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.
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 monitor girder to steel truss. The floor in this case rested directly on the top chord, which therefore brought bending strains as well as direct compression; 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
Fig. 271 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 laterally except at the center where a steel strut is provided.
LA SALLE STATION, L. S. & M. S. AND C, R. I. & P. RAILROADS, CHICAGO.
Frost & Granger, Architects; E. C. & R. M. Shankland, Engineers Steel trusses over train shed; span of truss, 215 feet. Note the traveling crane, with three derricks on it, used in setting these trusses
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 equations 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 rotation 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 forces must be zero.
Both these latter conditions are evidently essential for the equilibrium of the structure.
In a truss loaded solely with vertical forces, the first two conditions are the only ones which would be used. If the truss is acted on by a wind load which has a vertical and horizontal component, then the third condition needs to be considered.
In the strain sheet given in Fig. 272, the first thing to determine 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 the reaction of the portion of the truss on one side of the section AB, acting upon the portion on the other side along the lines of the different members, which holds the truss in equilibrium. If therefore the portion of the truss to the right of AB is considered as taken away, and if, along the lines of the top and bottom chords and the diagonal, forces are applied of the same intensity as the forces which resulted from the reaction of the portion on the right and which held the truss in equilibrium, then these forces can for the time being be considered as external forces, and the intensity of them will be such as to fulfill the three conditions of equilibrium as regards the external forces. This condition is indicated in Fig. 273. It will be seen that these forces acting along the lines of the members of the truss cut by the section are the actual stress in these members necessary to maintain the truss in equilibrium. The stresses produced in the members of a structure by the action of the loads, are called the "internal" or "inner" forces, in distinction from the "external" forces or "loads."
Any section, such as AB, cutting three members, gives three stresses to be determined. The top and bottom chord stresses are determined by using the condition that the algebraic sum of the moments about any point is zero. For the top chord, the point chosen is the intersection of the bottom chord and the diagonal. The moment of the stress in these two members about this point, is therefore zero, and this leaves only the moment of the top chord stress, which must then be equal to the moment of the loads about this point.