Standards in Detailing Trusses. Figs. 265, 266, and 267 show details of various types of trusses. The same remarks made previous ly for girders apply to trusses wherever they are symmetrical about the center line.
Fig. 277 shows a strain sheet of the truss detailed in Fig. 266. This is the form in which the information is g:znerally given to the draftsman for detailing. At other times the information may be given only by the general drawings, in which case the loads and measure ments would have to be obtained from them.
It will be noted that the same general method of detailing and dimensioning is followed in all cases. The strain lines are laid out first; these lines should always intersect at the panel points; and the strain lines of the members over a point of support should intersect over the center of bearing. The strain lines should be theoretically the center of gravity lines of the members; it is more common practice, however, to use the pitch lines of the angles as the strain lines, as these lines do not vary materially from the center of gravity lines, and much confusion is thus saved. In heavy trusses, however, where the chords are made up of side plates and angles, the strain lines for the chords should be the center of gravity lines, as the difference between these lines and the pitch line of the angle would be considerable.
Many times the position of one or more panel points will be fixed by some features of construction such as a monitor or a hanger for shafting, or rod for balcony, etc., as illustrated by Figs. 267 and 2S0. Wherever such concentrated loads are fixed, there should be a panel point, if possible, as otherwise the chord must be materially increased to provide for the bending strains produced by the load acting between panel points. The panel points being fixed, and the strain lines drawn, the lines showing the size and shape of each member are drawn.
Completeness of Measurements. In dimensioning a detail the draftsman should bear in mind all the steps he has to take to fully lay out and fix all the members and connections, and should remember that information must be given to enable the templet maker to go through the same operations.
1. There should be measurements center to center of each panel point along each member. These are calculated, never scaled.
2. There should be a line of measurements along each member from panel point to panel point, fixing each rivet or hole with respect to this panel point.
3. There should be a measurement center to center of the end panel points along the top and bottom chords and the vertical or in clined end members.
4. There should be over-all measurements of the above mem bers.
5. There should be a measurement from the end of each piece to the first rivet or hole, and each piece should have its size and over all length specified.
6. Each sloping member should have its slope indicated by a triangle of which one side is 12 inches and the other side inches and sixteenths.
7. Each piece should preferably be given a shop mark, to facili tate assembling.
To fix the measurements noted under (2), it is often necessary to make a full-sized or large-scale layout drawn very accurately so as to be able to scale closely the distance from panel point to first rivet, and to be sure of plenty of clearance and yet have the members fit closely.
After the first hole is fixed, the others are spaced 2.4 or 3 inches apart for the gusset connections. The number of rivets is of course determined from the strain sheet and the value of the rivet; i-inch rivets are generally used, and gusset plates or Where strains are very heavy and it is desired to avoid larger gussets, thicker plates can be used.
The measurements noted under (5) will be fixed by the above full-sized layout. It should be carefully borne in mind that such a layout is worse than useless unless it is very accurate, and therefore care should be taken to insure accuracy.
Special Notes and Details. As regards the shop marks noted under (7), each shop has a different practice. A convenient form, however, is to call the top chord "T. C. 1," "T. C. 2," etc.; the bottom chord "L. C. 1," "L. C. 2," etc.; the verticals "V 1," "V 2," etc.; the diagonals "D 1," "D 2," etc.