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.
Plate IV Work-Shop Drawing Showing Detail of Top and Bottom Lateral Bracing of a Roof Truss.
21'-10 1/2"+2 (1 1/2")-l'-1 1/2"=21'-0"
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 Pls. 8, 9, and 10 in Plate III.
At point U4 as many shop rivets are put in as there are 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 L0 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. 68, in which case the number of rivets L0 L2 may be calculated from the equation: n2v-Rn = 6Re/p in which n = number of rivets required; v= allowable stress on one rivet; R = the vertical reaction; p=the rivet spacing in inches; and c= distance shown in Fig. G8.
The number of rivets in L0 U1 may be determined from the equation: n2v-Sn = 6Se1/p in which S is the stress in Lo U1, and ei the distance shown in Fig. 68. 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.
Fig. 6S. Typical Detail for the End of a Roof Truss.
Fig. 69. Method of Riveting Clip Angles for Carrying Purlins.
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.
Also by this method the purlin may be put in place and used as support for erection apparatus. In Plate III, the additional pair of holes at panel points of the top chord are for these clip angles.
After the draftsman has finished his drawing he should carefully check up all dimensions and bevels and inspect the drawing for errors in rivet clearances. The passing in of accurate detail drawings will soon result in a promotion to checker, a more pleasant position, but one with greater responsibilities attached.
Detailing of Plate Girder Spans. The information which the draftsman has to start with is in the form of the stress sheet. This may be as Plate V which is the latest and most approved form, or it will be like Plate VI. In both cases the number of rivets for the lateral connections are given, but on Plate V the rivet curve for the spacing in the flanges is given and also the curve of the total and dead load shears and moments.
As soon as a plate-girder stress sheet is turned over to the draftsman, he should lay it out at once and determine the exact location of the web splices, the stiffeners, and the cover plates and their lengths (if not given), should decide upon the lengths of the panels of the lateral bracing, and should also make layouts of the lateral plates, if possible, so that the material can be ordered at once if necessary. In making the above layout the following should be observed:
(1) Be careful in locating splices to see that they come at a panel point of the lateral system.
(2) Locate all splices and stiffeners with a view of keeping the rivet spacing as regular as possible.
(3) Have the panels of the lateral systems equal if possible. If not, have a smaller one at the ends of the girder, the remainder being of equal length.
(6) It is always best to use as few sizes as possible for stiffeners, connection plates, etc., and avoid all unnecessary cutting of plates and angles.
(7) Locate the end holes for laterals and diagonals so that they can be sheared by a single operation, see Fig. 70. This will, as a rule, throw the end rivet further back from the working point, and may increase the size of the connection plate, but it is desirable.
(8) It is preferable to have an even number of panels in the lateral system since the girders can in most cases then be made symmetrical or nearly so about the center.