of transferring from one side to the other these forces tending to push together and tear apart, the truss would not stand. It is therefore Fig. 273.