After the center lines of the chords are drawn in, the angles themselves should be drawn on by laying of the gauge lines on one side and then the other'edge of the leg on the other side of the gauge line. After this the top chord should be divided into a certain num ber of equal parts at each of which a purlin is to be placed. This done, lines from these points should be drawn perpendicular to the top chord and their points of intersection with the bottom chord should be noted. From the intersection of the center one with the bottom chord to the apex or top, a line is now drawn, and this is the center line of the main interior tie, or tension member. The member itself should now be drawn on this gauge line. After this the other members should be drawn in as shown.
In order to proceed, the distances between the various points of intersection must be carefully computed, thus giving the ing data necessary to compute the bevels, which should now be done. In order to determine the length of the members and the sizes of the plates, it is now necessary to take each point of intersection where any members meet at any other than a right angle and make a layout of that joint to some large scale, say 1 to 2 inches to the foot. The customary 1-inch clearance should be allowed where there is any liability of pieces touching and, after the ends of the various angles are drawn in, the first rivet is set back IL I or inches as the sizes of the angle and of the rivet allow, and the other spacing is so arranged as to make the size and shape of the plate advantageous and economical. The distance from the first rivet to the intersection is measured id and noted. After the layout for each joint has been made and the necessary dimensions of the plates and the distance from each intersection to the first rivet has been determined, the length of each member may be computed. This is equal to the length, intersection to intersection, plus the sum of the distances from the first rivet at the ends to the end of the member, minus the sum of the distances from the first rivet to the nearest intersection. For example, in the main interior tie U4 L2, Plate I, the length, intersection to intersection, is 21'-101", the distance from each first rivet to the end of the angle is 1,4 inches, and the sum of the distances from each first rivet to the nearest intersection is (-J,+9) =13;',- inches, which is l'-11". The length of the member is: At the point L2, Plate I, a field connection must be made as well as at U4 on account of the fact that the truss must be shipped in part in case the span is larger than 30 feet, the length of an ordinary gondola freight car. At L2 both legs of the angle should be connected,
the horizontal leg connection being by a plate. In case of riveted lateral bracing such as is used here, the connection plate may also be used as a splice plate, see l'Is. S, 9, and 10 in Plate III.
At point U4 as many shop rivets are put in as there arc field rivets required. This will keep the plate symmetrical, and will allow the same templets to be used for the top chord and main interior tie on both sides of the truss. This more than overbalances the cost of driving the few additional shop rivets.
At Lo in this case the truss has been designed so that the rivets are symmetrical about the point of intersection and, therefore, only a sufficient number are required to take up the direct stress in the top and bottom chords. In many cases the end of a roof truss is as shown in Fig. OS, in which case the number of rivets Lo L2 may he calculated from the equation: — lin = in which n= number of rivets required ; v= allowable stress on one rivet; R= the vertical reaction; p= the rivet spacing in inches; and e= distance shown in Fig. GS.
The number of rivets in Lo U1 may be determined from the equation: in which S is the stress in Lo Lro and el the distance shown in Fig. GS. These formulas allow- for the stress due to eccentricity. The rivet spacing p is usually taken as 3 inches, although it may be taken as any value permissible by the specifications.
In the detailing of the lateral systems, Plate III, the same method of procedure as above mentioned should be followed. Care should be exercised in making the layouts for the lateral plates so that sufficient clearances are allowed, both in regards to clearances between members and clearances in rivet driving.
The purlins, or rafters, may be detailed directly upon the main sheet with the bracing or truss, or upon a beam sheet, preferably the latter. In Plate III they are upon the lateral sheet. These purlins should be riveted, not bolted to the chords of the trusses. In order to facilitate erection, clip angles should be riveted to the top chord as shown in Fig. 69 so that the purlin may be put in place and riveted up without having to hold it in place with ropes or chains.